JMS and Message-Driven Beans in Java

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JMS and Message-Driven Beans
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All EJB 3.x vendors must support a JMS provider. Most vendors supply a built-in provider, though it s possible to integrate a third-party implementation instead. Regardless, a JMS provider is an absolute necessity for supporting message-driven beans. By forcing the adoption of JMS, the EJB expert group has guaranteed that EJB developers can expect to have a working JMS provider to which messages can be both sent and received.
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JMS as a Resource
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JMS is a vendor-neutral API that can be used to access enterprise messaging systems. Enterprise messaging systems (aka message-oriented middleware) facilitate the exchange of messages between software applications, usually over a network. The role of JMS isn t unlike the role of JDBC; just as JDBC provides a common API for accessing many different relational databases, JMS provides vendor-independent access to enterprise messaging systems. Although messaging products aren t as familiar as database products, there s no shortage of messaging systems that support JMS, including OpenJMS, HornetQ, ActiveMQ, Qpid, and SonicMQ. Software applications that use the JMS API for sending or receiving messages are portable from one JMS vendor to another. Applications that use JMS are called JMS clients, and the messaging system that handles routing and delivery of messages is called the JMS provider. A JMS application is a business system composed of many JMS clients and, generally, one JMS provider. A JMS client that sends a message is called a producer, and a client that receives a message is called a consumer. A single JMS client can be both a producer and a consumer. In EJB, enterprise beans of all types can use JMS to send messages. These are later consumed by other applications or by message-driven beans. JMS facilitates sending messages from enterprise beans using a messaging service, sometimes called a message
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broker or router. Message brokers have been around for a couple of decades the oldest and most established is IBM s MQSeries but JMS is relatively new, and it is specifically designed to deliver a variety of message types from one Java application to another.
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JMS Is Asynchronous
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One of the principal advantages of JMS messaging is that it s asynchronous. In other words, a JMS client can send a message without having to wait for a reply. Contrast this flexibility with the synchronous messaging of Java RMI or JAX-RPC, in which each client invocation blocks the current thread until the method completes execution. This lockstep processing makes the client dependent on the availability of the EJB server, resulting in a tight coupling between the client and the enterprise bean. JMS clients send messages asynchronously to a destination (Topic or Queue) from which other JMS clients can also receive messages. When a JMS client sends a message, it doesn t wait for a reply; it sends the message to a router, which is responsible for forwarding the message to other clients. There s no effect on the client if one or more recipients are unavailable; once delivered, the sender will continue with its work. It s the router s responsibility to make sure that the message eventually reaches its destination. Clients sending messages are decoupled from the clients receiving them; senders are not dependent on the availability of receivers. The limitations of RMI make JMS an attractive alternative for communicating with other applications. Using the standard JNDI naming context, an enterprise bean can obtain a JMS connection to a JMS provider and use it to deliver asynchronous messages to other Java applications. For example, a fictional UserRegistrationEJB may want to notify arbitrary systems of a new user, but it doesn t want to be coupled. By sending a JMS message to a system that is listening for incoming registration events, the UserRegistrationEJB can dispatch the request and continue along its way (Figure 8-2). In this example, the applications receiving JMS messages initiated from the UserRegistrationEJB may be message-driven beans, other Java applications in the enterprise, or applications in other organizations that benefit from being notified that a new registration has been processed. Examples might include sending an email confirmation or an internal marketing application that adds customers to a catalog mailing list, and these may be plugged in or turned off at runtime without affecting the registration process itself. Because messaging is inherently decoupled and asynchronous, the transactions and security contexts of the sender are not propagated to the receiver. For example, when the UserRegistrationEJB sends the message, the JMS provider may authenticate it, but the message s security context won t be propagated to the JMS client that received the message. When a JMS client receives the message, it has no idea about the security
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context under which the message was sent. This is how it should be, because the sender and receiver often operate in environments with different security domains. Similarly, transactions are never propagated from the sender to the receiver. For one thing, the sender has no idea who the receivers of the message will be. There could be one receiver or thousands; managing a distributed transaction under such ambiguous circumstances is not tenable. In addition, the clients receiving the message may not get it for a long time after it is sent. There may be a network problem, the client may be down, or there may be some other delay. Transactions are designed to be executed quickly because they lock up resources, and applications can t tolerate the possibility of a long transaction with an unpredictable end. A JMS client can, however, have a distributed transaction with the JMS provider so that it manages the send or receive operation in the context of a transaction. For example, if the UserRegistrationEJB s transaction fails for any reason, the JMS provider discards the message. Transactions and JMS are covered in more detail in 17.
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