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[PDF version]

Developer’s Guide

 P. Garcia     (INRIA)
 M. Leclerq     (ST Micro-Electronics)
 V. Quéma     (CNRS)
 J.-B. Stefani     (INRIA)

Released January 2008
Status Draft
Version 2.0

General Information

Please send comments on this document to Authors would be glad to hear from people using or extending Dream.

Copyright 2003-2004 INRIA.
655 avenue de l’Europe, ZIRST, Montbonnot St Martin, 38334 Saint-Ismier Cedex, France.

All rights reserved.


All product names mentioned herein are trademarks of their respective owners.


Before reading this document, be sure you have following knowledge :


1  Configuring Eclipse and getting Dream sources

1.1  Getting Eclipse and required plugins

The latest eclipse release is available at the following URL:

We advice you to install the following plugins:

  • Subclipse is a plug-in providing support for Subversion within the Eclipse IDE. Installation instructions are available at the following URL:
  • Checkstyle is a plugin that helps you ensure that your Java code adheres to a set of coding standards. Installation instructions are available at the following URL:
  • Maven plugin provides integration for Maven, an open-source build manager for java projects. Installation instructions are available at the following URL:

1.2  Configuring the SVN repository

  • Open the “SVN perspective”: Window > Open Perspective > Others > SVN Repository Exploring.
  • In the “SVN Repositories” view, click on the Add SVN Repository button (see figure 1).

    Figure 1: Adding a SVN repository.

  • Fill in the dialog box shown in figure 2 with :
    • svn:// for anonymous (read-only) access.
    • svn+ssh:// for developer access.

    Figure 2: “Add a new SVN Repository” dialog box.

1.3  Checking out the DREAM trunk module

The trunk module contains everything you need to develop dream : DREAM modules and a tools module to configure eclipse. The dream module is an alias of the three base modules (dreamcore, dreamadl, and dreamlib).

N.B. : the trunk module is the main development branch of the DREAM project. To develop a specific module, the best practice is to develop first in a separate branch and then merge the branch to the trunk. For more information about branching under SVN, see

To check out the trunk module, proceed as follows:

  • In the “SVN Repositories” view, right click on the trunk directory, select Check Out (see figure 3).

    Figure 3: Checkout trunk module

A new project, called trunk, has been created. You can browse it using the “Java” perspective.

1.4  Configuring Eclipse

1.4.1  Configuring the CheckStyle plugin

The CheckStyle plugin is configured as follows:

  • Open the Preferences dialog box (Window > Preferences).
  • Select CheckStyle in the list (if it is not present, see section  1.1 to install the plugin).
  • Click on the New button to add the DREAM configuration.
  • Fill in the dialog box (see figure 4).

    Figure 4: CheckStyle configuration

1.4.2  Configuring the code formatter

The code formatter is a code beautifier. It must be configured as follows:

  • Open the Preferences dialog box (Window > Preferences).
  • Select Java > Code Style > Code Formatter in the list (see figure 5).

    Figure 5: Code formatter preferences

  • Click on the Import button.
  • Go into your workspace directory.
  • Select the code_formater.xml file in the tools/eclipse_settings directory.

1.4.3  Using the code formatter

The code formatter can be used when editing Java source files with the following shortcut: Ctrl-Shift-F.

2  Inside Dream modules

2.1  Module structure

2.1.1  Multi-Module Maven Project

Each multi-module project has the same structure as shows the picture 6.

Figure 6: Multi-Module Directory Layout.

  • The pom.xml file which contains a detailled description of the multi-module project and in particular the list of sub-modules.
  • Directories corresponding to sub-modules. On the pictures 6, the sub-modules are dreamadl, dreamannotation, dreamcore and dreamlib.

2.1.2  Single Module Maven Project

Each Dream module has approximately the same structure as shows the picture 7. It is conform to maven standard directory layout.

Figure 7: Single-Module Directory Layout.

