Enabling SOA Using WebSphere Messaging

Chapter 4. Application design 85

the listener port automatically stops listening. Unfortunately, this stops all

processing. In fact, there are two more properties needed, and provided by JMS:

򐂰 The

redelivery count message property

This property defines how many times a message has already been read

from the queue.

򐂰 The

backout threshold destination property

When the same message has been read a number of times equal to the

backout threshold, the infrastructure moves the message to a default queue

for messages that could not be delivered. The queue is called the dead letter

queue.

A rollback behavior that supports multiple (but not unlimited) processing retries

without stopping the listener can be achieved by setting the maximum retries

higher than the backout threshold. This allows the consumer to try to process a

message for the number of times specified by the backout threshold without

reaching the maximum retries number. Each time the redelivery fails the

redelivery count is incremented. When the redelivery count equals the backout

threshold, the message is moved to the dead letter queue. Maximum retries is

never reached and the listener port is not stopped.

4.5.3 Message producers

Message producers are triggered by programs to deliver messages. Producers

wrap messages they get from the application as payload in messaging-specific

messages; make sure the header attributes like expiry, reply address, message

ID, and maybe correlation ID are set correctly; and put the messages onto a

queue.

Example 4-5 shows a code sample of a simple message producer that does not

set any header attributes on the message.

Example 4-5 Simple message producer

public class JmsProducer {

public void sendMessage(String msg, String conFactoryName, String destName)

throws NamingException, JMSException {

Session session = null;

Connection connection = null;

MessageProducer producer = null;

try {

//Get the specified connection factory and queue

Context jndiConext = new InitialContext();

ConnectionFactory factory = (ConnectionFactory)

86 Enabling SOA Using WebSphere Messaging

jndiContext.lookup(conFactoryName);

Destination destination = (Destination)jndiContext.lookup(destName);

//Create the connection and session

Connection conenction = factory.createConnection();

Session session = connection.createSession(false,

Session.AUTO_ACKNOWLEDGE);

//Use the session and destination to create the producer

producer = session.createProducer(destination);

//Create and send the message

TextMessage message = session.createTextMessage(msg);

message.setJMSMessageID(messageId);

producer.send(message);

}

finally {

if (producer != null) {

producer.close();

}

if (session != null) {

session.close();

}

if (connection != null) {

connection.close();

}

}

}

}

Example 4-6 shows an extension of the previous example by setting some

header attributes on the message. This producer implementation could be used

as a sender on the service consumer side to support a pseudo-synchronous

request-reply scenario. Note the reply queue and the message ID are set.

Example 4-6 Message producer setting message header attributes

public class JmsProducer {

public void sendMessage(String msg, String conFactoryName, String destName,

String replyDestName, long messageTimeout, String messageId)

throws NamingException, JMSException {

...

//Set header properties

Destination replyDestination = (Destination)

jndiContext.lookup(replyDestName);

message.setJMSReplyTo(replyDestination)