Writing Simple JMS Applications

This section shows how to create, package, and run simple JMS clients that are packaged as application clients.

The following topics are addressed here:

Overview of Writing Simple JMS Application

The clients demonstrate the basic tasks a JMS application must perform:

Creating a JMSContext
Creating message producers and consumers
Sending and receiving messages

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

When you write a JMS client to run in an enterprise bean application, you use many of the same methods in much the same sequence as for an application client. However, there are some significant differences. Using the JMS API in Java EE Applications describes these differences, and this chapter provides examples that illustrate them.

The examples for this section are in the tut-install/examples/jms/simple/ directory, under the following subdirectories:

producer/
synchconsumer/
asynchconsumer/
messagebrowser/
clientackconsumer/

Before running the examples, you need to start GlassFish Server and create administered objects.

Starting the JMS Provider

Creating JMS Administered Objects

This example uses the following JMS administered objects:

A connection factory
Two destination resources: a topic and a queue

Before you run the applications, you can use the asadmin add-resources command to create needed JMS resources, specifying as the argument a file named glassfish-resources.xml. This file can be created in any project using NetBeans IDE, although you can also create it by hand. A file for the needed resources is present in the jms/simple/producer/src/main/setup/ directory.

The JMS examples use a connection factory with the logical JNDI lookup name java:comp/DefaultJMSConnectionFactory, which is preconfigured in GlassFish Server.

You can also use the asadmin create-jms-resource command to create resources, the asadmin list-jms-resources command to display their names, and the asadmin delete-jms-resource command to remove them.

To Create Resources for the Simple Examples

A glassfish-resources.xml file in one of the Maven projects can create all the resources needed for the simple examples.

Make sure that GlassFish Server has been started (see Starting and Stopping GlassFish Server).
In a command window, go to the Producer example.
```
cd tut-install/jms/simple/producer
```

Create the resources using the asadmin add-resources command:

asadmin add-resources src/main/setup/glassfish-resources.xml

Verify the creation of the resources:
```
asadmin list-jms-resources
```
The command lists the two destinations and connection factory specified in the glassfish-resources.xml file in addition to the platform default connection factory:
```
jms/MyQueue
jms/MyTopic
jms/__defaultConnectionFactory
Command list-jms-resources executed successfully.
```
In GlassFish Server, the Java EE java:comp/DefaultJMSConnectionFactory resource is mapped to a connection factory named jms/__defaultConnectionFactory.

Building All the Simple Examples

To run the simple examples using GlassFish Server, package each example in an application client JAR file. The application client JAR file requires a manifest file, located in the src/main/java/META-INF/ directory for each example, along with the .class file.

The pom.xml file for each example specifies a plugin that creates an application client JAR file. You can build the examples using either NetBeans IDE or Maven.

The following topics are addressed here:

To Build All the Simple Examples Using NetBeans IDE

From the File menu, choose Open Project.
In the Open Project dialog box, navigate to:
Expand the jms node and select the simple folder.
Click Open Project to open all the simple examples.
In the Projects tab, right-click the simple project and select Build to build all the examples.

This command places the application client JAR files in the target directories for the examples.

To Build All the Simple Examples Using Maven

In a terminal window, go to the simple directory:
```
cd tut-install/jms/simple/
```
Enter the following command to build all the projects:

This command places the application client JAR files in the target directories for the examples.

Sending Messages

This section describes how to use a client to send messages. The Producer.java client will send messages in all of these examples.

The following topics are addressed here:

General Steps Performed in the Example

General steps this example performs are as follows.

Inject resources for the administered objects used by the example.
Accept and verify command-line arguments. You can use this example to send any number of messages to either a queue or a topic, so you specify the destination type and the number of messages on the command line when you run the program.
Create a JMSContext, then send the specified number of text messages in the form of strings, as described in Message Bodies.
Send a final message of type Message to indicate that the consumer should expect no more messages.
Catch any exceptions.

The Producer.java Client

The sending client, Producer.java, performs the following steps.