  • The src/main/java directory contains java sources of the module.
  • The src/main/resources directory contains other resources of the module. These resources are mostly fractal ADL files.
  • The src/test/java directory contains java sources of junit tests.
  • The src/test/resources directory contains other resources of junit tests. These resources are mostly fractal ADL files.
  • The src/main/javadoc directory contains files used to generate javadoc of the module.
  • The src/main/config directory contains configuration files (ex : to configure logger).
  • Every file generated during the maven build process are put inside the target directory. It contains :
    • a generated-sources directory containing java source files generated by spoon.
    • a classes directory containing compiled classes and fractal files generated by spoon.
    • an apidocs directory containing generated javadoc.
    • a test-classes directory containing compiled java classes.
    • a surefire-reports directory containing reports of junit tests.
    • 3 jar files :
      • <artifactId>-<version-number>.jar : this jar contains compiled classes and fractal ADL definitions.
      • <artifactId>-<version-number>-sources.jar : this jar contains sources.
      • <artifactId>-<version-number>-javadoc.jar : this jar contains generated javadoc.

2.2  Make a distribution and run the examples


3  HelloWorld example

This section describes how to write a simple “HelloWorld” application with DREAM.

The example used throughout this tutorial is a very simple application made of two primitive user components encapsulated by a composite component (see the figure 8). The first primitive component is a message producer: an internal activity produces messages containing a string, and pushes them on the component’s push client interface. The second primitive component is a message consumer: it provides a push interface. When this component receives a message, it prints on the console the string contained in the message and then deletes the message. These two components have a messagemanager client interface since they create and delete messages. Finally the producer component has a org.objectweb.dream.control.activity.manager.TaskManager client interface in order to be able to execute tasks.

These two components can be bound directly (primitive binding) or using various components from the DREAM library (composite binding).

Figure 8: Simple Helloworld Example.

In this section we explain how to write the components described above and how to assemble and run them using the fractal ADL. The code and the ADL files described here can be found in the trunk/dreamlib/examples/helloworld directory of the dreamlib module.

3.1  Implementation

The interface used to exchange messages between client and server components is the org.objectweb.dream.Push interface of the dreamcore module.

3.1.1  The HelloWorldChunk implementation

In our example, producer and consumer components exchange messages containing a string. Since DREAM messages are composed of chunks, we need to write a class extending the org.objectweb.dream.message.AbstractChunk class and containing a String field. The code of this class is the following:

public class HelloWorldChunk extends AbstractChunk<HelloWorldChunk> { public static final String DEFAULT_NAME = "helloworld-chunk"; private String message; public final String getMessage() { return message; } public final void setMessage(String message) { this.message = message; } // Chunk Interface Implementation protected HelloWorldChunk newChunk() { return new HelloWorldChunk(); } protected void transfertStateTo(HelloWorldChunk newInstance) { newInstance.message = message; } public void recycle() { message = null; } // Externalizable Interface Implementation public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { message = in.readUTF(); } public void writeExternal(ObjectOutput out) throws IOException { out.writeUTF(message); } }

  • The Chunk interface is used by the associated chunk factory which manages chunk creation/destruction/recycling…
  • The Externalizable interface is used by message codecs when the message is serialized.

3.1.2  The consumer implementation

The consumer component receives message on its Push server interface, print it using the logger and delete it. Here is the code of the class :

@DreamComponent @Provides(interfaces={@Interface(name="in-push",signature=Push.class)}) public class Consumer implements Push { @Service Component ref; @DreamMonolog Logger logger; @Attribute(argument = "nbMessageToReceive") int nbMessageToReceive; // Client interfaces @Requires(name = "message-manager") MessageManagerType messageManagerItf; // Implementation of the Push interface public void push(Message message) throws PushException { HelloWorldChunk c = (HelloWorldChunk) messageManagerItf .getChunk(message, HelloWorldChunk.DEFAULT_NAME); if (c == null) { throw new PushException("Unable to find Helloworld chunk"); } logger.log(BasicLevel.INFO, c.getMessage()); messageManagerItf.deleteMessage(message); this.setNbMsgToReceive(--nbMessageToReceive); } public synchronized int getNbMsgToReceive() { return nbMsgToReceive; } public synchronized void setNbMsgToReceive(int nbMsgToReceive) { this.nbMsgToReceive = nbMsgToReceive; } }

