??本文介绍下Spring中的事件监听，其本质也就是观察者模型(发布/订阅模式),具体的观察者模式参考下文
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Java观察者模式(Observer)
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Spring事件监听

一、事件监听案例

1.事件类

/**
 * 事件类
 * @author 波波烤鸭
 * @email dengpbs@163.com
 *
 */
public class MyEvent extends ApplicationContextEvent {

    private static final long serialVersionUID = 1L;
    
    public MyEvent(ApplicationContext source) {
        super(source);
        System.out.println("myEvent 构造方法被执行了...");
    }
    
    public void out(String name){
        System.out.println("myEvent .... out方法执行了"+name);
    }
}

2.事件监听类

??事件监听器也就是我们的观察者。我们可以创建多个来观察。

/**
 * 监听器
 *    观察者
 * @author 波波烤鸭
 * @email dengpbs@163.com
 *
 */
public class MyListenerA implements ApplicationListener<MyEvent>{

    @Override
    public void onApplicationEvent(MyEvent event) {
        System.out.println("MyListenerA 监听器触发了...");
        // 执行事件中的特定方法
        event.out("AAAAA");
    }
}

/**
 * 监听器
 *    观察者
 * @author 波波烤鸭
 * @email dengpbs@163.com
 *
 */
public class MyListenerB implements ApplicationListener<MyEvent>{

    @Override
    public void onApplicationEvent(MyEvent event) {
        System.out.println("MyListenerB 监听器触发了...");
        // 执行事件中的特定方法
        event.out("BBBBB");
    }
}

3.事件发布者

/**
 * 事件发布类
 *   实现ApplicationContextAware接口用来感知ApplicationContext对象
 * @author 波波烤鸭
 * @email dengpbs@163.com
 *
 */
public class MyPublisher implements ApplicationContextAware{
    
    public ApplicationContext ac;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        // TODO Auto-generated method stub
        this.ac = applicationContext;
    }
    /**
     * 发布事件
     *    监听该事件的监听者都可以获取消息
     * @param event
     */
    public void publisherEvent(ApplicationEvent event){
        System.out.println("---发布事件---"+event);
        ac.publishEvent(event);
    }
}

4.配置文件中注册

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.3.xsd">
    
    <context:annotation-config/>

    <bean class="com.dpb.pojo.User" id="user"  >
        <property name="name" value="波波烤鸭"></property>
    </bean>
    
    <!-- 注册事件类 -->
    <bean class="com.dpb.event.MyEvent"></bean>
    
    <!-- 注册监听器 -->
    <bean class="com.dpb.listener.MyListenerA"></bean>
    <bean class="com.dpb.listener.MyListenerB"></bean>
    
    <!-- 注册发布者类 -->
    <bean class="com.dpb.publisher.MyPublisher"></bean>
</beans>

5.测试

@Test
public void test1() {
    ClassPathXmlApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext.xml");
    // 从Spring容器中获取发布者
    MyPublisher bean = ac.getBean(MyPublisher.class);
    // 从Spring容器中获取事件对象
    MyEvent event = ac.getBean(MyEvent.class);
    // 发布者发布事件
    bean.publisherEvent(event);
}

输出结果

myEvent 构造方法被执行了...
---发布事件---com.dpb.event.MyEvent[source=org.springframework.context.support.ClassPathXmlApplicationContext@311d617d: startup date [Wed Mar 06 13:04:57 CST 2019]; root of context hierarchy]
MyListenerA 监听器触发了...
myEvent .... out方法执行了AAAAA
MyListenerB 监听器触发了...
myEvent .... out方法执行了BBBBB

小结：通过案例我们实现了事件发生后注册的有此事件的监听者(观察者)监听到了此事件，并做出了响应的处理。

二、Spring中事件监听分析

1. Spring中事件监听的结构

2. 核心角色介绍

2.1 ApplicationEvent

??ApplicationEvent是所有事件对象的父类。ApplicationEvent继承自jdk的EventObject,所有的事件都需要继承ApplicationEvent,并且通过source得到事件源。

public abstract class ApplicationEvent extends EventObject {

    /** use serialVersionUID from Spring 1.2 for interoperability */
    private static final long serialVersionUID = 7099057708183571937L;

