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Writing Simple JMS Client Applications

This section shows how to create, package, and run simple JMS client programs packaged as stand-alone application clients. These clients access a J2EE server. The clients demonstrate the basic tasks that a JMS application must perform:

Creating a connection and a session
Creating message producers and consumers
Sending and receiving messages

In a J2EE application, some of these tasks are performed, in whole or in part, by the container. If you learn about these tasks, you will have a good basis for understanding how a JMS application works on the J2EE platform.

This section covers the following topics:

An example that uses synchronous message receives
An example that uses a message listener
Running JMS clients on multiple systems

Each example uses two programs: one that sends messages and one that receives them. You can run the programs in two terminal windows.

When you write a JMS application to run in a J2EE application, you use many of the same methods in much the same sequence as you do for a stand-alone application client. However, there are some significant differences. Using the JMS API in a J2EE Application describes these differences, and Chapter 34 provides examples that illustrate them.

The examples for this section are in the following directory:

<INSTALL>/j2eetutorial14/examples/jms/simple/ 

A Simple Example of Synchronous Message Receives

This section describes the sending and receiving programs in an example that uses the receive method to consume messages synchronously. This section then explains how to compile, package, and run the programs using the Application Server.

The following sections describe the steps in creating and running the example:

Writing the Client Programs
Compiling the Clients
Starting the JMS Provider
Creating JMS Administered Objects
Packaging the Clients
Running the Clients

Writing the Client Programs

The sending program, src/SimpleProducer.java, performs the following steps:

Retrieves command-line arguments that specify the destination name and type and the number of arguments:

final int NUM_MSGS;
String destName = new String(args[0]);
String destType = new String(args[1]);
System.out.println("Destination name is " + destName +
  ", type is " + destType);
if (args.length == 3){
  NUM_MSGS = (new Integer(args[2])).intValue();
} else {
  NUM_MSGS = 1;
}

Performs a JNDI lookup of the ConnectionFactory and Destination:

/*
* Create a JNDI API InitialContext object if none exists
* yet.
*/
Context jndiContext = null;
try {
  jndiContext = new InitialContext();
} catch (NamingException e) {
  System.out.println("Could not create JNDI API " +
    "context: " + e.toString());
  System.exit(1);
}

/*
* Look up connection factory and destination. If either
* does not exist, exit. If you look up a
* TopicConnectionFactory instead of a
* QueueConnectionFactory, program behavior is the same.
*/
ConnectionFactory connectionFactory = null;
Destination dest = null;
try {
  connectionFactory = (ConnectionFactory)
    jndiContext.lookup("jms/QueueConnectionFactory");
  if (destType.equals("queue")) {
    dest = (Queue) jndiContext.lookup(destName);
  } else if (destType.equals("topic")) {
    dest = (Topic) jndiContext.lookup(destName);
  } else {
    throw new Exception("Invalid destination type" +
      "; must be queue or topic");
  }
} catch (Exception e) {
  System.out.println("JNDI API lookup failed: " +
    e.toString());
  System.exit(1);
}

Creates a Connection and a Session:

Connection connection =
  connectionFactory.createConnection();
Session session = connection.createSession(false,
  Session.AUTO_ACKNOWLEDGE);

Creates a MessageProducer and a TextMessage:

MessageProducer producer =
  session.createProducer(dest);
TextMessage message = session.createTextMessage();

Sends one or more messages to the destination:

for (int i = 0; i < NUM_MSGS; i++) {
  message.setText("This is message " + (i + 1));
  System.out.println("Sending message: " +
    message.getText());
  producer.send(message);
}

Sends an empty control message to indicate the end of the message stream:

producer.send(session.createMessage());

Sending an empty message of no specified type is a convenient way to indicate to the consumer that the final message has arrived.

Closes the connection in a finally block, automatically closing the session and MessageProducer:

} finally {
  if (connection != null) {
    try {
      connection.close();
    } catch (JMSException e) {}
  }
}

The receiving program, src/SimpleSynchConsumer.java, performs the following steps:

Performs a JNDI lookup of the ConnectionFactory and Destination.
Creates a Connection and a Session.
Creates a MessageConsumer:

consumer = session.createConsumer(dest);

Starts the connection, causing message delivery to begin:

connection.start();

Receives the messages sent to the destination until the end-of-message-stream control message is received:

while (true) {
  Message m = consumer.receive(1);
  if (m != null) {
    if (m instanceof TextMessage) {
      message = (TextMessage) m;
      System.out.println("Reading message: " +
        message.getText());
    } else {
      break;
    }
  }
}

Because the control message is not a TextMessage, the receiving program terminates the while loop and stops receiving messages after the control message arrives.