To lightweight the code, annotations (from dream-annotation and fractal-spoonlet modules) are used in java implementation of fractal components :

  • @DreamComponent declares the component.
  • @Provides declares server interfaces of the component. This component provide one Push interface where messages are received.
  • @Service declares the reflexive reference to the component. This field is required to register the logger declared with the @DreamMonolog annotation
  • @Requires declares client interfaces of the component. Here the messageManagerItf is needed to delete processed messages.
  • @Attribute declares a fractal attribute for the consumer component. It represents the number of message that the consumer still wants to receive.

These annotations let the programmer write almost only functionnal code, i.e. the Push interface implementation.

3.1.3  The producer implementation

The producer component defines a periodic task which pushes a message containing a HelloWorldChunk. This component has two client interfaces: the push interface used to send produced messages via the messageManagerItf interface. It has no server interface. Here is the code corresponding to the component declaration :

/** Producer component implementation. */ @DreamComponent(controllerDesc = "activeDreamPrimitive") public class Producer { private ChunkFactoryReference<HelloWorldChunk> hcFactory; @Service private Component ref; @DreamMonolog private Logger logger; @Requires(name = "out-push") private Push outPushItf; @Requires(name = "message-manager") private MessageManagerType messageManagerItf; @Requires(name = "task-manager") private TaskManager taskManagerItf;

Notes :

  • the TaskManager.ITF_NAME is not used directly by the component implementation. It is used only by the task controller of the component at start time to register the task.

We now implement the inner task. A DREAM task implements the org.objectweb.dream.control.activity.task.Task interface. DREAM provides an abstract implementation of this interface. So to implement our task, we define an inner class extending org.objectweb.dream.control.activity.task.AbstractTask:

private class ClientTask extends AbstractTask { private String message; int seqno; ChunkFactoryReference<HelloWorldChunk> chunkFactory; public ClientTask(String message) { super(message); this.message = message; seqno = 0; chunkFactory = messageManagerItf .getChunkFactory(HelloWorldChunk.class); } public Object execute(Object hints) throws InterruptedException { try { Message msg = messageManagerItf.createMessage(); HelloWorldChunk chunk = messageManagerItf .createChunk(chunkFactory); logger.log(BasicLevel.DEBUG, "pushing message containing " + message + " seqno=" + (seqno)); chunk.setMessage(message + " seqno=" + (seqno++)); messageManagerItf.addChunk(msg, HelloWorldChunk.DEFAULT_NAME, chunk); outPushItf.push(msg); } catch (Exception e) { e.printStackTrace(); return STOP_EXECUTING; } return EXECUTE_AGAIN; } }

The execute method creates a message and a HelloWorldChunk, sets the chunk’s string, adds the chunk in the message and pushes the message.

To make this task executed, we need to register it with the task controller. This is done in the beforeFirstStart method. This method which is annotated with the (on=DreamLifeCycleType.FIRST_START) is called the first time the component is started. The code of this method is the following :

@DreamLifeCycle(on = DreamLifeCycleType.FIRST_START) protected void beforeFirstStart(Component componentItf) throws IllegalLifeCycleException { try { Map<String, Object> hints = new HashMap<String, Object>(); hints.put("period", new Long(1000)); Util.addTask(componentItf, new ClientTask("Hello World task1"), hints); logger.log(BasicLevel.DEBUG, "tasks added"); } catch (Exception e) { throw new IllegalLifeCycleException("Can't add task"); } }

The helloworld task is defined as periodic by adding a "period" key in the hints map. The unit of this period is millisecond.

3.2  ADL

In this section we will see the generated ADL of primitive components and how to write ADL files that allow binding these components.

