Spring 应用上下文支持基于事件的Bean间通信。在基于事件的通信模式中，事件的发送者不需要关系，事件的监听者。这样可以使消息的发送者和监听者进行解耦。
在Spring中所有事件类必须继承自ApplicationEvent,这样任何bean都可以调用事件发布者的publishEvent()方法，发布一个事件。
public class MyEvent extends ApplicationEvent {
/** */
private static final long serialVersionUID = 1L;
/**
* @param source
*/
public MyEvent(Object source) {
super(source);
}
}
@Component("eventPublisher")
public class EventPublisher implements ApplicationEventPublisherAware {
private ApplicationEventPublisher applicationEventPublisher;
public void ckeckout(){
applicationEventPublisher.publishEvent(new MyEvent(this));
}
/**
* @see org.springframework.context.ApplicationEventPublisherAware#setApplicationEventPublisher(org.springframework.context.ApplicationEventPublisher)
*/
@Override
public void setApplicationEventPublisher(ApplicationEventPublisher applicationEventPublisher) {
this.applicationEventPublisher=applicationEventPublisher;
}
}
@Component
public class MyListener implements ApplicationListener<MyEvent> {
/**
* @see org.springframework.context.ApplicationListener#onApplicationEvent(org.springframework.context.ApplicationEvent)
*/
@Override
public void onApplicationEvent(MyEvent event) {
System.out.println(Thread.currentThread().getName()+";"+event.getTimestamp());
}
}
public class Test {
/**
*
* @param args
* @author zhangwei<wei.zw@corp.netease.com>
*/
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
EventPublisher cashier = (EventPublisher) context.getBean("eventPublisher");
for(int i=0;i<20;i++) {
cashier.ckeckout();
}
}
}
<bean id="taskExecutor" class="org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor"> <property name="corePoolSize" value="5" /> <property name="keepAliveSeconds" value="30000" /> <property name="maxPoolSize" value="1000" /> <property name="queueCapacity" value="200" /> </bean> <bean id="applicationEventMulticaster" class="org.springframework.context.event.SimpleApplicationEventMulticaster"> <property name="taskExecutor" ref="taskExecutor" /> </bean>
taskExecutor-1;1454033201241 taskExecutor-2;1454033201242 taskExecutor-3;1454033201242 taskExecutor-4;1454033201242 taskExecutor-5;1454033201243 taskExecutor-1;1454033201243 taskExecutor-3;1454033201243 taskExecutor-3;1454033201243 taskExecutor-2;1454033201243 taskExecutor-5;1454033201243 taskExecutor-1;1454033201243 taskExecutor-4;1454033201243 taskExecutor-3;1454033201243 taskExecutor-2;1454033201244 taskExecutor-5;1454033201244 taskExecutor-5;1454033201244 taskExecutor-4;1454033201244 taskExecutor-3;1454033201244 taskExecutor-2;1454033201244 taskExecutor-5;1454033201244
通过上面的一个示例，实现了异步的基于事件通信。
在AbstractApplicationContext中发布事件的实现如下,首先获取applicationEventMulticaster,通过其发布事件。
/**
* Publish the given event to all listeners.
* <p>Note: Listeners get initialized after the MessageSource, to be able
* to access it within listener implementations. Thus, MessageSource
* implementations cannot publish events.
* @param event the event to publish (may be application-specific or a
* standard framework event)
*/
public void publishEvent(ApplicationEvent event) {
Assert.notNull(event, "Event must not be null");
if (logger.isTraceEnabled()) {
logger.trace("Publishing event in " + getDisplayName() + ": " + event);
}
getApplicationEventMulticaster().multicastEvent(event);
if (this.parent != null) {
this.parent.publishEvent(event);
}
}
在看看ApplicaitonEventMulticaster的初始化逻辑，如果在有配置过applicationEventMulticaster则直接使用，否则创建一个；注意，配置是ID必须是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 + "]");
}
}
}
再看看ApplicationEventMulticaster中又是如何发布事件的。如果有配置线程池，则异步处理，否则同步处理。
public void multicastEvent(final ApplicationEvent event) {
for (final ApplicationListener listener : getApplicationListeners(event)) {
Executor executor = getTaskExecutor();
if (executor != null) {
executor.execute(new Runnable() {
@SuppressWarnings("unchecked")
public void run() {
listener.onApplicationEvent(event);
}
});
}
else {
listener.onApplicationEvent(event);
}
}
}