The Message-Driven Bean Class - The J2EE 1.4 Tutorial

The Message-Driven Bean Class

The code for the SimpleMessageBean class illustrates the requirements of a message-driven bean class:

It must implement the MessageDrivenBean and MessageListener interfaces.
The class must be defined as public.
The class cannot be defined as abstract or final.
It must implement one onMessage method.
It must implement one ejbCreate method and one ejbRemove method.
It must contain a public constructor with no arguments.
It must not define the finalize method.

Unlike session and entity beans, message-driven beans do not have the remote or local interfaces that define client access. Client components do not locate message-driven beans and invoke methods on them. Although message-driven beans do not have business methods, they may contain helper methods that are invoked internally by the onMessage method.

The onMessage Method

When the queue receives a message, the EJB container invokes the onMessage method of the message-driven bean.

The onMessage method is called by the bean's container when a message has arrived for the bean to service. This method contains the business logic that handles the processing of the message. It is the message-driven bean's responsibility to parse the message and perform the necessary business logic.

The onMessage method has a single argument: the incoming message.

The message-driven bean class defines one onMessage method, whose signature must follow these rules:

The method must be declared as public and must not be declared as final or static.
The return type must be void.
The method must have a single argument of type javax.jms.Message.
The throws clause must not define any application exceptions.
The onMessage method must be invoked in the scope of a transaction that is determined by the transaction attribute specified in the deployment descriptor.

In the SimpleMessageBean class, the onMessage method casts the incoming message to a TextMessage and displays the text:

public void onMessage(Message inMessage) {
  TextMessage msg = null;

  try {
    if (inMessage instanceof TextMessage) {
      msg = (TextMessage) inMessage;
      logger.info("MESSAGE BEAN: Message received: " +
        msg.getText());
    } else {
      logger.warning("Message of wrong type: " +
        inMessage.getClass().getName());
    }
  } catch (JMSException e) {
    e.printStackTrace();
    mdc.setRollbackOnly();
  } catch (Throwable te) {
    te.printStackTrace();
  }
} 

The ejbCreate and ejbRemove Methods

The signatures of these methods have the following requirements:

The access control modifier must be public.
The return type must be void.
The modifier cannot be static or final.
The throws clause must not define any application exceptions.
The method has no arguments.

In the SimpleMessageBean class, the ejbCreate and ejbRemove methods are empty.

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