    /** System time when the event happened */
    private final long timestamp;


    /**
     * Create a new ApplicationEvent.
     * @param source the object on which the event initially occurred (never {@code null})
     */
    public ApplicationEvent(Object source) {
        super(source);
        this.timestamp = System.currentTimeMillis();
    }
    /**
     * Return the system time in milliseconds when the event happened.
     */
    public final long getTimestamp() {
        return this.timestamp;
    }
}

实现类：

2.2 ApplicationListener

??ApplicationListener事件监听器，也就是观察者。继承自jdk的EventListener，该类中只有一个方法onApplicationEvent。当监听的事件发生后该方法会被执行。

public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
    /**
     * Handle an application event.
     * @param event the event to respond to
     */
    void onApplicationEvent(E event);
}

实现类

2.3 ApplicationContext

??ApplicationContext是Spring中的核心容器，在事件监听中ApplicationContext可以作为事件的发布者，也就是事件源。因为ApplicationContext继承自ApplicationEventPublisher。在ApplicationEventPublisher中定义了事件发布的方法

public interface ApplicationEventPublisher {

    /**
     * Notify all <strong>matching</strong> listeners registered with this
     * application of an application event. Events may be framework events
     * (such as RequestHandledEvent) or application-specific events.
     * @param event the event to publish
     * @see org.springframework.web.context.support.RequestHandledEvent
     */
    void publishEvent(ApplicationEvent event);

    /**
     * Notify all <strong>matching</strong> listeners registered with this
     * application of an event.
     * <p>If the specified {@code event} is not an {@link ApplicationEvent},
     * it is wrapped in a {@link PayloadApplicationEvent}.
     * @param event the event to publish
     * @since 4.2
     * @see PayloadApplicationEvent
     */
    void publishEvent(Object event);

}

具体发布消息的方法实现:AbstractApplicationContext中

protected void publishEvent(Object event, ResolvableType eventType) {
    Assert.notNull(event, "Event must not be null");
    if (logger.isTraceEnabled()) {
        logger.trace("Publishing event in " + getDisplayName() + ": " + event);
    }

    // Decorate event as an ApplicationEvent if necessary
    ApplicationEvent applicationEvent;
    if (event instanceof ApplicationEvent) {
        applicationEvent = (ApplicationEvent) event;
    }
    else {
        applicationEvent = new PayloadApplicationEvent<Object>(this, event);
        if (eventType == null) {
            eventType = ((PayloadApplicationEvent) applicationEvent).getResolvableType();
        }
    }

    // Multicast right now if possible - or lazily once the multicaster is initialized
    if (this.earlyApplicationEvents != null) {
        this.earlyApplicationEvents.add(applicationEvent);
    }
    else {
        getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
    }

    // Publish event via parent context as well...
    if (this.parent != null) {
        if (this.parent instanceof AbstractApplicationContext) {
            ((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
        }
        else {
            this.parent.publishEvent(event);
        }
    }
}

getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);这行代码的作用是获取ApplicationEventMulticaster来广播事件给所有的监听器。

2.4 ApplicationEventMulticaster

??事件广播器,它的作用是把Applicationcontext发布的Event广播给所有的监听器.

具体的注册监听是在AbstractApplicationContext中实现的。

@Override
public void addApplicationListener(ApplicationListener<?> listener) {
    Assert.notNull(listener, "ApplicationListener must not be null");
    if (this.applicationEventMulticaster != null) {
        this.applicationEventMulticaster.addApplicationListener(listener);
    }
    else {
        this.applicationListeners.add(listener);
    }
}

三、总结

1. Spring中的事件监听使用的是观察者模式
2. 所有事件需要继承ApplicationEvent父类
3. 所有的监听器需要实现ApplicationListener接口
4. 事件发布需要通过ApplicationContext中的publisherEvent方法实现
5. 监听器的注册是ApplicationEventMulticaster提供的，但我们并不需要实现。

Spring之事件监听(观察者模型)

原文：https://www.cnblogs.com/dengpengbo/p/10482891.html