Closes the connection in a finally block, automatically closing the session and MessageConsumer.

The receive method can be used in several ways to perform a synchronous receive. If you specify no arguments or an argument of 0, the method blocks indefinitely until a message arrives:

Message m = consumer.receive(); 
Message m = consumer.receive(0); 

For a simple client program, this may not matter. But if you do not want your program to consume system resources unnecessarily, use a timed synchronous receive. Do one of the following:

Call the receive method with a timeout argument greater than 0:

Message m = consumer.receive(1); // 1 millisecond

Call the receiveNoWait method, which receives a message only if one is available:

Message m = consumer.receiveNoWait();

The SimpleSynchConsumer program uses an indefinite while loop to receive messages, calling receive with a timeout argument. Calling receiveNoWait would have the same effect.

Compiling the Clients

You can compile the examples using the asant tool, as described in Building the Examples.

To compile the examples, do the following:

In a terminal window, go to the following directory:

<INSTALL>/j2eetutorial14/examples/jms/simple/

Type the following command:

asant build

This command uses the build.xml file in the simple directory to compile all the source files in the directory. The class files are placed in the build directory.

Starting the JMS Provider

When you use the Application Server, your JMS provider is the Application Server. Start the server as described in Starting and Stopping the Application Server.

Creating JMS Administered Objects

Creating the JMS administered objects for this section involves the following:

Starting the Admin Console
Creating two connection factories
Creating two physical destinations
Creating two destination resources

If you built and ran the SimpleMessage example in Chapter 28 and did not delete the resources afterward, you need to create only half of these resources: those that involve topics.

To start the Admin Console, follow the instructions in Starting the Admin Console.

To create the connection factories, perform the following steps:

In the tree component, expand the Java Message Service node.
Select the Connection Factories node.
On the JMS Connection Factories page, click New. The Create JMS Connection Factory page appears.
In the JNDI Name field, type jms/QueueConnectionFactory.
Choose javax.jms.QueueConnectionFactory from the Type combo box.
Select the Enabled checkbox. The Admin Console appears as shown in Figure 33-6.
Click OK to save the connection factory.
Click New again.
In the JNDI Name field, type jms/TopicConnectionFactory.
Choose javax.jms.TopicConnectionFactory from the Type combo box.
Select the Enabled checkbox.
Click OK.

Figure 33-6 Creating a JMS Connection Factory

To create the physical destinations, perform the following steps:

Select the Physical Destinations node.
On the Physical Destinations page, click New. The Create Physical Destination page appears.
In the Physical Destination Name field, type PhysicalQueue.
Choose queue from the Type combo box.
Click OK.
Click New again.
In the Physical Destination Name field, type PhysicalTopic.
Choose topic from the Type combo box.
Click OK.

To create the destination resources and link them to the physical destinations, perform the following steps:

In the tree component, expand Destination Resources.
On the JMS Destination Resources page, click New. The Create JMS Destination Resource page appears.
In the JNDI Name field, type jms/Queue.
Choose javax.jms.Queue from the Type combo box.
Select the Enabled checkbox.
Under Additional Properties, click Add.
Type Name in the Name field.
Type PhysicalQueue in the Value field.
Click OK.
Click New again.
In the JNDI Name field, type jms/Topic.
Choose javax.jms.Topic from the Type combo box.
Select the Enabled checkbox.
Under Additional Properties, click Add.
Enter Name in the Name field.
Enter PhysicalTopic in the Value field. The Admin Console appears as shown in Figure 33-7.
Click OK to save the resource.

Figure 33-7 Creating a JMS Destination Resource

Packaging the Clients

The simplest way to run these examples using the Application Server is to package each one in an application client JAR file.

First, start deploytool:

On Windows systems, choose StartProgramsSun MicrosystemsJ2EE 1.4 SDKDeploytool.
On UNIX systems, use the deploytool command.

