db4o tutorial

Welcome

db4o is the native Java, .NET and Mono open source object database.

This tutorial was written to get you started with db4o as quickly as possible. Before you start, please

make sure that you have downloaded the latest db4o distribution from the db4objects website.

developer.db4o.com

You are invited to join the db4o community in the public db4o forums to ask for help at any time.

Please also try out the keyword search functionality on the db4o knowledgebase.

Links

In order to access free developer resources you may want to register on db4o developer website.

Feel free to check the membership benefits online.

And here are some further links on developer.db4o.com that you may find useful:

All Downloads

Release Note Blog

SVN Access

Design Wiki

Community Projects

www.db4o.com

Download Contents

The db4o Java distribution comes as one zip file, db4o-7.10-java.zip. When you unzip this file, you get

the following directory structure:

Please take a look at all the supplied documentation formats to choose the one that works best for

www.db4o.com

you:

.

db4o-7.10/doc/api/index.html

The API documentation for db4o is supplied as JavaDocs HTML files. While you read through this

tutorial it may be helpful to look into the API documentation occasionally.

db4o-7.10/doc/reference/index.html

The reference documentation is a complete compilation for experienced db4o users. It is maintained

online.

db4o-7.10/doc/tutorial/index.html

This is the interactive HTML tutorial. Examples can be run "live" against a db4o database from within

the browser. In order to use the interactive functionality a Java JRE 1.3 or above needs to be installed

and integrated into the browser. Java security settings have to allow applets to be run. If you have

problems using the interactive functionality, please refer to Tutorial Troubleshooting.

db4o-7.10/doc/tutorial/db4o-7.10-tutorial.pdf

The PDF version of the tutorial allows best fulltext search capabilities.

www.db4o.com

.

1. First Glance

Before diving straight into the first source code samples let's get you familiar with some basics.

1.1. The db4o engine

The db4o object database engine consists of one single core jar file. In addition you may want to use

client/server library or optional components. You may also get it all in a single jar "db4o-all":

db4o-7.10-core-java1.1.jar

will run with most Java JDKs that supply JDK 1.1.x functionality such as reflection and Exception

handling. That includes many IBM J9 configurations, Symbian and Savaje.

db4o-7.10-core-java1.2.jar

is built for all Java JDKs between 1.2 and 1.4.

db4o-7.10-core-java5.jar

is built for Java JDK 5 and JDK 6

If you intend to use client/server version of db4o you will additionally need client/server library

matching your JDK version:

db4o-7.10-cs-java1.1.jar

db4o-7.10-cs-java1.2.jar

db4o-7.10-cs-java5.jar

Some advanced functionality such as cluster support, platform-specific IO adapters,

statistic tools etc can be added by including db4o optional library:

db4o-7.10-optional-java1.1.jar

db4o-7.10-optional-java1.2.jar

db4o-7.10-optional-java5.jar

You can also get all of the above in a single jar:

db4o-7.10-all-java1.1.jar

db4o-7.10-all-java1.2.jar

db4o-7.10-all-java5.jar

www.db4o.com

1.2. Installation

If you add one of the above db4o-*.jar files to your CLASSPATH db4o is installed. For beginners it is

recommended to use "db4o-all" library to avoid confusion with the location of certain classes. In case

you work with an integrated development environment like Eclipse you would copy the db4o-*.jar to

the /lib/ folder under your project and add db4o to your project as a library.

1.3. Object Manager Enterprise installation

Object Manager Enterprise (OME) is an object browser for db4o databases. OME installation can be

found in /ome folder of the distribution. The zip file in this folder contains the Eclipse plugin version of

OME.

To install the plugin, you need to have a version of Eclipse >= 3.3 installed. Unzip the file to a folder of

your choice. Then open Eclipse, select 'Help' -> 'Software Updates...' -> 'Available Software' from the

menu. Choose 'Add Site...' -> 'Local...' and select the unzipped folder. Follow the Eclipse Update

Manager instructions for the OME feature from here on.

The actual menu structure may vary over Eclipse versions. (The above applies to Eclipse 3.4

Ganymede.) When in doubt, please refer to the Eclipse documentation on Software Updates.

Alternatively, you can install the plugin manually by simply copying the contents of the 'plugins' and

'features' folders from the unzipped folder to the corresponding subfolders in the root folder of your

Eclipse installation.

www.db4o.com

1.4. API Overview

Do not forget the API documentation while reading through this tutorial. It provides an organized view

of the API, looking from a java package perspective and you may find related functionality to the

theme you are currently reading up on.

For starters, the java packages com.db4o and com.db4o.query are all that you need to worry about.

com.db4o

The com.db4o java package contains almost all of the functionality you will commonly need when using

db4o. Two objects of note are com.db4o.Db4oEmbedded, and the com.db4o.ObjectContainer interface.

The com.db4o.Db4o factory is your starting point. Static methods in this class allow you to open a

database file and create an initial configuration. For client/server environment you will need to use

com.db4o.cs.Db4oClientServer factory class to start a server, or connect to an existing server, but this

will be discussed later , start a server, or connect to an existing server. The most important interface,

and the one that you will be using 99% of the time is

com.db4o.ObjectContainer: This is your db4o database.

- An ObjectContainer can either be a database in single-user mode or a client connection to a db4o

server.

- Every ObjectContainer owns one transaction. All work is transactional. When you open an

ObjectContainer, you are in a transaction, when you commit() or rollback(), the next transaction is

started immediately.

- Every ObjectContainer maintains it's own references to stored and instantiated objects. In doing so, it

manages object identities, and is able to achieve a high level of performance.

- ObjectContainers are intended to be kept open as long as you work against them. When you close an

ObjectContainer, all database references to objects in RAM will be discarded.

com.db4o.ext

In case you wonder why you only see very few methods in an ObjectContainer, here is why: The db4o

interface is supplied in two steps in two java packages, com.db4o and com.db4o.ext for the following

reasons:

- It's easier to get started, because the important methods are emphasized.

- It will be easier for other products to copy the basic db4o interface.

- It is an example of how a lightweight version of db4o could look.

Every com.db4o.ObjectContainer object is also an com.db4o.ext.ExtObjectContainer. You can cast it to

ExtObjectContainer or you can use the method to get to the advanced features.

com.db4o.config

www.db4o.com

The com.db4o.config java package contains types and classes necessary to configure db4o. The objects

and interfaces within are discussed in the Configuration section.

com.db4o.query

The com.db4o.query java package contains the Predicate class to construct Native Queries. The Native

Query interface is the primary db4o querying interface and should be preferred over the Soda Query

API.

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2. First Steps

Let's get started as simple as possible. We are going to demonstrate how to store, retrieve, update and

delete instances of a single class that only contains primitive and String members. In our example this

will be a Formula One (F1) pilot whose attributes are his name and the F1 points he has already gained

this season.

First we create a class to hold our data. It looks like this:

package com.db4odoc.f1.chapter1;

public class Pilot {

private String name;

private int points;

public Pilot(String name,int points) {

this.name=name;

this.points=points;

}

public int getPoints() {

return points;

}

public void addPoints(int points) {

this.points+=points;

}

public String getName() {

return name;

}

public String toString() {

return name+"/"+points;

}

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Notice that this class does not contain any db4o-related code.

2.1. Opening the database

To access a db4o database file or create a new one, call Db4oEmbedded.openFile() and provide

Db4oEmbedded.newConfiguration() as a configuration template and the path to your database file as

the second parameter, to obtain an ObjectContainer instance. ObjectContainer represents "The

Database", and will be your primary interface to db4o. Closing the ObjectContainer with the #close()

method will close the database file and release all resources associated with it.

// accessDb4o

ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

try {

// do something with db4o

} finally {

db.close();

}

DB4OFILENAME is just a string value representing any filename. If the file with this name already

exists, it will be opened as db4o database, otherwise a new db4o database will be created.

For the following examples we will assume that our environment takes care of opening and closing the

ObjectContainer automagically, and stores the reference in a variable named 'db'.

2.2. Storing objects

To store an object, we simply call #store() on our database, passing any object as a parameter.

// storeFirstPilot

Pilot pilot1 = new Pilot("Michael Schumacher", 100);

db.store(pilot1);

System.out.println("Stored " + pilot1);

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OUTPUT:

Stored Michael Schumacher/100

We'll need a second pilot, too.

// storeSecondPilot

Pilot pilot2 = new Pilot("Rubens Barrichello", 99);

db.store(pilot2);

System.out.println("Stored " + pilot2);

OUTPUT:

Stored Rubens Barrichello/99

2.3. Retrieving objects

The easiest way to see the content of our database is to use Object Manager Enterprise, which will be

introduced in the next chapter . For now let's continue with the API overview and learn how to build

db4o queries.

db4o supplies several different querying systems,

Query by Example

(QBE), ,

Native Queries

(NQ) and

the

SODA Query API

(SODA). In this first example we will introduce QBE. Once you are familiar with

storing objects, we encourage you to use Native Queries .

When using Query-By-Example, you create a prototypical object for db4o to use as an example of what

you wish to retrieve. db4o will retrieve all objects of the given type that contain the same (non-

default) field values as the example. The results will be returned as an ObjectSet instance. We will use

a convenience method #listResult() to display the contents of our result ObjectSet :

public static void listResult(ObjectSet result) {

System.out.println(result.size());

while(result.hasNext()) {

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System.out.println(result.next());

}

To retrieve all pilots from our database, we provide an 'empty' prototype:

// retrieveAllPilotQBE

Pilot proto = new Pilot(null, 0);

ObjectSet result = db.queryByExample(proto);

listResult(result);

OUTPUT:

2

Rubens Barrichello/99

Michael Schumacher/100

Note that we specify 0 points, but our results were not constrained to only those Pilots with 0 points; 0

is the default value for int fields.

db4o also supplies a shortcut to retrieve all instances of a class:

// retrieveAllPilots

ObjectSet result = db.queryByExample(Pilot.class);

listResult(result);

OUTPUT:

2

Rubens Barrichello/99

Michael Schumacher/100

www.db4o.com

For JDK 5 there also is a generics shortcut, using the query method:

List <Pilot> pilots = db.query(Pilot.class);

To query for a pilot by name:

// retrievePilotByName

Pilot proto = new Pilot("Michael Schumacher", 0);

ObjectSet result = db.queryByExample(proto);

listResult(result);

OUTPUT:

1

Michael Schumacher/100

And to query for Pilots with a specific number of points:

// retrievePilotByExactPoints

Pilot proto = new Pilot(null, 100);

ObjectSet result = db.queryByExample(proto);

listResult(result);

OUTPUT:

1

Michael Schumacher/100

www.db4o.com

Of course there's much more to db4o queries. They will be covered in more depth in later chapters.

2.4. Updating objects

Updating objects is just as easy as storing them. In fact, you use the same #store() method to update

your objects: just call #store() again after modifying any object.

// updatePilot

ObjectSet result = db

.queryByExample(new Pilot("Michael Schumacher", 0));

Pilot found = (Pilot) result.next();

found.addPoints(11);

db.store(found);

System.out.println("Added 11 points for " + found);

retrieveAllPilots(db);

OUTPUT:

Added 11 points for Michael Schumacher/111

2

Rubens Barrichello/99

Michael Schumacher/111

Notice that we query for the object first. This is an importaint point. When you call #store() to modify

a stored object, if the object is not 'known' (having been previously stored or retrieved during the

current session), db4o will insert a new object. db4o does this because it does not automatically match

up objects to be stored, with objects previously stored. It assumes you are inserting a second object

which happens to have the same field values.

To make sure you've updated the pilot, please return to any of the retrieval examples above and run

them again.

2.5. Deleting objects

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Objects are removed from the database using the #delete() method.

// deleteFirstPilotByName

ObjectSet result = db

.queryByExample(new Pilot("Michael Schumacher", 0));

Pilot found = (Pilot) result.next();

db.delete(found);

System.out.println("Deleted " + found);

retrieveAllPilots(db);

OUTPUT:

Deleted Michael Schumacher/111

1

Rubens Barrichello/99

Let's delete the other one, too.

// deleteSecondPilotByName

ObjectSet result = db

.queryByExample(new Pilot("Rubens Barrichello", 0));

Pilot found = (Pilot) result.next();

db.delete(found);

System.out.println("Deleted " + found);

retrieveAllPilots(db);

OUTPUT:

Deleted Rubens Barrichello/99

0

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Please check the deletion with the retrieval examples above.

As with updating objects, the object to be deleted has to be 'known' to db4o. It is not sufficient to

provide a prototype object with the same field values.

2.6. Conclusion

That was easy, wasn't it? We have stored, retrieved, updated and deleted objects with a few lines of

code. Now you are probably interested to see how the database looks like. Let's have a look using db4o

graphical tool - Object Manager in the next chapter .

2.7. Full source

package com.db4odoc.f1.chapter1;

import java.io.*;

import com.db4o.*;

import com.db4odoc.f1.*;

public class FirstStepsExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home")

+ "/formula1.db4o";

public static void main(String[] args) {

new File(DB4OFILENAME).delete();

accessDb4o();

new File(DB4OFILENAME).delete();

ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

try {

storeFirstPilot(db);

storeSecondPilot(db);

retrieveAllPilots(db);

retrievePilotByName(db);

retrievePilotByExactPoints(db);

updatePilot(db);

deleteFirstPilotByName(db);

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deleteSecondPilotByName(db);

} finally {

db.close();

}

public static void accessDb4o() {

ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

try {

// do something with db4o

} finally {

db.close();

}

public static void storeFirstPilot(ObjectContainer db) {

Pilot pilot1 = new Pilot("Michael Schumacher", 100);

db.store(pilot1);

System.out.println("Stored " + pilot1);

}

public static void storeSecondPilot(ObjectContainer db) {

Pilot pilot2 = new Pilot("Rubens Barrichello", 99);

db.store(pilot2);

System.out.println("Stored " + pilot2);

}

public static void retrieveAllPilotQBE(ObjectContainer db) {

Pilot proto = new Pilot(null, 0);

ObjectSet result = db.queryByExample(proto);

listResult(result);

}

public static void retrieveAllPilots(ObjectContainer db) {

ObjectSet result = db.queryByExample(Pilot.class);

listResult(result);

}

public static void retrievePilotByName(ObjectContainer db) {

Pilot proto = new Pilot("Michael Schumacher", 0);

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ObjectSet result = db.queryByExample(proto);

listResult(result);

}

public static void retrievePilotByExactPoints(ObjectContainer db)

{

Pilot proto = new Pilot(null, 100);

ObjectSet result = db.queryByExample(proto);

listResult(result);

}

public static void updatePilot(ObjectContainer db) {

ObjectSet result = db

.queryByExample(new Pilot("Michael Schumacher", 0));

Pilot found = (Pilot) result.next();

found.addPoints(11);

db.store(found);

System.out.println("Added 11 points for " + found);

retrieveAllPilots(db);

}

public static void deleteFirstPilotByName(ObjectContainer db) {

ObjectSet result = db

.queryByExample(new Pilot("Michael Schumacher", 0));

Pilot found = (Pilot) result.next();

db.delete(found);

System.out.println("Deleted " + found);

retrieveAllPilots(db);

}

public static void deleteSecondPilotByName(ObjectContainer db) {

ObjectSet result = db

.queryByExample(new Pilot("Rubens Barrichello", 0));

Pilot found = (Pilot) result.next();

db.delete(found);

System.out.println("Deleted " + found);

retrieveAllPilots(db);

}

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.

3. Object Manager Enterprise Overview

If you did not install the Object Manager Enterprise yet, please, return to FirstGlance chapter and

follow the installation instructions there.

Once the Object Manager Enterprise (OME) is installed you can see it in Eclipse by selecting Window-

>Open Perspective->Other and choosing "OME". Typically, OME window should look similar to this:

www.db4o.com

In the OME perspective you can see:

- OME menu:

- OME toolbar buttons to access the frequently used functionality fast

- Db4o Browser: window displaying the contents of the open db4o database

- Property Viewer: window displaying the properties of the open database or the properties of the

selected database class

- Build Query: windows allowing to build a query using drag&drop functionality

- Query Results: window to browse the results of the query execution

3.1. Browsing the database

In the previous chapter we've learned how to create a new database and store objects in it. Let's

create a new database and store some objects in it now:

// storePilots

ObjectContainer db=Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

try {

Pilot pilot1=new Pilot("Michael Schumacher",100);

db.store(pilot1);

System.out.println("Stored "+pilot1);

Pilot pilot2=new Pilot("Rubens Barrichello",99);

db.store(pilot2);

System.out.println("Stored "+pilot2);

}

finally {

db.close();

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}

OUTPUT:

Stored Michael Schumacher/100

Stored Rubens Barrichello/99

Now we can see the resultant database in the Object Manager. Please select OME-

>Connect/Disconnect DB (or use a shortcut button from the toolbar menu) and browse to ome.db4o

file in your user folder. (normally it is c:/Documents and Settings/[user name] on Windows XP/2000,

c:/Users/[user name] on Windows Vista and /home/[user_name] on *nix systems).

Once you've connected you will see a screen similar to this:

The db4o Browser window shows that there is 1 class in the database (Pilot), which contains 2 fields:

name and points. In the Property Viewer you can see more information about the class fields. You can

also change "Is indexed" field and add the index to the database by pressing "Save Index" button.

www.db4o.com

The filter panel on the top of the view allows easier navigation through the database with lots of

different classes. You can use wildcard searches and benefit from the search history to make the

selection faster.

3.2. Querying

It is easy to retrieve all of the Pilot instances from the database: just right-click the Pilot class in db4o

Browser and select "View All Objects". The list of the Pilot objects will be shown in the Query Result

view:

You can see object details in the detailed view below. Try to change any values and use Save button to

persist the changes to the database. You can also use Delete button to delete objects from the

database. For the objects containing field objects you will be prompted to use cascade on delete.

www.db4o.com

More complex queries can be done using Build Query view:

Drag "points" field from the db4oBrowser view into the Build Query view, set condition "Greater Than",

put a value "99" and run the query. You can return to the Built Query tab and modify the query later

on again. For example: add "AND" operator, drag "name" field and set the value to "Michael

Schumacher". Re-run the query.

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When the new query is created, the previous query is stored and can be selected from the history

drop-down:

More sophisticated queries can be build by joining grouped constraints using "Add Group" button.

When you are done working with the database in OME, you can close the connection by using OME-

>Connect/Disconnect DB menu command or by using the equivalent button on the toolbar.

In the next chapter we will explore more about db4o querying capabilities. You can continue using

OME while going through the rest of this tutorial. If you are using the interactive version you will find

the database in formula1.db4o file in your user folder. (normally it is c:/Documents and Settings/[user

name] on Windows XP/2000, c:/Users/[user name] on Windows Vista and /home/[user_name] on *nix

systems). Please, remember to disconnect OME before running any of the tutorial examples, as only

one active connection is allowed to the database file.

www.db4o.com

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4. Querying

db4o supplies three querying systems, Query-By-Example (QBE) Native Queries (NQ), and the SODA

API. In the previous chapter, you were briefly introduced to

Query By Example

(QBE).

Query-By-Example (QBE) is appropriate as a quick start for users who are still acclimating to storing

and retrieving objects with db4o.

Native Queries (NQ) are the main db4o query interface, recommended for general use.

SODA is the underlying internal API. It is provided for backward compatibility and it can be useful for

dynamic generation of queries, where NQ are too strongly typed.

www.db4o.com

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4.1. Query by Example (QBE)

When using

Query By Example

(QBE) you provide db4o with a template object. db4o will return all of

the objects which match all non-default field values. This is done via reflecting all of the fields and

building a query expression where all non-default-value fields are combined with AND expressions.

Here's an example from the previous chapter:

// retrievePilotByName

Pilot proto = new Pilot("Michael Schumacher", 0);

ObjectSet result = db.queryByExample(proto);

listResult(result);

Querying this way has some obvious limitations:

- db4o must reflect all members of your example object.

- You cannot perform advanced query expressions. (AND, OR, NOT, etc.)

- You cannot constrain on values like 0 (integers), "" (empty strings), or nulls (reference types)

because they would be interpreted as unconstrained.

- You need to be able to create objects without initialized fields. That means you can not initialize fields

where they are declared. You can not enforce contracts that objects of a class are only allowed in a

well-defined initialized state.

- You need a constructor to create objects without initialized fields.

