45.2 Basic JMS API Concepts

45.2.1 JMS API Architecture

A JMS application is composed of the following parts.

• A JMS provider is a messaging system that implements the JMS interfaces and provides administrative and control features. An implementation of the Java EE platform that supports the full profile includes a JMS provider.

• JMS clients are the programs or components, written in the Java programming language, that produce and consume messages. Any Java EE application component can act as a JMS client.

  Java SE applications can also act as JMS clients; the Message Queue Developer's Guide for Java Clients in the GlassFish Server documentation (https://glassfish.java.net/docs/) explains how to make this work.

• Messages are the objects that communicate information between JMS clients.

• Administered objects are JMS objects configured for the use of clients. The two kinds of JMS administered objects are destinations and connection factories, described in JMS Administered Objects. An administrator can create objects that are available to all applications that use a particular installation of GlassFish Server; alternatively, a developer can use annotations to create objects that are specific to a particular application.

Figure 45-2 illustrates the way these parts interact. Administrative tools or annotations allow you to bind destinations and connection factories into a JNDI namespace. A JMS client can then use resource injection to access the administered objects in the namespace and then establish a logical connection to the same objects through the JMS provider.

Figure 45-2 JMS API Architecture

Description of "Figure 45-2 JMS API Architecture"

45.2.2 Messaging Styles

Before the JMS API existed, most messaging products supported either the point-to-point or the publish/subscribe style of messaging. The JMS specification defines compliance for each style. A JMS provider must implement both styles, and the JMS API provides interfaces that are specific to each. The following subsections describe these messaging styles.

The JMS API, however, makes it unnecessary to use only one of the two styles. It allows you to use the same code to send and receive messages using either the PTP or the pub/sub style. The destinations you use remain specific to one style, and the behavior of the application will depend in part on whether you are using a queue or a topic. However, the code itself can be common to both styles, making your applications flexible and reusable. This tutorial describes and illustrates this coding approach, using the greatly simplified API provided by JMS 2.0.

45.2.2.1 Point-to-Point Messaging Style

A point-to-point (PTP) product or application is built on the concept of message queues, senders, and receivers. Each message is addressed to a specific queue, and receiving clients extract messages from the queues established to hold their messages. Queues retain all messages sent to them until the messages are consumed or expire.

PTP messaging, illustrated in Figure 45-3, has the following characteristics.

• Each message has only one consumer.

• The receiver can fetch the message whether or not it was running when the client sent the message.

Figure 45-3 Point-to-Point Messaging

Description of "Figure 45-3 Point-to-Point Messaging"

Use PTP messaging when every message you send must be processed successfully by one consumer.

45.2.2.2 Publish/Subscribe Messaging Style

In a publish/subscribe (pub/sub) product or application, clients address messages to a topic, which functions somewhat like a bulletin board. Publishers and subscribers can dynamically publish or subscribe to the topic. The system takes care of distributing the messages arriving from a topic's multiple publishers to its multiple subscribers. Topics retain messages only as long as it takes to distribute them to subscribers.

With pub/sub messaging, it is important to distinguish between the consumer that subscribes to a topic (the subscriber) and the subscription that is created. The consumer is a JMS object within an application, while the subscription is an entity within the JMS provider. Normally, a topic can have many consumers, but a subscription has only one subscriber. It is possible, however, to create shared subscriptions; see Creating Shared Subscriptions for details. See Consuming Messages from Topics for details on the semantics of pub/sub messaging.

Pub/sub messaging has the following characteristics.

• Each message can have multiple consumers.

• A client that subscribes to a topic can consume only messages sent after the client has created a subscription, and the consumer must continue to be active in order for it to consume messages.

  The JMS API relaxes this requirement to some extent by allowing applications to create durable subscriptions, which receive messages sent while the consumers are not active. Durable subscriptions provide the flexibility and reliability of queues but still allow clients to send messages to many recipients. For more information about durable subscriptions, see Creating Durable Subscriptions.

Use pub/sub messaging when each message can be processed by any number of consumers (or none). Figure 45-4 illustrates pub/sub messaging.

Figure 45-4 Publish/Subscribe Messaging

Description of "Figure 45-4 Publish/Subscribe Messaging"

45.2.3 Message Consumption

Messaging products are inherently asynchronous: There is no fundamental timing dependency between the production and the consumption of a message. However, the JMS specification uses this term in a more precise sense. Messages can be consumed in either of two ways.

• Synchronously: A consumer explicitly fetches the message from the destination by calling the receive method. The receive method can block until a message arrives or can time out if a message does not arrive within a specified time limit.

• Asynchronously: An application client or a Java SE client can register a message listener with a consumer. A message listener is similar to an event listener. Whenever a message arrives at the destination, the JMS provider delivers the message by calling the listener's onMessage method, which acts on the contents of the message. In a Java EE application, a message-driven bean serves as a message listener (it too has an onMessage method), but a client does not need to register it with a consumer.