【Spring源码分析】20-ApplicationEvent事件机制

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ApplicationContext提供事件处理通过ApplicationEvent类和ApplicationListener接口。如果一个bean实现ApplicationListener接口在容器中,每次一个ApplicationEvent被发布到ApplicationContext中,这类bean就会收到这些通知。从本质上讲,这是标准的观察者设计模式。

实现Spring事件机制主要有4个类:

  • ApplicationEvent:事件,每个实现类表示一类事件,可携带数据。
  • ApplicationListener:事件监听器,用于接收事件处理时间。
  • ApplicationEventMulticaster:事件管理者,用于事件监听器的注册和事件的广播。
  • ApplicationEventPublisher:事件发布者,委托ApplicationEventMulticaster完成事件发布。

1、ApplicationEvent

事件就是一个包含了任意对象并含有事件对象创建时间戳的类。

    public abstract class ApplicationEvent extends EventObject {
       private static final long serialVersionUID = 7099057708183571937L;
       private final long timestamp;
       public ApplicationEvent(Object source) {
          super(source);
          this.timestamp = System.currentTimeMillis();
       }
       public final long getTimestamp() {
          return this.timestamp;
       }
    }

下表描述了Spring提供的标准事件:

事件 描述
ContextRefreshedEvent 事件发布在ApplicationContext初始化或刷新时(例如,通过在ConfigurableApplicationContext接口使用refresh()方法)。这里,“初始化”意味着所有bean加载,post-processorbean被检测到并且激活,单例预先实例化,ApplicationContext对象可以使用了。只要上下文没有关闭,可以触发多次刷新,ApplicationContext提供了一种可选择的支持这种“热”刷新。例如,XmlWebApplicationContext支持热刷新,但GenericApplicationContext并非如此。具体是在AbstractApplicationContext的finishRefresh()方法中。
ContextStartedEvent 事件发布在ApplicationContext开始使用ConfigurableApplicationContext接口start()方法。这里,“开始”意味着所有生命周期bean接收到一个明确的起始信号。通常,这个信号用于明确停止后重新启动,但它也可以用于启动组件没有被配置为自动运行(例如,组件还没有开始初始化)。
ContextStoppedEvent 事件发布在ApplicationContext停止时通过使用ConfigurableApplicationContext接口上的stop()方法。在这里,“停止”意味着所有生命周期bean接收一个显式的停止信号。停止上下文可以通过重新调用start()方法。
ContextClosedEvent 事件发布在ApplicationContext关闭时通过关闭ConfigurableApplicationContext接口()方法。这里,“封闭”意味着所有单例bean被摧毁。一个封闭的环境达到生命的终结。它不能刷新或重启。
RequestHandledEvent 一个特定的web事件告诉所有能处理HTTP请求的bean。这个事件是在请求完成后发布的。这个事件只适用于使用Spring的DispatcherServlet的web应用程序。

2、ApplicationListener

    @FunctionalInterface
    public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
       void onApplicationEvent(E event);
    }

当事件监听器接收到它可以处理的事件,会调用onApplicationEvent()方法。注意到ApplicationListener是泛型参数的这样可以参数化的定制事件。这意味着onApplicationEvent()方法可以保持类型安全,避免任何需要向下类型转换。你可以尽可能多的注册你希望事件侦听器,但是注意,默认情况下,事件监听器同步接收事件。这意味着publishEvent()方法会阻塞直到所有的事件监听器成处理完事件。这种单线程同步方法的一个特点是,当一个监听器接收到一个事件时,它运行在事务上下文的发布者线程上如果事务上下文可用。如果事件的发布需要另一种策略(譬如多线程)需要实现自己的 ApplicationEventMulticaster接口类。

3、ApplicationEventMulticaster

ApplicationEventMulticaster接口方法分为三类,注册事件监听器、移除事件监听器、发布事件。

    public interface ApplicationEventMulticaster {
       void addApplicationListener(ApplicationListener<?> listener);
       void addApplicationListenerBean(String listenerBeanName);
       void removeApplicationListener(ApplicationListener<?> listener);
       void removeApplicationListenerBean(String listenerBeanName);
       void removeAllListeners();
       void multicastEvent(ApplicationEvent event);
       void multicastEvent(ApplicationEvent event, @Nullable ResolvableType eventType);
    }

