rabbit MQ (1)

http://www.rabbitmq.com/tutorials/tutorial-one-java.html

RabbitMQ is a message broker. The principle is very simple, that is, to realize the acceptance and forwarding of messages. RabbitMQ is like a post office, when we deliver mail to a mailbox, we trust the postman to deliver our letter to the destination. RabbitMQ is like a mailbox, post office and postman. The main difference between RabbitMQ and the post office is that there is no paper letter, RabbitMQ receives, stores, and forwards binary data - messages.

RabbitMQ usually uses the following technical terms.

Producing means to send. The program that sends the message is a producer, as shown in the following figure.

Queues are similar to mailboxes. Queues are the core of RabbitMQ. Although messages are transmitted between applications through RabbitMQ, they can only be stored in queues. The queue is not limited in any way, it can store many messages, the queue is an unbounded buffer. Many producers can send messages to a queue, and many consumers can receive messages from a queue. Queues can be depicted as shown below, with the queue name above the queue.

Consumer means to receive messages. A consumer program is mainly waiting to receive messages, which can be represented by the letter C.

Note that the producer, consumer, and message service (broker) do not need to be on the same machine, and will not exist in most scenarios.

In the following tutorial we will write two programs in java, a simple producer that sends messages and a consumer that receives and prints messages. We'll discuss the details of the Java API d, focusing on simple examples at first. Create a "hello world".

In the figure below, "P" represents our producer, "C" represents our consumer, and the box in the middle represents the queue, the message buffer that RabbitMQ reserves for consumers.

java client code base

RabbitMQ has multiple protocols. Our tutorial uses AMQP0-9-1, which is an open, versatile messaging protocol. There are multiple client implementations in different languages for RabbitMQ. We are using the java client provided by RabbitMQ.

message sending

We call our message sender send and message receiver Recv. The sender connects to RabbitMQ, sends a short message and then exits.

在发送者Send.java中我们需要引入下面一些类：

import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.Channel;

创建类同时明明队列

public class Send {
  private final static String QUEUE_NAME = "hello";

  public static void main(String[] argv)
      throws java.io.IOException {
      ...
  }
}

然后创建连接：

  ConnectionFactory factory = new ConnectionFactory();
    factory.setHost("localhost");
    Connection connection = factory.newConnection();
    Channel channel = connection.createChannel();

连接抽象套接字连接，并为我们处理协议版本协商和认证等。这里我们连接到本地机器上的代理，因此用的是localhost。如果想连接不同机器的消息服务器，只需要指定机器名或者IP即可。

接下来我们创建一个通道，这是大部分的API完成驻留的地方。

为了发送消息，我们必须声明一个队列用来发送消息；之后我们可以把消息放入队列：

  channel.queueDeclare(QUEUE_NAME, false, false, false, null);
    String message = "Hello World!";
    channel.basicPublish("", QUEUE_NAME, null, message.getBytes());
    System.out.println(" [x] Sent '" + message + "'");

声明队列是一个幂等的，即只有队列不存在时候才能够创建队列。消息内容是字节数组，所以可用用你喜欢的进行编码。

关闭通道和连接。

    channel.close();
    connection.close();

上面是整个发送消息代码

Sending doesn't work!

If this is your first time using RabbitMQ and you don't see the "Sent" message then you may be left scratching your head wondering what could be wrong. Maybe the broker was started without enough free disk space (by default it needs at least 1Gb free) and is therefore refusing to accept messages. Check the broker logfile to confirm and reduce the limit if necessary. The configuration file documentation will show you how to set disk_free_limit.

如果发送者不发送消息，可能是消息服务器启动时没有足够的可用空间因此拒绝接收消息。可以查看服务器配置如果必要的话可以修改配置。配置文件告诉你如何修改可用自盘限制

接收

接收消息是为发送者创建的。我们的接收者接收RabbitMQ的消息，所以她和发送者向服务器发送消息不同，我们保持接收者持续运行并且监听消息，同时把消息打印出

接收者代码引用的类基本和发送者相同：

import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Consumer;
import com.rabbitmq.client.DefaultConsumer;

额外的默认消费这是实现了Consumer接口的类，我们将使用它缓存消费服务器推送过来的消息。

实现方式和发送者相同，打开一个连接和一个通道，同时声明我们消费的队列。注意，接收者监听的是发送者推送消息的队列。

public class Recv {
  private final static String QUEUE_NAME = "hello";

  public static void main(String[] argv)
      throws java.io.IOException,
             java.lang.InterruptedException {

    ConnectionFactory factory = new ConnectionFactory();
    factory.setHost("localhost");
    Connection connection = factory.newConnection();
    Channel channel = connection.createChannel();

    channel.queueDeclare(QUEUE_NAME, false, false, false, null);
    System.out.println(" [*] Waiting for messages. To exit press CTRL+C");
    ...
    }
}

注意我们在接受者端也声明的队列。这是因为我们可能在发送者之前启动接收者，我们要确保从队列消费之前队列是已经存在的。

我们告诉服务器把队列的消息发送给我们。因此服务器将异步的向我们推送消息。我们以对象形式提供了一个回调，用来缓存消息，等待我们进行消费。这就是DefaultConsumer子类做的。

 Consumer consumer = new DefaultConsumer(channel) {
      @Override
      public void handleDelivery(String consumerTag, Envelope envelope, AMQP.BasicProperties properties, byte[] body)
          throws IOException {
        String message = new String(body, "UTF-8");
        System.out.println(" [x] Received '" + message + "'");
      }
    };
    channel.basicConsume(QUEUE_NAME, true, consumer);

可以在classpath中增加RabbitMQ java 客户端来编译发送者和接收者代码：

$ javac -cp amqp-client-4.0.2.jar Send.java Recv.java

为了运行上述代码，需要 rabbitmq-client.jar 同时添加到classpath中，运营发送者：

$ java -cp .:amqp-client-4.0.2.jar:slf4j-api-1.7.21.jar:slf4j-simple-1.7.22.jar Send

运行接收者：

$ java -cp .:amqp-client-4.0.2.jar:slf4j-api-1.7.21.jar:slf4j-simple-1.7.22.jar Recv

在windows系统中，classpath中用分号代替冒号

接收者会打印通过RabbitMQ接收的消息，接收者会一直运行等待消息（可以通过Ctrl+C停止），所有可以从其他终端运行发送者。

如果想查看队列，可以使用rabbitmqctl list_queues

提示：

为了少打字，可以设置一个类似下面的环境变量：

 $ export CP=.:amqp-client-4.0.2.jar:slf4j-api-1.7.21.jar:slf4j-simple-1.7.22.jar
 $ java -cp $CP Send

或者在windows上可以这样操作：

> set CP=.;amqp-client-4.0.2.jar;slf4j-api-1.7.21.jar;slf4j-simple-1.7.22.jar
 > java -cp %CP% Send

Sending doesn't work!

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