【基础系列】事件机制的两种消费姿势

借助Spring可以非常简单的实现事件监听机制，本文简单介绍下面向接口与注解监听的两种姿势

# I. 项目环境

本项目借助SpringBoot 2.2.1.RELEASE + maven 3.5.3 + IDEA进行开发

为了后面的发布事件验证，起一个web服务

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

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# II. 事件机制

# 1. 事件对象

在Spring中，所有的事件需要继承自ApplicationEvent，一个最基础的MsgEvent如下

public class MsgEvent extends ApplicationEvent {
    private String msg;

    /**
     * Create a new {@code ApplicationEvent}.
     *
     * @param source the object on which the event initially occurred or with
     *               which the event is associated (never {@code null})
     */
    public MsgEvent(Object source, String msg) {
        super(source);
        this.msg = msg;
    }

    @Override
    public String toString() {
        return "MsgEvent{" +
                "msg='" + msg + '\'' +
                '}';
    }
}

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# 2. 接口方式消费

消费事件有两种方式，接口的声明，主要是实现ApplicationListener接口；注意需要将listener声明为Spring的bean对象

@Service
public class MsgEventListener implements ApplicationListener<MsgEvent> {
    @Override
    public void onApplicationEvent(MsgEvent event) {
        System.out.println("receive msg event: " + event);
    }
}

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# 3. 注解方式消费

实现接口需要新建实现类，更简单的方法是直接在消费方法上加一个注解@EventListener

@EventListener(MsgEvent.class)
public void consumer(MsgEvent msgEvent) {
    System.out.println("receive msg by @anno: " + msgEvent);
}

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这个注解，支持根据Event参数类型进行匹配，即上面的实例中，方法上直接加@EventListener不指定圆括号内部的也没关系

# 4. 发布事件

前面是消费事件，消费的前提是有事件产生，在Spring中，发布事件主要需要借助ApplicationContext来实现

@Service
public class MsgPublisher implements ApplicationContextAware {
    private ApplicationContext applicationContext;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }

    public void publish(String msg) {
        applicationContext.publishEvent(new MsgEvent(this, msg));
    }
}

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# 5. 测试

一个简单的测试demo

@RestController
public class IndexController {
    @Autowired
    private MsgPublisher msgPublisher;

    @GetMapping(path = "pub")
    public String publish(String msg) {
        msgPublisher.publish(msg);
        return "ok";
    }
}

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访问: curl http://localhost:8082/pub?msg=一灰灰blog

输出日志:

receive msg by @anno: MsgEvent{msg='一灰灰blog'}
receive msg event: MsgEvent{msg='一灰灰blog'}

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上面这个测试两种消费方式都可以成功，但是，在实测的过程中发现一种case，注解消费方式不生效，测试姿势如下

@SpringBootApplication
public class Application {

    public Application(MsgPublisher msgPublisher) {
        msgPublisher.publish("一灰灰blog");
    }
    public static void main(String[] args) {
        SpringApplication.run(Application.class);
    }

}