Spring筆記(8) - @EventListener註解探究 Spring的BeanFactoryPostProcessor探究

　　在上文中講了Spring的事件監聽機制，流程是：定義事件、監聽器，發佈事件，控制檯輸出監聽到的事件內容。

　　在上文的擴展中 使用 @EventListener 註解來自定義監聽器，監聽指定的事件，比如下面的案例：

@Component
public class UserManagerListener {
    //ApplicationEvent能監聽到所有的事件,如果爲EmailEvent.class則只能監聽到關於郵件的事件
    //EventListener有兩個參數（可以不寫，直接在方法參數裏面寫上要監聽的事件即可）：
    //    classes：表示哪一個事件類
    //    condition：當滿足什麼條件是會調用該方法
    @EventListener(classes = ApplicationEvent.class)
    public void listen(ApplicationEvent event){
        System.out.println("用戶管理功能監聽到的事件。。。。"+event);
    }
}

　　通過上面的案例發現代碼比實現 ApplicationListener 接口更簡潔，那這個註解爲什麼功能這麼強大呢，接下來我們來分析它的源碼：

/**
 * @author Stephane Nicoll
 * @author Sam Brannen
 * @since 4.2
 * @see EventListenerMethodProcessor
 */
@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface EventListener {
.......
}

　　通過源碼發現是通過使用 EventListenerMethodProcessor 處理器來解析該註解，將標註了 @EventListener 的方法進行解析, 獲取攔截方法，對攔截方法進行轉換，變成 ApplicationListener 然

後放入到 IOC 容器中，在publishEvent 時，通過 getApplicationEventMulticaster().multicastEvent(a

pplicationEvent, eventType)方法，獲取到 ApplicationListener 對象，通過反射調用方法。下面是 EventListenerMethodProcessor 處理器的類圖關係：

　　　　可以看到 EventListenerMethodProcessor 是實現了 BeanFactoryPostProcessor 和 SmartInitializingSingleton 這兩個接口。在前文 Spring的BeanFactoryPostProcessor探究 中講述了

 BeanFactoryPostProcessor 的執行時機，在 refresh 容器的時候, 調用 invokeBeanFactoryPostProce

ssors() 方法時, 會執行 BeanFactoryPostProcessor#postProcessBeanFactory() 方法設置一個默認的監聽器工廠 ：DefaultEventListenerFactory。

/**
 * Registers {@link EventListener} methods as individual {@link ApplicationListener} instances.
 * Implements {@link BeanFactoryPostProcessor} (as of 5.1) primarily for early retrieval,
 * avoiding AOP checks for this processor bean and its {@link EventListenerFactory} delegates.
 *
 * @author Stephane Nicoll
 * @author Juergen Hoeller
 * @since 4.2
 * @see EventListenerFactory
 * @see DefaultEventListenerFactory
 */
    public class EventListenerMethodProcessor
        implements SmartInitializingSingleton, ApplicationContextAware, BeanFactoryPostProcessor {
    ......
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        this.beanFactory = beanFactory;

        Map<String, EventListenerFactory> beans = beanFactory.getBeansOfType(EventListenerFactory.class, false, false);
        List<EventListenerFactory> factories = new ArrayList<>(beans.values());
        AnnotationAwareOrderComparator.sort(factories);
        //DefaultEventListenerFactory
        this.eventListenerFactories = factories;
    }
    .......
}

　　而SmartInitializingSingleton 這個接口只有一個方法 afterSingletonsInstantiated()，這個方法在全部單實例創建完成之後執行，接下來對這個方法進行深入探討，我們對 EventListenerMethodProcessor裏面的實現方法 afterSingletonsInstantiated 打一個斷點 debug 運行下：

　　　　從中我們可以看到，流程是： IOC容器創建對象 --> refresh() --> finishBeanFactoryInitialization(beanFactory)(初始化剩下的所有單實例bean） --> beanFactory.preInstantiateSingletons()(初始化剩下的所有單實例bean）--> smartSingleton.afterSingletonsInstantiated()

    @Override
    public void preInstantiateSingletons() throws BeansException {
        if (logger.isTraceEnabled()) {
            logger.trace("Pre-instantiating singletons in " + this);
        }

        // Iterate over a copy to allow for init methods which in turn register new bean definitions.
        // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
        //獲取所有的註冊bean名稱
        List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

        // Trigger initialization of all non-lazy singleton beans...
        //先創建所有的單實例bean
        for (String beanName : beanNames) {
            RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
                if (isFactoryBean(beanName)) {
                    Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                    if (bean instanceof FactoryBean) {
                        final FactoryBean<?> factory = (FactoryBean<?>) bean;
                        boolean isEagerInit;
                        if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                            isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
                                            ((SmartFactoryBean<?>) factory)::isEagerInit,
                                    getAccessControlContext());
                        }
                        else {
                            isEagerInit = (factory instanceof SmartFactoryBean &&
                                    ((SmartFactoryBean<?>) factory).isEagerInit());
                        }
                        if (isEagerInit) {
                            getBean(beanName);//創建bean實例
                        }
                    }
                }
                else {
                    getBean(beanName);//創建bean實例
                }
            }
        }