执行AbstractApplicationContext.initApplicationEventMulticaster() 方法时会实例化一个bean name为applicationEventMulticaster的SimpleApplicationEventMulticaster,它的父类实现了前5个方法依靠一个内部类ListenerRetriever维护了一个Set<ApplicationListener<?>>,本质事件监听器的注册或移除就是对这个Set的添加和移除操作。

    public abstract class AbstractApplicationEventMulticaster
          implements ApplicationEventMulticaster, BeanClassLoaderAware, BeanFactoryAware {
       private final ListenerRetriever defaultRetriever = new ListenerRetriever(false);

       @Override
       public void addApplicationListener(ApplicationListener<?> listener) {
          synchronized (this.retrievalMutex) {
             // Explicitly remove target for a proxy, if registered already,
             // in order to avoid double invocations of the same listener.
             Object singletonTarget = AopProxyUtils.getSingletonTarget(listener);
             if (singletonTarget instanceof ApplicationListener) {
                this.defaultRetriever.applicationListeners.remove(singletonTarget);
             }
             this.defaultRetriever.applicationListeners.add(listener);
             this.retrieverCache.clear();
          }
       }

       @Override
       public void removeApplicationListener(ApplicationListener<?> listener) {
          synchronized (this.retrievalMutex) {
             this.defaultRetriever.applicationListeners.remove(listener);
             this.retrieverCache.clear();
          }
       }

       private class ListenerRetriever {
          public final Set<ApplicationListener<?>> applicationListeners = new LinkedHashSet<>();
          public final Set<String> applicationListenerBeans = new LinkedHashSet<>();
          private final boolean preFiltered;
          public ListenerRetriever(boolean preFiltered) {
             this.preFiltered = preFiltered;
          }
          public Collection<ApplicationListener<?>> getApplicationListeners() {
             List<ApplicationListener<?>> allListeners = new ArrayList<>(
                   this.applicationListeners.size() + this.applicationListenerBeans.size());
             allListeners.addAll(this.applicationListeners);
             if (!this.applicationListenerBeans.isEmpty()) {
                BeanFactory beanFactory = getBeanFactory();
                for (String listenerBeanName : this.applicationListenerBeans) {
                   try {
                      ApplicationListener<?> listener = beanFactory.getBean(listenerBeanName, ApplicationListener.class);
                      if (this.preFiltered || !allListeners.contains(listener)) {
                         allListeners.add(listener);
                      }
                   }
                   catch (NoSuchBeanDefinitionException ex) {
                      // Singleton listener instance (without backing bean definition) disappeared -
                      // probably in the middle of the destruction phase
                   }
                }
             }
             if (!this.preFiltered || !this.applicationListenerBeans.isEmpty()) {
                AnnotationAwareOrderComparator.sort(allListeners);
             }
             return allListeners;
          }
       }

    }

接口后两个方法由子类实现,可以看到SimpleApplicationEventMulticaster拥有一个Executor和ErrorHandler,分表表示监听器的调用线程池(如果不想使用单线程同步处理则可以设置一个线程池)和监听器处理事件失败的处理者(如果设置了的话)否则抛异常。

    public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {
       @Nullable
       private Executor taskExecutor;
       @Nullable
       private ErrorHandler errorHandler;
       public SimpleApplicationEventMulticaster() {
       }
       public SimpleApplicationEventMulticaster(BeanFactory beanFactory) {
          setBeanFactory(beanFactory);
       }
       public void setTaskExecutor(@Nullable Executor taskExecutor) {
          this.taskExecutor = taskExecutor;
       }
       public void setErrorHandler(@Nullable ErrorHandler errorHandler) {
          this.errorHandler = errorHandler;
       }
       @Override
       public void multicastEvent(ApplicationEvent event) {
          //广播事件,可以自动分析出ApplicationEvent是那种事件类型
          multicastEvent(event, resolveDefaultEventType(event));
       }
       @Override
       public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
          ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
          //调用父类方法getApplicationListeners只取得能处理此类事件的时间监听器,依次处理
          for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
             Executor executor = getTaskExecutor();
             if (executor != null) {
                executor.execute(() -> invokeListener(listener, event));
             }
             else {
                invokeListener(listener, event);
             }
          }
       }

       private ResolvableType resolveDefaultEventType(ApplicationEvent event) {
          return ResolvableType.forInstance(event);
       }

