开篇

• spring事件分为事件发布者(EventPublisher)、事件监听者(EventListener),还包括一个事件广播者(这个是spring实现相关，这一节不讨论)。
• 使用spring事件机制，需要自定义事件发布者和监听者。

使用案例

// 定义事件
public class ContentEvent extends ApplicationEvent {  
    public ContentEvent(final String content) {  
        super(content);  
    }  
}  

// 定义事件监听器
@Component  
public class UserOneListener implements ApplicationListener<ApplicationEvent> {  
    @Override  
    public void onApplicationEvent(final ApplicationEvent event) {  
        if(event instanceof ContentEvent) {  
            System.out.println("[UserOneListener]msg：" + event.getSource());  
        }  
    }  
}

// 测试消息发布
@RunWith(SpringJUnit4ClassRunner.class)  
@ContextConfiguration(locations={"classpath:spring-config-hello.xml"})  
public class HelloIT {  
  
    @Autowired  
    private ApplicationContext applicationContext;  
    @Test  
    public void testPublishEvent() {  
        applicationContext.publishEvent(new ContentEvent("news is  update"));  
    }  
}  

• 定义事件对象ContentEvent。
• 定义事件监听器UserOneListener。
• 通过applicationContext.publishEvent模拟事件发布。

源码分析

public abstract class AbstractApplicationContext extends DefaultResourceLoader
        implements ConfigurableApplicationContext, DisposableBean {
        // 事件广播器
    private ApplicationEventMulticaster applicationEventMulticaster;
        // 事件监听者
    private final Set<ApplicationListener<?>> applicationListeners = new LinkedHashSet<ApplicationListener<?>>();
    private Set<ApplicationEvent> earlyApplicationEvents;

    @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();

                // 初始化Spring的事件广播器
                initApplicationEventMulticaster();

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

                // 注册Spring事件监听器
                registerListeners();

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

                // Last step: publish corresponding event.
                finishRefresh();
            }
            catch (BeansException ex) {

            }
            finally {

            }
        }
    }


    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);
        }
        else {
            this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
            beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
        }
    }

        // 将所有listener注册到applicationEventMulticaster当中
    protected void registerListeners() {

        for (ApplicationListener<?> listener : getApplicationListeners()) {
            getApplicationEventMulticaster().addApplicationListener(listener);
        }

        String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
        for (String listenerBeanName : listenerBeanNames) {
            getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
        }

        Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
        this.earlyApplicationEvents = null;
        if (earlyEventsToProcess != null) {
            for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
                getApplicationEventMulticaster().multicastEvent(earlyEvent);
            }
        }
    }

        // 获取所有的listener
    public Collection<ApplicationListener<?>> getApplicationListeners() {
        return this.applicationListeners;
    }

    @Override
    public void addApplicationListener(ApplicationListener<?> listener) {
        if (this.applicationEventMulticaster != null) {
            this.applicationEventMulticaster.addApplicationListener(listener);
        }
        else {
            this.applicationListeners.add(listener);
        }
    }
}

• AbstractApplicationContext作为ApplicationContext的基类，提供广播对象的创建、listener的添加、listener的注册。
• addApplicationListener负责把通用的listener添加到applicationEventMulticaster当中。
• initApplicationEventMulticaster负责创建SimpleApplicationEventMulticaster广播对象。
• registerListeners把添加的listener绑定到SimpleApplicationEventMulticaster当中。
• 整个核心是把listener绑定到广播对象SimpleApplicationEventMulticaster当中。

public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {

    private Executor taskExecutor;
    private ErrorHandler errorHandler;

    @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);
            }
        }
    }

    @SuppressWarnings({"unchecked", "rawtypes"})
    protected void invokeListener(ApplicationListener listener, ApplicationEvent event) {
        ErrorHandler errorHandler = getErrorHandler();
        if (errorHandler != null) {
            try {
                listener.onApplicationEvent(event);
            }
            catch (Throwable err) {
            }
        }
        else {
            try {
                listener.onApplicationEvent(event);
            }
            catch (ClassCastException ex) {
            }
        }
    }
}

