企业中各项目中相互协作的时候可能用得到消息通知机制。比如有东西更新了，可以通知做索引。
在 Java 里有 JMS 的多个实现。其中 apache 下的 ActiveMQ 就是不错的选择。ActiveMQ 是Apache出品，最流行的，能力强劲的开源消息总线。ActiveMQ 是一个完全支持JMS1.1和J2EE 1.4规范的 JMS Provider实现。这里示例下使用 ActiveMQ
用 ActiveMQ 最好还是了解下 JMS
| JMS 公共 | 点对点域 | 发布/订阅域 |
| ConnectionFactory | QueueConnectionFactory | TopicConnectionFactory |
| Connection | QueueConnection | TopicConnection |
| Destination | Queue | Topic |
| Session | QueueSession | TopicSession |
| MessageProducer | QueueSender | TopicPublisher |
| MessageConsumer | QueueReceiver | TopicSubscriber |
JMS 定义了两种方式：Quere（点对点）；Topic（发布/订阅）。
ConnectionFactory 是连接工厂，负责创建Connection。
Connection 负责创建 Session。
Session 创建 MessageProducer（用来发消息） 和 MessageConsumer（用来接收消息）。
Destination 是消息的目的地。
详细的可以网上找些 JMS 规范（有中文版）。
下载 apache-activemq-5.3.0。http://activemq.apache.org/download.html ，解压，然后双击 bin/activemq.bat。运行后，可以在 http://localhost:8161/admin 观察。也有 demo， http://localhost:8161/demo 。把 activemq-all-5.3.0.jar 加入 classpath。
Jms 发送 代码：
public static void main(String[] args) throws Exception {
ConnectionFactory connectionFactory = new ActiveMQConnectionFactory();
Connection connection = connectionFactory.createConnection();
connection.start();
Session session = connection.createSession(Boolean.TRUE, Session.AUTO_ACKNOWLEDGE);
Destination destination = session.createQueue("my-queue");
MessageProducer producer = session.createProducer(destination);
for(int i=0; i<3; i++) {
MapMessage message = session.createMapMessage();
message.setLong("count", new Date().getTime());
Thread.sleep(1000);
//通过消息生产者发出消息
producer.send(message);
}
session.commit();
session.close();
connection.close();
}
Jms 接收代码：
public static void main(String[] args) throws Exception {
ConnectionFactory connectionFactory = new ActiveMQConnectionFactory();
Connection connection = connectionFactory.createConnection();
connection.start();
final Session session = connection.createSession(Boolean.TRUE, Session.AUTO_ACKNOWLEDGE);
Destination destination = session.createQueue("my-queue");
MessageConsumer consumer = session.createConsumer(destination);
int i=0;
while(i<3) {
i++;
MapMessage message = (MapMessage) consumer.receive();
session.commit();
//TODO something....
System.out.println("收到消息：" + new Date(message.getLong("count")));
}
session.close();
connection.close();
}
JMS五种消息的发送/接收的例子
转自：http://chenjumin.javaeye.com/blog/687124
1、消息发送
//连接工厂
ConnectionFactory connFactory = new ActiveMQConnectionFactory(
ActiveMQConnection.DEFAULT_USER,
ActiveMQConnection.DEFAULT_PASSWORD,
"tcp://localhost:61616");
//连接到JMS提供者
Connection conn = connFactory.createConnection();
conn.start();
//事务性会话，自动确认消息
Session session = conn.createSession(true, Session.AUTO_ACKNOWLEDGE);
//消息的目的地
Destination destination = session.createQueue("queue.hello");
//消息生产者
MessageProducer producer = session.createProducer(destination);
producer.setDeliveryMode(DeliveryMode.NON_PERSISTENT); //不持久化
//文本消息
TextMessage textMessage = session.createTextMessage("文本消息");
producer.send(textMessage);
//键值对消息
MapMessage mapMessage = session.createMapMessage();
mapMessage.setLong("age", new Long(32));
mapMessage.setDouble("sarray", new Double(5867.15));
mapMessage.setString("username", "键值对消息");
producer.send(mapMessage);
//流消息
StreamMessage streamMessage = session.createStreamMessage();
streamMessage.writeString("streamMessage流消息");
streamMessage.writeLong(55);
producer.send(streamMessage);
//字节消息
String s = "BytesMessage字节消息";
BytesMessage bytesMessage = session.createBytesMessage();
bytesMessage.writeBytes(s.getBytes());
producer.send(bytesMessage);
//对象消息
User user = new User("cjm", "对象消息"); //User对象必须实现Serializable接口
ObjectMessage objectMessage = session.createObjectMessage();
objectMessage.setObject(user);
producer.send(objectMessage);
session.commit(); //在事务性会话中，只有commit之后，消息才会真正到达目的地
producer.close();
session.close();
conn.close();
2、消息接收：通过消息监听器的方式接收消息
public class Receiver implements MessageListener{
private boolean stop = false;
public void execute() throws Exception {
//连接工厂
ConnectionFactory connFactory = new ActiveMQConnectionFactory(
ActiveMQConnection.DEFAULT_USER,
ActiveMQConnection.DEFAULT_PASSWORD,
"tcp://localhost:61616");
//连接到JMS提供者
Connection conn = connFactory.createConnection();
conn.start();
//事务性会话，自动确认消息
Session session = conn.createSession(true, Session.AUTO_ACKNOWLEDGE);
//消息的来源地
Destination destination = session.createQueue("queue.hello");
//消息消费者
MessageConsumer consumer = session.createConsumer(destination);
consumer.setMessageListener(this);
//等待接收消息
while(!stop){
Thread.sleep(5000);
}
session.commit();
consumer.close();
session.close();
conn.close();
}
public void onMessage(Message m) {
try{
if(m instanceof TextMessage){ //接收文本消息
TextMessage message = (TextMessage)m;
System.out.println(message.getText());
}else if(m instanceof MapMessage){ //接收键值对消息
MapMessage message = (MapMessage)m;
System.out.println(message.getLong("age"));
System.out.println(message.getDouble("sarray"));
System.out.println(message.getString("username"));
}else if(m instanceof StreamMessage){ //接收流消息
StreamMessage message = (StreamMessage)m;
System.out.println(message.readString());
System.out.println(message.readLong());
}else if(m instanceof BytesMessage){ //接收字节消息
byte[] b = new byte[1024];
int len = -1;
BytesMessage message = (BytesMessage)m;
while((len=message.readBytes(b))!=-1){
System.out.println(new String(b, 0, len));
}
}else if(m instanceof ObjectMessage){ //接收对象消息
ObjectMessage message = (ObjectMessage)m;
User user = (User)message.getObject();
System.out.println(user.getUsername() + " _ " + user.getPassword());
}else{
System.out.println(m);
}
stop = true;
}catch(JMSException e){
stop = true;
e.printStackTrace();
}
}
}