Spring Integration --- Message Channel

什么是Message

Message是在各个Component之间传输的消息。Message包括两个部分，Header和Playload，Header包含了Message的元数据，用户可以把一些额外信息添加到Header里面，Payload是消息的实体，是消息传输的实际内容，一般是一个java object

什么是Channel

Channel用来连接component，例如把生产者和消费者给连接起来，生产者把生产出来的Message放到Channel中，消费者通过主动或被动的方式取出Channel里面的Message

讲多无用，用代码解析

一个Hello World例子

首先看看context.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
 * Copyright 2012 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
-->
<beans:beans xmlns:beans="http://www.springframework.org/schema/beans"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xmlns="http://www.springframework.org/schema/integration"
             xsi:schemaLocation="http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/integration
			http://www.springframework.org/schema/integration/spring-integration.xsd">

  <channel id="names"/>


  <service-activator input-channel="names" ref="helloService"
                     method="sayHello"/>

  <beans:bean id="helloService"
              class="siia.helloworld.channel.MyHelloService"/>

</beans:beans>

首先在spring里定义了一个名字为“names”的channel，然后定义了一个消费者<service-activator>，该消费者会调用“helloService”bean的sayHello方法来消费channel里面的消息

接下来看看HelloWorldExample的代码

public class HelloWorldExample {

  public static void main(String args[]) {
	String cfg = "siia/helloworld/channel/context.xml";
    ApplicationContext context = new ClassPathXmlApplicationContext(cfg);
    MessageChannel channel = context.getBean("names", MessageChannel.class);
    Message<String> message = MessageBuilder.withPayload("World").build();
    channel.send(message);
  }
}

代码不难理解，首先拿到spring的上下文，得到channel的实例；然后利用MessageBuilder来协助新建一个payload为“World”字符串，Header为默认的Message；最后把该message发送到channel上。

当channel接受到消息后，会主动通知（还有被动的方式，以后会解析）service－activator，service－activator会调用MyHelloService实例的sayHello方法，并把消息作为实参传给sayHello。MyHelloService的sayHello逻辑很简单，就是把消息的payload打印出来。

public interface HelloService {

  void sayHello(String name);
}

public class MyHelloService implements HelloService {

  @Override
  public void sayHello(String name) {
    System.out.println("Hello " + name);
  }
}

最后看看maven使用到的依赖

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>siia-parent</artifactId>
        <groupId>com.manning.siia</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>
    <artifactId>hello-world</artifactId>
    <name>hello-world</name>
    <description>Supporting code for Chapter 1</description>
    <dependencies>
        <dependency>
            <groupId>org.springframework.integration</groupId>
            <artifactId>spring-integration-core</artifactId>
        </dependency>
    </dependencies>
</project>

运行Main方法

可以看到我们把“World”传到channel里，等到的是“Hello World”