To get around all of these constraints, db4o provides the Native Query (NQ) system.

www.db4o.com

.

4.2. Native Queries

Wouldn't it be nice to pose queries in the programming language that you are using? Wouldn't it be

nice if all your query code was 100% typesafe, 100% compile-time checked and 100% refactorable?

Wouldn't it be nice if the full power of object-orientation could be used by calling methods from within

queries? Enter Native Queries.

Native queries are the main db4o query interface and they are the recommended way to query

databases from your application. Because native queries simply use the semantics of your

programming language, they are perfectly standardized and a safe choice for the future.

Native Queries are available for all platforms supported by db4o.

4.2.1. Concept

The concept of native queries is taken from the following two papers:

- Cook/Rosenberger, Native Queries for Persistent Objects, A Design White Paper

- Cook/Rai, Safe Query Objects: Statically Typed Objects as Remotely Executable Queries

4.2.2. Principle

Native Queries provide the ability to run one or more lines of code against all instances of a class.

Native query expressions should return true to mark specific instances as part of the result set. db4o

will attempt to optimize native query expressions and run them against indexes and without

instantiating actual objects, where this is possible.

4.2.3. Simple Example

Let's look at how a simple native query will look like in some of the programming languages and

dialects that db4o supports:

C# .NET (using generics)

IList <Pilot> pilots = db.Query <Pilot> (delegate(Pilot pilot) {

return pilot.Points == 100;

});

Java JDK 5

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List <Pilot> pilots = db.query(new Predicate<Pilot>() {

public boolean match(Pilot pilot) {

return pilot.getPoints() == 100;

}

});

Java JDK 1.2 to 1.4

List pilots = db.query(new Predicate() {

public boolean match(Pilot pilot) {

return pilot.getPoints() == 100;

}

});

Java JDK 1.1

ObjectSet pilots = db.query(new PilotHundredPoints());

public static class PilotHundredPoints extends Predicate {

public boolean match(Pilot pilot) {

return pilot.getPoints() == 100;

}

C# .NET (not using generics)

IList pilots = db.Query(new PilotHundredPoints());

public class PilotHundredPoints : Predicate {

public boolean Match(Pilot pilot) {

return pilot.Points == 100;

}

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}

VB .NET (not using generics)

Dim pilots As IList = db.Query(new PilotHundredPoints())

Public Class PilotHundredPoints

Inherits Predicate

Public Function Match (pilot As Pilot) as Boolean

If pilot.Points = 100 Then

Return True

Else

Return False

End Function

End Class

A side note on the above syntax:

For all dialects without support for generics, Native Queries work by convention. A class that extends

the com.db4o.Predicate class is expected to have a boolean #match() method with one parameter to

describe the class extent:

boolean match(Pilot candidate);

When using native queries, don't forget that modern integrated development environments (IDEs) can

do all the typing work around the native query expression for you, if you use templates and auto-

completion.

Here is how to configure a Native Query template with Eclipse 3.1:

From the menu, choose Window + Preferences + Java + Editor + Templates + New

As the name type "nq". Make sure that "java" is selected as the context on the right. Paste the

following into the pattern field:

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List <${extent}> list = db.query(new Predicate <${extent}> () {

public boolean match(${extent} candidate){

return true;

}

});

Now you can create a native query with three keys: n + q + Control-Space.

Similar features are available in most modern IDEs.

4.2.4. Advanced Example

For complex queries, the native syntax is very precise and quick to write. Let's compare to a SODA

query that finds all pilots with a given name or a score within a given range:

// storePilots

db.store(new Pilot("Michael Schumacher",100));

db.store(new Pilot("Rubens Barrichello",99));

// retrieveComplexSODA

Query query=db.query();

query.constrain(Pilot.class);

Query pointQuery=query.descend("points");

query.descend("name").constrain("Rubens Barrichello")

.or(pointQuery.constrain(new Integer(99)).greater()

.and(pointQuery.constrain(new Integer(199)).smaller()));

ObjectSet result=query.execute();

listResult(result);

OUTPUT:

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2

Michael Schumacher/100

Rubens Barrichello/99

Here is how the same query will look like with native query syntax, fully accessible to autocompletion,

refactoring and other IDE features, fully checked at compile time:

C# .NET 2.0

IList <Pilot> result = db.Query<Pilot> (delegate(Pilot pilot) {

return pilot.Points > 99

&& pilot.Points < 199

|| pilot.Name == "Rubens Barrichello";

});

Java JDK 5

List <Pilot> result = db.query(new Predicate<Pilot>() {

public boolean match(Pilot pilot) {

return pilot.getPoints() > 99

&& pilot.getPoints() < 199

|| pilot.getName().equals("Rubens Barrichello");

}

});

4.2.5. Arbitrary Code

Basically that's all there is to know about native queries to be able to use them efficiently. In principle

you can run arbitrary code as native queries, you just have to be very careful with side effects -

especially those that might affect persistent objects.

Let's run an example that involves some more of the language features available.

// retrieveArbitraryCodeNQ

www.db4o.com

final int[] points={1,100};

List<Pilot> result=db.query(new Predicate<Pilot>() {

public boolean match(Pilot pilot) {

for(int i=0;i<points.length;i++) {

if(pilot.getPoints()==points[i]) {

return true;

}

return pilot.getName().startsWith("Rubens");

}

});

listResult(result);

OUTPUT:

2

Michael Schumacher/100

Rubens Barrichello/99

4.2.6. Native Query Performance

One drawback of native queries has to be pointed out: Under the hood db4o tries to analyze native

queries to convert them to SODA. This is not possible for all queries. For some queries it is very

difficult to analyze the flowgraph. In this case db4o will have to instantiate some of the persistent

objects to actually run the native query code. db4o will try to analyze parts of native query expressions

to keep object instantiation to the minimum.

The development of the native query optimization processor will be an ongoing process in a close

dialog with the db4o community. Feel free to contribute your results and your needs by providing

feedback to our db4o forums(Forums are accessible through free db4o membership ).

The current state of the query optimization process is detailed in the chapter on Native Query

Optimization

With the current implementation, all above examples will run optimized, except for the "Arbitrary

Code" example - we are working on it.

4.2.7. Full source

www.db4o.com

package com.db4odoc.f1.chapter1;

import java.util.*;

import com.db4o.*;

import com.db4o.query.*;

import com.db4odoc.f1.*;

public class NQExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home") + "/formula1.db4o";

public static void main(String[] args) {

ObjectContainer db=Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

try {

storePilots(db);

retrieveComplexSODA(db);

retrieveComplexNQ(db);

retrieveArbitraryCodeNQ(db);

clearDatabase(db);

}

finally {

db.close();

}

public static void storePilots(ObjectContainer db) {

db.store(new Pilot("Michael Schumacher",100));

db.store(new Pilot("Rubens Barrichello",99));

}

public static void retrieveComplexSODA(ObjectContainer db) {

Query query=db.query();

query.constrain(Pilot.class);

Query pointQuery=query.descend("points");

query.descend("name").constrain("Rubens Barrichello")

.or(pointQuery.constrain(new Integer(99)).greater()

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.and(pointQuery.constrain(new

Integer(199)).smaller()));

ObjectSet result=query.execute();

listResult(result);

}

public static void retrieveComplexNQ(ObjectContainer db) {

List<Pilot> result=db.query(new Predicate<Pilot>() {

public boolean match(Pilot pilot) {

return pilot.getPoints()>99

&& pilot.getPoints()<199

|| pilot.getName().equals("Rubens Barrichello");

}

});

listResult(result);

}

public static void retrieveArbitraryCodeNQ(ObjectContainer db) {

final int[] points={1,100};

List<Pilot> result=db.query(new Predicate<Pilot>() {

public boolean match(Pilot pilot) {

for(int i=0;i<points.length;i++) {

if(pilot.getPoints()==points[i]) {

return true;

}

return pilot.getName().startsWith("Rubens");

}

});

listResult(result);

}

public static void clearDatabase(ObjectContainer db) {

ObjectSet result=db.queryByExample(Pilot.class);

while(result.hasNext()) {

db.delete(result.next());

}

www.db4o.com

.

4.3. SODA Query API

The SODA query API is db4o's low level querying API, allowing direct access to nodes of query graphs.

Since SODA uses strings to identify fields, it is neither perfectly typesafe nor compile-time checked and

it also is quite verbose to write.

For most applications Native Queries will be the better querying interface.

However there can be applications where dynamic generation of queries is required, that's why SODA

is explained here.

4.3.1. Simple queries

Let's see how our familiar QBE queries are expressed with SODA. A new Query object is created

through the #query() method of the ObjectContainer and we can add Constraint instances to it. To find

all Pilot instances, we constrain the query with the Pilot class object.

// retrieveAllPilots

Query query=db.query();

query.constrain(Pilot.class);

ObjectSet result=query.execute();

listResult(result);

OUTPUT:

2

Michael Schumacher/100

Rubens Barrichello/99

Basically, we are exchanging our 'real' prototype for a meta description of the objects we'd like to hunt

down: a query graph made up of query nodes and constraints. A query node is a placeholder for a

candidate object, a constraint decides whether to add or exclude candidates from the result.

Our first simple graph looks like this.

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We're just asking any candidate object (here: any object in the database) to be of type Pilot to

aggregate our result.

To retrieve a pilot by name, we have to further constrain the candidate pilots by descending to their

name field and constraining this with the respective candidate String.

// retrievePilotByName

Query query=db.query();

query.constrain(Pilot.class);

query.descend("name").constrain("Michael Schumacher");

ObjectSet result=query.execute();

listResult(result);

OUTPUT:

1

Michael Schumacher/100

What does #descend mean here? Well, just as we did in our 'real' prototypes, we can attach

constraints to child members of our candidates.

So a candidate needs to be of type Pilot and have a member named 'name' that is equal to the given

String to be accepted for the result.

www.db4o.com

Note that the class constraint is not required: If we left it out, we would query for all objects that

contain a 'name' member with the given value. In most cases this will not be the desired behavior,

though.

Finding a pilot by exact points is analogous.We just have to cross the Java primitive/object divide.

// retrievePilotByExactPoints

Query query=db.query();

query.constrain(Pilot.class);

query.descend("points").constrain(new Integer(100));

ObjectSet result=query.execute();

listResult(result);

OUTPUT:

1

Michael Schumacher/100

4.3.2. Advanced queries

Now there are occasions when we don't want to query for exact field values, but rather for value

ranges, objects not containing given member values, etc. This functionality is provided by the

Constraint API.

First, let's negate a query to find all pilots who are not Michael Schumacher:

// retrieveByNegation

Query query=db.query();

query.constrain(Pilot.class);

query.descend("name").constrain("Michael Schumacher").not();

ObjectSet result=query.execute();

listResult(result);

www.db4o.com

OUTPUT:

1

Rubens Barrichello/99

Where there is negation, the other boolean operators can't be too far.

// retrieveByConjunction

Query query=db.query();

query.constrain(Pilot.class);

Constraint constr=query.descend("name")

.constrain("Michael Schumacher");

query.descend("points")

.constrain(new Integer(99)).and(constr);

ObjectSet result=query.execute();

listResult(result);

OUTPUT:

0

// retrieveByDisjunction

Query query=db.query();

query.constrain(Pilot.class);

Constraint constr=query.descend("name")

.constrain("Michael Schumacher");

query.descend("points")

.constrain(new Integer(99)).or(constr);

ObjectSet result=query.execute();

listResult(result);

www.db4o.com

OUTPUT:

2

Michael Schumacher/100

Rubens Barrichello/99

We can also constrain to a comparison with a given value.

// retrieveByComparison

Query query=db.query();

query.constrain(Pilot.class);

query.descend("points")

.constrain(new Integer(99)).greater();

ObjectSet result=query.execute();

listResult(result);

OUTPUT:

1

Michael Schumacher/100

The query API also allows to query for field default values.

// retrieveByDefaultFieldValue

Pilot somebody=new Pilot("Somebody else",0);

db.store(somebody);

Query query=db.query();

query.constrain(Pilot.class);

query.descend("points").constrain(new Integer(0));

ObjectSet result=query.execute();

listResult(result);

db.delete(somebody);

www.db4o.com

OUTPUT:

1

Somebody else/0

It is also possible to have db4o sort the results.

// retrieveSorted

Query query=db.query();

query.constrain(Pilot.class);

query.descend("name").orderAscending();

ObjectSet result=query.execute();

listResult(result);

query.descend("name").orderDescending();

result=query.execute();

listResult(result);

OUTPUT:

2

Michael Schumacher/100

Rubens Barrichello/99

2

Rubens Barrichello/99

Michael Schumacher/100

All these techniques can be combined arbitrarily, of course. Please try it out. There still may be cases

left where the predefined query API constraints may not be sufficient - don't worry, you can always let

db4o run any arbitrary code that you provide in an Evaluation. Evaluations will be discussed in a later

chapter.

To prepare for the next chapter, let's clear the database.

www.db4o.com

// clearDatabase

ObjectSet result=db.queryByExample(Pilot.class);

while(result.hasNext()) {

db.delete(result.next());

}

OUTPUT:

4.3.3. Conclusion

Now you have been provided with the following alternative approaches to query db4o databases:

Query-By-Example,.? net LINQ, Native Queries, SODA.

Which one is the best to use? Some hints:

- Native queries are targeted to be the primary interface for db4o, so they should be preferred.

- With the current state of the db4o query optimizer there may be queries that will execute faster in

SODA style, so it can be used to tune applications. SODA can also be more convenient for constructing

dynamic queries at runtime.

- Query-By-Example is nice for simple one-liners, but restricted in functionality. If you like this

approach, use it as long as it suits your application's needs.

Of course you can mix these strategies as needed.

We have finished our walkthrough and seen the various ways db4o provides to pose queries. But our

domain model is not complex at all, consisting of one class only. Let's have a look at the way db4o

handles object associations in the next chapter .

4.3.4. Full source

package com.db4odoc.f1.chapter1;

import com.db4o.*;

import com.db4o.query.Constraint;

import com.db4o.query.Query;

www.db4o.com

import com.db4odoc.f1.*;

public class QueryExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home") + "/formula1.db4o";

public static void main(String[] args) {

ObjectContainer db=Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

try {

storeFirstPilot(db);

storeSecondPilot(db);

retrieveAllPilots(db);

retrievePilotByName(db);

retrievePilotByExactPoints(db);

retrieveByNegation(db);

retrieveByConjunction(db);

retrieveByDisjunction(db);

retrieveByComparison(db);

retrieveByDefaultFieldValue(db);

retrieveSorted(db);

clearDatabase(db);

}

finally {

db.close();

}

public static void storeFirstPilot(ObjectContainer db) {

Pilot pilot1=new Pilot("Michael Schumacher",100);

db.store(pilot1);

System.out.println("Stored "+pilot1);

}

public static void storeSecondPilot(ObjectContainer db) {

Pilot pilot2=new Pilot("Rubens Barrichello",99);

db.store(pilot2);

System.out.println("Stored "+pilot2);

}

www.db4o.com

public static void retrieveAllPilots(ObjectContainer db) {

Query query=db.query();

query.constrain(Pilot.class);

ObjectSet result=query.execute();

listResult(result);

}

public static void retrievePilotByName(ObjectContainer db) {

Query query=db.query();

query.constrain(Pilot.class);

query.descend("name").constrain("Michael Schumacher");

ObjectSet result=query.execute();

listResult(result);

}

public static void retrievePilotByExactPoints(

ObjectContainer db) {

Query query=db.query();

query.constrain(Pilot.class);

query.descend("points").constrain(new Integer(100));

ObjectSet result=query.execute();

listResult(result);

}

public static void retrieveByNegation(ObjectContainer db) {

Query query=db.query();

query.constrain(Pilot.class);

query.descend("name").constrain("Michael Schumacher").not();

ObjectSet result=query.execute();

listResult(result);

}

public static void retrieveByConjunction(ObjectContainer db) {

Query query=db.query();

query.constrain(Pilot.class);

Constraint constr=query.descend("name")

.constrain("Michael Schumacher");

query.descend("points")

.constrain(new Integer(99)).and(constr);

ObjectSet result=query.execute();

www.db4o.com

listResult(result);

}

public static void retrieveByDisjunction(ObjectContainer db) {

Query query=db.query();

query.constrain(Pilot.class);

Constraint constr=query.descend("name")

.constrain("Michael Schumacher");

query.descend("points")

.constrain(new Integer(99)).or(constr);

ObjectSet result=query.execute();

listResult(result);

}

public static void retrieveByComparison(ObjectContainer db) {

Query query=db.query();

query.constrain(Pilot.class);

query.descend("points")

.constrain(new Integer(99)).greater();

ObjectSet result=query.execute();

listResult(result);

}

public static void retrieveByDefaultFieldValue(

ObjectContainer db) {

Pilot somebody=new Pilot("Somebody else",0);

db.store(somebody);

Query query=db.query();

query.constrain(Pilot.class);

query.descend("points").constrain(new Integer(0));

ObjectSet result=query.execute();

listResult(result);

db.delete(somebody);

}

public static void retrieveSorted(ObjectContainer db) {

Query query=db.query();

query.constrain(Pilot.class);

query.descend("name").orderAscending();

ObjectSet result=query.execute();

listResult(result);

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query.descend("name").orderDescending();

result=query.execute();

listResult(result);

}

public static void clearDatabase(ObjectContainer db) {

ObjectSet result=db.queryByExample(Pilot.class);

while(result.hasNext()) {

db.delete(result.next());

}

www.db4o.com

.

5. Structured objects

It's time to extend our business domain with another class and see how db4o handles object

interrelations. Let's give our pilot a vehicle.

package com.db4odoc.f1.chapter2;

public class Car {

private String model;

private Pilot pilot;

public Car(String model) {

this.model=model;

this.pilot=null;

}

public Pilot getPilot() {

return pilot;

}

public void setPilot(Pilot pilot) {

this.pilot = pilot;

}

public String getModel() {

return model;

}

public String toString() {

return model+"["+pilot+"]";

}

5.1. Storing structured objects

To store a car with its pilot, we just call #store() on our top level object, the car. The pilot will be

www.db4o.com

stored implicitly.

// storeFirstCar

Car car1 = new Car("Ferrari");

Pilot pilot1 = new Pilot("Michael Schumacher", 100);

car1.setPilot(pilot1);

db.store(car1);

Of course, we need some competition here. This time we explicitly store the pilot before entering the

car - this makes no difference.

// storeSecondCar

Pilot pilot2 = new Pilot("Rubens Barrichello", 99);

db.store(pilot2);

Car car2 = new Car("BMW");

car2.setPilot(pilot2);

db.store(car2);

5.2. Retrieving structured objects

5.2.1. QBE

To retrieve all cars, we simply provide a 'blank' prototype.

// retrieveAllCarsQBE

Car proto = new Car(null);

ObjectSet result = db.queryByExample(proto);

listResult(result);

www.db4o.com

OUTPUT:

2

BMW[Rubens Barrichello/99]

Ferrari[Michael Schumacher/100]

We can also query for all pilots, of course.

// retrieveAllPilotsQBE

Pilot proto = new Pilot(null, 0);

ObjectSet result = db.queryByExample(proto);

listResult(result);

OUTPUT:

2

Michael Schumacher/100

Rubens Barrichello/99

Now let's initialize our prototype to specify all cars driven by Rubens Barrichello.

// retrieveCarByPilotQBE

Pilot pilotproto = new Pilot("Rubens Barrichello", 0);

Car carproto = new Car(null);

carproto.setPilot(pilotproto);

ObjectSet result = db.queryByExample(carproto);

listResult(result);

OUTPUT:

1

www.db4o.com

BMW[Rubens Barrichello/99]

What about retrieving a pilot by car? We simply don't need that - if we already know the car, we can

simply access the pilot field directly.

5.2.2. Native Queries

Using native queries with constraints on deep structured objects is straightforward, you can do it just

like you would in plain other code.

Let's constrain our query to only those cars driven by a Pilot with a specific name:

// retrieveCarsByPilotNameNative

final String pilotName = "Rubens Barrichello";

List<Car> results = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getPilot().getName().equals(pilotName);

}

});

listResult(results);

OUTPUT:

1

BMW[Rubens Barrichello/99]

5.2.3. SODA Query API

In order to use SODA for querying for a car given its pilot's name we have to descend two levels into

our query.

www.db4o.com

// retrieveCarByPilotNameQuery

Query query = db.query();

query.constrain(Car.class);

query.descend("pilot").descend("name").constrain("Rubens

Barrichello");

ObjectSet result = query.execute();

listResult(result);

OUTPUT:

1

BMW[Rubens Barrichello/99]

We can also constrain the pilot field with a prototype to achieve the same result.

