Saturday, March 8, 2008

Java Message Service

The Java Message Service (JMS) API is a Java Message Oriented Middleware (MOM) API for sending messages between two or more clients. JMS is a part of the Java Platform, Enterprise Edition, and is defined by a specification developed under the Java Community Process as JSR 914.

General idea of messaging

Messaging is a form of loosely coupled distributed communication, where in this context the term 'communication' can be understood as an exchange of messages between software components. Message-oriented technologies attempt to relax tightly coupled communication (such as TCP network sockets, CORBA or RMI) by the introduction of an intermediary component, which in this case would be a queue. The latter approach allow software components to communicate 'indirectly' with each other. Benefits of this include message senders not needing to have precise knowledge of their receivers, since communication is performed using the queue.


The following are JMS elements: [1]

JMS provider
An implementation of the JMS interface for a Message Oriented Middleware (MOM). Providers are implemented as either a Java JMS implementation or an adapter to a non-Java MOM.
JMS client
An application or process that produces and/or receives messages.
JMS producer
A JMS client that creates and sends messages.
JMS consumer
A JMS client that receives messages.
JMS message
An object that contains the data being transferred between JMS clients.
JMS queue
A staging area that contains messages that have been sent and are waiting to be read. As the name queue suggests, the messages are delivered in the order sent. A message is removed from the queue once it has been read.
JMS topic
A distribution mechanism for publishing messages that are delivered to multiple subscribers.


The JMS API supports two models:

point-to-point or queuing model
publish and subscribe model
In the point-to-point or queuing model, a producer posts messages to a particular queue and a consumer reads messages from the queue. Here, the producer knows the destination of the message and posts the message directly to the consumer's queue. It is characterized by following:

Only one consumer will get the message
The producer does not have to be running at the time the consumer consumes the message, nor does the consumer need to be running at the time the message is sent
Every message successfully processed is acknowledged by the consumer
The publish/subscribe model supports publishing messages to a particular message topic. Subscribers may register interest in receiving messages on a particular message topic. In this model, neither the publisher nor the subscriber know about each other. A good metaphor for it is anonymous bulletin board. The following are characteristics of this model:

Multiple consumers can get the message

There is a timing dependency between publishers and subscribers. The publisher has to create a subscription in order for clients to be able to subscribe. The subscriber has to remain continuously active to receive messages, unless it has established a durable subscription. In that case, messages published while the subscriber is not connected will be redistributed whenever it reconnects.
Using Java, JMS provides a way of separating the application from the transport layer of providing data. The same Java classes can be used to communicate with different JMS providers by using the JNDI information for the desired provider. The classes first use a connection factory to connect to the queue or topic, and then use populate and send or publish the messages. On the receiving side, the clients then receive or subscribe to the messages.

Application programming interface

The JMS API is provided in the Java package javax.jms.

ConnectionFactory interface
An administered object that a client uses to create a connection to the JMS provider. JMS clients access the connection factory through portable interfaces so the code does not need to be changed if the underlying implementation changes. Administrators configure the connection factory in the Java Naming and Directory Interface (JNDI) namespace so that JMS clients can look them up. Depending on the type of message, users will use either a queue connection factory or topic connection factory.

Connection interface
Once a connection factory is obtained, a connection to a JMS provider can be created. A connection represents a communication link between the application and the messaging server. Depending on the connection type, connections allow users to create sessions for sending and receiving messages from a queue or topic.

Destination interface
An administered object that encapsulates the identity of a message destination, which is where messages are delivered and consumed. It is either a queue or a topic. The JMS administrator creates these objects, and users discover them using JNDI. Like the connection factory, the administrator can create two types of destinations: queues for Point-to-Point and topics for Publish/Subscribe.

MessageConsumer interface
An object created by a session. It receives messages sent to a destination. The consumer can receive messages synchronously (blocking) or asynchronously (non-blocking) for both queue and topic-type messaging.

MessageProducer interface
An object created by a session that sends messages to a destination. The user can create a sender to a specific destination or create a generic sender that specifies the destination at the time the message is sent.

Message interface
An object that is sent between consumers and producers; that is, from one application to another. A message has three main parts:

A message header (required): Contains operational settings to identify and route messages
A set of message properties (optional): Contains additional properties to support compatibility with other providers or users. It can be used to create custom fields or filters (selectors).
A message body (optional): Allows users to create five types of messages (text message, map message, bytes message, stream message, and object message).
The message interface is extremely flexible and provides numerous ways to customize the contents of a message.

Session interface
Represents a single-threaded context for sending and receiving messages. A session is single-threaded so that messages are serialized, meaning that messages are received one-by-one in the order sent. The benefit of a session is that it supports transactions. If the user selects transaction support, the session context holds a group of messages until the transaction is committed, then delivers the messages. Before committing the transaction, the user can cancel the messages using a rollback operation. A session allows users to create message producers to send messages, and message consumers to receive messages. [2]