Spring事件监听机制和状态机

Spring事件监听机制和Spring StateMachine都是基于观察者模式实现的，帮助业务代码进行解耦合。

Spring事件监听实现

Spring可以帮助开发者自己实现事件监听，开发者通过实现ApplicationListener（观察者Observer）接口定义一个监听器，同时通过实现ApplicationEvent（主题Subject）接口定义一个事件。实现ApplicationContextAware（发布器）通过发布器发布时间到ApplicationContext中。

package com.souche.observer;
import org.springframework.context.ApplicationEvent;
/**
 * 定义一个事件（主题 subject）
 * define an event
 */
public class MyTestEvent extends ApplicationEvent {
    public MyTestEvent(Object source) {
        super(source);
    }
    @Override
    public Object getSource() {
        return super.getSource();
    }
}

package com.souche.observer;

import org.springframework.context.ApplicationListener;
import org.springframework.stereotype.Service;

/**
 * 创建另一个事件监听器
 * 对于继承了 ApplicationEvent的事件所有入参对应的监听器都会监听到，执行onApplicationEvent方法
 */
@Service("myTestApplictionListener")
public class MyTestApplicationListener implements ApplicationListener<MyTestEvent> {

    @Override
    public void onApplicationEvent(MyTestEvent applicationEvent) {
        System.out.println(applicationEvent.getClass());
        if(applicationEvent instanceof MyTestEvent){
            System.out.println("get to MyTestApplicationListener...");
        }
    }
}

package com.souche.observer;

import org.springframework.context.ApplicationListener;
import org.springframework.stereotype.Service;

/**
 * 创建一个事件监听器（观察者 observer）
 * listener defination
 */
@Service("myApplicationListener")
public class MyApplicationListener implements ApplicationListener<MyTestEvent> {


    @Override
    public void onApplicationEvent(MyTestEvent applicationEvent) {
        System.out.println(applicationEvent.getClass());
        if(applicationEvent instanceof  MyTestEvent){
            System.out.println("get to MyApplicationListener");
        }
    }
}

package com.souche.observer;

import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.ApplicationEvent;
import org.springframework.stereotype.Service;
/**
 * 创建一个事件发布器
 */
@Service("myPublisher")
public class MyPubisher implements ApplicationContextAware {

    private ApplicationContext applicationContext;
    
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext=applicationContext;
    }
    public void publishEvent(ApplicationEvent event){
        System.out.println("into My Publisher's method to add event");
        applicationContext.publishEvent(event);
    }
}

启动一下：

package com.souche.observer;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;

@SpringBootApplication
public class ObserverApplication {

	public static void main(String[] args) {
		ConfigurableApplicationContext configurableApplicationContext=SpringApplication.run(ObserverApplication.class, args);
		MyPubisher myPubisher=(MyPubisher) configurableApplicationContext.getBean("myPublisher");
		myPubisher.publishEvent(new MyTestEvent(1));
	}
}

结果：

into My Publisher's method to add event
class com.souche.observer.MyTestEvent
get to MyApplicationListener
class com.souche.observer.MyTestEvent
get to MyTestApplicationListener...

可以通过@Order注解指定监听者的执行顺序。改善代码如下：

package com.souche.observer;

import org.springframework.context.ApplicationListener;
import org.springframework.core.annotation.Order;
import org.springframework.stereotype.Service;

/**
 * 创建另一个事件监听器
 * 对于继承了 ApplicationEvent的事件所有入参对应的监听器都会监听到，执行onApplicationEvent方法
 */
@Order(1)
@Service("myTestApplictionListener")
public class MyTestApplicationListener implements ApplicationListener<MyTestEvent> {
    @Override
    public void onApplicationEvent(MyTestEvent applicationEvent) {
        System.out.println(applicationEvent.getClass());
        if(applicationEvent instanceof MyTestEvent){
            System.out.println("get to MyTestApplicationListener...");
        }
    }
}

package com.souche.observer;

import org.springframework.context.ApplicationListener;
import org.springframework.core.annotation.Order;
import org.springframework.stereotype.Service;

/**
 * 创建一个事件监听器（观察者 observer）
 * listener defination
 */
@Order(2)
@Service("myApplicationListener")
public class MyApplicationListener implements ApplicationListener<MyTestEvent> {
    @Override
    public void onApplicationEvent(MyTestEvent applicationEvent) {
        System.out.println(applicationEvent.getClass());
        if(applicationEvent instanceof  MyTestEvent){
            System.out.println("get to MyApplicationListener");
        }
    }
}

结果：

into My Publisher's method to add event
class com.souche.observer.MyTestEvent
get to MyTestApplicationListener...
class com.souche.observer.MyTestEvent
get to MyApplicationListener