// retrieveCarByPilotProtoQuery

Query query = db.query();

query.constrain(Car.class);

Pilot proto = new Pilot("Rubens Barrichello", 0);

query.descend("pilot").constrain(proto);

ObjectSet result = query.execute();

listResult(result);

OUTPUT:

1

BMW[Rubens Barrichello/99]

We have seen that descending into a query provides us with another query. Starting out from a query

root we can descend in multiple directions. In practice this is the same as ascending from one child to

www.db4o.com

a parent and descending to another child. We can conclude that queries turn one-directional references

in our objects into true relations. Here is an example that queries for "a Pilot that is being referenced

by a Car, where the Car model is 'Ferrari'":

// retrievePilotByCarModelQuery

Query carquery = db.query();

carquery.constrain(Car.class);

carquery.descend("model").constrain("Ferrari");

Query pilotquery = carquery.descend("pilot");

ObjectSet result = pilotquery.execute();

listResult(result);

OUTPUT:

1

Michael Schumacher/100

5.3. Updating structured objects

To update structured objects in db4o, we simply call #store() on them again.

// updateCar

www.db4o.com

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

Car found = (Car) result.get(0);

found.setPilot(new Pilot("Somebody else", 0));

db.store(found);

result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

listResult(result);

OUTPUT:

1

Ferrari[Somebody else/0]

Let's modify the pilot, too.

// updatePilotSingleSession

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

Car found = result.get(0);

found.getPilot().addPoints(1);

db.store(found);

result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

www.db4o.com

});

listResult(result);

OUTPUT:

1

Ferrari[Somebody else/1]

Nice and easy, isn't it? But wait, there's something evil lurking right behind the corner. Let's see what

happens if we split this task in two separate db4o sessions: In the first we modify our pilot and update

his car:

// updatePilotSeparateSessionsPart1

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

Car found = result.get(0);

found.getPilot().addPoints(1);

db.store(found);

And in the second, we'll double-check our modification:

// updatePilotSeparateSessionsPart2

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

listResult(result);

www.db4o.com

OUTPUT:

1

Ferrari[Somebody else/0]

Looks like we're in trouble: Why did the Pilot's points not change? What's happening here and what can

we do to fix it?

5.3.1. Update depth

Imagine a complex object with many members that have many members themselves. When updating

this object, db4o would have to update all its children, grandchildren, etc. This poses a severe

performance penalty and will not be necessary in most cases - sometimes, however, it will.

So, in our previous update example, we were modifying the Pilot child of a Car object. When we saved

the change, we told db4o to save our Car object and assumed that the modified Pilot would be

updated. But we were modifying and saving in the same manner as we were in the first update

sample, so why did it work before? The first time we made the modification, db4o never actually had to

retrieve the modified Pilot it returned the same one that was still in memory that we modified, but it

never actually updated the database. The fact that we saw the modified value was, in fact, a bug.

Restarting the application would show that the value was unchanged.

To be able to handle this dilemma as flexible as possible, db4o introduces the concept of update depth

to control how deep an object's member tree will be traversed on update. The default update depth for

all objects is 1, meaning that only primitive and String members will be updated, but changes in object

members will not be reflected.

db4o provides means to control update depth with very fine granularity. For our current problem we'll

advise db4o to update the full graph for Car objects by setting #cascadeOnUpdate() for this class

accordingly.

// updatePilotSeparateSessionsImprovedPart1

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().objectClass("com.db4o.f1.chapter2.Car").cascadeOnUpda

te(true);

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

www.db4o.com

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

if (result.size() > 0) {

Car found = result.get(0);

found.getPilot().addPoints(1);

db.store(found);

}

db.close();

// updatePilotSeparateSessionsImprovedPart2

ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

Car car = result.get(0);

listResult(result);

db.close();

OUTPUT:

1

Ferrari[Somebody else/0]

This looks much better.

Note that container configuration must be passed when the container is opened.

www.db4o.com

We'll cover update depth as well as other issues with complex object graphs and the respective db4o

configuration options in more detail in a later chapter.

5.4. Deleting structured objects

As we have already seen, we call #delete() on objects to get rid of them.

// deleteFlat

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

Car found = result.get(0);

db.delete(found);

result = db.queryByExample(new Car(null));

listResult(result);

OUTPUT:

1

BMW[Rubens Barrichello/99]

Fine, the car is gone. What about the pilots?

// retrieveAllPilotsQBE

Pilot proto = new Pilot(null, 0);

ObjectSet result = db.queryByExample(proto);

listResult(result);

OUTPUT:

3

www.db4o.com

Somebody else/0

Michael Schumacher/100

Rubens Barrichello/99

Ok, this is no real surprise - we don't expect a pilot to vanish when his car is disposed of in real life,

too. But what if we want an object's children to be thrown away on deletion, too?

5.4.1. Recursive deletion

You may already suspect that the problem of recursive deletion (and perhaps its solution, too) is quite

similar to our little update problem, and you're right. Let's configure db4o to delete a car's pilot, too,

when the car is deleted.

// deleteDeep

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().objectClass("com.db4o.f1.chapter2.Car").cascadeOnDele

te(true);

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("BMW");

}

});

if (result.size() > 0) {

Car found = result.get(0);

db.delete(found);

}

result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return true;

}

});

listResult(result);

db.close();

www.db4o.com

OUTPUT:

0

Again: Note that configuration should be passed to the call opening ObjectContainer.

Let's have a look at our pilots again.

// retrieveAllPilots

Pilot proto = new Pilot(null, 0);

ObjectSet result = db.queryByExample(proto);

listResult(result);

OUTPUT:

3

Somebody else/0

Michael Schumacher/100

Rubens Barrichello/99

5.4.2. Recursive deletion revisited

But wait - what happens if the children of a removed object are still referenced by other objects?

// deleteDeepRevisited

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().objectClass("com.db4o.f1.chapter2.Car").cascadeOnDele

te(true);

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

ObjectSet<Pilot> result = db.query(new Predicate<Pilot>() {

public boolean match(Pilot pilot) {

www.db4o.com

return pilot.getName().equals("Michael Schumacher");

}

});

if (!result.hasNext()) {

System.out.println("Pilot not found!");

db.close();

return;

}

Pilot pilot = (Pilot) result.next();

Car car1 = new Car("Ferrari");

Car car2 = new Car("BMW");

car1.setPilot(pilot);

car2.setPilot(pilot);

db.store(car1);

db.store(car2);

db.delete(car2);

List<Car> cars = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return true;

}

});

listResult(cars);

db.close();

OUTPUT:

1

Ferrari[Michael Schumacher/100]

// retrieveAllPilots

Pilot proto = new Pilot(null, 0);

ObjectSet result = db.queryByExample(proto);

listResult(result);

OUTPUT:

www.db4o.com

3

Somebody else/0

Michael Schumacher/100

Rubens Barrichello/99

Houston, we have a problem - and there's no simple solution at hand. Currently db4o does not check

whether objects to be deleted are referenced anywhere else, so please be very careful when using this

feature.

Let's clear our database for the next chapter.

// deleteAll

ObjectSet result=db.queryByExample(new Object());

while(result.hasNext()) {

db.delete(result.next());

}

5.5. Conclusion

So much for object associations: We can hook into a root object and climb down its reference graph to

specify queries. But what about multi-valued objects like arrays and collections? We will cover this in

the next chapter .

5.6. Full source

package com.db4odoc.f1.chapter2;

import java.io.*;

import java.util.*;

import com.db4o.*;

import com.db4o.config.*;

import com.db4o.query.*;

www.db4o.com

import com.db4odoc.f1.*;

public class StructuredExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home") + "/formula1.db4o";

public static void main(String[] args) {

new File(DB4OFILENAME).delete();

ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

storeFirstCar(db);

storeSecondCar(db);

retrieveAllCarsQBE(db);

retrieveAllPilotsQBE(db);

retrieveCarByPilotQBE(db);

retrieveCarByPilotNameQuery(db);

retrieveCarByPilotProtoQuery(db);

retrievePilotByCarModelQuery(db);

updateCar(db);

updatePilotSingleSession(db);

updatePilotSeparateSessionsPart1(db);

db.close();

db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

updatePilotSeparateSessionsPart2(db);

db.close();

updatePilotSeparateSessionsImprovedPart1();

updatePilotSeparateSessionsImprovedPart2();

db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

deleteFlat(db);

db.close();

deleteDeep();

deleteDeepRevisited();

}

public static void storeFirstCar(ObjectContainer db) {

Car car1 = new Car("Ferrari");

Pilot pilot1 = new Pilot("Michael Schumacher", 100);

www.db4o.com

car1.setPilot(pilot1);

db.store(car1);

}

public static void storeSecondCar(ObjectContainer db) {

Pilot pilot2 = new Pilot("Rubens Barrichello", 99);

db.store(pilot2);

Car car2 = new Car("BMW");

car2.setPilot(pilot2);

db.store(car2);

}

public static void retrieveAllCarsQBE(ObjectContainer db) {

Car proto = new Car(null);

ObjectSet result = db.queryByExample(proto);

listResult(result);

}

public static void retrieveAllPilotsQBE(ObjectContainer db) {

Pilot proto = new Pilot(null, 0);

ObjectSet result = db.queryByExample(proto);

listResult(result);

}

public static void retrieveAllPilots(ObjectContainer db) {

ObjectSet result = db.queryByExample(Pilot.class);

listResult(result);

}

public static void retrieveCarByPilotQBE(ObjectContainer db) {

Pilot pilotproto = new Pilot("Rubens Barrichello", 0);

Car carproto = new Car(null);

carproto.setPilot(pilotproto);

ObjectSet result = db.queryByExample(carproto);

listResult(result);

}

public static void retrieveCarByPilotNameQuery(ObjectContainer

db) {

Query query = db.query();

query.constrain(Car.class);

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query.descend("pilot").descend("name").constrain("Rubens

Barrichello");

ObjectSet result = query.execute();

listResult(result);

}

public static void retrieveCarByPilotProtoQuery(ObjectContainer

db) {

Query query = db.query();

query.constrain(Car.class);

Pilot proto = new Pilot("Rubens Barrichello", 0);

query.descend("pilot").constrain(proto);

ObjectSet result = query.execute();

listResult(result);

}

public static void retrievePilotByCarModelQuery(ObjectContainer

db) {

Query carquery = db.query();

carquery.constrain(Car.class);

carquery.descend("model").constrain("Ferrari");

Query pilotquery = carquery.descend("pilot");

ObjectSet result = pilotquery.execute();

listResult(result);

}

public static void retrieveAllPilotsNative(ObjectContainer db) {

List<Pilot> results = db.query(new Predicate<Pilot>() {

public boolean match(Pilot pilot) {

return true;

}

});

listResult(results);

}

public static void retrieveAllCars(ObjectContainer db) {

ObjectSet results = db.queryByExample(Car.class);

listResult(results);

}

public static void retrieveCarsByPilotNameNative(ObjectContainer

www.db4o.com

db) {

final String pilotName = "Rubens Barrichello";

List<Car> results = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getPilot().getName().equals(pilotName);

}

});

listResult(results);

}

public static void updateCar(ObjectContainer db) {

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

Car found = (Car) result.get(0);

found.setPilot(new Pilot("Somebody else", 0));

db.store(found);

result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

listResult(result);

}

public static void updatePilotSingleSession(ObjectContainer db) {

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

Car found = result.get(0);

found.getPilot().addPoints(1);

db.store(found);

result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

www.db4o.com

listResult(result);

}

public static void

updatePilotSeparateSessionsPart1(ObjectContainer db) {

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

Car found = result.get(0);

found.getPilot().addPoints(1);

db.store(found);

}

public static void

updatePilotSeparateSessionsPart2(ObjectContainer db) {

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

listResult(result);

}

public static void updatePilotSeparateSessionsImprovedPart1() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

config.common().objectClass("com.db4o.f1.chapter2.Car").cascadeOnUpda

te(true);

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

if (result.size() > 0) {

Car found = result.get(0);

found.getPilot().addPoints(1);

db.store(found);

www.db4o.com

}

db.close();

}

public static void updatePilotSeparateSessionsImprovedPart2() {

ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

Car car = result.get(0);

listResult(result);

db.close();

}

public static void deleteFlat(ObjectContainer db) {

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("Ferrari");

}

});

Car found = result.get(0);

db.delete(found);

result = db.queryByExample(new Car(null));

listResult(result);

}

public static void deleteDeep() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

config.common().objectClass("com.db4o.f1.chapter2.Car").cascadeOnDele

te(true);

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

List<Car> result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return car.getModel().equals("BMW");

}

www.db4o.com

});

if (result.size() > 0) {

Car found = result.get(0);

db.delete(found);

}

result = db.query(new Predicate<Car>() {

public boolean match(Car car) {

return true;

}

});

listResult(result);

db.close();

}

public static void deleteDeepRevisited() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

config.common().objectClass("com.db4o.f1.chapter2.Car").cascadeOnDele

te(true);

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

ObjectSet<Pilot> result = db.query(new Predicate<Pilot>() {

public boolean match(Pilot pilot) {

return pilot.getName().equals("Michael Schumacher");

}

});

if (!result.hasNext()) {

System.out.println("Pilot not found!");

db.close();

return;

}

Pilot pilot = (Pilot) result.next();

Car car1 = new Car("Ferrari");

Car car2 = new Car("BMW");

car1.setPilot(pilot);

car2.setPilot(pilot);

db.store(car1);

db.store(car2);

db.delete(car2);

List<Car> cars = db.query(new Predicate<Car>() {

public boolean match(Car car) {

www.db4o.com

return true;

}

});

listResult(cars);

db.close();

}

www.db4o.com

.

6. Collections and Arrays

We will slowly move towards real-time data processing now by installing sensors to our car and

collecting their output.

package com.db4odoc.f1.chapter4;

import java.util.*;

public class SensorReadout {

private double[] values;

private Date time;

private Car car;

public SensorReadout(double[] values,Date time,Car car) {

this.values=values;

this.time=time;

this.car=car;

}

public Car getCar() {

return car;

}

public Date getTime() {

return time;

}

public int getNumValues() {

return values.length;

}

public double[] getValues(){

return values;

}

public double getValue(int idx) {

return values[idx];

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}

public String toString() {

StringBuffer str=new StringBuffer();

str.append(car.toString())

.append(" : ")

.append(time.getTime())

.append(" : ");

for(int idx=0;idx<values.length;idx++) {

if(idx>0) {

str.append(',');

}

str.append(values[idx]);

}

return str.toString();

}

A car may produce its current sensor readout when requested and keep a list of readouts collected

during a race.

package com.db4odoc.f1.chapter4;

import java.util.*;

public class Car {

private String model;

private Pilot pilot;

private List history;

public Car(String model) {

this(model,new ArrayList());

}

public Car(String model,List history) {

this.model=model;

this.pilot=null;

www.db4o.com

this.history=history;

}

public Pilot getPilot() {

return pilot;

}

public void setPilot(Pilot pilot) {

this.pilot=pilot;

}

public String getModel() {

return model;

}

public List getHistory() {

return history;

}

public void snapshot() {

history.add(new SensorReadout(poll(),new Date(),this));

}

protected double[] poll() {

int factor=history.size()+1;

return new double[]{0.1d*factor,0.2d*factor,0.3d*factor};

}

public String toString() {

return model+"["+pilot+"]/"+history.size();

}

We will constrain ourselves to rather static data at the moment and add flexibility during the next

chapters.

6.1. Storing

This should be familiar by now.

www.db4o.com

// storeFirstCar

Car car1 = new Car("Ferrari");

Pilot pilot1 = new Pilot("Michael Schumacher", 100);

car1.setPilot(pilot1);

db.store(car1);

The second car will take two snapshots immediately at startup.

// storeSecondCar

Pilot pilot2 = new Pilot("Rubens Barrichello", 99);

Car car2 = new Car("BMW");

car2.setPilot(pilot2);

car2.snapshot();

db.store(car2);

6.2. Retrieving

6.2.1. QBE

First let us verify that we indeed have taken snapshots.

// retrieveAllSensorReadout

SensorReadout proto = new SensorReadout(null, null, null);

ObjectSet results = db.queryByExample(proto);

listResult(results);

www.db4o.com

OUTPUT:

2

BMW[Rubens Barrichello/99]/2 : 1244840365881 : 0.1,0.2,0.3

BMW[Rubens Barrichello/99]/2 : 1244840365881 : 0.2,0.4,0.6

As a prototype for an array, we provide an array of the same type, containing only the values we

expect the result to contain.

// retrieveSensorReadoutQBE

SensorReadout proto = new SensorReadout(new double[] { 0.3, 0.1 },

null, null);

ObjectSet results = db.queryByExample(proto);

listResult(results);

OUTPUT:

1

BMW[Rubens Barrichello/99]/2 : 1244840365881 : 0.1,0.2,0.3

Note that the actual position of the given elements in the prototype array is irrelevant.

To retrieve a car by its stored sensor readouts, we install a history containing the sought-after values.

// retrieveCarQBE

SensorReadout protoreadout = new SensorReadout(

new double[] { 0.6, 0.2 }, null, null);

List protohistory = new ArrayList();

protohistory.add(protoreadout);

Car protocar = new Car(null, protohistory);

ObjectSet result = db.queryByExample(protocar);

listResult(result);

www.db4o.com

OUTPUT:

1

BMW[Rubens Barrichello/99]/2

We can also query for the collections themselves, since they are first class objects.

// retrieveCollections

ObjectSet result = db.queryByExample(new ArrayList());

listResult(result);

OUTPUT:

2

[BMW[Rubens Barrichello/99]/2 : 1244840365881 : 0.1,0.2,0.3,

BMW[Rubens Barrichello/99]/2 : 1244840365881 : 0.2,0.4,0.6]

[]

This doesn't work with arrays, though.

// retrieveArrays

ObjectSet result = db.queryByExample(new double[] { 0.6, 0.4 });

listResult(result);

OUTPUT:

0

6.2.2. Native Queries

www.db4o.com

If we want to use Native Queries to find SensorReadouts with matching values, we simply write this as

if we would check every single instance:

// retrieveSensorReadoutNative

List<SensorReadout> results = db.query(new Predicate<SensorReadout>()

{

public boolean match(SensorReadout candidate) {

return Arrays.binarySearch(candidate.getValues(), 0.3) >= 0

&& Arrays.binarySearch(candidate.getValues(), 1.0) >=

0;

}

});

listResult(results);

OUTPUT:

0

And here's how we find Cars with matching readout values:

// retrieveCarNative

List<Car> results = db.query(new Predicate<Car>() {

public boolean match(Car candidate) {

List history = candidate.getHistory();

for (int i = 0; i < history.size(); i++) {

SensorReadout readout = (SensorReadout) history.get(i);

if (Arrays.binarySearch(readout.getValues(), 0.6) >= 0

|| Arrays.binarySearch(readout.getValues(), 0.2)

>= 0)

return true;

}

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return false;

}

});

listResult(results);

OUTPUT:

1

BMW[Rubens Barrichello/99]/2

6.2.3. Query API

Handling of arrays and collections is analogous to the previous example. First, lets retrieve only the

SensorReadouts with specific values:

// retrieveSensorReadoutQuery

Query query = db.query();

query.constrain(SensorReadout.class);

Query valuequery = query.descend("values");

valuequery.constrain(new Double(0.3));

valuequery.constrain(new Double(0.1));

ObjectSet result = query.execute();

listResult(result);

OUTPUT:

1

BMW[Rubens Barrichello/99]/2 : 1244840365881 : 0.1,0.2,0.3

Then let's get some Cars with matching Readout values:

// retrieveCarQuery

www.db4o.com

Query query = db.query();

query.constrain(Car.class);

Query historyquery = query.descend("history");

historyquery.constrain(SensorReadout.class);

Query valuequery = historyquery.descend("values");

valuequery.constrain(new Double(0.3));

valuequery.constrain(new Double(0.1));

ObjectSet result = query.execute();

listResult(result);

OUTPUT:

1

BMW[Rubens Barrichello/99]/2

6.3. Updating and deleting

This should be familiar, we just have to remember to take care of the update depth.

