EventListenerMethodProcessor 是 BeanFactory 的一個後置處理器, 用來對 @EventListener 提供支援.

主要是標註了 @EventListener 的方法進行解析, 然後轉換為一個ApplicationListener.

1. 在 refresh 容器的時候, 呼叫invokeBeanFactoryPostProcessors() 方法時, 會執行BeanFactoryPostProcessor#postProcessBeanFactory() 方法.

　　此方法中, 他只是設定了一個預設的監聽器工廠 :DefaultEventListenerFactory

2. 他實現了SmartInitializingSingleton 介面, 會在

　　org.springframework.context.support.AbstractApplicationContext#refresh

　　\|/

　　org.springframework.context.support.AbstractApplicationContext#finishBeanFactoryInitialization

　　\|/

　　org.springframework.beans.factory.support.DefaultListableBeanFactory#preInstantiateSingletons 方法中呼叫其 afterSingletonsInstantiated() 方法.

　　其呼叫時機是, 遍歷容器中註冊的 BeanDefinition, 呼叫所有 getBean() 方法之後, 才會開始遍歷執行afterSingletonsInstantiated() 方法.

　　這裡最終會呼叫processBean() 方法.

　　此方法中, 就會去容器中查詢標註了@EventListener 註解的方法, 然後進行轉換.

/**
 * 這個後置處理器, 主要是處理 @EventListener 註解的.
 * 1. 解析 @EventListener , 獲取攔截方法
 * 2. 對攔截方法進行轉換, 變成 ApplicationListener
 * 3. 將轉換的 ApplicationListener, 放到spring容器中
 * 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 {

    protected final Log logger = LogFactory.getLog(getClass());

    @Nullable
    private ConfigurableApplicationContext applicationContext;

    @Nullable
    private ConfigurableListableBeanFactory beanFactory;

    @Nullable
    private List<EventListenerFactory> eventListenerFactories;

    private final EventExpressionEvaluator evaluator = new EventExpressionEvaluator();

    private final Set<Class<?>> nonAnnotatedClasses = Collections.newSetFromMap(new ConcurrentHashMap<>(64));


    @Override
    public void setApplicationContext(ApplicationContext applicationContext) {
        Assert.isTrue(applicationContext instanceof ConfigurableApplicationContext,
                "ApplicationContext does not implement ConfigurableApplicationContext");
        this.applicationContext = (ConfigurableApplicationContext) applicationContext;
    }

    @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;
    }


    @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) {
        if (!this.nonAnnotatedClasses.contains(targetType) &&
                !targetType.getName().startsWith("java") &&
                !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);
                }
            }

            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) {
                        if (factory.supportsMethod(method)) {
                            Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName));
                            //為監聽方法建立 ApplicationListener
                            ApplicationListener<?> applicationListener =
                                    factory.createApplicationListener(beanName, targetType, methodToUse);
                            if (applicationListener instanceof ApplicationListenerMethodAdapter) {
                                ((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
                            }
                            //將建立的 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);
                }
            }
        }
    }

    /**
     * Determine whether the given class is an {@code org.springframework}
     * bean class that is not annotated as a user or test {@link Component}...
     * which indicates that there is no {@link EventListener} to be found there.
     * @since 5.1
     */
    private static boolean isSpringContainerClass(Class<?> clazz) {
        return (clazz.getName().startsWith("org.springframework.") &&
                !AnnotatedElementUtils.isAnnotated(ClassUtils.getUserClass(clazz), Component.class));
    }

}