一. 測試用例

package main.java.com.wenniuwuren.spring.event.listener;


import org.springframework.context.ApplicationEvent;

/**
 * 定義監聽事件
 * Created by hzzhuyibin on 2016/11/22.
 */
public class TestEvent extends ApplicationEvent {

public String msg;

 /**
 * Create a new ApplicationEvent.
 *
 * @param source the object on which the event initially occurred (never {@code null})
 */
 public TestEvent(Object source) {
super(source);
 }

public TestEvent(Object source, String msg) {
super(source);
 this.msg = msg;
 }

public void print() {
System.out.println(msg);
 }

}
 

package main.java.com.wenniuwuren.spring.event.listener;

import org.springframework.context.ApplicationEvent;
import org.springframework.context.ApplicationListener;

/**
 * 定義監聽器
 * Created by hzzhuyibin on 2016/11/22.
 */
public class TestListener implements ApplicationListener{
@Override
 public void onApplicationEvent(ApplicationEvent event) {

if (event instanceof TestEvent) {
TestEvent testEvent = (TestEvent)event;
 testEvent.print();
 }
}
}
 

listenerTest.xml:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    <bean name="testListener" class="main.java.com.wenniuwuren.spring.event.listener.TestListener"/>

</beans>
 

測試類：

package main.java.com.wenniuwuren.spring.event.listener;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

/**
 *
 * Created by Yibin_Zhu on 2016/11/21.
 */
public class Test{

    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("main/resources/listenerTest.xml");
        // 不僅僅可以指定 beanName，也可以有建構函式或者工廠方法的方法引數

        TestEvent testEvent = new TestEvent("hello", "wenniuwuren");

        context.publishEvent(testEvent);

    }
}

二. 原始碼解析 debug 到 AbstractApplicationContext 類的 initApplicationEventMulticaster() 方法 廣播器我們又經常成為事件

protected void initApplicationEventMulticaster() {
   ConfigurableListableBeanFactory beanFactory = getBeanFactory();
   // 使用者自定義事件廣播器
   if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
      this.applicationEventMulticaster =
            beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
      if (logger.isDebugEnabled()) {
         logger.debug("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
      }
   }
   else { // 預設事件廣播器
      this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
      beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
      if (logger.isDebugEnabled()) {
         logger.debug("Unable to locate ApplicationEventMulticaster with name '" +
               APPLICATION_EVENT_MULTICASTER_BEAN_NAME +
               "': using default [" + this.applicationEventMulticaster + "]");
      }
   }
}

我們關注的是 SimpleApplicationEventMulticaster 事件類的具體實現，繼續 debug 進去，看到此類繼承自 AbstractApplicationEventMulticaster， 而 AbstractApplicationEventMulticaster 中有許多 add**Listener、delete**Listener 之類的方法，可以知道這是典型的觀察者模式的應用。 繼續看如下方法：事件在這裡廣播，遍歷所有監聽器，通知監聽器

@Override
public void multicastEvent(ApplicationEvent event) {
   multicastEvent(event, resolveDefaultEventType(event));
}

@Override
public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {
   ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
   for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
      Executor executor = getTaskExecutor();
      if (executor != null) {
         executor.execute(new Runnable() {
            @Override
            public void run() {
               invokeListener(listener, event);
            }
         });
      }
      else {
         invokeListener(listener, event);
      }
   }
}

protected void invokeListener(ApplicationListener listener, ApplicationEvent event) {
   ErrorHandler errorHandler = getErrorHandler();
   if (errorHandler != null) {
      try {
         listener.onApplicationEvent(event);
      }
      catch (Throwable err) {
         errorHandler.handleError(err);
      }
   }
   else {
      listener.onApplicationEvent(event);
   }
}

這個方法 debug 結束，回到 AbstractApplicationContext 類，debug 進 registerListeners() 方法：註冊監聽器

protected void registerListeners() {
   // 硬編碼的方式註冊監聽器
   for (ApplicationListener<?> listener : getApplicationListeners()) {
      getApplicationEventMulticaster().addApplicationListener(listener);
   }

   // 配置檔案註冊監聽器
   String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
   for (String listenerBeanName : listenerBeanNames) {
      getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
   }

   // Publish early application events now that we finally have a multicaster...
   Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
   this.earlyApplicationEvents = null;
   if (earlyEventsToProcess != null) {
      for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
         getApplicationEventMulticaster().multicastEvent(earlyEvent);
      }
   }
}