3.2.1  The producer and consumer ADLs

As we have seen in 3.1.2 the consumer component provides a push interface and requires a message manager interface. So its generated ADL (in target/classes directory) is the following:

<definition name="helloworld.Consumer"> <interface name="in-push" role="server" signature="org.objectweb.dream.Push"/> <interface name="message-manager" role="client" signature="org.objectweb.dream.message.MessageManager"/> <content class="helloworld.ConsumerImpl"/> <controller desc="dreamPrimitive"/> </definition>

The generated ADL of the producer component is the following:

<definition name="helloworld.Producer"> <interface name="out-push" role="client" signature="org.objectweb.dream.Push"/> <interface name="message-manager" role="client" signature="org.objectweb.dream.message.MessageManager"/> <interface name="task-manager" role="client" signature= "org.objectweb.dream.control.activity.manager.TaskManager"/> <content class="helloworld.ProducerImpl"/> <controller desc="activeDreamPrimitive"/> </definition>

Note the controller element. Since the DREAM framework defines its own controllers (logger, activities …), we need to specify the controller description. The available controller descriptors are:

  • dreamPrimitive for simple primitive component
  • dreamUnstoppablePrimitive for primitive component that cannot be stopped alone (without life cycle interceptor)
  • activeDreamPrimitive for primitive component containing activities.
  • activeDreamUnstoppablePrimitive for primitive component that cannot be stopped alone and containing activities
  • dreamComposite for simple composite component
  • dreamUnstoppableComposite for composite component that cannot be stopped alone

Note : the life cycle interceptor also blocks incoming calls on server interface until the component is started.

3.2.2  The simple HelloWorld ADL

In this ADL we describe a composite component containing the producer and consumer components, plus two other required components (message manager, activity manager). The producer is bound directly to the consumer:

<definition name="helloworld.HelloWorldSimple"> <component name="Producer" definition="helloworld.Producer(5)"/> <component name="Consumer" definition="helloworld.Consumer"/> <component name="MessageManager" definition="org.objectweb.dream.message.MessageManager"/> <component name="ActivityManager" definition="org.objectweb.dream.control.activity .manager.ActivityManager"/> <binding client="Producer.out-push" server=""/> <binding client="Producer.message-manager" server="MessageManager.message-manager"/> <binding client="Consumer.message-manager" server="MessageManager.message-manager"/> <binding client="Producer.task-manager" server="ActivityManager.task-manager"/> <controller desc="dreamComposite"/> </definition>

We can execute this application by running mvn in the trunk/dreamlib/examples/helloworld directory. The application is deployed and started by the Fractal ADL factory. This results in a message being printed on the console every second:

  [java] [INFO] thread=10 helloworld.HelloWorldSimple.Consumer.impl
ConsumerImpl.push: Hello World task1 seqno=0
  [java] [INFO] thread=10 helloworld.HelloWorldSimple.Consumer.impl
ConsumerImpl.push: Hello World task1 seqno=1
  [java] [INFO] thread=10 helloworld.HelloWorldSimple.Consumer.impl
ConsumerImpl.push: Hello World task1 seqno=2
  [java] [INFO] thread=10 helloworld.HelloWorldSimple.Consumer.impl
ConsumerImpl.push: Hello World task1 seqno=3

3.2.3  More sophisticated HelloWorld

We now add some components from the DREAM component library to build a composite binding from the message producer to the consumer. The first component we add is a Queue. We choose to add a “Push/Push active queue”, this queue provides a push interface used to add incoming messages. These messages are then pushed on a push client interface by the internal activity of the queue. The ADL describing our new application is the following:

<definition name="helloworld.HelloWorldQueue" extends="helloworld.HelloWorldSimple"> <component name="Queue" definition="org.objectweb.dream.queue. PushPushQueueActive(10,ExceptionThrowing,FIFO,1)"> <component name="ActivityManager" definition="./ActivityManager" /> <component name="MessageManager" definition="./MessageManager" /> </component> <binding client="Producer.out-push" server="" /> <binding client="Queue.out-push" server="" /> <controller desc="dreamComposite" /> </definition>

This ADL extends the previous one. It adds the Queue component with the specified definition and arguments. This component is a composite component, it must contain an activity manager and a message manager, so the activity manager and the message manager defined in the HelloworldSimple ADL are shared with this composite. Then the binding from the producer to the consumer is overridden and the producer is bound to the in-push Queue interface and the out-push interface of the Queue is bound to the message consumer.