Package the SimpleProducer example as follows:

Choose FileNewApplication Client to start the Application Client wizard.
In the JAR File Contents screen, select the radio button labeled Create New Stand-Alone AppClient Module.
Click Browse next to the AppClient Location field and navigate to the <INSTALL>/j2eetutorial14/examples/jms/simple/ directory.
Type SimpleProducer in the File Name field, and click Create Module File.
Verify that SimpleProducer appears in the AppClient Name field.
Click the Edit button next to the Contents text area.
In the dialog box, locate the build directory. Select SimpleProducer.class from the Available Files tree. Click Add and then OK.
In the General screen, select SimpleProducer in the Main Class combo box.
Click Next.
Click Finish.

Package the SimpleSynchConsumer example in the same way, except for the values listed in Table 33-3.

Table 33-3 Application Values for SimpleSynchConsumer 

Wizard Field or Area	Value
File Name	SimpleSynchConsumer.jar
AppClient Name	SimpleSynchConsumer
Available Files class	build/SimpleSynchConsumer.class
Main Class	SimpleSynchConsumer

Running the Clients

You run the sample programs using the appclient command. Each of the programs takes command-line arguments: a destination name, a destination type, and, for SimpleProducer, a number of messages.

Run the clients as follows.

Run the SimpleProducer program, sending three messages to the queue jms/Queue:

appclient -client SimpleProducer.jar jms/Queue queue 3

The output of the program looks like this:

Destination name is jms/Queue, type is queue
Sending message: This is message 1
Sending message: This is message 2
Sending message: This is message 3

The messages are now in the queue, waiting to be received.

In the same window, run the SimpleSynchConsumer program, specifying the queue name and type:

appclient -client SimpleSynchConsumer.jar jms/Queue queue

The output of the program looks like this:

Destination name is jms/Queue, type is queue
Reading message: This is message 1
Reading message: This is message 2
Reading message: This is message 3

Now try running the programs in the opposite order. Run the SimpleSynchConsumer program. It displays the queue name and then appears to hang, waiting for messages.
In a different terminal window, run the SimpleProducer program. When the messages have been sent, the SimpleSynchConsumer program receives them and exits.
Now run the SimpleProducer program using a topic instead of a queue:

appclient -client SimpleProducer.jar jms/Topic topic 3

The output of the program looks like this:

Destination name is jms/Topic, type is topic
Sending message: This is message 1
Sending message: This is message 2
Sending message: This is message 3

Now run the SimpleSynchConsumer program using the topic:

appclient -client SimpleSynchConsumer.jar jms/Topic topic

The result, however, is different. Because you are using a topic, messages that were sent before you started the consumer cannot be received. (See Publish/Subscribe Messaging Domain, for details.) Instead of receiving the messages, the program appears to hang.

Run the SimpleProducer program again in another terminal window. Now the SimpleSynchConsumer program receives the messages:

Destination name is jms/Topic, type is topic
Reading message: This is message 1
Reading message: This is message 2
Reading message: This is message 3

Because the examples use the common interfaces, you can run them using either a queue or a topic.

A Simple Example of Asynchronous Message Consumption

This section describes the receiving programs in an example that uses a message listener to consume messages asynchronously. This section then explains how to compile and run the programs using the Application Server.

The following sections describe the steps in creating and running the example:

Writing the Client Programs
Compiling the Clients
Starting the JMS Provider
Packaging the SimpleAsynchConsumer Client
Running the Clients

Writing the Client Programs

The sending program is src/SimpleProducer.java, the same program used in the example in A Simple Example of Synchronous Message Receives. You may, however, want to comment out the following line of code, where the producer sends a nontext control message to indicate the end of the messages:

producer.send(session.createMessage()); 

An asynchronous consumer normally runs indefinitely. This one runs until the user types the letter q or Q to stop the program, so it does not use the nontext control message.

The receiving program, src/SimpleAsynchConsumer.java, performs the following steps:

Performs a JNDI lookup of the ConnectionFactory and Destination.
Creates a Connection and a Session.
Creates a MessageConsumer.
Creates an instance of the TextListener class and registers it as the message listener for the MessageConsumer:

listener = new TextListener();
consumer.setMessageListener(listener);

Starts the connection, causing message delivery to begin.
Listens for the messages published to the destination, stopping when the user types the character q or Q:

System.out.println("To end program, type Q or q, " +
  "then <return>");
inputStreamReader = new InputStreamReader(System.in);
while (!((answer == 'q') || (answer == 'Q'))) {
  try {
    answer = (char) inputStreamReader.read();
  } catch (IOException e) {
    System.out.println("I/O exception: "
      + e.toString());
  }
}

Closes the connection, which automatically closes the session and MessageConsumer.

