ApplicationListener源码解读（异步通知、统一ErrorHandler）

简单使用（老司机直接跳过～）

定义一个实体类

@Data@AllArgsConstructor@NoArgsConstructorpublicclassUserInfo{
​
   private Integer age;
​
   private String name;
​
}

定义一个事件，需要继承ApplicationEvent

@Getter@SetterpublicclassCustomEventextendsApplicationEvent{
​
   private UserInfo userInfo;
​
   publicCustomEvent(UserInfo userInfo){
      super(userInfo);
      this.userInfo = userInfo;
   }
​
}

定义一个监听器，需要实现ApplicationListener接口

@ComponentpublicclassCustomListenerimplementsApplicationListener<CustomEvent> {
​
   @OverridepublicvoidonApplicationEvent(@NonNull CustomEvent event){
      System.out.println("CustomListener接收到事件～");
      System.out.println(JSONObject.toJSONString(event.getUserInfo()));
   }
​
}

测试

@SpringBootTestpublicclassListenerTest{
​
  @Resourceprivate ApplicationContext applicationContext;
​
  @Testpublicvoidtest(){
    UserInfo userInfo = new UserInfo(1, "1");
    // 发布事件
    applicationContext.publishEvent(new CustomEvent(userInfo));
  }
​
}

可见，监听器成功监听并处理了我们发布的事件～

---

源码分析

通过上面的案例，我们了解到，发布事件是调用的publishEvent方法，所以我们直接从publishEvent开始看～

publishEvent

protectedvoidpublishEvent(Object event, @Nullable ResolvableType eventType){
   Assert.notNull(event, "Event must not be null");
​
   // 如果event是ApplicationEvent实例，则向上转型
   ApplicationEvent applicationEvent;
   if (event instanceof ApplicationEvent) {
      applicationEvent = (ApplicationEvent) event;
   }
   else {
      applicationEvent = new PayloadApplicationEvent<>(this, event);
      if (eventType == null) {
         eventType = ((PayloadApplicationEvent<?>) applicationEvent).getResolvableType();
      }
   }
​
   // 早期发布的事件，此时listener还未注册，所以先将event存起来if (this.earlyApplicationEvents != null) {
      this.earlyApplicationEvents.add(applicationEvent);
   }
   else {
      // 获取事件发布器，并进行发布
      getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
   }
​
   // 如果父context不为空，则要通过父context再发布一次if (this.parent != null) {
      if (this.parent instanceof AbstractApplicationContext) {
         ((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
      }
      else {
         this.parent.publishEvent(event);
      }
   }
}

publishEvent方法，核心逻辑在于获取时间发布器进行发布，即下面这行代码

getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);

---

applicationEventMulticaster（事件多播器的获取和初始化）

getApplicationEventMulticaster只是简单的获取applicationEventMulticaster，并做一下参数校验，没什么好说的。

@Nullableprivate ApplicationEventMulticaster applicationEventMulticaster;
​
ApplicationEventMulticaster getApplicationEventMulticaster()throws IllegalStateException {
   if (this.applicationEventMulticaster == null) {
      thrownew IllegalStateException("ApplicationEventMulticaster not initialized - " +
            "call 'refresh' before multicasting events via the context: " + this);
   }
   returnthis.applicationEventMulticaster;
}

重点在于，applicationEventMulticaster是怎么初始化的？

熟悉Spring源码的小伙伴们，对下面这张图肯定不会陌生～

没错，正是Spring初始化过程中非常重要的refresh方法 ，在refresh方法中，会调用initApplicationEventMulticaster方法初始化事件多播器

initApplicationEventMulticaster源码如下:

publicstaticfinal String APPLICATION_EVENT_MULTICASTER_BEAN_NAME = "applicationEventMulticaster";
​
protectedvoidin