Injects resources for a connection factory, queue, and topic:

@Resource(lookup = "java:comp/DefaultJMSConnectionFactory")
private static ConnectionFactory connectionFactory;
@Resource(lookup = "jms/MyQueue")
private static Queue queue;
@Resource(lookup = "jms/MyTopic")
private static Topic topic;

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

final int NUM_MSGS;
String destType = args[0];
System.out.println("Destination type is " + destType);
if ( ! ( destType.equals("queue") || destType.equals("topic") ) ) {
    System.err.println("Argument must be \"queue\" or " + "\"topic\"");
    System.exit(1);
}
if (args.length == 2){
    NUM_MSGS = (new Integer(args[1])).intValue();
} else {
    NUM_MSGS = 1;
}

Assigns either the queue or the topic to a destination object, based on the specified destination type:

Destination dest = null;
try {
    if (destType.equals("queue")) {
        dest = (Destination) queue;
    } else {
        dest = (Destination) topic;
    }
} catch (Exception e) {
    System.err.println("Error setting destination: " + e.toString());
    System.exit(1);
}

Within a try-with-resources block, creates a JMSContext:

try (JMSContext context = connectionFactory.createContext();) {

Sets the message count to zero, then creates a JMSProducer and sends one or more messages to the destination and increments the count. Messages in the form of strings are of the TextMessage message type:

    int count = 0;
    for (int i = 0; i < NUM_MSGS; i++) {
        String message = "This is message " + (i + 1)
                + " from producer";
        // Comment out the following line to send many messages
        System.out.println("Sending message: " + message);
        context.createProducer().send(dest, message);
        count += 1;
    }
    System.out.println("Text messages sent: " + count);

Sends an empty control message to indicate the end of the message stream:
```
    context.createProducer().send(dest, context.createMessage());
```
Sending an empty message of no specified type is a convenient way for an application to indicate to the consumer that the final message has arrived.

Catches and handles any exceptions. The end of the try-with-resources block automatically causes the JMSContext to be closed:

} catch (Exception e) {
    System.err.println("Exception occurred: " + e.toString());
    System.exit(1);
}
System.exit(0);

To Run the Producer Client

You can run the client using the appclient command. The Producer client takes one or two command-line arguments: a destination type and, optionally, a number of messages. If you do not specify a number of messages, the client sends one message.

You will use the client to send three messages to a queue.

Make sure that GlassFish Server has been started (see Starting and Stopping GlassFish Server) and that you have created resources and built the simple JMS examples (see Creating JMS Administered Objects and Building All the Simple Examples).
In a terminal window, go to the producer directory:
Run the Producer program, sending three messages to the queue:
```
appclient -client target/producer.jar queue 3
```
The output of the program looks like this (along with some additional output):
```
Destination type is queue
Sending message: This is message 1 from producer
Sending message: This is message 2 from producer
Sending message: This is message 3 from producer
Text messages sent: 3
```
The messages are now in the queue, waiting to be received.

Note:

When you run an application client, the command may take a long time to complete.

Receiving Messages Synchronously

This section describes the receiving client, which uses the receive method to consume messages synchronously. This section then explains how to run the clients using GlassFish Server.

The following topics are addressed here:

The SynchConsumer.java Client

The receiving client, SynchConsumer.java, performs the following steps.

Injects resources for a connection factory, queue, and topic.
Assigns either the queue or the topic to a destination object, based on the specified destination type.
Within a try-with-resources block, creates a JMSContext.
Creates a JMSConsumer, starting message delivery:
```
consumer = context.createConsumer(dest);
```

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

int count = 0;
while (true) {
    Message m = consumer.receive(1000);
    if (m != null) {
        if (m instanceof TextMessage) {
            System.out.println(
                    "Reading message: " + m.getBody(String.class));
            count += 1;
        } else {
            break;
        }
    }
}
System.out.println("Messages received: " + count);

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

Catches and handles any exceptions. The end of the try-with-resources block automatically causes the JMSContext to be closed.

The SynchConsumer client uses an indefinite while loop to receive messages, calling receive with a timeout argument.

To Run the SynchConsumer and Producer Clients

You can run the client using the appclient command. The SynchConsumer client takes one command-line argument, the destination type.

These steps show how to receive and send messages synchronously using both a queue and a topic. The steps assume you already ran the Producer client and have three messages waiting in the queue.

In the same terminal window where you ran Producer, go to the synchconsumer directory:

Run the SynchConsumer client, specifying the queue:

appclient -client target/synchconsumer.jar queue

The output of the client looks like this (along with some additional output):

Destination type is queue
Reading message: This is message 1 from producer
Reading message: This is message 2 from producer
Reading message: This is message 3 from producer
Messages received: 3

Now try running the clients in the opposite order. Run the SynchConsumer client:
```
appclient -client target/synchconsumer.jar queue
```
The client displays the destination type and then waits for messages.
Open a new terminal window and run the Producer client:
```
cd tut-install/jms/simple/producer
appclient -client target/producer.jar queue 3
```
When the messages have been sent, the SynchConsumer client receives them and exits.

