JMS (Java Message Service)

What is messaging?

Messaging is a method of communicating between software components and applications. Each client can register to a messaging service which provides facilities for creating, sending, receiving and reading messages.
Messaging enables "loose coupling" in distributed computing.

What is JMS?

The Java Message Service is an API. It is not an implementation.
It minimizes the concepts programmer must know to write a rich messaging application.
JMS enables communication

Asynchronous: Deliver messages as the client becomes alive;client does not have to request to receive messages
Reliable: Ensure that a message is delivered once and only once.

JMS API Architecture and Concepts

JMS provider: a messaging system that "implements" JMS interface and provides administrative and control features.
JMS client: a program or a component that produces or consumes messages
Administrated objects: Pre-configured JMS objects created by an "administrator" to be used by a JMS client
Naive clients: all programs that use native client API for messaging instead of JMS.

Has message senders,receivers and "queues".
Messages producers send a message to a specific queue.
receiving clients exact messages from queues.
Each message has only one consumer.
When the message is sent receiver may or may not present. But when it come alive messages are delivered in order.

2. Publish/subscribe messaging domain

Message produces sent them to a "topic".
Publishers and subscribers can dynamically come and go. But messaging system is capable of distributing messages from multiple publishers to multiple subscribers.
Publishers and subscribers have a time dependency because subscribers can only consume messages that are published after the subscription has done.

Synchronously: Explicitly fetch messages by calling "receive" method.
Asynchronously: Whenever a message is addressed, JMS provider delivers it to the "message listener" which is subscribed with that particular consumer using "onMessage" method.

Basic Programming hierarchy

Create a Contex.
Create a QueueConnectionFactory or a TopicConnectionFactory using a contex lookup (Java Naming and Directory Interface /"JNID" lookup).
Create a topic or a queue. Can use multiple topics or queues.
Create a "connection" (a virtual connection with JMS provider) using connection factory created.
Create a "session" with the connection created.
Create a queuesender or a topicproducer using the session created. You have to specify the topic or queue it should send messages to.
Send or publish the message by queueSender.send(message) or topicPublisher.publish(message);
Create queueReceiver or topicSubscriber using the session.
Receive messages by

queueConnection.start();

Message m = queueReceiver.receive();

topicConnection.start();

Message m = topicSubscriber.receive(1000); //timeout time

Additionally JMS API enables you to,

Subscribe message listeners with subscribers/receivers
Filter messages using message selectors
Write custom messages agreed with the API
Handle exceptions ( The root class for exceptions thrown by JMS API methods is JMSException. Catching JMSException provides a generic way of handling all exceptions related to the JMS API.)

With the next post I will introduce you with "Hello World example"

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