The message listener, src/TextListener.java, follows these steps:

When a message arrives, the onMessage method is called automatically.
The onMessage method converts the incoming message to a TextMessage and displays its content. If the message is not a text message, it reports this fact:

public void onMessage(Message message) {
  TextMessage msg = null;

  try {
    if (message instanceof TextMessage) {
      msg = (TextMessage) message;
      System.out.println("Reading message: " +
        msg.getText());
    } else {
      System.out.println("Message is not a " +
        "TextMessage");
    }
  } catch (JMSException e) {
    System.out.println("JMSException in onMessage(): " +
      e.toString());
  } catch (Throwable t) {
    System.out.println("Exception in onMessage():" +
      t.getMessage());
  }
}

Compiling the Clients

Compile the programs if you did not do so before or if you edited SimpleProducer.java as described in Writing the Client Programs:

asant build 

Starting the JMS Provider

If you did not do so before, start the Application Server in another terminal window.

You will use the connection factories and destinations you created in Creating JMS Administered Objects.

Packaging the SimpleAsynchConsumer Client

If you did not do so before, start deploytool.

If you did not package the SimpleProducer example, follow the instructions in Packaging the Clients to do so. Package the SimpleAsynchConsumer example in the same way as SimpleProducer, except for the values listed in Table 33-4.

Table 33-4 Application Values for SimpleAsynchConsumer 

Wizard Field or Area	Value
File Name	SimpleAsynchConsumer.jar
AppClient Name	SimpleAsynchConsumer
Available Files classes	build/SimpleAsynchConsumer.class build/TextListener.class
Main Class	SimpleAsynchConsumer

Running the Clients

As before, you run the sample programs using the appclient command.

Run the clients as follows.

Run the SimpleAsynchConsumer program, specifying the topic jms/Topic and its type.

appclient -client SimpleAsynchConsumer.jar jms/Topic topic

The program displays the following lines and appears to hang:

Destination name is jms/Topic, type is topic
To end program, type Q or q, then <return>

In another terminal window, run the SimpleProducer program, sending three messages. The commands look like this:

appclient -client SimpleProducer.jar jms/Topic topic 3

The output of the program looks like this:

Destination name is jms/Topic, type is topic
Sending message: This is message 1
Sending message: This is message 2
Sending message: This is message 3

In the other window, the SimpleAsynchConsumer program displays the following:

Destination name is jms/Topic, type is topic
To end program, type Q or q, then <return>
Reading message: This is message 1
Reading message: This is message 2
Reading message: This is message 3

If you did not edit SimpleProducer.java, the following line also appears:

Message is not a TextMessage

Type Q or q to stop the program.
Now run the programs using a queue. In this case, as with the synchronous example, you can run the SimpleProducer program first, because there is no timing dependency between the sender and receiver:

appclient -client SimpleProducer.jar jms/Queue queue 3

The output of the program looks like this:

Destination name is jms/Queue, type is queue
Sending message: This is message 1
Sending message: This is message 2
Sending message: This is message 3

Run the SimpleAsynchConsumer program:

appclient -client SimpleAsynchConsumer.jar jms/Queue queue

The output of the program looks like this:

Destination name is jms/Queue, type is queue
To end program, type Q or q, then <return>
Reading message: This is message 1
Reading message: This is message 2
Reading message: This is message 3

Type Q or q to stop the program.

Running JMS Client Programs on Multiple Systems

JMS client programs using the Application Server can exchange messages with each other when they are running on different systems in a network. The systems must be visible to each other by name--the UNIX host name or the Microsoft Windows computer name--and must both be running the Application Server. You do not have to install the tutorial examples on both systems; you can use the examples installed on one system if you can access its file system from the other system.

---

Note: Any mechanism for exchanging messages between systems is specific to the J2EE server implementation. This tutorial describes how to use the Application Server for this purpose.