Now run the Producer client using a topic instead of a queue:

appclient -client target/producer.jar topic 3

The output of the client looks like this (along with some additional output):

Destination type is topic
Sending message: This is message 1 from producer
Sending message: This is message 2 from producer
Sending message: This is message 3 from producer
Text messages sent: 3

Now, in the other terminal window, run the SynchConsumer client using the topic:
```
appclient -client target/synchconsumer.jar topic
```
The result, however, is different. Because you are using a subscription on a topic, messages that were sent before you created the subscription on the topic will not be added to the subscription and delivered to the consumer. (See Publish/Subscribe Messaging Style and Consuming Messages from Topics for details.) Instead of receiving the messages, the client waits for messages to arrive.

Leave the SynchConsumer client running and run the Producer client again:

appclient -client target/producer.jar topic 3

Now the SynchConsumer client receives the messages:

Destination type is topic
Reading message: This is message 1 from producer
Reading message: This is message 2 from producer
Reading message: This is message 3 from producer
Messages received: 3

Because these messages were sent after the consumer was started, the client receives them.

Using a Message Listener for Asynchronous Message Delivery

This section describes the receiving clients in an example that uses a message listener for asynchronous message delivery. This section then explains how to compile and run the clients using GlassFish Server.

Note:

In the Java EE platform, message listeners can be used only in application clients, as in this example. To allow asynchronous message delivery in a web or enterprise bean application, you use a message-driven bean, shown in later examples in this chapter.

The following topics are addressed here:

Writing the AsynchConsumer.java and TextListener.java Clients

The sending client is Producer.java, the same client used in Sending Messages and Receiving Messages Synchronously.

An asynchronous consumer normally runs indefinitely. This one runs until the user types the character q or Q to stop the client.

The client, AsynchConsumer.java, performs the following steps.
Injects resources for a connection factory, queue, and topic.
Assigns either the queue or the topic to a destination object, based on the specified destination type.
In a try-with-resources block, creates a JMSContext.
Creates a JMSConsumer.
Creates an instance of the TextListener class and registers it as the message listener for the JMSConsumer:
```
listener = new TextListener();
consumer.setMessageListener(listener);
```
Listens for the messages sent to the destination, stopping when the user types the character q or Q (it uses a java.io.InputStreamReader to do this).
Catches and handles any exceptions. The end of the try-with-resources block automatically causes the JMSContext to be closed, thus stopping delivery of messages to the message listener.
The message listener, TextListener.java, follows these steps:
When a message arrives, the onMessage method is called automatically.

If the message is a TextMessage, the onMessage method displays its content as a string value. If the message is not a text message, it reports this fact:

public void onMessage(Message m) {
    try {
        if (m instanceof TextMessage) {
            System.out.println(
                    "Reading message: " + m.getBody(String.class));
        } else {
             System.out.println("Message is not a TextMessage");
        }
    } catch (JMSException | JMSRuntimeException e) {
        System.err.println("JMSException in onMessage(): " + e.toString());
    }
}

For this example, you will use the same connection factory and destinations you created in To Create Resources for the Simple Examples.

The steps assume that you have already built and packaged all the examples using NetBeans IDE or Maven.

To Run the AsynchConsumer and Producer Clients

You will need two terminal windows, as you did in Receiving Messages Synchronously.

In the terminal window where you ran the SynchConsumer client, go to the asynchconsumer example directory:
```
cd tut-install/jms/simple/asynchconsumer
```
Run the AsynchConsumer client, specifying the topic destination type:
```
appclient -client target/asynchconsumer.jar topic
```
The client displays the following lines (along with some additional output) and then waits for messages:
```
Destination type is topic
To end program, enter Q or q, then <return>
```

In the terminal window where you ran the Producer client previously, run the client again, sending three messages:

appclient -client target/producer.jar topic 3

The output of the client looks like this (along with some additional output):

Destination type is topic
Sending message: This is message 1 from producer
Sending message: This is message 2 from producer
Sending message: This is message 3 from producer
Text messages sent: 3

In the other window, the AsynchConsumer client displays the following (along with some additional output):

Destination type is topic
To end program, enter Q or q, then <return>
Reading message: This is message 1 from producer
Reading message: This is message 2 from producer
Reading message: This is message 3 from producer
Message is not a TextMessage

The last line appears because the client has received the non-text control message sent by the Producer client.

Enter Q or q and press Return to stop the AsynchConsumer client.

Now run the clients using a queue.