// updateCar

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().objectClass(Car.class).cascadeOnUpdate(true);

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

List<Car> results = db.query(new Predicate<Car>() {

public boolean match(Car candidate) {

return true;

}

});

if (results.size() > 0) {

Car car = results.get(0);

car.snapshot();

db.store(car);

retrieveAllSensorReadoutNative(db);

}

db.close();

www.db4o.com

OUTPUT:

3

BMW[Rubens Barrichello/99]/3 : 1244840365881 : 0.1,0.2,0.3

BMW[Rubens Barrichello/99]/3 : 1244840365881 : 0.2,0.4,0.6

BMW[Rubens Barrichello/99]/3 : 1244840366179 :

0.30000000000000004,0.6000000000000001,0.8999999999999999

There's nothing special about deleting arrays and collections, too.

Deleting an object from a collection is an update, too, of course.

// updateCollection

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().objectClass(Car.class).cascadeOnUpdate(true);

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

ObjectSet<Car> results = db.query(new Predicate<Car>() {

public boolean match(Car candidate) {

return true;

}

});

if (results.hasNext()) {

Car car = (Car) results.next();

car.getHistory().remove(0);

db.store(car.getHistory());

results = db.query(new Predicate<Car>() {

public boolean match(Car candidate) {

return true;

}

});

while (results.hasNext()) {

car = results.next();

for (int idx = 0; idx < car.getHistory().size(); idx++) {

System.out.println(car.getHistory().get(idx));

}

www.db4o.com

}

db.close();

OUTPUT:

BMW[Rubens Barrichello/99]/2 : 1244840365881 : 0.2,0.4,0.6

BMW[Rubens Barrichello/99]/2 : 1244840366179 :

0.30000000000000004,0.6000000000000001,0.8999999999999999

(This example also shows that with db4o it is quite easy to access object internals we were never

meant to see. Please keep this always in mind and be careful.)

We will delete all cars from the database again to prepare for the next chapter.

// deleteAll

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().objectClass(Car.class).cascadeOnDelete(true);

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

ObjectSet<Car> cars = db.query(new Predicate<Car>() {

public boolean match(Car candidate) {

return true;

}

});

while (cars.hasNext()) {

db.delete(cars.next());

}

ObjectSet<SensorReadout> readouts = db

.query(new Predicate<SensorReadout>() {

public boolean match(SensorReadout candidate) {

return true;

}

});

while (readouts.hasNext()) {

db.delete(readouts.next());

}

www.db4o.com

db.close();

6.4. Conclusion

Ok, collections are just objects. But why did we have to specify the concrete ArrayList type all the way?

Was that necessary? How does db4o handle inheritance? We will cover that in the next chapter.

6.5. Full source

package com.db4odoc.f1.chapter4;

import java.io.*;

import java.util.*;

import com.db4o.*;

import com.db4o.config.*;

import com.db4o.query.*;

import com.db4odoc.f1.*;

public class CollectionsExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home") + "/formula1.db4o";

public static void main(String[] args) {

new File(DB4OFILENAME).delete();

ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

try {

storeFirstCar(db);

storeSecondCar(db);

retrieveAllSensorReadout(db);

retrieveSensorReadoutQBE(db);

retrieveCarQBE(db);

retrieveCollections(db);

retrieveArrays(db);

retrieveAllSensorReadoutNative(db);

retrieveSensorReadoutNative(db);

www.db4o.com

retrieveCarNative(db);

retrieveSensorReadoutQuery(db);

retrieveCarQuery(db);

db.close();

updateCar();

updateCollection();

deleteAll();

} finally {

db.close();

}

public static void storeFirstCar(ObjectContainer db) {

Car car1 = new Car("Ferrari");

Pilot pilot1 = new Pilot("Michael Schumacher", 100);

car1.setPilot(pilot1);

db.store(car1);

}

public static void storeSecondCar(ObjectContainer db) {

Pilot pilot2 = new Pilot("Rubens Barrichello", 99);

Car car2 = new Car("BMW");

car2.setPilot(pilot2);

car2.snapshot();

db.store(car2);

}

public static void retrieveAllSensorReadout(ObjectContainer db) {

SensorReadout proto = new SensorReadout(null, null, null);

ObjectSet results = db.queryByExample(proto);

listResult(results);

}

public static void retrieveAllSensorReadoutNative(ObjectContainer

db) {

List<SensorReadout> results = db.query(new

Predicate<SensorReadout>() {

public boolean match(SensorReadout candidate) {

return true;

www.db4o.com

}

});

listResult(results);

}

public static void retrieveSensorReadoutQBE(ObjectContainer db) {

SensorReadout proto = new SensorReadout(new double[] { 0.3,

0.1 },

null, null);

ObjectSet results = db.queryByExample(proto);

listResult(results);

}

public static void retrieveSensorReadoutNative(ObjectContainer

db) {

List<SensorReadout> results = db.query(new

Predicate<SensorReadout>() {

public boolean match(SensorReadout candidate) {

return Arrays.binarySearch(candidate.getValues(),

0.3) >= 0

&& Arrays.binarySearch(candidate.getValues(),

1.0) >= 0;

}

});

listResult(results);

}

public static void retrieveCarQBE(ObjectContainer db) {

SensorReadout protoreadout = new SensorReadout(

new double[] { 0.6, 0.2 }, null, null);

List protohistory = new ArrayList();

protohistory.add(protoreadout);

Car protocar = new Car(null, protohistory);

ObjectSet result = db.queryByExample(protocar);

listResult(result);

}

public static void retrieveCarNative(ObjectContainer db) {

List<Car> results = db.query(new Predicate<Car>() {

public boolean match(Car candidate) {

List history = candidate.getHistory();

www.db4o.com

for (int i = 0; i < history.size(); i++) {

SensorReadout readout = (SensorReadout)

history.get(i);

if (Arrays.binarySearch(readout.getValues(), 0.6)

>= 0

||

Arrays.binarySearch(readout.getValues(), 0.2) >= 0)

return true;

}

return false;

}

});

listResult(results);

}

public static void retrieveCollections(ObjectContainer db) {

ObjectSet result = db.queryByExample(new ArrayList());

listResult(result);

}

public static void retrieveArrays(ObjectContainer db) {

ObjectSet result = db.queryByExample(new double[] { 0.6, 0.4

});

listResult(result);

}

public static void retrieveSensorReadoutQuery(ObjectContainer db)

{

Query query = db.query();

query.constrain(SensorReadout.class);

Query valuequery = query.descend("values");

valuequery.constrain(new Double(0.3));

valuequery.constrain(new Double(0.1));

ObjectSet result = query.execute();

listResult(result);

}

public static void retrieveCarQuery(ObjectContainer db) {

Query query = db.query();

query.constrain(Car.class);

Query historyquery = query.descend("history");

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historyquery.constrain(SensorReadout.class);

Query valuequery = historyquery.descend("values");

valuequery.constrain(new Double(0.3));

valuequery.constrain(new Double(0.1));

ObjectSet result = query.execute();

listResult(result);

}

public static void updateCar() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

config.common().objectClass(Car.class).cascadeOnUpdate(true);

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

List<Car> results = db.query(new Predicate<Car>() {

public boolean match(Car candidate) {

return true;

}

});

if (results.size() > 0) {

Car car = results.get(0);

car.snapshot();

db.store(car);

retrieveAllSensorReadoutNative(db);

}

db.close();

}

public static void updateCollection() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

config.common().objectClass(Car.class).cascadeOnUpdate(true);

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

ObjectSet<Car> results = db.query(new Predicate<Car>() {

public boolean match(Car candidate) {

return true;

}

});

if (results.hasNext()) {

Car car = (Car) results.next();

www.db4o.com

car.getHistory().remove(0);

db.store(car.getHistory());

results = db.query(new Predicate<Car>() {

public boolean match(Car candidate) {

return true;

}

});

while (results.hasNext()) {

car = results.next();

for (int idx = 0; idx < car.getHistory().size();

idx++) {

System.out.println(car.getHistory().get(idx));

}

db.close();

}

public static void deleteAll() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

config.common().objectClass(Car.class).cascadeOnDelete(true);

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

ObjectSet<Car> cars = db.query(new Predicate<Car>() {

public boolean match(Car candidate) {

return true;

}

});

while (cars.hasNext()) {

db.delete(cars.next());

}

ObjectSet<SensorReadout> readouts = db

.query(new Predicate<SensorReadout>() {

public boolean match(SensorReadout candidate) {

return true;

}

});

while (readouts.hasNext()) {

db.delete(readouts.next());

www.db4o.com

}

db.close();

}

www.db4o.com

.

7. Inheritance

So far we have always been working with the concrete (i.e. most specific type of an object. What about

subclassing and interfaces?

To explore this, we will differentiate between different kinds of sensors.

package com.db4odoc.f1.chapter5;

import java.util.*;

public class SensorReadout {

private Date time;

private Car car;

private String description;

protected SensorReadout(Date time,Car car,String description) {

this.time=time;

this.car=car;

this.description=description;

}

public Car getCar() {

return car;

}

public Date getTime() {

return time;

}

public String getDescription() {

return description;

}

public String toString() {

return car+" : "+time+" : "+description;

}

www.db4o.com

package com.db4odoc.f1.chapter5;

import java.util.*;

public class TemperatureSensorReadout extends SensorReadout {

private double temperature;

public TemperatureSensorReadout(

Date time,Car car,

String description,double temperature) {

super(time,car,description);

this.temperature=temperature;

}

public double getTemperature() {

return temperature;

}

public String toString() {

return super.toString()+" temp : "+temperature;

}

package com.db4odoc.f1.chapter5;

import java.util.*;

public class PressureSensorReadout extends SensorReadout {

private double pressure;

public PressureSensorReadout(

Date time,Car car,

String description,double pressure) {

www.db4o.com

super(time,car,description);

this.pressure=pressure;

}

public double getPressure() {

return pressure;

}

public String toString() {

return super.toString()+" pressure : "+pressure;

}

Our car's snapshot mechanism is changed accordingly.

package com.db4odoc.f1.chapter5;

import java.util.*;

public class Car {

private String model;

private Pilot pilot;

private List history;

public Car(String model) {

this.model=model;

this.pilot=null;

this.history=new ArrayList();

}

public Pilot getPilot() {

return pilot;

}

public void setPilot(Pilot pilot) {

this.pilot=pilot;

}

www.db4o.com

public String getModel() {

return model;

}

public SensorReadout[] getHistory() {

return (SensorReadout[])history.toArray(new

SensorReadout[history.size()]);

}

public void snapshot() {

history.add(new TemperatureSensorReadout(

new Date(),this,"oil",pollOilTemperature()));

history.add(new TemperatureSensorReadout(

new Date(),this,"water",pollWaterTemperature()));

history.add(new PressureSensorReadout(

new Date(),this,"oil",pollOilPressure()));

}

protected double pollOilTemperature() {

return 0.1*history.size();

}

protected double pollWaterTemperature() {

return 0.2*history.size();

}

protected double pollOilPressure() {

return 0.3*history.size();

}

public String toString() {

return model+"["+pilot+"]/"+history.size();

}

7.1. Storing

Our setup code has not changed at all, just the internal workings of a snapshot.

www.db4o.com

// storeFirstCar

Car car1=new Car("Ferrari");

Pilot pilot1=new Pilot("Michael Schumacher",100);

car1.setPilot(pilot1);

db.store(car1);

// storeSecondCar

Pilot pilot2=new Pilot("Rubens Barrichello",99);

Car car2=new Car("BMW");

car2.setPilot(pilot2);

car2.snapshot();

db.store(car2);

7.2. Retrieving

db4o will provide us with all objects of the given type. To collect all instances of a given class, no

matter whether they are subclass members or direct instances, we just provide a corresponding

prototype.

// retrieveTemperatureReadoutsQBE

SensorReadout proto=

new TemperatureSensorReadout(null,null,null,0.0);

ObjectSet result=db.queryByExample(proto);

listResult(result);

OUTPUT:

www.db4o.com

4

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.8

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.30000000000000004

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.0

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.2

// retrieveAllSensorReadoutsQBE

SensorReadout proto=new SensorReadout(null,null,null);

ObjectSet result=db.queryByExample(proto);

listResult(result);

OUTPUT:

6

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.8

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 1.5

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 0.6

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.30000000000000004

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.0

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.2

This is one more situation where QBE might not be applicable: What if the given type is an interface or

an abstract class? Well, there's a little trick to keep in mind: Class objects receive special handling with

QBE.

www.db4o.com

// retrieveAllSensorReadoutsQBEAlternative

ObjectSet result=db.queryByExample(SensorReadout.class);

listResult(result);

OUTPUT:

6

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.8

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 1.5

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 0.6

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.30000000000000004

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.0

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.2

And of course there's our SODA API:

// retrieveAllSensorReadoutsQuery

Query query=db.query();

query.constrain(SensorReadout.class);

ObjectSet result=query.execute();

listResult(result);

OUTPUT:

6

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

www.db4o.com

temp : 0.8

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 1.5

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 0.6

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.30000000000000004

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.0

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.2

7.3. Updating and deleting

is just the same for all objects, no matter where they are situated in the inheritance tree.

Just like we retrieved all objects from the database above, we can delete all stored objects to prepare

for the next chapter.

// deleteAll

ObjectSet result=db.queryByExample(new Object());

while(result.hasNext()) {

db.delete(result.next());

}

7.4. Conclusion

Now we have covered all basic OO features and the way they are handled by db4o. We will complete

the first part of our db4o walkthrough in the next chapter by looking at deep object graphs, including

recursive structures.

7.5. Full source

www.db4o.com

package com.db4odoc.f1.chapter5;

import java.io.*;

import com.db4o.*;

import com.db4o.query.*;

import com.db4odoc.f1.*;

public class InheritanceExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home") + "/formula1.db4o";

public static void main(String[] args) {

new File(DB4OFILENAME).delete();

ObjectContainer db=Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

try {

storeFirstCar(db);

storeSecondCar(db);

retrieveTemperatureReadoutsQBE(db);

retrieveAllSensorReadoutsQBE(db);

retrieveAllSensorReadoutsQBEAlternative(db);

retrieveAllSensorReadoutsQuery(db);

retrieveAllObjectsQBE(db);

}

finally {

db.close();

}

public static void storeFirstCar(ObjectContainer db) {

Car car1=new Car("Ferrari");

Pilot pilot1=new Pilot("Michael Schumacher",100);

car1.setPilot(pilot1);

db.store(car1);

}

www.db4o.com

public static void storeSecondCar(ObjectContainer db) {

Pilot pilot2=new Pilot("Rubens Barrichello",99);

Car car2=new Car("BMW");

car2.setPilot(pilot2);

car2.snapshot();

db.store(car2);

}

public static void retrieveAllSensorReadoutsQBE(

ObjectContainer db) {

SensorReadout proto=new SensorReadout(null,null,null);

ObjectSet result=db.queryByExample(proto);

listResult(result);

}

public static void retrieveTemperatureReadoutsQBE(

ObjectContainer db) {

SensorReadout proto=

new TemperatureSensorReadout(null,null,null,0.0);

ObjectSet result=db.queryByExample(proto);

listResult(result);

}

public static void retrieveAllSensorReadoutsQBEAlternative(

ObjectContainer db) {

ObjectSet result=db.queryByExample(SensorReadout.class);

listResult(result);

}

public static void retrieveAllSensorReadoutsQuery(

ObjectContainer db) {

Query query=db.query();

query.constrain(SensorReadout.class);

ObjectSet result=query.execute();

listResult(result);

}

public static void retrieveAllObjectsQBE(ObjectContainer db) {

ObjectSet result=db.queryByExample(new Object());

listResult(result);

www.db4o.com

}

www.db4o.com

.

8. Deep graphs

We have already seen how db4o handles object associations, but our running example is still quite flat

and simple, compared to real-world domain models. In particular we haven't seen how db4o behaves

in the presence of recursive structures. We will emulate such a structure by replacing our history list

with a linked list implicitely provided by the SensorReadout class.

package com.db4odoc.f1.chapter6;

import java.util.*;

public class SensorReadout {

private Date time;

private Car car;

private String description;

private SensorReadout next;

protected SensorReadout(Date time,Car car,String description) {

this.time=time;

this.car=car;

this.description=description;

this.next=null;

}

public Car getCar() {

return car;

}

public Date getTime() {

return time;

}

public String getDescription() {

return description;

}

public SensorReadout getNext() {

return next;

www.db4o.com

}

public void append(SensorReadout readout) {

if(next==null) {

next=readout;

}

else {

next.append(readout);

}

public int countElements() {

return (next==null ? 1 : next.countElements()+1);

}

public String toString() {

return car+" : "+time+" : "+description;

}

Our car only maintains an association to a 'head' sensor readout now.

package com.db4odoc.f1.chapter6;

import java.util.*;

public class Car {

private String model;

private Pilot pilot;

private SensorReadout history;

public Car(String model) {

this.model=model;

this.pilot=null;

this.history=null;

}

www.db4o.com

public Pilot getPilot() {

return pilot;

}

public void setPilot(Pilot pilot) {

this.pilot=pilot;

}

public String getModel() {

return model;

}

public SensorReadout getHistory() {

return history;

}

public void snapshot() {

appendToHistory(new TemperatureSensorReadout(

new Date(),this,"oil",pollOilTemperature()));

appendToHistory(new TemperatureSensorReadout(

new Date(),this,"water",pollWaterTemperature()));

appendToHistory(new PressureSensorReadout(

new Date(),this,"oil",pollOilPressure()));

}

protected double pollOilTemperature() {

return 0.1*countHistoryElements();

}

protected double pollWaterTemperature() {

return 0.2*countHistoryElements();

}

protected double pollOilPressure() {

return 0.3*countHistoryElements();

}

public String toString() {

return model+"["+pilot+"]/"+countHistoryElements();

}

www.db4o.com

private int countHistoryElements() {

return (history==null ? 0 : history.countElements());

}

private void appendToHistory(SensorReadout readout) {

if(history==null) {

history=readout;

}

else {

history.append(readout);

}

8.1. Storing and updating

No surprises here.

// storeCar

Pilot pilot = new Pilot("Rubens Barrichello", 99);

Car car = new Car("BMW");

car.setPilot(pilot);

db.store(car);

Now we would like to build a sensor readout chain. We already know about the update depth trap, so

we configure this first.

// takeManySnapshots

Configuration config = Db4o.newConfiguration();

config.objectClass(Car.class).cascadeOnUpdate(true);

ObjectContainer db = Db4o.openFile(config, DB4OFILENAME);

ObjectSet result = db.queryByExample(Car.class);

www.db4o.com

if (result.hasNext()) {

Car car = (Car) result.next();

for (int i = 0; i < 5; i++) {

car.snapshot();

}

db.store(car);

}

db.close();

8.2. Retrieving

Now that we have a sufficiently deep structure, we'll retrieve it from the database and traverse it.

First let's verify that we indeed have taken lots of snapshots.

// retrieveAllSnapshots

ObjectSet result = db.queryByExample(SensorReadout.class);

while (result.hasNext()) {

System.out.println(result.next());

}

OUTPUT:

BMW[Rubens Barrichello/99]/9 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.30000000000000004

BMW[Rubens Barrichello/99]/10 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.8

BMW[Rubens Barrichello/99]/11 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 1.5

BMW[Rubens Barrichello/99]/12 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.6000000000000001

BMW[Rubens Barrichello/99]/13 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 1.4000000000000001

BMW[Rubens Barrichello/99]/14 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 2.4

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

www.db4o.com

temp : 0.9

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.0

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.2

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 0.6

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 2.0

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 3.3

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 1.2000000000000002

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 2.6

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 4.2

All these readouts belong to one linked list, so we should be able to access them all by just traversing

our list structure.

// retrieveSnapshotsSequentially

ObjectSet result = db.queryByExample(Car.class);

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout = readout.getNext();

}

OUTPUT:

BMW[Rubens Barrichello/99]/5 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.0

BMW[Rubens Barrichello/99]/5 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.2

www.db4o.com

BMW[Rubens Barrichello/99]/5 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 0.6

BMW[Rubens Barrichello/99]/5 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.30000000000000004

null : null : null temp : 0.0

Ouch! What's happening here?

.