• SimpleApplicationEventMulticaster的multicastEvent负责同步或者多线程发布事件。
• invokeListener支持同步或者多线程发布事件。
• 广播过程中会通过getApplicationListeners(event, type)来获取该事件对象的listener。
• SimpleApplicationEventMulticaster包含了所有的事件监听器，但是广播的时候需要根据事件类型找到对应的listener进行广播。

public abstract class AbstractApplicationEventMulticaster
        implements ApplicationEventMulticaster, BeanClassLoaderAware, BeanFactoryAware {

    private final ListenerRetriever defaultRetriever = new ListenerRetriever(false);
    final Map<ListenerCacheKey, ListenerRetriever> retrieverCache =
            new ConcurrentHashMap<ListenerCacheKey, ListenerRetriever>(64);
    private ClassLoader beanClassLoader;
    private BeanFactory beanFactory;
    private Object retrievalMutex = this.defaultRetriever;

    @Override
    public void addApplicationListener(ApplicationListener<?> listener) {
        synchronized (this.retrievalMutex) {
            Object singletonTarget = AopProxyUtils.getSingletonTarget(listener);
            if (singletonTarget instanceof ApplicationListener) {
                this.defaultRetriever.applicationListeners.remove(singletonTarget);
            }
            this.defaultRetriever.applicationListeners.add(listener);
            this.retrieverCache.clear();
        }
    }

    protected Collection<ApplicationListener<?>> getApplicationListeners(
            ApplicationEvent event, ResolvableType eventType) {

        Object source = event.getSource();
        Class<?> sourceType = (source != null ? source.getClass() : null);
        ListenerCacheKey cacheKey = new ListenerCacheKey(eventType, sourceType);

        ListenerRetriever retriever = this.retrieverCache.get(cacheKey);
        if (retriever != null) {
            return retriever.getApplicationListeners();
        }

        if (this.beanClassLoader == null ||
                (ClassUtils.isCacheSafe(event.getClass(), this.beanClassLoader) &&
                        (sourceType == null || ClassUtils.isCacheSafe(sourceType, this.beanClassLoader)))) {
            synchronized (this.retrievalMutex) {
                retriever = this.retrieverCache.get(cacheKey);
                if (retriever != null) {
                    return retriever.getApplicationListeners();
                }
                retriever = new ListenerRetriever(true);
                Collection<ApplicationListener<?>> listeners =
                        retrieveApplicationListeners(eventType, sourceType, retriever);
                this.retrieverCache.put(cacheKey, retriever);
                return listeners;
            }
        }
        else {
            return retrieveApplicationListeners(eventType, sourceType, null);
        }
    }


    private Collection<ApplicationListener<?>> retrieveApplicationListeners(
            ResolvableType eventType, Class<?> sourceType, ListenerRetriever retriever) {

        LinkedList<ApplicationListener<?>> allListeners = new LinkedList<ApplicationListener<?>>();
        Set<ApplicationListener<?>> listeners;
        Set<String> listenerBeans;
        synchronized (this.retrievalMutex) {
            listeners = new LinkedHashSet<ApplicationListener<?>>(this.defaultRetriever.applicationListeners);
            listenerBeans = new LinkedHashSet<String>(this.defaultRetriever.applicationListenerBeans);
        }

        for (ApplicationListener<?> listener : listeners) {
            if (supportsEvent(listener, eventType, sourceType)) {
                if (retriever != null) {
                    retriever.applicationListeners.add(listener);
                }
                allListeners.add(listener);
            }
        }
        if (!listenerBeans.isEmpty()) {
            BeanFactory beanFactory = getBeanFactory();
            for (String listenerBeanName : listenerBeans) {
                try {
                    Class<?> listenerType = beanFactory.getType(listenerBeanName);
                    if (listenerType == null || supportsEvent(listenerType, eventType)) {
                        ApplicationListener<?> listener =
                                beanFactory.getBean(listenerBeanName, ApplicationListener.class);
                        if (!allListeners.contains(listener) && supportsEvent(listener, eventType, sourceType)) {
                            if (retriever != null) {
                                retriever.applicationListenerBeans.add(listenerBeanName);
                            }
                            allListeners.add(listener);
                        }
                    }
                }
                catch (NoSuchBeanDefinitionException ex) {
                }
            }
        }
        AnnotationAwareOrderComparator.sort(allListeners);
        return allListeners;
    }
}

• retrieveApplicationListeners根据EventType筛选Listener进行广播。