In this case, as with the synchronous example, you can run the Producer client first, because there is no timing dependency between the sender and receiver:

appclient -client target/producer.jar queue 3

The output of the client looks like this:

Destination type is queue
Sending message: This is message 1 from producer
Sending message: This is message 2 from producer
Sending message: This is message 3 from producer
Text messages sent: 3

In the other window, run the AsynchConsumer client:

appclient -client target/asynchconsumer.jar queue

The output of the client looks like this (along with some additional output):

Destination type is queue
To end program, enter Q or q, then <return>
Reading message: This is message 1 from producer
Reading message: This is message 2 from producer
Reading message: This is message 3 from producer
Message is not a TextMessage

Enter Q or q and press Return to stop the client.

Browsing Messages on a Queue

This section describes an example that creates a QueueBrowser object to examine messages on a queue, as described in JMS Queue Browsers. This section then explains how to compile, package, and run the example using GlassFish Server.

The following topics are addressed here:

The MessageBrowser.java Client

To create a QueueBrowser for a queue, you call the JMSContext.createBrowser method with the queue as the argument. You obtain the messages in the queue as an Enumeration object. You can then iterate through the Enumeration object and display the contents of each message.

The MessageBrowser.java client performs the following steps.

Injects resources for a connection factory and a queue.
In a try-with-resources block, creates a JMSContext.

Creates a QueueBrowser:

QueueBrowser browser = context.createBrowser(queue);

Retrieves the Enumeration that contains the messages:
```
Enumeration msgs = browser.getEnumeration();
```

Verifies that the Enumeration contains messages, then displays the contents of the messages:

if ( !msgs.hasMoreElements() ) {
    System.out.println("No messages in queue");
} else {
    while (msgs.hasMoreElements()) {
        Message tempMsg = (Message)msgs.nextElement();
        System.out.println("Message: " + tempMsg);
    }
}

Catches and handles any exceptions. The end of the try-with-resources block automatically causes the JMSContext to be closed.

Dumping the message contents to standard output retrieves the message body and properties in a format that depends on the implementation of the toString method. In GlassFish Server, the message format looks something like this:

Text:   This is message 3 from producer
Class:                  com.sun.messaging.jmq.jmsclient.TextMessageImpl
getJMSMessageID():      ID:8-10.152.23.26(bf:27:4:e:e7:ec)-55645-1363100335526
getJMSTimestamp():      1129061034355
getJMSCorrelationID():  null
JMSReplyTo:             null
JMSDestination:         PhysicalQueue
getJMSDeliveryMode():   PERSISTENT
getJMSRedelivered():    false
getJMSType():           null
getJMSExpiration():     0
getJMSPriority():       4
Properties:             {JMSXDeliveryCount=0}

Instead of displaying the message contents this way, you can call some of the Message interface’s getter methods to retrieve the parts of the message you want to see.

For this example, you will use the connection factory and queue you created for Receiving Messages Synchronously. It is assumed that you have already built and packaged all the examples.

To Run the QueueBrowser Client

To run the MessageBrowser example using the appclient command, follow these steps.

You also need the Producer example to send the message to the queue, and one of the consumer clients to consume the messages after you inspect them.

To run the clients, you need two terminal windows.

In a terminal window, go to the producer directory:
```
cd tut-install/examples/jms/simple/producer/
```
Run the Producer client, sending one message to the queue, along with the non-text control message:
```
appclient -client target/producer.jar queue
```
The output of the client looks like this (along with some additional output):
```
Destination type is queue
Sending message: This is message 1 from producer
Text messages sent: 1
```
In another terminal window, go to the messagebrowser directory:
```
cd tut-install/jms/simple/messagebrowser
```

Run the MessageBrowser client using the following command:

appclient -client target/messagebrowser.jar

The output of the client looks something like this (along with some additional output):

Message:
Text:   This is message 1 from producer
Class:                  com.sun.messaging.jmq.jmsclient.TextMessageImpl
getJMSMessageID():      ID:9-10.152.23.26(bf:27:4:e:e7:ec)-55645-1363100335526
getJMSTimestamp():      1363100335526
getJMSCorrelationID():  null
JMSReplyTo:             null
JMSDestination:         PhysicalQueue
getJMSDeliveryMode():   PERSISTENT
getJMSRedelivered():    false
getJMSType():           null
getJMSExpiration():     0
getJMSPriority():       4
Properties:             {JMSXDeliveryCount=0}

Message:
Class:                  com.sun.messaging.jmq.jmsclient.MessageImpl
getJMSMessageID():      ID:10-10.152.23.26(bf:27:4:e:e7:ec)-55645-1363100335526
getJMSTimestamp():      1363100335526
getJMSCorrelationID():  null
JMSReplyTo:             null
JMSDestination:         PhysicalQueue
getJMSDeliveryMode():   PERSISTENT
getJMSRedelivered():    false
getJMSType():           null
getJMSExpiration():     0
getJMSPriority():       4
Properties:             {JMSXDeliveryCount=0}

The first message is the TextMessage, and the second is the non-text control message.

