A message listener is an object that acts as an asynchronous event handler for messages. This object implements the MessageListener interface, which contains one method, onMessage. In the onMessage method, you define the actions to be taken when a message arrives.
You register the message listener with a specific MessageConsumer by using the setMessageListener method. For example, if you define a class named Listener that implements the MessageListener interface, you can register the message listener as follows:
Listener myListener = new Listener(); consumer.setMessageListener(myListener);
After you register the message listener, you call the start method on the Connection to begin message delivery. (If you call start before you register the message listener, you are likely to miss messages.)
When message delivery begins, the JMS provider automatically calls the message listener’s onMessage method whenever a message is delivered. The onMessage method takes one argument of type Message, which your implementation of the method can cast to any of the other message types (see Message Bodies).
A message listener is not specific to a particular destination type. The same listener can obtain messages from either a queue or a topic, depending on the type of destination for which the message consumer was created. A message listener does, however, usually expect a specific message type and format.
Your onMessage method should handle all exceptions. It must not throw checked exceptions, and throwing a RuntimeException is considered a programming error.
The session used to create the message consumer serializes the execution of all message listeners registered with the session. At any time, only one of the session’s message listeners is running.
In the Java EE platform, a message-driven bean is a special kind of message listener. For details, see Using Message-Driven Beans to Receive Messages Asynchronously.