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

       @SuppressWarnings({"unchecked", "rawtypes"})
       private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
          try {
             listener.onApplicationEvent(event);
          }
          catch (ClassCastException ex) {
             String msg = ex.getMessage();
             if (msg == null || matchesClassCastMessage(msg, event.getClass())) {
                // Possibly a lambda-defined listener which we could not resolve the generic event type for
                // -> let's suppress the exception and just log a debug message.
                Log logger = LogFactory.getLog(getClass());
                if (logger.isDebugEnabled()) {
                   logger.debug("Non-matching event type for listener: " + listener, ex);
                }
             }
             else {
                throw ex;
             }
          }
       }

       private boolean matchesClassCastMessage(String classCastMessage, Class<?> eventClass) {
          // On Java 8, the message starts with the class name: "java.lang.String cannot be cast..."
          if (classCastMessage.startsWith(eventClass.getName())) {
             return true;
          }
          // On Java 11, the message starts with "class ..." a.k.a. Class.toString()
          if (classCastMessage.startsWith(eventClass.toString())) {
             return true;
          }
          // On Java 9, the message used to contain the module name: "java.base/java.lang.String cannot be cast..."
          int moduleSeparatorIndex = classCastMessage.indexOf('/');
          if (moduleSeparatorIndex != -1 && classCastMessage.startsWith(eventClass.getName(), moduleSeparatorIndex + 1)) {
             return true;
          }
          // Assuming an unrelated class cast failure...
          return false;
       }

    }

4、ApplicationEventPublisher

    @FunctionalInterface
    public interface ApplicationEventPublisher {
       default void publishEvent(ApplicationEvent event) {
          publishEvent((Object) event);
       }
       void publishEvent(Object event);
    }

ApplicationEventPublisher很简单只有两个发布事件的方法,AbstractApplicationContext是它的默认实现类,下面是具体实现。

    @Override
    public void publishEvent(ApplicationEvent event) {
       publishEvent(event, null);
    }
    @Override
    public void publishEvent(Object event) {
       publishEvent(event, null);
    }
    protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
       Assert.notNull(event, "Event must not be null");

       // 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);
          }
       }
    }

可以看到事件的发布依赖于前面提到的bean name是applicationEventMulticaster的SimpleApplicationEventMulticaster。

5、事件监听器的注册

在AbstractApplicationContext.prepareBeanFactory()方法执行的时候会添加一个BeanPostProcessor–>ApplicationListenerDetector,它实现了MergedBeanDefinitionPostProcessor接口,在postProcessAfterInitialization()方法执行的时候检测已注册到容器中的bean如果是ApplicationListener,就通过applicationContext添加。

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) {
       if (bean instanceof ApplicationListener) {
          // potentially not detected as a listener by getBeanNamesForType retrieval
          Boolean flag = this.singletonNames.get(beanName);
          if (Boolean.TRUE.equals(flag)) {
             // singleton bean (top-level or inner): register on the fly
             this.applicationContext.addApplicationListener((ApplicationListener<?>) bean);
          }
          else if (Boolean.FALSE.equals(flag)) {
             if (logger.isWarnEnabled() && !this.applicationContext.containsBean(beanName)) {
                // inner bean with other scope - can't reliably process events
                logger.warn("Inner bean '" + beanName + "' implements ApplicationListener interface " +
                      "but is not reachable for event multicasting by its containing ApplicationContext " +
                      "because it does not have singleton scope. Only top-level listener beans are allowed " +
                      "to be of non-singleton scope.");
             }
             this.singletonNames.remove(beanName);
          }
       }
       return bean;
    }

自Spring 4.2起,事件机制明显改善,提供了一个基于注解的模型以及发布任意事件的能力。基于注解的Spring启动形式会在AnnotationConfigBeanDefinitionParser.parse(Element, ParserContext)方法内部注册一个bean name为org.springframework.context.event.internalEventListenerProcessor的EventListenerMethodProcessor。这个bean实现了SmartInitializingSingleton接口,在接口方法afterSingletonsInstantiated()调用时会检测容器内所有的bean,如果发现有@EventListener方法将使用容器内EventListenerFactory将此@EventListener方法包装一个ApplicationListener放入容器中。