Go to the synchconsumer directory.

Run the SynchConsumer client to consume the messages:

appclient -client target/synchconsumer.jar queue

The output of the client looks like this (along with some additional output):

Destination type is queue
Reading message: This is message 1 from producer
Messages received: 1

Running Multiple Consumers on the Same Destination

To illustrate further the way point-to-point and publish/subscribe messaging works, you can use the Producer and SynchConsumer examples to send messages that are then consumed by two clients running simultaneously.

Open three command windows. In one, go to the producer directory. In the other two, go to the synchconsumer directory.
In each of the synchconsumer windows, start running the client, receiving messages from a queue:
```
appclient -client target/synchconsumer.jar queue
```
Wait until you see the "Destination type is queue" message in both windows.
In the producer window, run the client, sending 20 or so messages to the queue:
```
appclient -client target/producer.jar queue 20
```
Look at the output in the synchconsumer windows. In point-to-point messaging, each message can have only one consumer. Therefore, each of the clients receives some of the messages. One of the clients receives the non-text control message, reports the number of messages received, and exits.
In the window of the client that did not receive the non-text control message, enter Control-C to exit the program.
Next, run the synchconsumer clients using a topic. In each window, run the following command:
```
appclient -client target/synchconsumer.jar topic
```
Wait until you see the "Destination type is topic" message in both windows.
In the producer window, run the client, sending 20 or so messages to the topic:
```
appclient -client target/producer.jar topic 20
```
Again, look at the output in the synchconsumer windows. In publish/subscribe messaging, a copy of every message is sent to each subscription on the topic. Therefore, each of the clients receives all 20 text messages as well as the non-text control message.

Acknowledging Messages

JMS provides two alternative ways for a consuming client to ensure that a message is not acknowledged until the application has finished processing the message:

Using a synchronous consumer in a JMSContext that has been configured to use the CLIENT_ACKNOWLEDGE setting
Using a message listener for asynchronous message delivery in a JMSContext that has been configured to use the default AUTO_ACKNOWLEDGE setting

Note:

In the Java EE platform, CLIENT_ACKNOWLEDGE sessions can be used only in application clients, as in this example.

The clientackconsumer example demonstrates the first alternative, in which a synchronous consumer uses client acknowledgment. The asynchconsumer example described in Using a Message Listener for Asynchronous Message Delivery demonstrates the second alternative.

The following table describes four possible interactions between types of consumers and types of acknowledgment.

Table 49-3 Message Acknowledgment with Synchronous and Asynchronous Consumers

Consumer Type	Acknowledgment Type	Behavior
Synchronous	Client	Client acknowledges message after processing is complete
Asynchronous	Client	Client acknowledges message after processing is complete
Synchronous	Auto	Acknowledgment happens immediately after `receive` call; message cannot be redelivered if any subsequent processing steps fail
Asynchronous	Auto	Message is automatically acknowledged when `onMessage` method returns

The example is under the tut-install`/examples/jms/simple/clientackconsumer/` directory.

The example client, ClientAckConsumer.java, creates a JMSContext that specifies client acknowledgment:

try (JMSContext context =
      connectionFactory.createContext(JMSContext.CLIENT_ACKNOWLEDGE);) {
    ...

The client uses a while loop almost identical to that used by SynchConsumer.java, with the exception that after processing each message, it calls the acknowledge method on the JMSContext:

The example uses the following objects:

The jms/MyQueue resource that you created for Receiving Messages Synchronously.
java:comp/DefaultJMSConnectionFactory, the platform default connection factory preconfigured with GlassFish Server

To Run the ClientAckConsumer Client

In a terminal window, go to the following directory:
```
tut-install/examples/jms/simple/producer/
```
Run the Producer client, sending some messages to the queue:
```
appclient -client target/producer.jar queue 3
```
In another terminal window, go to the following directory:
```
tut-install/examples/jms/simple/clientackconsumer/
```
To run the client, use the following command:
```
appclient -client target/clientackconsumer.jar
```
The client output looks like this (along with some additional output):
```
Created client-acknowledge JMSContext
Reading message: This is message 1 from producer
Acknowledging TextMessage
Reading message: This is message 2 from producer
Acknowledging TextMessage
Reading message: This is message 3 from producer
Acknowledging TextMessage
Acknowledging non-text control message
```
The client acknowledges each message explicitly after processing it, just as a JMSContext configured to use AUTO_ACKNOWLEDGE does automatically after a MessageListener returns successfully from processing a message received asynchronously.