To run this application you can executing the following command:

mvn -Prun -Dcom=Queue

The output on the console will not differ from the previous example since only the consumer logs messages on the console. To see the queue’s debug messages uncomment the following line in the src/main/resources/ file :

#logger.helloworld.HelloWorldQueue.level  DEBUG

This line sets the debug level to loggers of the Queue component. Loggers are named using the architecture of the application. The Queue component is encapsulated in the helloworld.HelloWorldQueue composite, which is the root component, so each logger of the queue component (controller’s logger and sub-components’ logger) has a name beginning with helloworld.HelloWorldQueue.Queue

A DREAM component can have multiple loggers for controllers and implementation. Each one can have a specific level defined in the file. For instance if you want to activate the debug level of the logger of the life cycle controller of the consumer component you add the following line to the file: DEBUG

We now add channel components between the producer and the consumer. Channel components allow to exchange messages between different address spaces. Various channel implementations are provided by the DREAM library. In this example, we use the TCP based channel.

  • The TCPChannelOut component allows to send messages to a given destination using a TCP socket. This destination is found in each outgoing message as a ExportIdentifierChunk. This chunk contains the hostname and the listening port of the destination. It can be added to outgoing messages by the AddIPExportIdChunk component.
  • The TCPChannelInOut component can receive incoming messages sent by remote TCPChannelOut components. In addition, this compoent can also send outgoing messages as the TCPChannelOut component.

In our example we deploy both ChannelIn and ChannelOut in the same JVM. The ADL describing our new application is the following:

<definition name="helloworld.HelloWorldTCPChannel" extends="helloworld.HelloWorldSimple"> <component name="AddDestination" definition= ", localhost,1600)"/> <component name="ChannelOut" definition= ",true,2)"> <component name="MessageManager" definition="./MessageManager"/> <component name="ActivityManager" definition="./ActivityManager"/> <component name="MessageCodec" definition= "org.objectweb.dream.message.codec.MessageCodecObjectStream(false)"> <component name="MessageManager" definition="./MessageManager"/> </component> </component> <component name="ChannelIn" definition= ",true, null,2,1600,localhost)"> <component name="MessageManager" definition="./MessageManager"/> <component name="ActivityManager" definition="./ActivityManager"/> <component name="MessageCodec" definition= "org.objectweb.dream.message.codec.MessageCodecObjectStream(false)"> <component name="MessageManager" definition="./MessageManager"/> </component> </component> <binding client="Producer.out-push" server=""/> <binding client="AddDestination.out-push" server=""/> <binding client="ChannelIn.out-push" server=""/> </definition>

This ADL extends the previous one. It adds tree components:

  • the AddDestination component add a ExportIdentifierChunk in every messages pushed by the producer. The name of the added chunk, hostname and port are specified as ADL arguments (in our case "destination", "localhost" and 1600).
  • the ChannelOut composite component is responsible for sending messages created by the producer. The first argument ("destination" here) specifies the name of the chunk that contains the identifier of the destination of the message. The second one specifies if this chunk should be removed before the message is sent. As we seen in the queue example, this composite component shares MessageManager and ActivityManager components. It also contains a MessageCodec component. This latter is responsible for serializing messages.
  • the ChannelIn composite component accept incoming connections on the port 1600 (as it is specified as argument) and push received messages to the consumer.

To run this application you can executing the following command:

mvn -Prun -Dcom=TCPChannel

An UDP example with the same architecture can be launched with the following command :

mvn -Prun -Dcom=UDPChannel

This document was translated from LATEX by HEVEA.

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