8.2.1. Activation depth

Deja vu - this is just the other side of the update depth issue.

db4o cannot track when you are traversing references from objects retrieved from the database. So it

would always have to return 'complete' object graphs on retrieval - in the worst case this would boil

down to pulling the whole database content into memory for a single query.

This is absolutely undesirable in most situations, so db4o provides a mechanism to give the client fine-

grained control over how much he wants to pull out of the database when asking for an object. This

mechanism is called

activation depth

and works quite similar to our familiar update depth.

The default activation depth for any object is 5, so our example above runs into nulls after traversing 5

references.

We can dynamically ask objects to activate their member references. This allows us to retrieve each

single sensor readout in the list from the database just as needed.

// retrieveSnapshotsSequentiallyImproved

ObjectSet result = db.queryByExample(Car.class);

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

db.activate(readout, 1);

System.out.println(readout);

readout = readout.getNext();

}

www.db4o.com

OUTPUT:

BMW[Rubens Barrichello/99]/5 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.0

BMW[Rubens Barrichello/99]/5 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.2

BMW[Rubens Barrichello/99]/5 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 0.6

BMW[Rubens Barrichello/99]/5 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.30000000000000004

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.8

BMW[Rubens Barrichello/99]/7 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 1.5

BMW[Rubens Barrichello/99]/8 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.6000000000000001

BMW[Rubens Barrichello/99]/9 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 1.4000000000000001

BMW[Rubens Barrichello/99]/10 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 2.4

BMW[Rubens Barrichello/99]/11 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.9

BMW[Rubens Barrichello/99]/12 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 2.0

BMW[Rubens Barrichello/99]/13 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 3.3

BMW[Rubens Barrichello/99]/14 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 1.2000000000000002

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 2.6

BMW[Rubens Barrichello/99]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 4.2

Note that 'cut' references may also influence the behavior of your objects: in this case the length of the

list is calculated dynamically, and therefor constrained by activation depth.

Instead of dynamically activating subgraph elements, you can configure activation depth statically, too.

We can tell our SensorReadout class objects to cascade activation automatically, for example.

www.db4o.com

// retrieveSnapshotsSequentiallyCascade

Configuration config = Db4o.newConfiguration();

config.objectClass(TemperatureSensorReadout.class).cascadeOnActivate(

true);

ObjectContainer db = Db4o.openFile(config, DB4OFILENAME);

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout = readout.getNext();

}

db.close();

OUTPUT:

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.0

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.2

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 0.6

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.30000000000000004

BMW[Rubens Barrichello/99]/6 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.8

null : null : null pressure : 0.0

You have to be very careful, though. Activation issues are tricky. Db4o provides a wide range of

configuration features to control activation depth at a very fine-grained level. You'll find those triggers

in com.db4o.config.Configuration and the associated ObjectClass and ObjectField classes.

www.db4o.com

Don't forget to clean up the database.

// deleteAll

ObjectSet result=db.queryByExample(new Object());

while(result.hasNext()) {

db.delete(result.next());

}

8.3. Conclusion

Now we should have the tools at hand to work with arbitrarily complex object graphs. But so far we

have only been working forward, hoping that the changes we apply to our precious data pool are

correct. What if we have to roll back to a previous state due to some failure? In the next chapter we

will introduce the db4o transaction concept.

8.4. Full source

package com.db4odoc.f1.chapter6;

import java.io.*;

import com.db4o.*;

import com.db4o.config.*;

import com.db4odoc.f1.*;

public class DeepExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home") + "/formula1.db4o";

public static void main(String[] args) {

new File(DB4OFILENAME).delete();

ObjectContainer db = Db4o.openFile(DB4OFILENAME);

storeCar(db);

db.close();

db = Db4o.openFile(DB4OFILENAME);

www.db4o.com

db.close();

takeManySnapshots();

db = Db4o.openFile(DB4OFILENAME);

retrieveAllSnapshots(db);

db.close();

db = Db4o.openFile(DB4OFILENAME);

retrieveSnapshotsSequentially(db);

retrieveSnapshotsSequentiallyImproved(db);

db.close();

retrieveSnapshotsSequentiallyCascade();

}

public static void storeCar(ObjectContainer db) {

Pilot pilot = new Pilot("Rubens Barrichello", 99);

Car car = new Car("BMW");

car.setPilot(pilot);

db.store(car);

}

public static void takeManySnapshots() {

Configuration config = Db4o.newConfiguration();

config.objectClass(Car.class).cascadeOnUpdate(true);

ObjectContainer db = Db4o.openFile(config, DB4OFILENAME);

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

for (int i = 0; i < 5; i++) {

car.snapshot();

}

db.store(car);

}

db.close();

}

public static void retrieveAllSnapshots(ObjectContainer db) {

ObjectSet result = db.queryByExample(SensorReadout.class);

while (result.hasNext()) {

System.out.println(result.next());

}

www.db4o.com

public static void retrieveSnapshotsSequentially(ObjectContainer

db) {

ObjectSet result = db.queryByExample(Car.class);

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout = readout.getNext();

}

public static void retrieveSnapshotsSequentiallyCascade() {

Configuration config = Db4o.newConfiguration();

config.objectClass(TemperatureSensorReadout.class).cascadeOnActivate(

true);

ObjectContainer db = Db4o.openFile(config, DB4OFILENAME);

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout = readout.getNext();

}

db.close();

}

public static void

retrieveSnapshotsSequentiallyImproved(ObjectContainer db) {

ObjectSet result = db.queryByExample(Car.class);

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

db.activate(readout, 1);

System.out.println(readout);

readout = readout.getNext();

}

www.db4o.com

}

www.db4o.com

.

9. Transactions

Probably you have already wondered how db4o handles concurrent access to a single database. Just as

any other DBMS, db4o provides a transaction mechanism. Before we take a look at multiple, perhaps

even remote, clients accessing a db4o instance in parallel, we will introduce db4o transaction concepts

in isolation.

9.1. Commit and rollback

You may not have noticed it, but we have already been working with transactions from the first chapter

on. By definition, you are always working inside a transaction when interacting with db4o. A

transaction is implicitly started when you open a container, and the current transaction is implicitly

committed when you close it again. So the following code snippet to store a car is semantically

identical to the ones we have seen before; it just makes the commit explicit.

// storeCarCommit

Pilot pilot=new Pilot("Rubens Barrichello",99);

Car car=new Car("BMW");

car.setPilot(pilot);

db.store(car);

db.commit();

// listAllCars

ObjectSet result=db.queryByExample(Car.class);

listResult(result);

OUTPUT:

1

BMW[Rubens Barrichello/99]/0

www.db4o.com

However, we can also rollback the current transaction, resetting the state of our database to the last

commit point.

// storeCarRollback

Pilot pilot=new Pilot("Michael Schumacher",100);

Car car=new Car("Ferrari");

car.setPilot(pilot);

db.store(car);

db.rollback();

// listAllCars

ObjectSet result=db.queryByExample(Car.class);

listResult(result);

OUTPUT:

1

BMW[Rubens Barrichello/99]/0

9.2. Refresh live objects

There's one problem, though: We can roll back our database, but this cannot automagically trigger a

rollback for our live objects.

// carSnapshotRollback

ObjectSet result=db.queryByExample(new Car("BMW"));

Car car=(Car)result.next();

car.snapshot();

db.store(car);

www.db4o.com

db.rollback();

System.out.println(car);

OUTPUT:

BMW[Rubens Barrichello/99]/3

We will have to explicitly refresh our live objects when we suspect they may have participated in a

rollback transaction.

// carSnapshotRollbackRefresh

ObjectSet result=db.queryByExample(new Car("BMW"));

Car car=(Car)result.next();

car.snapshot();

db.store(car);

db.rollback();

db.ext().refresh(car,Integer.MAX_VALUE);

System.out.println(car);

OUTPUT:

BMW[Rubens Barrichello/99]/0

What is this #ext() construct good for? Well, it provides some functionality that is in itself stable, but

the API may still be subject to change. As soon as we are confident that no more changes will occur,

#ext() functionality will be transferred to the common ObjectContainer API.

Finally, we clean up again.

// deleteAll

ObjectSet result=db.queryByExample(new Object());

www.db4o.com

while(result.hasNext()) {

db.delete(result.next());

}

9.3. Conclusion

We have seen how transactions work for a single client. In the Client/Server chapter we will see how

the transaction concept extends to multiple clients, whether they are located within the same VM or on

a remote machine.

Let't first revisit Activation again in the next chapter and take a look at how db4o can take care of our

Object lifecycle automatically.

9.4. Full source

package com.db4odoc.f1.chapter6;

import java.io.*;

import com.db4o.*;

import com.db4odoc.f1.*;

public class TransactionExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home") + "/formula1.db4o";

public static void main(String[] args) {

new File(DB4OFILENAME).delete();

ObjectContainer

db=Db4oEmbedded.openFile(Db4oEmbedded.newConfiguration(),

DB4OFILENAME);

try {

storeCarCommit(db);

db.close();

db=Db4oEmbedded.openFile(Db4oEmbedded.newConfiguration(),

DB4OFILENAME);

www.db4o.com

listAllCars(db);

storeCarRollback(db);

db.close();

db=Db4oEmbedded.openFile(Db4oEmbedded.newConfiguration(),

DB4OFILENAME);

listAllCars(db);

carSnapshotRollback(db);

carSnapshotRollbackRefresh(db);

}

finally {

db.close();

}

public static void storeCarCommit(ObjectContainer db) {

Pilot pilot=new Pilot("Rubens Barrichello",99);

Car car=new Car("BMW");

car.setPilot(pilot);

db.store(car);

db.commit();

}

public static void listAllCars(ObjectContainer db) {

ObjectSet result=db.queryByExample(Car.class);

listResult(result);

}

public static void storeCarRollback(ObjectContainer db) {

Pilot pilot=new Pilot("Michael Schumacher",100);

Car car=new Car("Ferrari");

car.setPilot(pilot);

db.store(car);

db.rollback();

}

public static void carSnapshotRollback(ObjectContainer db) {

ObjectSet result=db.queryByExample(new Car("BMW"));

Car car=(Car)result.next();

car.snapshot();

db.store(car);

db.rollback();

www.db4o.com

System.out.println(car);

}

public static void carSnapshotRollbackRefresh(ObjectContainer db)

{

ObjectSet result=db.queryByExample(new Car("BMW"));

Car car=(Car)result.next();

car.snapshot();

db.store(car);

db.rollback();

db.ext().refresh(car,Integer.MAX_VALUE);

System.out.println(car);

}

www.db4o.com

.

10. Transparent Activation

Let's take a second look at the concept of Activation. We have seen how db4o uses a "depth" concept

by default to activate objects to a specific depth when they are returned from a query.

Wouldn't it be a lot nicer, if an application would never have to worry about activating objects and if

db4o could handle things transparently for us? This is what Transparent Activation was developed for.

10.1. The Activation Problem

We can reuse most of the code from the Deep Graphs chapter and get it to work with Transparent

Activation.

As a first step we should fill up our database with Car, Pilot and SensorReadout objects, so we have

some objects to work with.

// storeCarAndSnapshots

Pilot pilot = new Pilot("Kimi Raikkonen", 110);

Car car = new Car("Ferrari");

car.setPilot(pilot);

for (int i = 0; i < 5; i++) {

car.snapshot();

}

db.store(car);

If we now rerun the code to traverse all cars and their sensor readings, we are again confronted with

the same problem that we had before, we end up with some leaves of our object graph being null.

// retrieveSnapshotsSequentially

ObjectSet result = db.queryByExample(Car.class);

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

www.db4o.com

System.out.println(readout);

readout = readout.getNext();

}

OUTPUT:

Ferrari[Kimi Raikkonen/110]/5 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.0

Ferrari[Kimi Raikkonen/110]/5 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.2

Ferrari[Kimi Raikkonen/110]/5 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 0.6

Ferrari[Kimi Raikkonen/110]/5 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.30000000000000004

null : null : null temp : 0.0

10.2. Turning On Transparent Activation

Let's configure db4o to run in Transparent Activation mode and let's try again:

// retrieveSnapshotsSequentiallyTA

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().add(new TransparentActivationSupport());

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout = readout.getNext();

}

db.close();

www.db4o.com

OUTPUT:

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.0

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.2

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 0.6

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.30000000000000004

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 0.8

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 1.5

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.6000000000000001

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 1.4000000000000001

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 2.4

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 0.9

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 2.0

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 3.3

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

temp : 1.2000000000000002

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : water

temp : 2.6

Ferrari[Kimi Raikkonen/110]/15 : Fri Jun 12 20:59:26 GMT 2009 : oil

pressure : 4.2

Wow it worked! Is it really that easy? Principally yes. When db4o is run in Transparent Activation mode

there are no surprises with null members that have not yet been read from the database.

10.3. Implementing Activatable

www.db4o.com

When Transparent Activation is turned on, all objects that do not implement the com.db4o.ta.

Activatable interface will be fully activated when they are used.

Although we wont get any surprises with null objects when we work in this mode, access to the root of

a deep graph may load the entire graph into memory and that can take a long time and use a lot of

memory.

To prevent immediate activation of a class you can implement the Activatable interface. Whenever

db4o comes across an Activatable object while activating a graph of objects it will stop traversing the

graph any deeper. db4o will "know" that it can activate Activatable objects on demand, so there is no

reason to continue activation until these objects are really needed.

For demonstration purposes we have made all classes used in this example Activatable and we have

also added all the code required to activate by hand.

Let's take a look at the Activatable version of our Car class:

package com.db4odoc.f1.chapter8;

import java.util.*;

import com.db4o.activation.*;

import com.db4o.ta.*;

public class Car implements Activatable {

private String model;

private Pilot pilot;

private SensorReadout history;

private transient Activator _activator;

public Car(String model) {

this.model=model;

this.pilot=null;

this.history=null;

}

public Pilot getPilot() {

activate(ActivationPurpose.READ);

www.db4o.com

return pilot;

}

public Pilot getPilotWithoutActivation() {

return pilot;

}

public void setPilot(Pilot pilot) {

this.pilot=pilot;

}

public String getModel() {

activate(ActivationPurpose.READ);

return model;

}

public SensorReadout getHistory() {

activate(ActivationPurpose.READ);

return history;

}

public void snapshot() {

activate(ActivationPurpose.WRITE);

appendToHistory(new TemperatureSensorReadout(

new Date(),this,"oil",pollOilTemperature()));

appendToHistory(new TemperatureSensorReadout(

new Date(),this,"water",pollWaterTemperature()));

appendToHistory(new PressureSensorReadout(

new Date(),this,"oil",pollOilPressure()));

}

protected double pollOilTemperature() {

return 0.1*countHistoryElements();

}

protected double pollWaterTemperature() {

return 0.2*countHistoryElements();

}

protected double pollOilPressure() {

return 0.3*countHistoryElements();

www.db4o.com

}

public String toString() {

activate(ActivationPurpose.READ);

return model+"["+pilot+"]/"+countHistoryElements();

}

private int countHistoryElements() {

activate(ActivationPurpose.READ);

return (history==null ? 0 : history.countElements());

}

private void appendToHistory(SensorReadout readout) {

activate(ActivationPurpose.WRITE);

if(history==null) {

history=readout;

}

else {

history.append(readout);

}

public void activate(ActivationPurpose purpose) {

if(_activator != null) {

_activator.activate(purpose);

}

public void bind(Activator activator) {

if (_activator == activator) {

return;

}

if (activator != null && _activator != null) {

throw new IllegalStateException();

}

_activator = activator;

}

www.db4o.com

Can you spot the member _activator, all the # activate() calls and the two methods # activate() and #

bind(Activator) at the end?

An Activatable class should store the Activator that db4o provides to the bind method in a transient

variable and call # activate() on this Activator before any field is accessed.

If the object is already activated, the method will return immediately. If it is not, activation will happen

at this time.

We have added the #getPilotWithoutActivation() method to the Car class to be able to demonstrate.

// demonstrateTransparentActivation

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().add(new TransparentActivationSupport());

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

System.out

.println("#getPilotWithoutActivation() before the car is

activated");

System.out.println(car.getPilotWithoutActivation());

System.out.println("calling #getPilot() activates the car

object");

System.out.println(car.getPilot());

System.out

.println("#getPilotWithoutActivation() after the car is

activated");

System.out.println(car.getPilotWithoutActivation());

}

db.close();

www.db4o.com

OUTPUT:

#getPilotWithoutActivation() before the car is activated

null

calling #getPilot() activates the car object

Kimi Raikkonen/110

#getPilotWithoutActivation() after the car is activated

Kimi Raikkonen/110

10.4. Where Enhancement can help

If all this Activatable code in a persistent class looked like a lot of typing work, do not worry: db4o

comes with a tool to add this code automatically to all of your persistent classes. Read more about it in

the chapter on Enhancement .

As a final step we should clean up the database again.

// deleteAll

ObjectSet result=db.queryByExample(new Object());

while(result.hasNext()) {

db.delete(result.next());

}

10.5. Conclusion

This was just a short introduction to Transparent Activation and what it can do for you. For more

detailed information please see the pages on Transparent Activation in our online reference or in your

offline copy of the Reference documentation.

10.6. Full source

package com.db4odoc.f1.chapter8;

www.db4o.com

import java.io.*;

import com.db4o.*;

import com.db4o.config.*;

import com.db4o.ta.*;

import com.db4odoc.f1.*;

public class TransparentActivationExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home")

+ "/formula1.db4o";

public static void main(String[] args) throws Exception {

new File(DB4OFILENAME).delete();

ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

try {

storeCarAndSnapshots(db);

db.close();

db =

Db4oEmbedded.openFile(Db4oEmbedded.newConfiguration(),

DB4OFILENAME);

retrieveSnapshotsSequentially(db);

db.close();

retrieveSnapshotsSequentiallyTA();

demonstrateTransparentActivation();

} finally {

db.close();

}

public static void storeCarAndSnapshots(ObjectContainer db) {

Pilot pilot = new Pilot("Kimi Raikkonen", 110);

Car car = new Car("Ferrari");

car.setPilot(pilot);

for (int i = 0; i < 5; i++) {

car.snapshot();

}

db.store(car);

www.db4o.com

}

public static void retrieveSnapshotsSequentially(ObjectContainer

db) {

ObjectSet result = db.queryByExample(Car.class);

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout = readout.getNext();

}

public static void retrieveSnapshotsSequentiallyTA() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

config.common().add(new TransparentActivationSupport());

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout = readout.getNext();

}

db.close();

}

public static void demonstrateTransparentActivation() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

config.common().add(new TransparentActivationSupport());

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

www.db4o.com

System.out

.println("#getPilotWithoutActivation() before the

car is activated");

System.out.println(car.getPilotWithoutActivation());

System.out.println("calling #getPilot() activates the car

object");

System.out.println(car.getPilot());

System.out

.println("#getPilotWithoutActivation() after the

car is activated");

System.out.println(car.getPilotWithoutActivation());

}

db.close();

}

www.db4o.com

.

11. Transparent Persistence

The problem of updating deep object structures was briefly outlined in Structured objects chapter .

Update Depth configuration setting gives a user a certain control over the object updating and allows

to find a balance between performance and convenient object storage code. However, this balance is

far from ideal:

- when update depth is zero, each piece of code should "know" how many levels of objects should be

updated; this potentially creates lots of problems when the objects are refactored;

- when update depth is maximum performance can become very poor as many unchanged objects will

be stored unnecessary.

The solution to this problem is to let db4o engine decide, which objects were modified and should be

stored. This feature was introduced in db4o version 7.1 and was named Transparent Persistence. So

how does it work?

1. Database should be configured to use TransparentPersistenceSupport.

2. Persistent classes available for Transparent Persistence must implement Activatable interface. This

interface provides a #bind() method to bind an object to the object container's activator.

3. The object is bound to the ObjectContainer when it is first stored or instantiated from the database.

4. When an object field is modified in the runtime, #activate() method is called to register the object

to be stored with the next commit. The difference from Transparent Activation is in the activation

purpose:

ActivationPurpose.WRITE

is used for TP.

5. When the transaction is committed or the database is closed, db4o traverses the list of modified

Activatable objects and persists them.

Well, that's enough of theory, let's look at an example.