---

Suppose that you want to run the SimpleProducer program on one system, earth, and the SimpleSynchConsumer program on another system, jupiter. Before you can do so, you need to perform these tasks:

Create two new connection factories
Edit the source code
Recompile the source code and update the client JAR files

---

Note: A limitation in the JMS provider in the Application Server may cause a runtime failure to create a connection to systems that use the Dynamic Host Configuration Protocol (DHCP) to obtain an IP address. You can, however, create a connection from a system that uses DHCP to a system that does not use DHCP. In the examples in this tutorial, earth can be a system that uses DHCP, and jupiter can be a system that does not use DHCP.

---

Before you begin, start the server on both systems:

Start the Application Server on earth and log in to the Admin Console.
Start the Application Server on jupiter and log in to the Admin Console.

Creating Administered Objects for Multiple Systems

To run these programs, you must do the following:

Create a new connection factory on both earth and jupiter
Create a destination resource and physical destination on both earth and jupiter

Create a new connection factory on jupiter as follows:

In the Admin Console, expand the Java Message Service node.
Select the Connection Factories node.
On the JMS Connection Factories page, click New. The Create JMS Connection Factory page appears.
In the JNDI Name field, type jms/JupiterQueueConnectionFactory.
Choose javax.jms.QueueConnectionFactory from the Type combo box.
Select the Enabled checkbox.
Click OK.

Create a new connection factory with the same name on earth as follows:

In the Admin Console, expand the Java Message Service node.
Select the Connection Factories node.
On the JMS Connection Factories page, click New. The Create JMS Connection Factory page appears.
In the JNDI Name field, type jms/JupiterQueueConnectionFactory.
Choose javax.jms.QueueConnectionFactory from the Type combo box.
Select the Enabled checkbox.
Click Add in the Additional Properties area. A Name/Value line appears.
In the Name field, type MessageServiceAddressList.
In the Value field, type the name of the remote system (whatever the real name of jupiter is). If the JMS service on the remote system uses a port number other than the default (7676), specify the port number also, using the syntax sys-name:port-number.
Click OK.

If you have already been working on either earth or jupiter, you have the queue on one system. On the system that does not have the queue, perform the following steps:

Use the Admin Console to create a physical destination named PhysicalQueue, just as you did in Creating JMS Administered Objects.
Use the Admin Console to create a destination resource named jms/Queue and set its Name property to the value PhysicalQueue.

When you run the programs, they will work as shown in Figure 33-8. The program run on earth needs the queue on earth only in order that the JNDI lookup will succeed. The connection, session, and message producer are all created on jupiter using the connection factory that points to jupiter. The messages sent from earth will be received on jupiter.

Figure 33-8 Sending Messages from One System to Another

Running the Programs

These steps assume that you have the tutorial installed on only one of the two systems you are using.

To edit, update, and run the programs, perform the following steps on the system where you first ran them:

In both SimpleProducer.java and SimpleSynchConsumer.java, change the line that looks up the connection factory so that it refers to the new connection factory:

connectionFactory = (ConnectionFactory)
  jndiContext.lookup("jms/JupiterQueueConnectionFactory");

Recompile the programs:

asant build

In deploytool, choose ToolsUpdate Module Files to add the recompiled source files to the SimpleProducer.jar and SimpleSynchConsumer.jar files.
Save the changed JAR files.
Run SimpleProducer on earth:

appclient -client SimpleProducer.jar jms/Queue queue 3

Run SimpleSynchConsumer on jupiter:

appclient -client SimpleSynchConsumer.jar jms/Queue queue

Because both connection factories have the same name, you can run either the producer or the consumer on either system.

For examples showing how to deploy J2EE applications on two different systems, see An Application Example That Consumes Messages from a Remote J2EE Server and An Application Example That Deploys a Message-Driven Bean on Two J2EE Servers.

Deleting the Connection Factory and Stopping the Server

You will need the connection factory jms/JupiterQueueConnectionFactory in Chapter 34. However, if you wish to delete it, perform the following steps in the Admin Console:

Expand the Java Message Service node and click Connection Factories.
Select the checkbox next to jms/JupiterQueueConnectionFactory and click Delete.

Remember to delete the connection factory on both systems.

You can also use the Admin Console to delete the destinations and connection factories you created in Creating JMS Administered Objects. However, we recommend that you keep them, because they will be used in most of the examples in Chapter 34. After you have created them, they will be available whenever you restart the Application Server.

Delete the class files for the programs as follows:

asant clean 

If you wish, you can manually delete the client JAR files.

You can also stop the Application Server, but you will need it to run the sample programs in the next section.

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