Point to Point Messaging Program
The Hello World application consists of a sender application that sends a
"Hello" message to a queue. This message will be received by one
queue receiver connected to the queue in question. If no receivers are
connected, the message will be retained on the queue. If more queue receivers
are connected, they will receive messages in a round-robin fashion
There are four sample programs for this section:
- Queue Sender
- Synchronous Queue Receiver
- Asynchronous Queue Receiver
- Queue Browser
Note that none of the examples in this section show code for handling
exceptions. Although this improves the readability of the example code,
application programmers should notice that almost all methods in the JMS
API's may raise a
JMSException
if the JMS provider fails.
Queue Sender
The queue sender application performs the following steps:
- Obtain an
InitialContext object for the JMS server.
- Use the context object to lookup a specific queue, in this case,
queue0.
- Use the context object to lookup the queue connection factory. You only
need to specify the
queue/connectionFactory with the lookup because
the batch file that you run this sample from has set the System
properties to point to the appropriate root context for the System
namespace. If you were using the JMS server with the
Novell exteNd Application Server, you would have to specify the lookup
as follows:
QueueConnectionFactory connFactory = (QueueConnectionFactory) ctx.
lookup("iiop://localhost:53506/queue/connectionFactory");
- Use the
QueueConnectionFactory to create a QueueConnection. The
QueueConnection represents a physical connection to the JMS
server.
- Create a queue session. The first parameter in the
createQueueSession
method decides whether or not the session is transacted. Here, we
use a non-transacted session. The second parameter decided the
delivery mode, which is never used for sending applications.
- Create a queue sender for
queue0 and create a message.
- Send the "Hello" message to
queue0.
- Close the queue connection. This will in turn close both the session
and the
QueueSender.
The full source code for the sender application is shown below:
package pointToPoint;
import javax.naming.InitialContext;
import javax.jms.Queue;
import javax.jms.Session;
import javax.jms.TextMessage;
import javax.jms.QueueSender;
import javax.jms.DeliveryMode;
import javax.jms.QueueSession;
import javax.jms.QueueConnection;
import javax.jms.QueueConnectionFactory;
public class Sender
{
public static void main(String[] args) throws Exception
{
// get the initial context
InitialContext ctx = new InitialContext();
// lookup the queue object
Queue queue = (Queue) ctx.lookup("queue/queue0");
// lookup the queue connection factory
QueueConnectionFactory connFactory = (QueueConnectionFactory) ctx.
lookup("queue/connectionFactory");
// create a queue connection
QueueConnection queueConn = connFactory.createQueueConnection();
// create a queue session
QueueSession queueSession = queueConn.createQueueSession(false,
Session.DUPS_OK_ACKNOWLEDGE);
// create a queue sender
QueueSender queueSender = queueSession.createSender(queue);
queueSender.setDeliveryMode(DeliveryMode.NON_PERSISTENT);
// create a simple message to say "Hello"
TextMessage message = queueSession.createTextMessage("Hello");
// send the message
queueSender.send(message);
// print what we did
System.out.println("sent: " + message.getText());
// close the queue connection
queueConn.close();
}
}
The
Sender class sets the delivery mode to
NON_PERSISTENT before
sending the message. This means that the message will be lost in case the JMS server
crashes. Since
NON_PERSISTENT messages giver better performance than
PERSISTENT messages, applications should set the delivery mode to
NON_PERSISTENT whenever guaranteed delivery is not a requirement.
Synchronous Queue Receiver
The receive application performs the same initial steps as the queue sender
because you always have to find a queue object using the initial context,
connect to the queue and create a session as shown here.
Instead of a
QueueSender object, the receiver application creates a
QueueReceiver from which messages can be received synchronously.
Note that the receiver application
must start the connection before
any messages can be received.
The receiver application uses a non-transacted session with automatic
message acknowledgement. This means that message will automatically be
acknowledged by the session right before the receive method returns the
message to the application.
Below is the source for the
Receiver class:
package pointToPoint;
import javax.naming.InitialContext;
import javax.jms.Queue;
import javax.jms.Session;
import javax.jms.TextMessage;
import javax.jms.QueueSession;
import javax.jms.QueueReceiver;
import javax.jms.QueueConnection;
import javax.jms.QueueConnectionFactory;
public class Receiver
{
public static void main(String[] args) throws Exception
{
// get the initial context
InitialContext ctx = new InitialContext();
// lookup the queue object
Queue queue = (Queue) ctx.lookup("queue/queue0");
// lookup the queue connection factory
QueueConnectionFactory connFactory = (QueueConnectionFactory) ctx.
lookup("queue/connectionFactory");
// create a queue connection
QueueConnection queueConn = connFactory.createQueueConnection();
// create a queue session
QueueSession queueSession = queueConn.createQueueSession(false,
Session.AUTO_ACKNOWLEDGE);
// create a queue receiver
QueueReceiver queueReceiver = queueSession.createReceiver(queue);
// start the connection
queueConn.start();
// receive a message
TextMessage message = (TextMessage) queueReceiver.receive();
// print the message
System.out.println("received: " + message.getText());
// close the queue connection
queueConn.close();
}
}
If the connection is not started, the
receive method will
block forever (or until some other thread starts the connection). If a
client want to temporarily stop delivery of messages, the connection can
be stopped and then re-started later.
Asynchronous Queue Receiver
The
AsyncReceiver class illustrates the use of message listeners. A message
listener is a regular Java class that implements the
MessageListener interface.
This interface has a single
onMessage method, which is called by JMS when
messages arrive at a destination.