11.1. Transparent Persistence Example

We will use Car and SensorReadout classes from Deep Graphs chapter. These are persistent classes, so

if we want to use Transparent Persistence, we should make them "visible" to Transparent Persistence

by implementing

Activatable

interface.

package com.db4odoc.f1.chapter9;

import java.util.*;

import com.db4o.activation.*;

import com.db4o.ta.*;

www.db4o.com

public class Car implements Activatable {

private String model;

private SensorReadout history;

private transient Activator _activator;

public Car(String model) {

this.model=model;

this.history=null;

}

public String getModel() {

activate(ActivationPurpose.READ);

return model;

}

public SensorReadout getHistory() {

activate(ActivationPurpose.READ);

return history;

}

public void snapshot() {

activate(ActivationPurpose.WRITE);

appendToHistory(new TemperatureSensorReadout(

new Date(),this,"oil",pollOilTemperature()));

appendToHistory(new TemperatureSensorReadout(

new Date(),this,"water",pollWaterTemperature()));

}

protected double pollOilTemperature() {

return 0.1*countHistoryElements();

}

protected double pollWaterTemperature() {

return 0.2*countHistoryElements();

}

public String toString() {

activate(ActivationPurpose.READ);

return model+"/"+countHistoryElements();

}

www.db4o.com

private int countHistoryElements() {

activate(ActivationPurpose.READ);

return (history==null ? 0 : history.countElements());

}

private void appendToHistory(SensorReadout readout) {

activate(ActivationPurpose.WRITE);

if(history==null) {

history=readout;

}

else {

history.append(readout);

}

public void activate(ActivationPurpose purpose) {

if(_activator != null) {

_activator.activate(purpose);

}

public void bind(Activator activator) {

if (_activator == activator) {

return;

}

if (activator != null && _activator != null) {

throw new IllegalStateException();

}

_activator = activator;

}

Note, that we've added an

activator

field,

bind

and

activate

methods to implement

Activatable

interface. In addition to that all methods that read or write object fields has got

activate

calls with a

corresponding purpose.

Similar modifications should be done to the SensorReadout class.

Now we are ready to test how Transparent Persistence work. First we should configure the database to

www.db4o.com

use TransparentPersistenceSupport before storing objects:

// storeCarAndSnapshots

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().add(new TransparentPersistenceSupport());

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

Car car = new Car("Ferrari");

for (int i = 0; i < 3; i++) {

car.snapshot();

}

db.store(car);

db.close();

Ok, all the objects are stored.

Now, let's retrieve all the stored objects and modify them:

// modifySnapshotHistory

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().add(new TransparentPersistenceSupport());

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

System.out.println("Read all sensors and modify the description:");

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout.setDescription("Modified: " +

readout.getDescription());

readout = readout.getNext();

}

db.commit();

}

db.close();

www.db4o.com

You can see that we do not have to call

#store

any more - all the objects are stored when

#commit

is

called.

Let's test that the modifications actually reached the database:

// readSnapshotHistory

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

config.common().add(new TransparentPersistenceSupport());

ObjectContainer db = Db4oEmbedded.openFile(config, DB4OFILENAME);

System.out.println("Read all modified sensors:");

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout = readout.getNext();

}

db.close();

Yes, it is all as it should be. If you want to see the difference without Transparent Persistence, run the

same examples without

TransparentPersistenceSupport

.

11.2. Transparent Persistence Enhancement

As we saw before enhancement tools can simplify the process for Transparent Activation. The same

applies to Transparent Persistence. Actually Transparent Persistence enhancement implicitly provides

TA for enhanced classes.

For more information please refer to Enhancement chapter .

11.3. Conclusion

Transparent Persistence can considerably simplify the development process at the same time providing

considerable performance benefits. For more information on Transparent Persistence please refer to

our online reference or your offline copy of the Reference documentation.

11.4. Full source

www.db4o.com

package com.db4odoc.f1.chapter9;

import java.io.*;

import com.db4o.*;

import com.db4o.config.*;

import com.db4o.ta.*;

import com.db4odoc.f1.*;

public class TransparentPersistenceExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home") + "/formula1.db4o";

public static void main(String[] args) throws Exception {

new File(DB4OFILENAME).delete();

storeCarAndSnapshots();

modifySnapshotHistory();

readSnapshotHistory();

}

public static void storeCarAndSnapshots() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

config.common().add(new TransparentPersistenceSupport());

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

Car car = new Car("Ferrari");

for (int i = 0; i < 3; i++) {

car.snapshot();

}

db.store(car);

db.close();

}

public static void modifySnapshotHistory() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

www.db4o.com

config.common().add(new TransparentPersistenceSupport());

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

System.out.println("Read all sensors and modify the

description:");

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout.setDescription("Modified: " +

readout.getDescription());

readout = readout.getNext();

}

db.commit();

}

db.close();

}

public static void readSnapshotHistory() {

EmbeddedConfiguration config =

Db4oEmbedded.newConfiguration();

config.common().add(new TransparentPersistenceSupport());

ObjectContainer db = Db4oEmbedded.openFile(config,

DB4OFILENAME);

System.out.println("Read all modified sensors:");

ObjectSet result = db.queryByExample(Car.class);

if (result.hasNext()) {

Car car = (Car) result.next();

SensorReadout readout = car.getHistory();

while (readout != null) {

System.out.println(readout);

readout = readout.getNext();

}

db.close();

}

www.db4o.com

}

package com.db4odoc.f1.chapter9;

import java.util.*;

import com.db4o.activation.*;

import com.db4o.ta.*;

public class Car implements Activatable {

private String model;

private SensorReadout history;

private transient Activator _activator;

public Car(String model) {

this.model=model;

this.history=null;

}

public String getModel() {

activate(ActivationPurpose.READ);

return model;

}

public SensorReadout getHistory() {

activate(ActivationPurpose.READ);

return history;

}

public void snapshot() {

activate(ActivationPurpose.WRITE);

appendToHistory(new TemperatureSensorReadout(

new Date(),this,"oil",pollOilTemperature()));

appendToHistory(new TemperatureSensorReadout(

new Date(),this,"water",pollWaterTemperature()));

}

protected double pollOilTemperature() {

www.db4o.com

return 0.1*countHistoryElements();

}

protected double pollWaterTemperature() {

return 0.2*countHistoryElements();

}

public String toString() {

activate(ActivationPurpose.READ);

return model+"/"+countHistoryElements();

}

private int countHistoryElements() {

activate(ActivationPurpose.READ);

return (history==null ? 0 : history.countElements());

}

private void appendToHistory(SensorReadout readout) {

activate(ActivationPurpose.WRITE);

if(history==null) {

history=readout;

}

else {

history.append(readout);

}

public void activate(ActivationPurpose purpose) {

if(_activator != null) {

_activator.activate(purpose);

}

public void bind(Activator activator) {

if (_activator == activator) {

return;

}

if (activator != null && _activator != null) {

throw new IllegalStateException();

}

_activator = activator;

www.db4o.com

}

package com.db4odoc.f1.chapter9;

import java.util.*;

import com.db4o.activation.*;

import com.db4o.ta.*;

public class SensorReadout implements Activatable {

private Date time;

private Car car;

private String description;

private SensorReadout next;

private transient Activator _activator;

protected SensorReadout(Date time,Car car,String description) {

this.time=time;

this.car=car;

this.description=description;

this.next=null;

}

public Car getCar() {

activate(ActivationPurpose.READ);

return car;

}

public Date getTime() {

activate(ActivationPurpose.READ);

return time;

}

public String getDescription() {

activate(ActivationPurpose.READ);

return description;

www.db4o.com

}

public void setDescription(String description) {

activate(ActivationPurpose.WRITE);

this.description = description;

}

public SensorReadout getNext() {

activate(ActivationPurpose.READ);

return next;

}

public void append(SensorReadout readout) {

activate(ActivationPurpose.WRITE);

if(next==null) {

next=readout;

}

else {

next.append(readout);

}

public int countElements() {

activate(ActivationPurpose.READ);

return (next==null ? 1 : next.countElements()+1);

}

public String toString() {

activate(ActivationPurpose.READ);

return car+" : "+time+" : "+description;

}

public void activate(ActivationPurpose purpose) {

if(_activator != null) {

_activator.activate(purpose);

}

public void bind(Activator activator) {

if (_activator == activator) {

return;

www.db4o.com

}

if (activator != null && _activator != null) {

throw new IllegalStateException();

}

_activator = activator;

}

package com.db4odoc.f1.chapter9;

import java.util.*;

import com.db4o.activation.*;

public class TemperatureSensorReadout extends SensorReadout {

private double temperature;

public TemperatureSensorReadout(

Date time,Car car,

String description,double temperature) {

super(time,car,description);

this.temperature=temperature;

}

public double getTemperature() {

activate(ActivationPurpose.READ);

return temperature;

}

public String toString() {

return super.toString()+" temp : "+temperature;

}

www.db4o.com

.

12. Client/Server

Now that we have seen how transactions work in db4o conceptually, we are prepared to tackle

concurrently executing transactions.

We start by preparing our database in the familiar way.

// setFirstCar

Pilot pilot = new Pilot("Rubens Barrichello", 99);

Car car = new Car("BMW");

car.setPilot(pilot);

db.store(car);

// setSecondCar

Pilot pilot = new Pilot("Michael Schumacher", 100);

Car car = new Car("Ferrari");

car.setPilot(pilot);

db.store(car);

12.1. Embedded server

From the API side, there's no real difference between transactions executing concurrently within the

same VM and transactions executed against a remote server. To use concurrent transactions within a

single VM , we just open a db4o server on our database file, directing it to run on port 0, thereby

declaring that no networking will take place.

// accessLocalServer

ObjectServer server = Db4oClientServer.openServer(Db4oClientServer

.newServerConfiguration(), DB4OFILENAME, 0);

www.db4o.com

try {

ObjectContainer client = server.openClient();

// Do something with this client, or open more clients

client.close();

} finally {

server.close();

}

Again, we will delegate opening and closing the server to our environment to focus on client

interactions.

// queryLocalServer

ObjectContainer client = server.openClient();

listResult(client.queryByExample(new Car(null)));

client.close();

OUTPUT:

2

Ferrari[Michael Schumacher/100]/0

BMW[Rubens Barrichello/99]/0

The transaction level in db4o is

read committed

. However, each client container maintains its own

weak reference cache of already known objects. To make all changes committed by other clients

immediately, we have to explicitly refresh known objects from the server. We will delegate this task to

a specialized version of our #listResult() method.

public static void listRefreshedResult(ObjectContainer

container,ObjectSet result,int depth) {

System.out.println(result.size());

while(result.hasNext()) {

Object obj = result.next();

container.ext().refresh(obj, depth);

www.db4o.com

System.out.println(obj);

}

// demonstrateLocalReadCommitted

ObjectContainer client1 = server.openClient();

ObjectContainer client2 = server.openClient();

Pilot pilot = new Pilot("David Coulthard", 98);

ObjectSet result = client1.queryByExample(new Car("BMW"));

Car car = (Car) result.next();

car.setPilot(pilot);

client1.store(car);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.commit();

listResult(client1.queryByExample(Car.class));

listRefreshedResult(client2, client2.queryByExample(Car.class), 2);

client1.close();

client2.close();

OUTPUT:

2

Ferrari[Michael Schumacher/100]/0

BMW[David Coulthard/98]/0

2

Ferrari[Michael Schumacher/100]/0

BMW[Rubens Barrichello/99]/0

2

Ferrari[Michael Schumacher/100]/0

BMW[David Coulthard/98]/0

2

Ferrari[Michael Schumacher/100]/0

BMW[David Coulthard/98]/0

www.db4o.com

Simple rollbacks just work as you might expect now.

// demonstrateLocalRollback

ObjectContainer client1 = server.openClient();

ObjectContainer client2 = server.openClient();

ObjectSet result = client1.queryByExample(new Car("BMW"));

Car car = (Car) result.next();

car.setPilot(new Pilot("Someone else", 0));

client1.store(car);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.rollback();

client1.ext().refresh(car, 2);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.close();

client2.close();

OUTPUT:

2

Ferrari[Michael Schumacher/100]/0

BMW[Someone else/0]/0

2

Ferrari[Michael Schumacher/100]/0

BMW[David Coulthard/98]/0

2

Ferrari[Michael Schumacher/100]/0

BMW[David Coulthard/98]/0

2

Ferrari[Michael Schumacher/100]/0

BMW[David Coulthard/98]/0

12.2. Networking

www.db4o.com

From here it's only a small step towards operating db4o over a TCP/IP network. We just specify a port

number greater than zero and set up one or more accounts for our client(s).

// accessRemoteServer

ObjectServer server = Db4oClientServer.openServer(Db4oClientServer

.newServerConfiguration(), DB4OFILENAME, PORT);

server.grantAccess(USER, PASSWORD);

try {

ObjectContainer client = Db4oClientServer.openClient(

Db4oClientServer.newClientConfiguration(), "localhost",

PORT, USER, PASSWORD);

// Do something with this client, or open more clients

client.close();

} finally {

server.close();

}

The client connects providing host, port, user name and password.

// queryRemoteServer

ObjectContainer client = Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), "localhost", port, user,

password);

listResult(client.queryByExample(new Car(null)));

client.close();

OUTPUT:

2

Ferrari[Michael Schumacher/100]/0

BMW[David Coulthard/98]/0

www.db4o.com

Everything else is absolutely identical to the local server examples above.

// demonstrateRemoteReadCommitted

ObjectContainer client1 =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), "localhost", port, user,

password);

ObjectContainer client2 =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), "localhost", port, user,

password);

Pilot pilot = new Pilot("Jenson Button", 97);

ObjectSet result = client1.queryByExample(new Car(null));

Car car = (Car) result.next();

car.setPilot(pilot);

client1.store(car);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.commit();

listResult(client1.queryByExample(new Car(null)));

listRefreshedResult(client2, client2.queryByExample(Car.class), 2);

client1.close();

client2.close();

OUTPUT:

2

Ferrari[Jenson Button/97]/0

BMW[David Coulthard/98]/0

2

Ferrari[Michael Schumacher/100]/0

BMW[David Coulthard/98]/0

2

Ferrari[Jenson Button/97]/0

BMW[David Coulthard/98]/0

2

Ferrari[Jenson Button/97]/0

BMW[David Coulthard/98]/0

www.db4o.com

// demonstrateRemoteRollback

ObjectContainer client1 =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), "localhost", port, user,

password);

ObjectContainer client2 =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), "localhost", port, user,

password);

ObjectSet result = client1.queryByExample(new Car(null));

Car car = (Car) result.next();

car.setPilot(new Pilot("Someone else", 0));

client1.store(car);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.rollback();

client1.ext().refresh(car, 2);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.close();

client2.close();

OUTPUT:

2

Ferrari[Someone else/0]/0

BMW[David Coulthard/98]/0

2

Ferrari[Jenson Button/97]/0

BMW[David Coulthard/98]/0

2

Ferrari[Jenson Button/97]/0

BMW[David Coulthard/98]/0

2

Ferrari[Jenson Button/97]/0

BMW[David Coulthard/98]/0

www.db4o.com

12.3. Native Queries in Client/Server mode

A quite subtle problem may occur if you're using Native Queries as anonymous (or just non-static)

inner classes in Client/Server mode. Anonymous/non-static inner class instances carry a synthetic field

referencing their outer class instance - this is just Java's way of implementing inner class access to

fields or final method locals of the outer class without introducing any notion of inner classes at all at

the bytecode level. If such a non-static inner class predicate cannot be converted to S.O.D.A. form on

the client, it will be wrapped into an evaluation and passed to the server, introducing the same

problem already mentioned in the evaluation chapter : db4o will try to transfer the evaluation, using

the standard platform serialization mechanism. If this fails, it will just try to pass it to the server via

db4o marshalling. However, this may fail, too, for example if the outer class references any local db4o

objects like ObjectContainer, etc., resulting in an ObjectNotStorableException.

So to be on the safe side with NQs in C/S mode, you should declare Predicates as top-level or static

inner classes only. Alternatively, you could either make sure that the outer classes containing Predicate

definitions could principally be persisted to db4o, too, and don't add significant overhead to the

predicate (the appropriate value for 'significant'

being your choice) or ensure during development that all predicates used actually can be optimized to

S.O.D.A. form.

12.4. Out-of-band signalling

Sometimes a client needs to send a special message to a server in order to tell the server to do

something. The server may need to be signalled to perform a defragment or it may need to be

signalled to shut itself down gracefully.

This is configured bycalling #messageRecipient() , passing the object that will process client-initiated

messages.

public void runServer() {

synchronized (this) {

ServerConfiguration config =

Db4oClientServer.newServerConfiguration();

// Using the messaging functionality to redirect all

// messages to this.processMessage

config.networking().messageRecipient(this);

ObjectServer db4oServer = Db4oClientServer.openServer(config

www.db4o.com

, FILE, PORT);

db4oServer.grantAccess(USER, PASS);

// to identify the thread in a debugger

Thread.currentThread().setName(this.getClass().getName());

// We only need low priority since the db4o server has

// it's own thread.

Thread.currentThread().setPriority(Thread.MIN_PRIORITY);

try {

if (!stop) {

// wait forever for notify() from close()

this.wait(Long.MAX_VALUE);

}

} catch (Exception e) {

e.printStackTrace();

}

db4oServer.close();

}

The message is received and processed by a #processMessage() method:

public void processMessage(MessageContext con, Object message) {

if (message instanceof StopServer) {

close();

}

Db4o allows a client to send an arbitrary signal or message to a server by sending a plain object to the

server. The server will receive a callback message, including the object that came from the client. The

server can interpret this message however it wants.

www.db4o.com

public static void main(String[] args) {

ObjectContainer objectContainer = null;

try {

// connect to the server

objectContainer =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), HOST, PORT, USER, PASS);

} catch (Exception e) {

e.printStackTrace();

}

if (objectContainer != null) {

// get the messageSender for the ObjectContainer

MessageSender messageSender =

objectContainer.ext().configure()

.clientServer().getMessageSender();

// send an instance of a StopServer object

messageSender.send(new StopServer());

// close the ObjectContainer

objectContainer.close();

}

12.5. Putting it all together: a simple but complete db4o server

Let's put all of this information together now to implement a simple standalone db4o server with a

special client that can tell the server to shut itself down gracefully on demand.

First, both the client and the server need some shared configuration information. We will provide this

using an interface:

www.db4o.com

package com.db4odoc.f1.chapter6;

/**

* Configuration used for {@link StartServer} and {@link StopServer}.

*/

public interface ServerInfo {

/**

* the host to be used.

* If you want to run the client server examples on two

computers,

* enter the computer name of the one that you want to use as

server.

*/

public String HOST = "localhost";

/**

* the database file to be used by the server.

*/

public String FILE = "formula1.db4o";

/**

* the port to be used by the server.

*/

public int PORT = 4488;

/**

* the user name for access control.

*/

public String USER = "db4o";

/**

* the pasword for access control.

*/

public String PASS = "db4o";

}

www.db4o.com

Now we'll create the server:

package com.db4odoc.f1.chapter6;

import com.db4o.*;

import com.db4o.cs.*;

import com.db4o.cs.config.*;

import com.db4o.messaging.*;

/**

* starts a db4o server with the settings from {@link ServerInfo}.

*

* This is a typical setup for a long running server.

*

* The Server may be stopped from a remote location by running

StopServer. The

* StartServer instance is used as a MessageRecipient and reacts to

receiving an

* instance of a StopServer object.

*

* Note that all user classes need to be present on the server side

and that all

* possible Db4o.configure() calls to alter the db4o configuration

need to be

* executed on the client and on the server.

*/

public class StartServer implements ServerInfo, MessageRecipient {

/**

* setting the value to true denotes that the server should be

closed

*/

private boolean stop = false;

/**

* starts a db4o server using the configuration from {@link

ServerInfo}.

*/

www.db4o.com

public static void main(String[] arguments) {

new StartServer().runServer();

}

/**

* opens the ObjectServer, and waits forever until close() is

called or a

* StopServer message is being received.

*/

public void runServer() {

synchronized (this) {

ServerConfiguration config =

Db4oClientServer.newServerConfiguration();

// Using the messaging functionality to redirect all

// messages to this.processMessage

config.networking().messageRecipient(this);

ObjectServer db4oServer =

Db4oClientServer.openServer(config

, FILE, PORT);

db4oServer.grantAccess(USER, PASS);

// to identify the thread in a debugger

Thread.currentThread().setName(this.getClass().getName());

// We only need low priority since the db4o server has

// it's own thread.