        // Trigger post-initialization callback for all applicable beans...
        //創建完成後進行遍歷，如果bean是SmartInitializingSingleton類型，執行afterSingletonsInstantiated方法
        for (String beanName : beanNames) {
            Object singletonInstance = getSingleton(beanName);
            if (singletonInstance instanceof SmartInitializingSingleton) {
                final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
                if (System.getSecurityManager() != null) {
                    AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                        smartSingleton.afterSingletonsInstantiated();
                        return null;
                    }, getAccessControlContext());
                }
                else {
                    smartSingleton.afterSingletonsInstantiated();
                }
            }
        }
    }

　　從上面可以看出其調用時機是，遍歷容器中註冊的 BeanDefinition， 調用所有 getBean() 方法創建實例之後, 纔會開始遍歷執行 afterSingletonsInstantiated() 方法。

　　接下來詳細解析下 EventListenerMethodProcessor 的 afterSingletonsInstantiated 方法，重點看 processBean 方法：

    @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) {
            if (!ScopedProxyUtils.isScopedTarget(beanName)) {
                Class<?> type = null;
                try {
                    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 {
                        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) {
        //不包含沒有註解的class，註解是EventListener的類型，是Spring容器的類型
        if (!this.nonAnnotatedClasses.contains(targetType) &&
                AnnotationUtils.isCandidateClass(targetType, EventListener.class) &&
                !isSpringContainerClass(targetType)) {

            Map<Method, EventListener> annotatedMethods = null;
            try {
                //獲取標註了 @EventListener 註解的監聽方法
                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);
                }
            }
            //監聽方法添加到沒有註解的集合
            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) {
                        // 判斷是否支持該方法  在DefaultEventListenerFactory中寫死的返回true
                        if (factory.supportsMethod(method)) {
                            //選擇方法  beanName 這裏是AddDataEventListener的beanName 默認是addDataEventListener
                            Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName));
                            //將監聽方法轉換爲ApplicationListener（ApplicationListenerMethodAdapter）對象
                            ApplicationListener<?> applicationListener =
                                    factory.createApplicationListener(beanName, targetType, methodToUse);
                            // 如果是ApplicationListenerMethodAdapter對象 就把context和evaluator傳進去
                            if (applicationListener instanceof ApplicationListenerMethodAdapter) {
                                ((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
                            }
                            //將創建的 ApplicationListener 加入到容器中
                            context.addApplicationListener(applicationListener);
                            break;
                        }
                    }
                }
                if (logger.isDebugEnabled()) {
                    logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
                            beanName + "': " + annotatedMethods);
                }
            }
        }
    }
    //class是在org.springframework包下，註解類型不是組件
    private static boolean isSpringContainerClass(Class<?> clazz) {
        return (clazz.getName().startsWith("org.springframework.") &&
                !AnnotatedElementUtils.isAnnotated(ClassUtils.getUserClass(clazz), Component.class));
    }

 　　後面就是添加listener到Context中：

　　　　1）如果有applicationEventMulticaster，添加到ApplicationContext.applicationEventMulticas

ter中；

　　　　2）如果沒有applicationEventMulticaster，添加到ApplicationContext.applicationListeners中；

　　最後是觸發事件監聽了 AbstractApplicationContext.publishEvent --> SimpleApplicationEventMulticaster.multicastEvent --> invokeListener --> doInvokeListener --> ApplicationListenerMethodAdapter.onApplicationEvent

@Override
public void onApplicationEvent(ApplicationEvent event) {
   processEvent(event);
}
ApplicationListenerMethodAdapter#processEvent
 public void processEvent(ApplicationEvent event) {
   Object[] args = resolveArguments(event);
   if (shouldHandle(event, args)) {
      // 反射執行真正的方法
      Object result = doInvoke(args);
      if (result != null) {
         handleResult(result);
      }
      else {
         logger.trace("No result object given - no result to handle");
      }
   }
}

 　　在ApplicationListenerMethodAdapter.doInvoke中會反射執行真正的方法：

 protected Object doInvoke(Object... args) {
   //獲取目標對象
   Object bean = getTargetBean();
   ReflectionUtils.makeAccessible(this.method);
   try {
     //反射執行監聽方法
      return this.method.invoke(bean, args);
   }
   catch (IllegalArgumentException ex) {
      assertTargetBean(this.method, bean, args);
      throw new IllegalStateException(getInvocationErrorMessage(bean, ex.getMessage(), args), ex);
   }
   catch (IllegalAccessException ex) {
      throw new IllegalStateException(getInvocationErrorMessage(bean, ex.getMessage(), args), ex);
   }
   catch (InvocationTargetException ex) {
      // Throw underlying exception
      Throwable targetException = ex.getTargetException();
      if (targetException instanceof RuntimeException) {
         throw (RuntimeException) targetException;
      }
      else {
         String msg = getInvocationErrorMessage(bean, "Failed to invoke event listener method", args);
         throw new UndeclaredThrowableException(targetException, msg);
      }
   }
}