    @Override
    public void afterSingletonsInstantiated() {
       ConfigurableListableBeanFactory beanFactory = this.beanFactory;
       Assert.state(this.beanFactory != null, "No ConfigurableListableBeanFactory set");
       String[] beanNames = beanFactory.getBeanNamesForType(Object.class);
       for (String beanName : beanNames) {
          //如果一个类被代理,那么处理它的代理类即可
          if (!ScopedProxyUtils.isScopedTarget(beanName)) {
             Class<?> type = null;
             try {
                //代理模式需要取得目标类,然后取得目标类上@EventListener方法
                type = AutoProxyUtils.determineTargetClass(beanFactory, beanName);
             }
             catch (Throwable ex) {
                // An unresolvable bean type, probably from a lazy bean - let's ignore it.
                if (logger.isDebugEnabled()) {
                   logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex);
                }
             }
             if (type != null) {
                if (ScopedObject.class.isAssignableFrom(type)) {
                   try {
                      Class<?> targetClass = AutoProxyUtils.determineTargetClass(
                            beanFactory, ScopedProxyUtils.getTargetBeanName(beanName));
                      if (targetClass != null) {
                         type = targetClass;
                      }
                   }
                   catch (Throwable ex) {
                      // An invalid scoped proxy arrangement - let's ignore it.
                      if (logger.isDebugEnabled()) {
                         logger.debug("Could not resolve target bean for scoped proxy '" + beanName + "'", ex);
                      }
                   }
                }
                try {
                   //解析@EventListener方法的类,注册事件监听器
                   processBean(beanName, type);
                }
                catch (Throwable ex) {
                   throw new BeanInitializationException("Failed to process @EventListener " +
                         "annotation on bean with name '" + beanName + "'", ex);
                }
             }
          }
       }
    }

    private void processBean(final String beanName, final Class<?> targetType) {
       //缓存防止重复注册
       //只解析用户定义的类
       if (!this.nonAnnotatedClasses.contains(targetType) && !isSpringContainerClass(targetType)) {
          Map<Method, EventListener> annotatedMethods = null;
          try {
             //所有@EventListener方法
             annotatedMethods = MethodIntrospector.selectMethods(targetType,
                   (MethodIntrospector.MetadataLookup<EventListener>) method ->
                         AnnotatedElementUtils.findMergedAnnotation(method, EventListener.class));
          }
          catch (Throwable ex) {
             // An unresolvable type in a method signature, probably from a lazy bean - let's ignore it.
             if (logger.isDebugEnabled()) {
                logger.debug("Could not resolve methods for bean with name '" + beanName + "'", ex);
             }
          }
          if (CollectionUtils.isEmpty(annotatedMethods)) {
             this.nonAnnotatedClasses.add(targetType);
             if (logger.isTraceEnabled()) {
                logger.trace("No @EventListener annotations found on bean class: " + targetType.getName());
             }
          }
          else {
             // Non-empty set of methods
             ConfigurableApplicationContext context = this.applicationContext;
             Assert.state(context != null, "No ApplicationContext set");
             List<EventListenerFactory> factories = this.eventListenerFactories;
             Assert.state(factories != null, "EventListenerFactory List not initialized");
             for (Method method : annotatedMethods.keySet()) {
                for (EventListenerFactory factory : factories) {
                   //使用Spring容器中第一个能解析此@EventListener方法的EventListenerFactory
                   if (factory.supportsMethod(method)) {
                      Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName));
                      //使用EventListenerFactory将@EventListener解析为一个ApplicationListener后加入容器
                      ApplicationListener<?> applicationListener =
                            factory.createApplicationListener(beanName, targetType, methodToUse);
                      if (applicationListener instanceof ApplicationListenerMethodAdapter) {
                         ((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
                      }
                      context.addApplicationListener(applicationListener);
                      break;
                   }
                }
             }
             if (logger.isDebugEnabled()) {
                logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
                      beanName + "': " + annotatedMethods);
             }
          }
       }
    }

上面的代码核心部分就是processBean()方法,内部使用容器第一个可能解析当前@EventListener的EventListenerFactory创建一个ApplicationListener用来处理@EventListener指定的类型事件。下面就是看看Spring容器都有哪些EventListenerFactory,和EventListenerFactory是如何创建ApplicationListener的?

EventListenerFactory的注册和EventListenerMethodProcessor一样在AnnotationConfigBeanDefinitionParser.parse(Element, ParserContext)方法内,bean name为org.springframework.context.event.internalEventListenerFactory,是DefaultEventListenerFactory。

    public class DefaultEventListenerFactory implements EventListenerFactory, Ordered {
       private int order = LOWEST_PRECEDENCE;
       public void setOrder(int order) {
          this.order = order;
       }
       @Override
       public int getOrder() {
          return this.order;
       }
       public boolean supportsMethod(Method method) {
          return true;
       }
       @Override
       public ApplicationListener<?> createApplicationListener(String beanName, Class<?> type, Method method) {
          return new ApplicationListenerMethodAdapter(beanName, type, method);
       }
    }

DefaultEventListenerFactory支持所有的方法成为ApplicationListener,下面看createApplicationListener()方法返回的ApplicationListenerMethodAdapter是如何处理@EventListener指定的事件的。

    public ApplicationListenerMethodAdapter(String beanName, Class<?> targetClass, Method method) {
       this.beanName = beanName;
       this.method = BridgeMethodResolver.findBridgedMethod(method);
       this.targetMethod = (!Proxy.isProxyClass(targetClass) ?
             AopUtils.getMostSpecificMethod(method, targetClass) : this.method);
       this.methodKey = new AnnotatedElementKey(this.targetMethod, targetClass);