Thread.currentThread().setPriority(Thread.MIN_PRIORITY);

try {

if (!stop) {

// wait forever for notify() from close()

this.wait(Long.MAX_VALUE);

}

} catch (Exception e) {

e.printStackTrace();

}

db4oServer.close();

}

/**

* messaging callback

www.db4o.com

*

* @see

com.db4o.messaging.MessageRecipient#processMessage(MessageContext,

* Object)

*/

public void processMessage(MessageContext con, Object message) {

if (message instanceof StopServer) {

close();

}

/**

* closes this server.

*/

public void close() {

synchronized (this) {

stop = true;

this.notify();

}

And last but not least, the client that stops the server.

package com.db4odoc.f1.chapter6;

import com.db4o.*;

import com.db4o.cs.*;

import com.db4o.messaging.*;

/**

* stops the db4o Server started with {@link StartServer}.

*

* This is done by opening a client connection to the server and by

sending a

* StopServer object as a message. {@link StartServer} will react in

it's

www.db4o.com

* processMessage method.

*/

public class StopServer implements ServerInfo {

/**

* stops a db4o Server started with StartServer.

*

* @throws Exception

*/

public static void main(String[] args) {

ObjectContainer objectContainer = null;

try {

// connect to the server

objectContainer =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), HOST, PORT, USER,

PASS);

} catch (Exception e) {

e.printStackTrace();

}

if (objectContainer != null) {

// get the messageSender for the ObjectContainer

MessageSender messageSender =

objectContainer.ext().configure()

.clientServer().getMessageSender();

// send an instance of a StopServer object

messageSender.send(new StopServer());

// close the ObjectContainer

objectContainer.close();

}

www.db4o.com

12.6. Conclusion

That's it, folks. No, of course it isn't. There's much more to db4o we haven't covered yet: schema

evolution, custom persistence for your classes, writing your own query objects, etc. A much more

thorough documentation is provided in the reference that you should have also received with the

download or online.

We hope that this tutorial has helped to get you started with db4o. How should you continue now?

- Register on db4o developer website.

The list of the membership benefits is presented online.

- You could browse the remaining chapters. They are a selection of themes from the reference that

very frequently come up as questions in our http://forums.db4o.com/forums/.

-

(Interactive version only)

While this tutorial is basically sequential in nature, try to switch back and

forth between the chapters and execute the sample snippets in arbitrary order. You will be working

with the same database throughout; sometimes you may just get stuck or even induce exceptions, but

you can always reset the database via the console window.

- The examples we've worked through are included in your db4o distribution in full source code. Feel

free to experiment with it.

- If you're stuck, browse the information on our web site, check if your problem is submitted to Jira or

visit our forums at http://forums.db4o.com/forums/.

12.7. Full source

package com.db4odoc.f1.chapter6;

import java.io.*;

import com.db4o.*;

import com.db4o.cs.*;

import com.db4o.cs.config.*;

import com.db4odoc.f1.*;

public class ClientServerExample extends Util {

www.db4o.com

final static String DB4OFILENAME =

System.getProperty("user.home")

+ "/formula1.db4o";

private final static int PORT = 0xdb40;

private final static String USER = "user";

private final static String PASSWORD = "password";

public static void main(String[] args) throws IOException {

new File(DB4OFILENAME).delete();

accessLocalServer();

new File(DB4OFILENAME).delete();

ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded

.newConfiguration(), DB4OFILENAME);

try {

setFirstCar(db);

setSecondCar(db);

} finally {

db.close();

}

ServerConfiguration config =

Db4oClientServer.newServerConfiguration();

config.common().objectClass(Car.class).updateDepth(3);

ObjectServer server = Db4oClientServer.openServer(config,

DB4OFILENAME,

0);

try {

queryLocalServer(server);

demonstrateLocalReadCommitted(server);

demonstrateLocalRollback(server);

} finally {

server.close();

}

accessRemoteServer();

server = Db4oClientServer.openServer(Db4oClientServer

.newServerConfiguration(), DB4OFILENAME, PORT);

server.grantAccess(USER, PASSWORD);

try {

queryRemoteServer(PORT, USER, PASSWORD);

demonstrateRemoteReadCommitted(PORT, USER, PASSWORD);

demonstrateRemoteRollback(PORT, USER, PASSWORD);

www.db4o.com

} finally {

server.close();

}

public static void setFirstCar(ObjectContainer db) {

Pilot pilot = new Pilot("Rubens Barrichello", 99);

Car car = new Car("BMW");

car.setPilot(pilot);

db.store(car);

}

public static void setSecondCar(ObjectContainer db) {

Pilot pilot = new Pilot("Michael Schumacher", 100);

Car car = new Car("Ferrari");

car.setPilot(pilot);

db.store(car);

}

public static void accessLocalServer() {

ObjectServer server =

Db4oClientServer.openServer(Db4oClientServer

.newServerConfiguration(), DB4OFILENAME, 0);

try {

ObjectContainer client = server.openClient();

// Do something with this client, or open more clients

client.close();

} finally {

server.close();

}

public static void queryLocalServer(ObjectServer server) {

ObjectContainer client = server.openClient();

listResult(client.queryByExample(new Car(null)));

client.close();

}

public static void demonstrateLocalReadCommitted(ObjectServer

server) {

ObjectContainer client1 = server.openClient();

www.db4o.com

ObjectContainer client2 = server.openClient();

Pilot pilot = new Pilot("David Coulthard", 98);

ObjectSet result = client1.queryByExample(new Car("BMW"));

Car car = (Car) result.next();

car.setPilot(pilot);

client1.store(car);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.commit();

listResult(client1.queryByExample(Car.class));

listRefreshedResult(client2,

client2.queryByExample(Car.class), 2);

client1.close();

client2.close();

}

public static void demonstrateLocalRollback(ObjectServer server)

{

ObjectContainer client1 = server.openClient();

ObjectContainer client2 = server.openClient();

ObjectSet result = client1.queryByExample(new Car("BMW"));

Car car = (Car) result.next();

car.setPilot(new Pilot("Someone else", 0));

client1.store(car);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.rollback();

client1.ext().refresh(car, 2);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.close();

client2.close();

}

public static void accessRemoteServer() throws IOException {

ObjectServer server =

Db4oClientServer.openServer(Db4oClientServer

.newServerConfiguration(), DB4OFILENAME, PORT);

server.grantAccess(USER, PASSWORD);

try {

ObjectContainer client = Db4oClientServer.openClient(

www.db4o.com

Db4oClientServer.newClientConfiguration(),

"localhost",

PORT, USER, PASSWORD);

// Do something with this client, or open more clients

client.close();

} finally {

server.close();

}

public static void queryRemoteServer(int port, String user,

String password)

throws IOException {

ObjectContainer client =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), "localhost", port, user,

password);

listResult(client.queryByExample(new Car(null)));

client.close();

}

public static void demonstrateRemoteReadCommitted(int port,

String user,

String password) throws IOException {

ObjectContainer client1 =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), "localhost", port, user,

password);

ObjectContainer client2 =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), "localhost", port, user,

password);

Pilot pilot = new Pilot("Jenson Button", 97);

ObjectSet result = client1.queryByExample(new Car(null));

Car car = (Car) result.next();

car.setPilot(pilot);

client1.store(car);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.commit();

listResult(client1.queryByExample(new Car(null)));

www.db4o.com

listRefreshedResult(client2,

client2.queryByExample(Car.class), 2);

client1.close();

client2.close();

}

public static void demonstrateRemoteRollback(int port, String

user,

String password) throws IOException {

ObjectContainer client1 =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), "localhost", port, user,

password);

ObjectContainer client2 =

Db4oClientServer.openClient(Db4oClientServer

.newClientConfiguration(), "localhost", port, user,

password);

ObjectSet result = client1.queryByExample(new Car(null));

Car car = (Car) result.next();

car.setPilot(new Pilot("Someone else", 0));

client1.store(car);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.rollback();

client1.ext().refresh(car, 2);

listResult(client1.queryByExample(new Car(null)));

listResult(client2.queryByExample(new Car(null)));

client1.close();

client2.close();

}

www.db4o.com

.

13. SODA Evaluations

In the SODA API chapter we already mentioned

Evaluations

as a means of providing user-defined

custom constraints and as a means to run any arbitrary code in a SODA query. Let's have a closer

look.

13.1. Evaluation API

The evaluation API consists of two interfaces, Evaluation and Candidate . Evaluation

implementations are implemented by the user and injected into a query. During a query, they will be

called from db4o with a candidate instance in order to decide whether to include it into the current

(sub-)result.

The Evaluation interface contains a single method only:

public void evaluate(Candidate candidate);

This will be called by db4o to check whether the object encapsulated by this candidate should be

included into the current candidate set.

The Candidate interface provides three methods:

public Object getObject();

public void include(boolean flag);

public ObjectContainer objectContainer();

An Evaluation implementation may call #getObject() to retrieve the actual object instance to be

evaluated, it may call #include() to instruct db4o whether or not to include this object in the current

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candidate set, and finally it may access the current database directly by calling #objectContainer() .

13.2. Example

For a simple example, let's go back to our Pilot/Car implementation from the Collections chapter. Back

then, we kept a history of SensorReadout instances in a List member inside the car. Now imagine that

we wanted to retrieve all cars that have assembled an even number of history entries. A quite

contrived and seemingly trivial example, however, it gets us into trouble: Collections are transparent to

the query API, it just 'looks through' them at their respective members.

So how can we get this done? Let's implement an Evaluation that expects the objects passed in to be

instances of type Car and checks their history size.

package com.db4odoc.f1.chapter7;

import com.db4o.query.*;

import com.db4odoc.f1.chapter4.*;

public class EvenHistoryEvaluation implements Evaluation {

public void evaluate(Candidate candidate) {

Car car=(Car)candidate.getObject();

candidate.include(car.getHistory().size() % 2 == 0);

}

To test it, let's add two cars with history sizes of one, respectively two:

// storeCars

Pilot pilot1 = new Pilot("Michael Schumacher", 100);

Car car1 = new Car("Ferrari");

car1.setPilot(pilot1);

car1.snapshot();

db.store(car1);

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Pilot pilot2 = new Pilot("Rubens Barrichello", 99);

Car car2 = new Car("BMW");

car2.setPilot(pilot2);

car2.snapshot();

db.store(car2);

and run our evaluation against them:

// queryWithEvaluation

Query query = db.query();

query.constrain(Car.class);

query.constrain(new EvenHistoryEvaluation());

ObjectSet result = query.execute();

Util.listResult(result);

OUTPUT:

1

BMW[Rubens Barrichello/99]/2

13.3. Drawbacks

While evaluations offer you another degree of freedom for assembling queries, they come at a certain

cost: As you may already have noticed from the example, evaluations work on the fully instantiated

objects, while 'normal' queries peek into the database file directly. So there's a certain performance

penalty for the object instantiation, which is wasted if the object is not included into the candidate set.

Another restriction is that, while 'normal' queries can bypass encapsulation and access candidates'

private members directly, evaluations are bound to use their external API, just as in the language

itself.

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One last hint: Evaluations are expected to be serializable for client/server operation. So be careful

when implementing them as (anonymous) inner classes and keep in mind that those will carry an

implicit reference to their surrounding class and everything that belongs to it. Best practice is to always

implement evaluations as normal top level or static inner classes.

13.4. Conclusion

With the introduction of evaluations we finally completed our query toolbox. Evaluations provide a

simple way of assemble arbitrary custom query building blocks, however, they come at a price.

13.5. Full source

package com.db4odoc.f1.chapter7;

import java.io.*;

import com.db4o.*;

import com.db4o.query.*;

import com.db4odoc.f1.*;

import com.db4odoc.f1.chapter4.*;

public class EvaluationExample extends Util {

final static String DB4OFILENAME =

System.getProperty("user.home") + "/formula1.db4o";

public static void main(String[] args) {

new File(DB4OFILENAME).delete();

ObjectContainer db =

Db4oEmbedded.openFile(Db4oEmbedded.newConfiguration(), DB4OFILENAME);

try {

storeCars(db);

queryWithEvaluation(db);

} finally {

db.close();

}

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public static void storeCars(ObjectContainer db) {

Pilot pilot1 = new Pilot("Michael Schumacher", 100);

Car car1 = new Car("Ferrari");

car1.setPilot(pilot1);

car1.snapshot();

db.store(car1);

Pilot pilot2 = new Pilot("Rubens Barrichello", 99);

Car car2 = new Car("BMW");

car2.setPilot(pilot2);

car2.snapshot();

db.store(car2);

}

public static void queryWithEvaluation(ObjectContainer db) {

Query query = db.query();

query.constrain(Car.class);

query.constrain(new EvenHistoryEvaluation());

ObjectSet result = query.execute();

Util.listResult(result);

}

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.

14. Configuration

db4o provides a wide range of configuration methods to request special behaviour. For a complete list

of all available methods see the API documentation for thecom.db4o.config and com.db4o.cs.config

packages.

A more complete description of Configuration usage and scope can also be obtained from the

Reference documentation.

Some hints around using configuration calls:

14.1. Scope

Configuration should be created and passed to an opening ObjectContainer or ObjectServer:

EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();

ServerConfiguration serverConfig =

Db4oClientServer.newServerConfiguration();

ClientConfiguration clientConfig =

Db4oClientServer.newClientConfiguration();

Appropriate configuration should be submitted when an ObjectContainer ObjectServer is opened:

Db4oEmbedded.openFile(config, filename)

Db4oClientServer.openServer(serverConfig, filename, PORT)

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Db4oClientServer.openClient(clientConfig, HOST, PORT, USER, PASS);

A separate configuration instance should be created for each new ObjectContainer ObjectServer It is

recommended to use a factory method with all the necessary settings.

14.2. Calling Methods

Configurations that influence the database file format will have to take place, before a database is

created, before the first #openXXX() call. Some examples:

EmbeddedConfiguration conf = Db4oEmbedded.newConfiguration();

conf.blockSize(8);

conf.stringEncoding(new MyStringEncoding());

Configuration settings are not stored in db4o database files. Accordingly the same configuration has to

be submitted every time an ObjectContainer ObjectServer is opened.

www.db4o.com

.

15. Indexes

db4o allows to index fields to provide maximum querying performance. To request an index to be

created, you would issue the following API method call in your global db4o configuration method

before you open an ObjectContainer/ObjectServer:

// assuming

class Foo{

String bar;

}

CommonConfiguration#objectClass(Foo.class).objectField("bar").indexed

(true);

If the configuration is set in this way, an index on the Foo#bar field will be created (if not present

already) the next time you open an

ObjectContainer/ObjectServer and you use the Foo class the first time

in your application.

Contrary to all other configuration calls indexes - once created - will remain in a database even if the

index configuration call is not issued before opening an ObjectContainer/ObjectServer.

To drop an index you would also issue a configuration call in your db4o configuration method:

CommonConfiguration#objectClass(Foo.class).objectField("bar").indexed

(false);

Actually dropping the index will take place the next time the respective class is used.

db4o will tell you when it creates and drops indexes, if you choose a message level of 1 or higher:

CommonConfiguration#messageLevel(1);

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For creating and dropping indexes on large amounts of objects there are two possible strategies:

(1) Import all objects with indexing off, configure the index and reopen the

ObjectContainer/ObjectServer.

(2) Import all objects with indexing turned on and commit regularly for a fixed amount of objects

(~10,000).

(1) will be faster.

(2) will keep memory consumption lower.

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.

16. IDs

The db4o team recommends, not to use object IDs where this is not necessary. db4o keeps track of

object identities in a transparent way, by identifying "known" objects on updates. The reference

system also makes sure that every persistent object is instantiated only once, when a graph of objects

is retrieved from the database, no matter which access path is chosen. If an object is accessed by

multiple queries or by multiple navigation access paths, db4o will always return the one single object,

helping you to put your object graph together exactly the same way as it was when it was stored,

without having to use IDs.

The use of IDs does make sense when object and database are disconnected, for instance in stateless

applications.

db4o provides two types of ID systems.

16.1. Internal IDs

The internal db4o ID is a physical pointer into the database with only one indirection in the file to the

actual object so it is the fastest external access to an object db4o provides. The internal ID of an object

is available with

objectContainer.ext().getID(object);

To get an object for an internal ID use

objectContainer.ext().getByID(id);

Note that #getByID() does not activate objects. If you want to work with objects that you get with

#getByID(), your code would have to make sure the object is activated by calling

objectContainer.activate(object, depth);

db4o assigns internal IDs to any stored first class object. These internal IDs are guaranteed to be

unique within one ObjectContainer/ObjectServer and they will stay the same for every object when an

ObjectContainer/ObjectServer is closed and reopened. Internal IDs will change when an object is

moved from one ObjectContainer to another, as it happens during Defragment.

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16.2. Unique Universal IDs (UUIDs)

For long term external references and to identify an object even after it has been copied or moved to

another ObjectContainer, db4o supplies UUIDs. These UUIDs are not generated by default, since they

occupy some space and consume some performance for maintaining their index. UUIDs can be turned

on globally or for individual classes:

FileConfiguration#generateUUIDs(Integer.MAX_VALUE);

Db4o.configure().objectClass(Foo.class).generateUUIDs(true);

The respective methods for working with UUIDs are:

ExtObjectContainer#getObjectInfo(Object)

ObjectInfo#getUUID();

ExtObjectContainer#getByUUID(Db4oUUID);

www.db4o.com

.

17. Enhancement

As we have seen both Transparent Activation and Native Queries will produce correct results out-of-

the-box.

To get your application to run as fast as possible with db4o, you will want to make sure that only

required objects are loaded from the database and that Native Queries are optimized to use field

indexes where this is possible.

Theoretically you could write all the necessary code by hand but you will save yourself a lot of time if

you use automatic enhancement.

The db4o enhancer tools will add code to your persistent classes to make them work even better with

db4o.

db4o for Java supplies three different enhancer strategies:

Enhancing at Compile/Deployment Time

Enhancing at Class Loading Time

Native Query Optimization at Query Execution Time

17.1. Required Libraries

The enhancer tasks require the addition of the following libraries to your project:

ant.jar

for using the Ant enhancer task

bloat-1.0.jar

the bytecode optimization library

db4o-7.0-instrumentation.jar

instrumentation library on top of bloat

db4o-7.0-java5.jar

the db4o core engine

db4o-7.0-nqopt.jar

the Native Query Optimizer

db4o-7.0-taj.jar

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the Transparent Activation Framework for Java

db4o-7.0-tools.jar

the tools interface that provides the top level enhancer tasks.

Alternatively you can add db4o-7.0-all-java5.jarinstead, which contains the classes from all

the libraries listed above.

.

17.2. Enhancing at Compile Time

The db4o enhancer adds code to class files or jar files. Afterwards these files will be slightly different to

what the debugger expects.

In the setup the db4o team has been working with, using the Eclipse IDE, enhancing has worked

perfectly when files were instrumented in-place, directly in the /bin/ directory where the Eclipse

compiler places compiled class files. In all our tests the Eclipse debugger simply ignored additional

statements and stepped over them and debugging worked perfectly as usual.

The db4o enhancer technology is new. Not all possible development environment setups have been

checked by the db4o team and there may be incompatibility issues with your IDE when enhancing class

and library files in place. By trying out in-place enhancement with your setup and by reporting back

issues to the db4o forums you can help the db4o team to improve enhancement for your personal

setup. A possible strategy to resolve problems with in-place enhancement would be to use different

output directories than /bin and /lib as output paths for the enhancer and to run your enhanced

application from there.

17.2.1. Enhancing with Ant

db4otools.jar contains the main enhancer task:

com.db4o.enhance.Db4oEnhancerAntTask

For all possibilities of using this task see the topic on build time enhancement in the reference

documentation.

The simplest enhancer task can do the following:

1. optimize Native Queries 2. add the Activatable interface to your classes, making sure a persistent

object is activated whenever any field is read.