As with the synchronous receiver, the
AsyncReceiver class performs the same
initial steps to create a
QueueReceiver. Then, the
setMessageListener
method is called to register
this as a message listener. As with a
synchronous receiver, messages will not be delivered until the start method
is called on the connection.
Since acknowledge mode is set to automatic, JMS will acknowledge messages
right after calls to the
onMessage method returns. Note that
onMessage
is not allowed to throw any exceptions. You must catch all exceptions and deal
with them somehow in the
onMessage method.
In the synchronous receiver, the
receive method can raise an
exception if the JMS provider fails. Due to its asynchronous nature, this is
not possible with message listeners. Therefore, it is possible to register an
exception listener with the connection, which can pick up such exceptions.
Below is the full source for the
AsyncReceiver class:
package pointToPoint;
import javax.naming.InitialContext;
import javax.jms.Queue;
import javax.jms.Session;
import javax.jms.Message;
import javax.jms.TextMessage;
import javax.jms.MessageListener;
import javax.jms.JMSException;
import javax.jms.ExceptionListener;
import javax.jms.QueueSession;
import javax.jms.QueueReceiver;
import javax.jms.QueueConnection;
import javax.jms.QueueConnectionFactory;
public class AsyncReceiver implements MessageListener, ExceptionListener
{
public static void main(String[] args) throws Exception
{
// get the initial context
InitialContext ctx = new InitialContext();
// lookup the queue object
Queue queue = (Queue) ctx.lookup("queue/queue0");
// lookup the queue connection factory
QueueConnectionFactory connFactory = (QueueConnectionFactory) ctx.
lookup("queue/connectionFactory");
// create a queue connection
QueueConnection queueConn = connFactory.createQueueConnection();
// create a queue session
QueueSession queueSession = queueConn.createQueueSession(false,
Session.AUTO_ACKNOWLEDGE);
// create a queue receiver
QueueReceiver queueReceiver = queueSession.createReceiver(queue);
// set an asynchronous message listener
AsyncReceiver asyncReceiver = new AsyncReceiver();
queueReceiver.setMessageListener(asyncReceiver);
// set an asynchronous exception listener on the connection
queueConn.setExceptionListener(asyncReceiver);
// start the connection
queueConn.start();
// wait for messages
System.out.print("waiting for messages");
for (int i = 0; i < 10; i++) {
Thread.sleep(1000);
System.out.print(".");
}
System.out.println();
// close the queue connection
queueConn.close();
}
/**
This method is called asynchronously by JMS when a message arrives
at the queue. Client applications must not throw any exceptions in
the onMessage method.
@param message A JMS message.
*/
public void onMessage(Message message)
{
TextMessage msg = (TextMessage) message;
try {
System.out.println("received: " + msg.getText());
} catch (JMSException ex) {
ex.printStackTrace();
}
}
/**
This method is called asynchronously by JMS when some error occurs.
When using an asynchronous message listener it is recommended to use
an exception listener also since JMS have no way to report errors
otherwise.
@param exception A JMS exception.
*/
public void onException(JMSException exception)
{
System.err.println("an error occurred: " + exception);
}
}
As documented in the source code, it is recommended to always set a
connection exception listener when using asynchronous message listeners.
This will allow you to detect any runtime problems, including a crash of
the JMS server.
The JMSException
API supports the
getLinkedException method,
which can be used to get the root cause of the exception (if any). As an
example, if you raise a
RuntimeException in the
onMessage
method, the linked exception will be this runtime exception when
onException is called.
Queue Browser
A queue browser can be used to look at a queue without consuming any messages.
The queue browser must perform the same initial steps as any other JMS client
application, i.e. get a session object, which is a factory for QueueBrowser
objects.
The QueueBrowser
supports an iterator, which can be used to enumerate the
messages on a queue. The following example shows how to count the number
of messages on a queue. Note that acknowledge mode is not meaningful to a
queue browser:
package pointToPoint;
import java.util.Enumeration;
import javax.naming.InitialContext;
import javax.jms.Queue;
import javax.jms.Session;
import javax.jms.Message;
import javax.jms.QueueSession;
import javax.jms.QueueBrowser;
import javax.jms.QueueConnection;
import javax.jms.QueueConnectionFactory;
public class Browser
{
public static void main(String[] args) throws Exception
{
// get the initial context
InitialContext ctx = new InitialContext();
// lookup the queue object
Queue queue = (Queue) ctx.lookup("queue/queue0");
// lookup the queue connection factory
QueueConnectionFactory connFactory = (QueueConnectionFactory) ctx.
lookup("queue/connectionFactory");
// create a queue connection
QueueConnection queueConn = connFactory.createQueueConnection();
// create a queue session
QueueSession queueSession = queueConn.createQueueSession(false,
Session.AUTO_ACKNOWLEDGE);
// create a queue browser
QueueBrowser queueBrowser = queueSession.createBrowser(queue);
// start the connection
queueConn.start();
// browse the messages
Enumeration e = queueBrowser.getEnumeration();
int numMsgs = 0;
// count number of messages
while (e.hasMoreElements()) {
Message message = (Message) e.nextElement();
numMsgs++;
}
System.out.println(queue + " has " + numMsgs + " messages");
// close the queue connection
queueConn.close();
}
}
The order of messages returned by the enumeration reflects the order of
messages a regular message receiver would see. Note that a queue browser
represents a static snapshop of the queue. If more messages are added to
the queue while browsing, this will not be available to the queue browser.
Thanks to
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