ActiveMQ of java message service

JMS, the Java Message Service (Java Message Service) application program interface, is an API for message-oriented middleware (MOM) in the Java platform, which is used to send messages between two applications or in a distributed system for asynchronous communication. Java Message Service is a platform-independent API, and most MOM providers support JMS.

 

ActiveMQ is the most popular and powerful open source message bus produced by Apache. ActiveMQ is a JMS Provider implementation that fully supports the JMS1.1 and J2EE 1.4 specifications. Although the JMS specification has been published for a long time, JMS still plays a special role in today's J2EE applications.

 

 

 

ActiveMQ development package download and running environment construction

 

Homepage: http://activemq.apache.org/

The latest version: 5.11.1 Development kit and source code download address: http://activemq.apache.org/activemq-5111-release.html

ActiveMQ service startup address: http://127.0.0.1:8161/admin/ username/password admin/admin

 

ActiveMQ method:

    1. Direct Receive method

          Session.AUTO_ACKNOWLEDGE. When the client successfully returns from the receive method, or from the MessageListener.onMessage method, the session automatically acknowledges the message received by the client.

Session.CLIENT_ACKNOWLEDGE. The client acknowledges the message through the message's acknowledge method. Note that, in this mode, acknowledgment is done at the session layer: acknowledging a consumed message will automatically acknowledgment all messages that have been consumed by the session. For example, if a message consumer consumes 10 messages and then acknowledges the 5th message, all 10 messages are acknowledged.

Session.DUPS_ACKNOWLEDGE. The selection is only the submission of the session sluggish acknowledgment message. If the JMS provider fails, some duplicate messages may result. If it is a duplicate message, then the JMS provider must set the JMSRedelivered field of the message header to true.

 

    2. Use Listener to listen

 

 

1. basic concept

   JMS is a Java message service, and asynchronous message transmission can be performed between JMS clients through the JMS service.

2. message model

   ○ Point-to-Point(P2P)
   ○ Publish/Subscribe(Pub/Sub)
   

   i.e. peer-to-peer and publish-subscribe models

3. P2P

   1. P2P Mode Diagram 
      
   2. concepts involved 
      
      1. Message queue (Queue)
      2. Sender
      3. Receiver
      4. Each message is sent to a specific queue, and the receiver gets the message from the queue. Queues hold messages until they are consumed or time out.

   3. Features of P2P

      1. There is only one consumer per message (that is, once consumed, the message is no longer in the message queue)
      2. There is no time dependency between the sender and the receiver, that is, when the sender sends a message, whether the receiver is running or not, it will not affect the message being sent to the queue
      3. The receiver needs to reply to the queue successfully after receiving the message successfully

      You need P2P mode if you want every message you send should be processed successfully.

4. Pub/Sub

   1. Pub/Sub Mode Diagram 
      
   2. concepts involved 
      
      1. Topic
      2. Publisher
      3. Subscriber 
         clients send messages to topics. Multiple publishers send messages to topics, and the system delivers these messages to multiple subscribers.

   3. Features of Pub/Sub

      1. Each message can have multiple consumers
      2. There is a time dependency between publishers and subscribers. For a subscriber of a topic (Topic), it must create a subscriber before it can consume the message of the publisher, and in order to consume the message, the subscriber must keep the running state.
      3. To ease such strict time dependencies, JMS allows subscribers to create a durable subscription. This way, even if the subscriber is not activated (running), it can receive messages from the publisher.

      If you want to send a message that can be processed without any processing, or processed by one messager, or can be processed by multiple consumers, then you can use the Pub/Sub model

5. 消息的消费 
   在JMS中，消息的产生和消息是异步的。对于消费来说，JMS的消息者可以通过两种方式来消费消息。 
   ○ 同步 
   订阅者或接收者调用receive方法来接收消息，receive方法在能够接收到消息之前（或超时之前）将一直阻塞 
   ○ 异步 
   订阅者或接收者可以注册为一个消息监听器。当消息到达之后，系统自动调用监听器的onMessage方法。

6. JMS编程模型

   (1) ConnectionFactory

   创建Connection对象的工厂，针对两种不同的jms消息模型，分别有QueueConnectionFactory和TopicConnectionFactory两种。可以通过JNDI来查找ConnectionFactory对象。

   (2) Destination

   Destination的意思是消息生产者的消息发送目标或者说消息消费者的消息来源。对于消息生产者来说，它的Destination是某个队列（Queue）或某个主题（Topic）;对于消息消费者来说，它的Destination也是某个队列或主题（即消息来源）。

   所以，Destination实际上就是两种类型的对象：Queue、Topic可以通过JNDI来查找Destination。

   (3) Connection

   Connection表示在客户端和JMS系统之间建立的链接（对TCP/IP socket的包装）。Connection可以产生一个或多个Session。跟ConnectionFactory一样，Connection也有两种类型：QueueConnection和TopicConnection。

   (4) Session

   Session是我们操作消息的接口。可以通过session创建生产者、消费者、消息等。Session提供了事务的功能。当我们需要使用session发送/接收多个消息时，可以将这些发送/接收动作放到一个事务中。同样，也分QueueSession和TopicSession。

   (5) 消息的生产者

   消息生产者由Session创建，并用于将消息发送到Destination。同样，消息生产者分两种类型：QueueSender和TopicPublisher。可以调用消息生产者的方法（send或publish方法）发送消息。

   (6) 消息消费者

   消息消费者由Session创建，用于接收被发送到Destination的消息。两种类型：QueueReceiver和TopicSubscriber。可分别通过session的createReceiver(Queue)或createSubscriber(Topic)来创建。当然，也可以session的creatDurableSubscriber方法来创建持久化的订阅者。

   (7) MessageListener

   消息监听器。如果注册了消息监听器，一旦消息到达，将自动调用监听器的onMessage方法。EJB中的MDB（Message-Driven Bean）就是一种MessageListener。

7. 企业消息系统的好处

我们先来看看下图，应用程序A将Message发送到服务器上，然后应用程序B从服务器中接收A发来的消息，通过这个图我们一起来分析一下JMS的好处： 

1. 提供消息灵活性
2. 松散耦合
3. 异步性