Spring源码学习--Spring事件体系

文章来源:

https://blog.csdn.net/caihaijiang/article/details/7460888


Spring事件体系包括三个组件:事件,事件监听器,事件广播器。


Spring源码学习--Spring事件体系_第1张图片

事件:ApplicationEvent

事件监听器:ApplicationListener,对监听到的事件进行处理。

事件广播器:ApplicationEventMulticaster,将Springpublish的事件广播给所有的监听器。

Spring在ApplicationContext接口的抽象实现类AbstractApplicationContext中完成了事件体系的搭建。

AbstractApplicationContext拥有一个applicationEventMulticaster成员变量,applicationEventMulticaster提供了容器监听器的注册表。

AbstractApplicationContext在refresh()这个容器启动方法中搭建了事件的基础设施,其中AbstractApplicationContext的refresh方法实现如下:

@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();

        // Tell the subclass to refresh the internal bean factory.
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Prepare the bean factory for use in this context.
        prepareBeanFactory(beanFactory);

        try {
            // Allows post-processing of the bean factory in context subclasses.
            postProcessBeanFactory(beanFactory);

            // Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);

            // Register bean processors that intercept bean creation.
            registerBeanPostProcessors(beanFactory);

            // Initialize message source for this context.
            initMessageSource();

            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses.
            onRefresh();

            // Check for listener beans and register them.
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);

            // Last step: publish corresponding event.
            finishRefresh();
        }

        catch (BeansException ex) {
            if (logger.isWarnEnabled()) {
                logger.warn("Exception encountered during context initialization - " +
                        "cancelling refresh attempt: " + ex);
            }

            // Destroy already created singletons to avoid dangling resources.
            destroyBeans();

            // Reset 'active' flag.
            cancelRefresh(ex);

            // Propagate exception to caller.
            throw ex;
        }

        finally {
            // Reset common introspection caches in Spring's core, since we
            // might not ever need metadata for singleton beans anymore...
            resetCommonCaches();
        }
    }
}

1 事件广播器的初始化

/**
 * 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 + "]");
        }
    }
}

用户可以在配置文件中为容器定义一个自定义的事件广播器,只要实现ApplicationEventMulticaster就可以了,Spring会通过 反射的机制将其注册成容器的事件广播器,如果没有找到配置的外部事件广播器,Spring自动使用 SimpleApplicationEventMulticaster作为事件广播器。

2 注册事件监听器

/**
 * 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 earlyEventsToProcess = this.earlyApplicationEvents;
    this.earlyApplicationEvents = null;
    if (earlyEventsToProcess != null) {
        for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
            getApplicationEventMulticaster().multicastEvent(earlyEvent);
        }
    }
}

Spring根据反射机制,使用ListableBeanFactory的getBeansOfType方法,从BeanDefinitionRegistry中找出所有实现 org.springframework.context.ApplicationListener的Bean,将它们注册为容器的事件监听器,实际的操作就是将其添加到事件广播器所提供的监听器注册表中。

3 发布事件

跟着 finishRefresh();方法进入publishEvent(new ContextRefreshedEvent(this));方法如下:

/**
 * Publish the given event to all listeners.
 * @param event the event to publish (may be an {@link ApplicationEvent}
 * or a payload object to be turned into a {@link PayloadApplicationEvent})
 * @param eventType the resolved event type, if known
 * @since 4.2
 */
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 necessary
    ApplicationEvent applicationEvent;
    if (event instanceof ApplicationEvent) {
        applicationEvent = (ApplicationEvent) event;
    }
    else {
        applicationEvent = new PayloadApplicationEvent(this, event);
        if (eventType == null) {
            eventType = ((PayloadApplicationEvent)applicationEvent).getResolvableType();
        }
    }

    // Multicast right now if possible - or lazily once the multicaster is initialized
    if (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);
        }
    }
} 
  

在AbstractApplicationContext的publishEvent方法中, Spring委托ApplicationEventMulticaster将事件通知给所有的事件监听器.

4 Spring默认的事件广播器SimpleApplicationEventMulticaster

@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);
        }
    }
}

/**
 * Invoke the given listener with the given event.
 * @param listener the ApplicationListener to invoke
 * @param event the current event to propagate
 * @since 4.1
 */
@SuppressWarnings({"unchecked", "rawtypes"})
protected void invokeListener(ApplicationListener listener, ApplicationEvent event) {
    ErrorHandler errorHandler = getErrorHandler();
    if (errorHandler != null) {
        try {
            listener.onApplicationEvent(event);
        }
        catch (Throwable err) {
            errorHandler.handleError(err);
        }
    }
    else {
        try {
            listener.onApplicationEvent(event);
        }
        catch (ClassCastException ex) {
            // Possibly a lambda-defined listener which we could not resolve the generic event type for
            LogFactory.getLog(getClass()).debug("Non-matching event type for listener: " + listener, ex);
        }
    }
}

遍历注册的每个监听器,并启动来调用每个监听器的onApplicationEvent方法。由于SimpleApplicationEventMulticaster的taskExecutor的实现类是SyncTaskExecutor,因此,事件监听器对事件的处理,是同步进行的。


从代码可以看出,applicationContext.publishEvent()方法,需要同步等待各个监听器处理完之后,才返回。
也就是说,Spring提供的事件机制,默认是同步的。如果想用异步的,可以自己实现ApplicationEventMulticaster接口,并在Spring容器中注册id为applicationEventMulticaster的Bean。例如下面所示:

public class AsyncApplicationEventMulticaster extends AbstractApplicationEventMulticaster {  
    private TaskExecutor taskExecutor = new SimpleAsyncTaskExecutor();  

    public void setTaskExecutor(TaskExecutor taskExecutor) {  
        this.taskExecutor = (taskExecutor != null ? taskExecutor : new SimpleAsyncTaskExecutor());  
    }  

    protected TaskExecutor getTaskExecutor() {  
        return this.taskExecutor;  
    }  

    @SuppressWarnings("unchecked")  
    public void multicastEvent(final ApplicationEvent event) {  
        for (Iterator it = getApplicationListeners().iterator(); it.hasNext();) {  
            final ApplicationListener listener =  it.next();  
            getTaskExecutor().execute(new Runnable() {  
                public void run() {  
                    listener.onApplicationEvent(event);  
                }  
            });  
        }  
    }  
}  

spring配置:

id="applicationEventMulticaster"   
          class="com.alibaba.chj.event.AsyncApplicationEventMulticaster" />  

Spring发布事件之后,所有注册的事件监听器,都会收到该事件,因此,事件监听器在处理事件时,需要先判断该事件是否是自己关心的。

Sping事件体系所使用的设计模式是:观察者模式。ApplicationListener是观察者接口,接口中定义了onApplicationEvent方法,该方法的作用是对ApplicationEvent事件进行处理。

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