Spring源码之ApplicationContext(八)消息广播

当我们定义某个类，并将这个类继承ApplicationEvent，那么这个类就是一个监听事件的类。如果我们定义了个类继承了ApplicatioListener的话，那么这个类就是个监听器了。我们可以在Spring的配置文件中加入：<bean id=”testListener” class=”com.test.event.TestListener”/>当程序运行时，Spring会将发出的事件转给我们定义的监听器进行处理。这个过程是如何实现，我们来看一下时序图。（相关资源可到这里下载：http://pan.baidu.com/s/1sjSo9a9）



我们跟踪代码到initApplicationEventMulticaster的函数中，我们可以看，这个函数无非就是做两件事的判断：

1) 如果用户自定义了广播器，那么使用用户自定义的事件广播器。

2) 如果用户没有自定义事件广播器，那么使用默认的ApplicationEventMulticaster.

/**
* Initialize the ApplicationEventMulticaster.
* Uses SimpleApplicationEventMulticaster if none defined in the context.
* @see org.springframework.context.event.SimpleApplicationEventMulticaster
*/
protected void initApplicationEventMulticaster() {
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
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 {
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 + "]");
}
}
}

对于上面的代码，我们可能会更关心的是SimpleApplicationEventMulticaster的类，我们跟踪代码进入，可能我们对于类中的multicastEvent的函数会更兴趣。

@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);
}
}
}

从这段代码，我们可以推断，当产生Spring事件的时候会默认使用SimpleApplicationEventMulticaster的multicastEvent来广播事件，遍历所有监听器，并使用监听中的onApplicationEvent方法来进行监听的处理。那么Spring注册监听器后又做了什么呢？我们来看下面代码。

/**
* Add beans that implement ApplicationListener as listeners.
* Doesn't affect other listeners, which can be added without being beans.
*/
protected void registerListeners() {
// Register statically specified listeners first.
for (ApplicationListener<?> listener : getApplicationListeners()) {
getApplicationEventMulticaster().addApplicationListener(listener);
}

// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let post-processors apply to them!
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);
}
}
}