       EventListener ann = AnnotatedElementUtils.findMergedAnnotation(this.targetMethod, EventListener.class);
       this.declaredEventTypes = resolveDeclaredEventTypes(method, ann);
       this.condition = (ann != null ? ann.condition() : null);
       this.order = resolveOrder(method);
    }

declaredEventTypes是指事件监听器能够支持的事件类型。

    private List<ResolvableType> resolveDeclaredEventTypes(Method method, @Nullable EventListener ann) {
       //注意方法参数最多只能有一个
       int count = method.getParameterCount();
       if (count > 1) {
          throw new IllegalStateException(
                "Maximum one parameter is allowed for event listener method: " + method);
       }

       if (ann != null) {
          Class<?>[] classes = ann.classes();
          if (classes.length > 0) {
             List<ResolvableType> types = new ArrayList<>(classes.length);
             for (Class<?> eventType : classes) {
                types.add(ResolvableType.forClass(eventType));
             }
             return types;
          }
       }

       if (count == 0) {
          throw new IllegalStateException(
                "Event parameter is mandatory for event listener method: " + method);
       }
       return Collections.singletonList(ResolvableType.forMethodParameter(method, 0));
    }

    ​​​​​​​@Override
    public boolean supportsEventType(ResolvableType eventType) {
       for (ResolvableType declaredEventType : this.declaredEventTypes) {
          if (declaredEventType.isAssignableFrom(eventType)) {
             return true;
          }
          if (PayloadApplicationEvent.class.isAssignableFrom(eventType.toClass())) {
             ResolvableType payloadType = eventType.as(PayloadApplicationEvent.class).getGeneric();
             if (declaredEventType.isAssignableFrom(payloadType)) {
                return true;
             }
          }
       }
       return eventType.hasUnresolvableGenerics();
    }

处理事件的大致逻辑为,事件作为@EventListener方法的参数(如果是PayloadApplicationEvent则使用payload作为参数),然后执行该方法,如果有返回值则继续讲返回值作为事件发布。注意返回值是数组或集合会将集合内所有的元素单独发布。

    @Override
    public void onApplicationEvent(ApplicationEvent event) {
       processEvent(event);
    }
    public void processEvent(ApplicationEvent event) {
       Object[] args = resolveArguments(event);
       if (shouldHandle(event, args)) {
          Object result = doInvoke(args);
          if (result != null) {
             handleResult(result);
          }
          else {
             logger.trace("No result object given - no result to handle");
          }
       }
    }
    private boolean shouldHandle(ApplicationEvent event, @Nullable Object[] args) {
       if (args == null) {
          return false;
       }
       String condition = getCondition();
       if (StringUtils.hasText(condition)) {
          Assert.notNull(this.evaluator, "EventExpressionEvaluator must not be null");
          return this.evaluator.condition(
                condition, event, this.targetMethod, this.methodKey, args, this.applicationContext);
       }
       return true;
    }
    protected void handleResult(Object result) {
       if (result.getClass().isArray()) {
          Object[] events = ObjectUtils.toObjectArray(result);
          for (Object event : events) {
             publishEvent(event);
          }
       }
       else if (result instanceof Collection<?>) {
          Collection<?> events = (Collection<?>) result;
          for (Object event : events) {
             publishEvent(event);
          }
       }
       else {
          publishEvent(result);
       }
    }

由shouldHandle()方法可知,@EventListener的condition属性决定着事件可否被处理。condition的值为SpEL,代表true才可被处理。下表列出了项目可用的上下文,这样您就可以使用这些条件事件处理:

名称 所在位置 描述 例子
Event rootobject 实际的ApplicationEvent. #root.event
Argumentsarray rootobject 执行目标方法的参数数组 #root.args[0]
Argumentname evaluationcontext 任何方法参数的名称。出于某种原因,如果名字不是可用的(例如,因为没有调试信息),参数名称也可以使用这种形式 #a<#arg>,#arg代表参数索引(从0开始)。 #blEvent or #a0 (也可使用 #p0 或 #p<#arg>符号作为一个别名)
    @EventListener(condition = "#blEvent.content == 'my-event'")
    public void processBlackListEvent(BlackListEvent blEvent) {
        // notify appropriate parties via notificationAddress...
    }

来源:[]()

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