You can group a series of operations into an atomic unit of work called a transaction. If any one of the operations fails, the transaction can be rolled back, and the operations can be attempted again from the beginning. If all the operations succeed, the transaction can be committed.
In a JMS client, you can use local transactions to group message sends and receives. The JMS API Session interface provides commit and rollback methods that you can use in a JMS client. A transaction commit means that all produced messages are sent and all consumed messages are acknowledged. A transaction rollback means that all produced messages are destroyed and all consumed messages are recovered and redelivered unless they have expired (see Allowing Messages to Expire).
A transacted session is always involved in a transaction. As soon as the commit or the rollback method is called, one transaction ends and another transaction begins. Closing a transacted session rolls back its transaction in progress, including any pending sends and receives.
In an Enterprise JavaBeans component, you cannot use the Session.commit and Session.rollback methods. Instead, you use distributed transactions, which are described in Using the JMS API in Java EE Applications.
You can combine several sends and receives in a single JMS API local transaction. If you do so, you need to be careful about the order of the operations. You will have no problems if the transaction consists of all sends or all receives or if the receives come before the sends. But if you try to use a request/reply mechanism, whereby you send a message and then try to receive a reply to the sent message in the same transaction, the program will hang, because the send cannot take place until the transaction is committed. The following code fragment illustrates the problem:
// Don’t do this! outMsg.setJMSReplyTo(replyQueue); producer.send(outQueue, outMsg); consumer = session.createConsumer(replyQueue); inMsg = consumer.receive(); session.commit();
Because a message sent during a transaction is not actually sent until the transaction is committed, the transaction cannot contain any receives that depend on that message’s having been sent.
In addition, the production and the consumption of a message cannot both be part of the same transaction. The reason is that the transactions take place between the clients and the JMS provider, which intervenes between the production and the consumption of the message. Figure 30–8 illustrates this interaction.
The sending of one or more messages to one or more destinations by client 1 can form a single transaction, because it forms a single set of interactions with the JMS provider using a single session. Similarly, the receiving of one or more messages from one or more destinations by client 2 also forms a single transaction using a single session. But because the two clients have no direct interaction and are using two different sessions, no transactions can take place between them.
Another way of putting this is that the act of producing and/or consuming messages in a session can be transactional, but the act of producing and consuming a specific message across different sessions cannot be transactional.
This is the fundamental difference between messaging and synchronized processing. Instead of tightly coupling the sending and receiving of data, message producers and consumers use an alternative approach to reliability, one that is built on a JMS provider’s ability to supply a once-and-only-once message delivery guarantee.
When you create a session, you specify whether it is transacted. The first argument to the createSession method is a boolean value. A value of true means that the session is transacted; a value of false means that it is not transacted. The second argument to this method is the acknowledgment mode, which is relevant only to nontransacted sessions (see Controlling Message Acknowledgment). If the session is transacted, the second argument is ignored, so it is a good idea to specify 0 to make the meaning of your code clear. For example:
session = connection.createSession(true, 0);
The commit and the rollback methods for local transactions are associated with the session. You can combine queue and topic operations in a single transaction if you use the same session to perform the operations. For example, you can use the same session to receive a message from a queue and send a message to a topic in the same transaction.
You can pass a client program’s session to a message listener’s constructor function and use it to create a message producer. In this way, you can use the same session for receives and sends in asynchronous message consumers.
A Local Transaction Example provides an example of the use of JMS API local transactions.