基于Spring的发布订阅模式 EventListener _生活百科

基于Spring的发布订阅模式在我们使用spring开发应用时，经常会碰到要去解耦合一些依赖调用，比如我们在做代码的发布流程中，需要去通知相关的测试，开发人员关注发布中的错误信息。而且通知这个操作又不希望强耦合在主业务流程中，这个时候我们很容易就想到了观察者设计模式，而spring恰好提供了事件-监听机制，让我们看一下他们是具体怎么实现的吧。
事件-监听机制：

首先是一种对象间的一对多的关系；最简单的如交通信号灯，信号灯是目标（一方），行人注视着信号灯（多方）；
当目标发送改变（发布），观察者（订阅者）就可以接收到改变；
观察者如何处理（如行人如何走，是快走/慢走/不走，目标不会管的），目标无需干涉；所以就松散耦合了它们之间的关系。

Spring提供的事件驱动模型/观察者抽象

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其实整个模型就有三个角色，事件，目标（发布者），监听者，我们看一下spring中如何实现这三者
事件：具体代表者是：ApplicationEvent：
1、我们可以看到spring中ApplicationEvent该抽象类继承自JDK的EventObject 。JDK要求所有事件将继承它，并通过source得到事件源。
package org.springframework.context;import java.util.EventObject;/** * Class to be extended by all application events. Abstract as it * doesn't make sense for generic events to be published directly. * * @author Rod Johnson * @author Juergen Hoeller */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; }}
目标(发布者)具体代表者是具体代表者是：ApplicationEventPublisher及ApplicationEventMulticaster 。ApplicationContext该接口继承了ApplicationEventPublisher，并在AbstractApplicationContext实现了具体代码，实际执行是委托给ApplicationEventMulticaster（可以认为是多播）：
ApplicationContext继承自ApplicationEventPublisher
public interface ApplicationContext extends EnvironmentCapable, ListableBeanFactory, HierarchicalBeanFactory,MessageSource, ApplicationEventPublisher, ResourcePatternResolver {}ApplicationEventPublisher定义了publishEvent方法
public interface ApplicationEventPublisher {/*** Notify all matching 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 matching listeners registered with this* application of an event.* 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);}在AbstractApplicationContext实现了具体代码,实际执行是委托给ApplicationEventMulticaster（可以认为是多播）
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 necessaryApplicationEvent 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 initializedif (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);}}}ApplicationContext自动到本地容器里找一个ApplicationEventMulticaster实现，如果没有自己new一个SimpleApplicationEventMulticaster 。