Use this task to develop a JMS client application to use messages
to communicate with enterprise applications.
About this task
This topic gives an overview of the steps needed to develop a
JMS client application. This topic only describes the JMS-related considerations;
it does not describe general client programming, which you should already
be familiar with. For detailed information about these steps, and for examples
of developing JMS clients, see the Java Message Service Documentation and
the WebSphere MQ Using Java book, SC34-5456.
A JMS client
assumes that the JMS resources (such as a queue connection factory and queue
destination) already exist. A client application can use JMS resources administered
by the application server or administered by the client container regardless
of whether the client application is running on the same machine as the server
or remotely.
For more information about developing client applications
and configuring JMS resources for them, see Developing J2EE application client code and
related tasks.
To use JMS, a typical JMS client program completes the
following general steps:
- Import JMS packages. An enterprise application that
uses JMS starts with a number of import statements for JMS; for example:
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.rmi.PortableRemoteObject;
import javax.jms.*;
- Get an initial context.
try {
ctx = new InitialContext(env);
...
- Define the parameters that the client wants to use; for example,
to identify the queue connection factory and to assemble a message to be sent.
public class JMSppSampleClient
{
public static void main(String[] args)
throws JMSException, Exception
{
String messageID = null;
String outString = null;
String qcfName = "java:comp/env/jms/ConnectionFactory";
String qnameIn = "java:comp/env/jms/Q1";
String qnameOut = "java:comp/env/jms/Q2";
boolean verbose = false;
QueueSession session = null;
QueueConnection connection = null;
Context ctx = null;
QueueConnectionFactory qcf = null;
Queue inQueue = null;
Queue outQueue = null;
...
- Retrieve administered objects from the JNDI namespace. The
InitialContext.lookup() method is used to retrieve administered objects (a
queue connection factory and the queue destinations):
qcf = (QueueConnectionFactory)ctx.lookup( qcfName );
...
inQueue = (Queue)ctx.lookup( qnameIn );
outQueue = (Queue)ctx.lookup( qnameOut );
...
- Create a connection to the messaging service provider. The
connection provides access to the underlying transport, and is used to create
sessions. The createQueueConnection() method on the factory object is used
to create the connection.
connection = qcf.createQueueConnection();
The JMS specification defines that connections should be created
in the stopped state. Until the connection starts, MessageConsumers that are
associated with the connection cannot receive any messages. To start the connection,
issue the following command:
connection.start();
- Create a session, for sending and receiving messages. The
session provides a context for producing and consuming messages, including
the methods used to create MessageProducers and MessageConsumers. The createQueueSession
method is used on the connection to obtain a session. The method takes two
parameters:
- A boolean that determines whether or not the session is transacted.
- A parameter that determines the acknowledge mode.
boolean transacted = false;
session = connection.createQueueSession( transacted,
Session.AUTO_ACKNOWLEDGE);
In this example, the session is not transacted, and it should automatically
acknowledge received messages. With these settings, a message is backed out
only after a system error or if the client application terminates unexpectedly.
- Send the message.
- Create MessageProducers to create messages. For point-to-point
the MessageProducer is a QueueSender that is created by passing an output
queue object (retrieved earlier) into the createSender method on the session.
A QueueSender is normally created for a specific queue, so that all messages
sent using that sender are sent to the same destination.
QueueSender queueSender = session.createSender(inQueue);
- Create the message. Use the session to create an
empty message and add the data passed.
JMS provides several message types,
each of which embodies some knowledge of its content. To avoid referencing
the vendor-specific class names for the message types, methods are provided
on the Session object for message creation.
In this example, a text
message is created from the outString property, which could be provided as
an input parameter on invocation of the client program or constructed in some
other way:
TextMessage outMessage = session.createTextMessage(outString);
- Send the message.
To send the message, the message
is passed to the send method on the QueueSender:
queueSender.send(outMessage);
- Receive replies.
- Create a correlation ID to link the message sent with any replies.
In this example, the client receives reply messages that are related
to the message that it has sent, by using a provider-specific message ID in
a JMSCorrelationID.
messageID = outMessage.getJMSMessageID();
The
correlation ID is then used in a message selector, to select only messages
that have that ID:
String selector = "JMSCorrelationID = '"+messageID+"'";
- Create a MessageReceiver to receive messages. For
point-to-point the MessageReceiver is a QueueReceiver that is created by passing
an input queue object (retrieved earlier) and the message selector into the
createReceiver method on the session.
QueueReceiver queueReceiver = session.createReceiver(outQueue, selector);
- Retrieve the reply message. To retrieve a reply message,
the receive method on the QueueReceiver is used:
Message inMessage = queueReceiver.receive(2000);
The
parameter in the receive call is a timeout in milliseconds. This parameter
defines how long the method should wait if there is no message available immediately.
If you omit this parameter, the call blocks indefinitely. If you do not want
any delay, use the receiveNoWait()method. In this example, the receive call
returns when the message arrives, or after 2000ms, whichever is sooner.
- Act on the message received. When a message is received,
you can act on it as needed by the business logic of the client. Some general
JMS actions are to check that the message is of the correct type and extract
the content of the message. To extract the content from the body of the message,
you need to cast from the generic Message class (which is the declared return
type of the receive methods) to the more specific subclass, such as TextMessage.
It is good practice always to test the message class before casting, so that
unexpected errors can be handled gracefully.
In this example, the instanceof
operator is used to check that the message received is of the TextMessage
type. The message content is then extracted by casting to the TextMessage
subclass.
if ( inMessage instanceof TextMessage )
...
String replyString = ((TextMessage) inMessage).getText();
- Closing down. If the application needs to create many
short-lived JMS objects at the Session level or lower, it is important to
close all the JMS resources used. To do this, you call the close() method
on the various classes (QueueConnection, QueueSession, QueueSender, and QueueReceiver)
when the resources are no longer required.
queueReceiver.close();
...
queueSender.close();
...
session.close();
session = null;
...
connection.close();
connection = null;
- Publishing and subscribing messages. To use publish/subscribe
support instead of point-to-point messaging, the general client actions are
the same; for example, to create a session and connection. The exceptions
are that topic resources are used instead of queue resources (such as TopicPublisher
instead of QueueSender), as shown in the following example to publish a message:
// Creating a TopicPublisher
TopicPublisher pub = session.createPublisher(topic);
...
pub.publish(outMessage);
...
// Closing TopicPublisher
pub.close();
- Handling errors Any JMS runtime errors are reported
by exceptions. The majority of methods in JMS throw JMSExceptions to indicate
errors. It is good programming practice to catch these exceptions and display
them on a suitable output.
Unlike normal Java exceptions, a JMSException
can contain another exception embedded in it. The implementation of JMSException
does not include the embedded exception in the output of its toString()method.
Therefore, you need to check explicitly for an embedded exception and print
it out, as shown in the following example:
catch (JMSException je)
{
System.out.println("JMS failed with "+je);
Exception le = je.getLinkedException();
if (le != null)
{
System.out.println("linked exception "+le);
}
}