3. replace constructor invocation for platform collections (ArrayList,...) with a custom activatable

version derived from the platform collection

www.db4o.com

Here is an example of how an Ant script could look like. This script enhances all class files and jar files

except for the db4o libraries in-place, in the /bin and /lib directory of your project:

<?xml version="1.0"?>

</fileset>

</path>

<taskdef name="db4o-enhance"

classname="com.db4o.enhance.Db4oEnhancerAntTask"

classpathref="project.classpath" />

<db4o-enhance classtargetdir="${basedir}/bin"

jartargetdir="${basedir}/lib" nq="true" ta="true" collections="true">

</jars>

</db4o-enhance>

</target>

</project>

The instrumentation process can be configured with the ollowing attributes:

nq: switch for native query optimization enhancement

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ta: switch for transparent activation/persistence enhancement

collections: switch for enhancement of platform collections for transparent activation/persistence (only

effective if ta is switched on)

By default, these all are set to true.

If you use Eclipse, there is a very convenient way to make sure an Ant script is executed every time

after compilation and before you run your application: You can add a builder to your project. To do

this, right click on your project, choose "Properties" + "Builders" + "New" + "Ant Builder" and enter the

name of the build file you created.

17.2.2. Programmatic Enhancement

The db4o enhancer is just another Java application so of course you can start it directly. The enhancer

user interface is a convenience interface to the underlying instrumentation layer. To take advantage of

all the possibilities you may want to consider a look at the sources of

com.db4o.instrumentation.main.Db4oFileInstrumentor and call instrumentation directly.

Here is a very basic example how you could enhance all the files in the /bin folder below the project

'myproject' using the Db4oFileEnhancer:

import com.db4o.enhance.*;

public static void main(String[] args) throws Exception {

String path = "../myproject/bin";

new Db4oFileEnhancer().enhance(path, path);

}

.

17.3. Enhancing at Load Time

As an alternative to enhancing classes at build time, it is also possible to run a db4o application using a

special launcher. Under the hood the Db4oEnhancedLauncher installs a special ClassLoader that

instruments classes for db4o on the fly, when they are loaded.

For using the launcher, please see the method signatures of

www.db4o.com

Db4oEnhancedLauncher#launch()

.

17.4. NQ Optimization at Querying Time

In case no other enhancer has been run previously, Native Queries will still be optimized automatically

if the following optimization libraries are found in the CLASSPATH:

bloat-1.0.jar, db4o-7.0-classedit.jar, db4o-7.0-nqopt.jar

The Native Query optimizer is still under development to eventually "understand" all Java constructs.

Current optimization supports the following constructs well:

- compile-time constants

- simple member access

- primitive comparisons

- #equals() on primitive wrappers and Strings

- #contains()/#startsWith()/#endsWith() for Strings

- arithmetic expressions

- boolean expressions

- static field access

- array access for static/predicate fields

- arbitrary method calls on static/predicate fields (without candidate based params)

- candidate methods composed of the above

- chained combinations of the above

Note that the current implementation does not support polymorphism and multiline methods yet.

17.5. Monitoring Native Query optimization

To find out if a Native Query runs optimized or unoptimized you can attach a query execution listener

as follows:

((ObjectContainerBase)db).getNativeQueryHandler().addListener(new

Db4oQueryExecutionListener() {

public void notifyQueryExecuted(NQOptimizationInfo info) {

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System.err.println(info);

}

});

The listener will be notified on each native query call and will be passed the Predicate object processed,

the optimized expression tree (if successful) and the success status of the optimization run:

NativeQueryHandler.UNOPTIMIZED ("UNOPTIMIZED")

if the predicate could not be optimized and is run in unoptimized mode

NativeQueryHandler.PREOPTIMIZED ("PREOPTIMIZED")

if the predicate already was optimized (due to class file or load time

instrumentation)

NativeQueryHandler.DYNOPTIMIZED ("DYNOPTIMIZED")

if the predicate was optimized at query execution time

www.db4o.com

.

18. Interactive Tutorial Troubleshooting

db4o interactive tutorial uses Java applets to execute code examples directly in your browser enabling

you to see the results immediately during reading.

In order to use the interactive functionality a Java JRE 1.3 or above needs to be installed and enabled

in the browser.

Normally this and db4o for java installation is all that is needed to start with the interactive examples.

However some system configurations can prevent the examples from running properly.

Use the following check-list to locate and fix the problem:

- JRE1.3 or above should be installed.

- JAVA_HOME environment variable should point to the correct JRE (or JDK).

- The Java control panel should be configured to use the web browser.

- Java security settings have to allow a user to grant permissions to signed content and to grant

permission to content from an untrusted authority.

- You should also check that your system firewalls do not block interactive content.

18.1. Configuring JRE on Windows

To see whether your browser is configured to use the Java Runtime Environment or not, open the

Windows Control Panel and check if the Java icon is present.

In the Java Control Panel:

- check that your browser is enabled ( For JRE1.4 and above set this parameter on Advanced Tab, for

older JRE versions check "Enable Java Plug-in")

- check that the user is granted permissions to signed content (untrusted authority as well).

18.2. Configuring JRE on Linux

18.2.1. Mozilla 1.4 and later

Go to the plugins sub-directory under the Mozilla installation directory:

cd <Mozilla installation directory>/plugins

In the current directory, create a symbolic link to the JRE ns7/libjavaplugin_oji.so file. Type:

ln -s <JRE installation

directory>/plugin/i386/ns7/libjavaplugin_oji.so

Example:

If Mozilla is installed in this directory:

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/usr/lib/mozilla-1.4/

and if the JRE is installed in this directory:

/usr/java/jre1.5.0

Then type at the terminal to go to the browser plug-in directory:

cd /usr/lib/mozilla-1.4/plugins

Enter the following command to create a symbolic link to the Java Plug-in for the Mozilla browser.

ln -s /usr/java/jre1.5.0/plugin/i386/ns7/libjavaplugin_oji.so .

Start Mozilla browser or restart it if it is already running. Note that if you have other Mozilla

components (ie: Messenger, Composer, etc) running, you will need to restart them as well.

Go to Edit > Preferences. Under Advanced category > Select Enable Java

18.2.2. Mozilla 1.2, Netscape 6 and later

Go to the plugins sub-directory under the Netscape directory

cd <Mozilla1.2>/plugins

Create a symbolic link to the ns7-gcc29/libjavaplugin_oji.so file:

ln -s <JRE>/plugin/i386/ns7-gcc29/libjavaplugin_oji.so

Example:

If Netscape is installed at this directory:

/usr/lib/Mozilla1.2/

And if the JRE is installed at this directory:

/usr/java/jre1.5.0

Then type at the terminal to go to the browser plug-in directory:

cd /usr/lib/Mozilla1.2/plugins

Enter the following command to create a symbolic link to the Java Plug-in for the Mozilla browser.

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ln -s /usr/java/jre1.5.0/plugin/i386/ns7-gcc29/libjavaplugin_oji.so .

Start Mozilla browser or restart it if it is already running. Note that if you have other Mozilla

components (ie: Messenger, Composer, etc) running, you will need to restart them as well.

Go to Edit > Preferences. Under Advanced category > Select Enable Java

www.db4o.com

.

19. IO Benchmark

I/O access times play a crucial role in the overall performance of a database. It is important to

estimate the performance before committing to a specific database. To help you with this we introduce

IO Benchmark tool with the following functionality:

1. Measuring the actual I/O performance of a system as seen by db4o

2. Simulating the behaviour of a slower system on a faster one

IO benchmark consists of db4o_bench.jar, with the source code in the db4otools project /

com.db4o.bench package.

The code can be compiled with JDK 1.3 and higher.

Benchmark runs in 3 stages:

1. Run a target application and log its I/O access pattern using LoggingIoAdapter:

// runTargetApplication

// Stage 1: running the application to record IO access

System.out.println("Running target application ...");

// Any custom application can be used instead

new CrudApplication().run(itemCount);

2. Replay the recorded I/O operations once to prepare a database file.

// prepareDbFile

// Stage 2:Replay the recorded IO to prepare a database file

System.out.println("Preparing DB file ...");

deleteFile(_dbFileName);

IoAdapter rafFactory = new RandomAccessFileAdapter();

IoAdapter raf = rafFactory.open(_dbFileName, false, 0, false);

// Use the file with the recorded operations from stage 1

LogReplayer replayer = new LogReplayer("simplecrud_" + itemCount +

".log", raf);

try {

replayer.replayLog();

} catch (IOException e) {

exitWithError("Error reading I/O operations log file");

} finally {

raf.close();

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}

This step is necessary to ensure that during the grouped replay in the next step, none of the accesses

will go beyond the currently existing file.

3. Replay the recorded I/O operations a second time. Operations are grouped by command type

(read, write, seek, sync), and the total time executing all operations of a specific command type is

measured. Grouping is necessary to avoid micro-benchmarking effects and to get time values above

timer resolution.

We divide the numbers collected in stage 3 by the respective number of operations and we calculate

the average time a particular command takes on the given system.

// runBenchmark

// Stage 3: Replay the recorded IO grouping command types

System.out.println("Running benchmark ...");

deleteBenchmarkResultsFile(itemCount);

PrintStream out = new PrintStream(new

FileOutputStream(resultsFileName(itemCount), true));

printRunHeader(itemCount, out);

// run all commands: READ_ENTRY, WRITE_ENTRY, SYNC_ENTRY, SEEK_ENTRY

for (int i = 0; i < LogConstants.ALL_CONSTANTS.length; i++) {

String currentCommand = LogConstants.ALL_CONSTANTS[i];

benchmarkCommand(currentCommand, itemCount, _dbFileName, out);

}

out.close();

deleteFile(_dbFileName);

deleteCrudLogFile(itemCount);

19.1. Benchmark Application

IO Benchmark can be used with any custom application. Here we will look at it on an example of a

simple CRUD application.

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package com.db4odoc.f1.chapter11;

import java.io.*;

import com.db4o.*;

import com.db4o.bench.logging.*;

import com.db4o.config.*;

import com.db4o.io.*;

/**

* Very simple CRUD (Create, Read, Update, Delete) application to

* produce log files as an input for I/O-benchmarking.

*/

public class CrudApplication {

private static final String DATABASE_FILE = "simplecrud.db4o";

public void run(final int itemCount) {

deleteDbFile();

final String logFileName = logFileName(itemCount);

create(itemCount, newConfigWithLogging(logFileName));

read(newConfigWithLogging(logFileName));

update(newConfigWithLogging(logFileName));

delete(newConfigWithLogging(logFileName));

deleteDbFile();

}

private Configuration newConfigWithLogging(final String

logFileName) {

RandomAccessFileAdapter rafAdapter = new

RandomAccessFileAdapter();

IoAdapter ioAdapter = new LoggingIoAdapter(rafAdapter,

logFileName);

Configuration config = Db4o.newConfiguration();

config.io(ioAdapter);

return config;

}

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private void create(int itemCount, Configuration config) {

ObjectContainer oc = open(config);

for (int i = 0; i < itemCount; i++) {

oc.store(Item.newItem(i));

// preventing heap space problems by committing from time

to time

if(i % 100000 == 0) {

oc.commit();

}

oc.commit();

oc.close();

}

private void read(Configuration config) {

ObjectContainer oc = open(config);

ObjectSet objectSet = oc.query(Item.class);

while(objectSet.hasNext()){

Item item = (Item) objectSet.next();

}

oc.close();

}

private void update(Configuration config) {

ObjectContainer oc = open(config);

ObjectSet objectSet = oc.query(Item.class);

while(objectSet.hasNext()){

Item item = (Item) objectSet.next();

item.change();

oc.store(item);

}

oc.close();

}

private void delete(Configuration config) {

ObjectContainer oc = open(config);

ObjectSet objectSet = oc.query(Item.class);;

while(objectSet.hasNext()){

oc.delete(objectSet.next());

// adding commit results in more syncs in the log,

// which is necessary for meaningful statistics!

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oc.commit();

}

oc.close();

}

private void deleteDbFile() {

new File(DATABASE_FILE).delete();

}

private ObjectContainer open(Configuration config) {

return Db4o.openFile(config, DATABASE_FILE);

}

public static String logFileName(int itemCount) {

return "simplecrud_" + itemCount + ".log";

}

Please, pay attention to prepare method, which configures the use of LoggingIoAdapter - this is the

only change that is required on your application's side to make it available for benchmarking.

LoggingIoAdapter will ensure that all IO access operations will be logged to the specified file. This

information can be used later (stage 2 and 3 of the benchmark) to replay the application's database

interaction and measure performance for this specific pattern on different environments.

19.2. Benchmark Example

You can try to run the benchmark immediately on our sample application. We use a very small number

of objects (10) for this example to make it faster:

// runNormal

printDoubleLine();

RandomAccessFileAdapter rafAdapter = new RandomAccessFileAdapter();

IoAdapter ioAdapter = new LoggingIoAdapter(rafAdapter, "test.log");

System.out.println("Running db4o IoBenchmark");

printDoubleLine();

// Run a target application and log its I/O access pattern

runTargetApplication(_objectCount);

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// Replay the recorded I/O operations once to prepare a database

file.

prepareDbFile(_objectCount);

// Replay the recorded I/O operations a second time.

// Operations are grouped by command type (read, write, seek, sync),

// and the total time executing all operations of a specific command

type is measured.

runBenchmark(_objectCount);

OUTPUT:

=============================================================

Running db4o IoBenchmark

=============================================================

Running target application ...

Preparing DB file ...

Running benchmark ...

-------------------------------------------------------------

db4o IoBenchmark results with 10 items

Statistics written to db4o-IoBenchmark-results-10.log

-------------------------------------------------------------

Results for READ

> operations executed: 83

> time elapsed: 0 ms

> operations per millisecond: Infinity

> average duration per operation: 0.0 ms

READ 0 ns

Results for WRITE

> operations executed: 85

> time elapsed: 0 ms

> operations per millisecond: Infinity

> average duration per operation: 0.0 ms

WRITE 0 ns

Results for SYNC

> operations executed: 45

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> time elapsed: 6 ms

> operations per millisecond: 7.5

> average duration per operation: 0.13333333333333333 ms

SYNC 133333 ns

Results for SEEK

> operations executed: 168

> time elapsed: 0 ms

> operations per millisecond: Infinity

> average duration per operation: 0.0 ms

SEEK 0 ns

You can use the above mentioned sequence of operations to benchmark any db4o application:

- make sure that your application uses LoggingIoAdapter for the benchmark run;

- modify runTargetApplication method to call your application.

The ns (nanosecond) values are our benchmark standard for the respective operations. Smaller

numbers are better.

Note: It may be possible, that you get some zero values for time elapsed, and therefore infinity for

operations per ms. This can occur if your machine is fast enough to execute all operations under 1 ms.

To overcome this you can run the run.benchmark.medium target which operates with more objects and

takes longer to complete.

19.3. Delayed Benchmark

IO Benchmark provides you with another helpful feature - delayed benchmark. It can be used to

benchmark target devices (with a slow IO) on your development workstation. In order to do so you will

need to run benchmark on your workstation and on target device, copy the result files to some folder

on your workstation and run delayed benchmark. The delayed benchmark will use benchmark results

files to analyze how much slower is the device than the workstation: a special delays will be

introduced for each operation. Let's look at an example.

You should have the benchmark results from the previous example run. If you did not run the previous

example, please, do it now. An example benchmark result data from a target device is already

prepared and will be saved into db4o-IoBenchmark-results-10-slower.log. Workstation and target

device results are defined in the benchmark example as:

private static final String _resultsFile2 = "db4o-IoBenchmark-

results-10-slower.log";

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private static final String _resultsFile1 = "db4o-IoBenchmark-

results-10.log";

Note, that the order of files does not matter - the delays will be calculated to match the slower one.

// runDelayed

printDoubleLine();

System.out.println("Running db4o IoBenchmark");

printDoubleLine();

// Write sample slow data to the test file

prepareDelayedFile(_resultsFile2);

// calculate the delays to match the slower device

processResultsFiles(_resultsFile1, _resultsFile2);

// Run a target application and log its I/O access pattern

runTargetApplication(_objectCount);

// Replay the recorded I/O operations once to prepare a database

file.

prepareDbFile(_objectCount);

// Replay the recorded I/O operations a second time.

// Operations are grouped by command type (read, write, seek, sync),

// and the total time executing all operations of a specific command

type is measured.

runBenchmark(_objectCount);

OUTPUT:

=============================================================

Running db4o IoBenchmark

=============================================================

Delaying:

> machine1 (db4o-IoBenchmark-results-10.log) is faster!

> Required delays:

> [delays in nanoseconds] read: 43956 | write: 47058 | seek: 11362 |

sync: 2044443

> Adjusting delay timer to match required delays...

> Adjusted delays:

> [delays in nanoseconds] read: 41076 | write: 44122 | seek: 8649 |

sync: 2038316

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Running target application ...

Preparing DB file ...

Running benchmark ...

-------------------------------------------------------------

db4o IoBenchmark results with 10 items

Statistics written to db4o-IoBenchmark-results-10-delayed.log

-------------------------------------------------------------

Results for READ

> operations executed: 83

> time elapsed: 4 ms

> operations per millisecond: 20.75

> average duration per operation: 0.04819277108433735 ms

READ 48192 ns

Results for WRITE

> operations executed: 85

> time elapsed: 4 ms

> operations per millisecond: 21.25

> average duration per operation: 0.047058823529411764 ms

WRITE 47058 ns

Results for SYNC

> operations executed: 45

> time elapsed: 99 ms

> operations per millisecond: 0.45454545454545453

> average duration per operation: 2.2 ms

SYNC 2200000 ns

Results for SEEK

> operations executed: 168

> time elapsed: 2 ms

> operations per millisecond: 84.0

> average duration per operation: 0.011904761904761904 ms

SEEK 11904 ns

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Now you are supposed to get slower results (be patient - it can take a while). The example in this

tutorial might be a bit confusing as it operates on very little amount of objects (thus time adjustments

can be largely rounded), and the slow results file contains test data, which might not be realistic for

any real device environment. There are some other pitfalls that you must remember when using

delayed benchmark:

- Delayed benchmark can only work if all measured operations (read, write, seek, sync) are slower on

one device and faster on another. If that is not the case an error will be returned.

- There is a minimum delay that can be achieved (a delay of processing operation). If your target

device requires a smaller than minimum delay - the benchmark results will be inaccurate. You can try

to improve the mesurements by running on a bigger amount of objects.

For any questions and improvement suggestions, please, use our forums or Jira.

www.db4o.com

.

20. License

Versant Inc. supplies the object database engine db4o under a triple licensing regime:

20.1. General Public License (GPL)

db4o is free to be used:

- for development,

- in-house as long as no deployment to third parties takes place,

- together with works that are placed under the GPL themselves.

You should have received a copy of the GPL in the file db4o.license.txt together with the db4o

distribution.

If you have questions about when a commercial license is required, please

read our GPL Interpretation policy for further detail, available at:

http://www.db4o.com/about/company/legalpolicies/gplinterpretation.aspx

20.2. Opensource Compatibility license (dOCL)

The db4o Opensource Compatibility License (dOCL) is designed for free/open source projects that want

to embed db4o but do not want to (or are not able to) license their derivative work under the GPL in its

entirety. This initiative aims to proliferate db4o into many more open source projects by providing

compatibility for projects licensed under Apache, LGPL, BSD, EPL, and others, as required by our users.

The terms of this license are available here: "dOCL" agreement.

20.3. Commercial License

For incorporation into own commercial products and for use together with redistributed software that is

not placed under the GPL, db4o is also available under a commercial license.

Visit the purchasing area on the db4o website or contact db4o sales for licensing terms and pricing.

20.4. Bundled 3rd Party Licenses

The db4o distribution comes with the following 3rd party libraries:

-Apache Ant(Apache Software License)

Files: lib/ant.jar, lib/ant.license.txt

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Ant can be used as a make tool for class file based optimization of native

queries at compile time.

This product includes software developed by the Apache Software Foundation

(http://www.apache.org/).

-BLOAT(GNU LGPL)

Files: lib/bloat-1.0.jar, lib/bloat.license.txt

Bloat is used for bytecode analysis during native queries optimization. It

needs to be on the classpath during runtime at load time or query execution

time for just-in-time optimization. Preoptimized class files are not dependent

on BLOAT at runtime.

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.

21. Contacting Versant

Versant Corporation

255 Shoreline Drive

Suite 450

Redwood City, CA 94065

USA

Phone

+1 (650) 232-2431

Fax

+1 (650) 232-2401

Sales

Fill out our sales contact form on the db4o website

or

mail to [email protected]

Support

Visit our free Community Forums

or log into your https://customer.db4o.com Customer Portal (dDN Members Only).

Careers

[email protected]

Partnering

[email protected]