SpringBoot源码分析(二)

SpringBoot源码分析(二)

refreshContext 方法

private void refreshContext(ConfigurableApplicationContext context) {
    //调用refresh方法
    refresh(context);
    if (this.registerShutdownHook) {
        try {
            //添加一个shutdown钩子
            context.registerShutdownHook();
        }
        catch (AccessControlException ex) {
            // Not allowed in some environments.
        }
    }
}

protected void refresh(ApplicationContext applicationContext) {
    Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
    //调用applicationContext的refresh方法，这里的applicationContext是AnnotationConfigServletWebServerApplicationContext对象
    ((AbstractApplicationContext) applicationContext).refresh();
}

AnnotationConfigServletWebServerApplicationContext 的 refresh 方法

@Override
public final void refresh() throws BeansException, IllegalStateException {
    try {
        //调用父类的refresh方法，父类是AbstractApplicationContext
        super.refresh();
    }
    catch (RuntimeException ex) {
        stopAndReleaseWebServer();
        throw ex;
    }
}

AbstractApplicationContext 的 refresh 方法

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

        // Tell the subclass to refresh the internal bean factory.
        //获取需要刷新的BeanFactory
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

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

        try {
            // Allows post-processing of the bean factory in context subclasses.
            //注册一些跟web有关的
            postProcessBeanFactory(beanFactory);

            // Invoke factory processors registered as beans in the context.
            //调用后置处理器BeanFactoryPostProcessor
            invokeBeanFactoryPostProcessors(beanFactory);

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

            // Initialize message source for this context.
            //初始化主题、国际化这些，跳过
            initMessageSource();

            // Initialize event multicaster for this context.
            //初始化事件广播器
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses.
            //初始化其它特殊的bean，这里创建了tomcat对象
            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();
        }
    }
}

prepareRefresh 方法

protected void prepareRefresh() {
    //记录开始时间
    this.startupDate = System.currentTimeMillis();
    //设置标志位
    this.closed.set(false);
    this.active.set(true);

    if (logger.isDebugEnabled()) {
        if (logger.isTraceEnabled()) {
            logger.trace("Refreshing " + this);
        }
        else {
            logger.debug("Refreshing " + getDisplayName());
        }
    }

    // Initialize any placeholder property sources in the context environment
    //初始化资源属性
    initPropertySources();

    // Validate that all properties marked as required are resolvable
    // see ConfigurablePropertyResolver#setRequiredProperties
    //验证环境
    getEnvironment().validateRequiredProperties();

    // Allow for the collection of early ApplicationEvents,
    // to be published once the multicaster is available...
    //初始化earlyApplicationEvents
    this.earlyApplicationEvents = new LinkedHashSet<>();
}

obtainFreshBeanFactory 方法

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
    //从下面可以看出，这个方法设置了一个序列化id，如果已经处于刷新状态抛出异常
    refreshBeanFactory();
    //直接返回BeanFactory对象，这个对象在当前对象构造方法中初始化
    return getBeanFactory();
}

protected final void refreshBeanFactory() throws IllegalStateException {
    if (!this.refreshed.compareAndSet(false, true)) {
        throw new IllegalStateException(
            "GenericApplicationContext does not support multiple refresh attempts: just call 'refresh' once");
    }
    this.beanFactory.setSerializationId(getId());
}

public final ConfigurableListableBeanFactory getBeanFactory() {
    return this.beanFactory;
}

prepareBeanFactory 方法

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
    // Tell the internal bean factory to use the context's class loader etc.
    //设置BeanFactory的类加载器
    beanFactory.setBeanClassLoader(getClassLoader());
    beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
    beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

    // Configure the bean factory with context callbacks.
    //这里添加了一个BeanPostProcessor--ApplicationContextAwareProcessor
    beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
    beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
    beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
    beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
    beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
    beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
    beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

    // BeanFactory interface not registered as resolvable type in a plain factory.
    // MessageSource registered (and found for autowiring) as a bean.
    beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
    beanFactory.registerResolvableDependency(ResourceLoader.class, this);
    beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
    beanFactory.registerResolvableDependency(ApplicationContext.class, this);

    // Register early post-processor for detecting inner beans as ApplicationListeners.
    beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

    // Detect a LoadTimeWeaver and prepare for weaving, if found.
    if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
        beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
        // Set a temporary ClassLoader for type matching.
        beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
    }

    // Register default environment beans.
    if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
        beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
    }
    if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
        beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
    }
    if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
        beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
    }
}

postProcessBeanFactory 方法

//这段代码位置：org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext#postProcessBeanFactory
protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
    //调用父类方法
    super.postProcessBeanFactory(beanFactory);
    //扫描包，默认basePackages是null
    if (this.basePackages != null && this.basePackages.length > 0) {
        this.scanner.scan(this.basePackages);
    }
    //注册注解类，默认annotatedClasses是空的
    if (!this.annotatedClasses.isEmpty()) {
        this.reader.register(ClassUtils.toClassArray(this.annotatedClasses));
    }
}
//这段代码位置：org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext#postProcessBeanFactory
protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
    //这里添加了一个BeanPostProcessor--WebApplicationContextServletContextAwareProcessor
    beanFactory.addBeanPostProcessor(
        new WebApplicationContextServletContextAwareProcessor(this));
    //忽略接口
    beanFactory.ignoreDependencyInterface(ServletContextAware.class);
    //注册web的scope。如quest、session等
    registerWebApplicationScopes();
}

invokeBeanFactoryPostProcessors 方法

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
    //这行代码很重要，跟进去
    PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

    // Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
    // (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
    if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
        beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
        beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
    }
}

//这行代码位置：org.springframework.context.support.PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors(org.springframework.beans.factory.config.ConfigurableListableBeanFactory, java.util.List<org.springframework.beans.factory.config.BeanFactoryPostProcessor>)
public static void invokeBeanFactoryPostProcessors(
    ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
	
    // Invoke BeanDefinitionRegistryPostProcessors first, if any.
    //这里保存已经执行了的BeanFactoryPostProcessor的beanName，在执行BeanFactoryPostProcessor的回调方法过程中，可能添加新的BeanFactoryPostProcessor对象。这里用Set可以达到去重的目的
    Set<String> processedBeans = new HashSet<>();
	//这里的beanFactory是org.springframework.beans.factory.support.DefaultListableBeanFactory对象
    if (beanFactory instanceof BeanDefinitionRegistry) {
        BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
        List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
        List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();

        for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
            if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                BeanDefinitionRegistryPostProcessor registryProcessor =
                    (BeanDefinitionRegistryPostProcessor) postProcessor;
                //实现了BeanDefinitionRegistryPostProcessor接口的BeanFactoryPostProcessor会在这里被调用
                registryProcessor.postProcessBeanDefinitionRegistry(registry);
                registryProcessors.add(registryProcessor);
            }
            else {
                regularPostProcessors.add(postProcessor);
            }
        }

        // Do not initialize FactoryBeans here: We need to leave all regular beans
        // uninitialized to let the bean factory post-processors apply to them!
        // Separate between BeanDefinitionRegistryPostProcessors that implement
        // PriorityOrdered, Ordered, and the rest.
        List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();
		//注意，以下为了防止重复执行，都使用了!processedBeans.contains(ppName)条件判断，所有执行过的BeanDefinitionRegistryPostProcessors 都添加在processedBeans里面
        // First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
        //先执行实现了PriorityOrdered接口的BeanDefinitionRegistryPostProcessors
        String[] postProcessorNames =
            beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
        for (String ppName : postProcessorNames) {
            if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                processedBeans.add(ppName);
            }
        }
        sortPostProcessors(currentRegistryProcessors, beanFactory);
        registryProcessors.addAll(currentRegistryProcessors);
        invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
        currentRegistryProcessors.clear();

        // Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
        //再执行实现了Ordered接口的BeanDefinitionRegistryPostProcessors
        postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
        for (String ppName : postProcessorNames) {
            if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
                currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                processedBeans.add(ppName);
            }
        }
        sortPostProcessors(currentRegistryProcessors, beanFactory);
        registryProcessors.addAll(currentRegistryProcessors);
        invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
        currentRegistryProcessors.clear();

        // Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
        //最后执行所有的BeanDefinitionRegistryPostProcessors
        boolean reiterate = true;
        while (reiterate) {
            reiterate = false;
            postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
            for (String ppName : postProcessorNames) {
                if (!processedBeans.contains(ppName)) {
                    currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                    reiterate = true;
                }
            }
            sortPostProcessors(currentRegistryProcessors, beanFactory);
            registryProcessors.addAll(currentRegistryProcessors);
            invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
            currentRegistryProcessors.clear();
        }

        // Now, invoke the postProcessBeanFactory callback of all processors handled so far.
        //这里调用BeanFactoryPostProcessor的回调方法
        invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
        invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
    }

    else {
        // Invoke factory processors registered with the context instance.
        invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
    }

    // Do not initialize FactoryBeans here: We need to leave all regular beans
    // uninitialized to let the bean factory post-processors apply to them!
    //获取所有的 BeanFactoryPostProcessor
    String[] postProcessorNames =
        beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

    // Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
    // Ordered, and the rest.
    List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
    List<String> orderedPostProcessorNames = new ArrayList<>();
    List<String> nonOrderedPostProcessorNames = new ArrayList<>();
    for (String ppName : postProcessorNames) {
        if (processedBeans.contains(ppName)) {
            //如果是已经执行了的BeanDefinitionRegistryPostProcessors直接跳过
            // skip - already processed in first phase above
        }
        //这里根据实现优先级的接口来区别对待
        else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
            priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
        }
        else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
            orderedPostProcessorNames.add(ppName);
        }
        else {
            nonOrderedPostProcessorNames.add(ppName);
        }
    }

    // First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
    //先执行实现了PriorityOrdered接口的BeanFactoryPostProcessor
    sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
    invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

    // Next, invoke the BeanFactoryPostProcessors that implement Ordered.
    //再执行实现了Ordered接口的BeanFactoryPostProcessor
    List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>();
    for (String postProcessorName : orderedPostProcessorNames) {
        orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
    }
    sortPostProcessors(orderedPostProcessors, beanFactory);
    invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

    // Finally, invoke all other BeanFactoryPostProcessors.
    //最后执行剩下的BeanFactoryPostProcessors
    List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
    for (String postProcessorName : nonOrderedPostProcessorNames) {
        nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
    }
    invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

    // Clear cached merged bean definitions since the post-processors might have
    // modified the original metadata, e.g. replacing placeholders in values...
    beanFactory.clearMetadataCache();
}

//执行BeanFactoryPostProcessor的方法
private static void invokeBeanFactoryPostProcessors(
    Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {
	//遍历执行BeanFactoryPostProcessor
    for (BeanFactoryPostProcessor postProcessor : postProcessors) {
        postProcessor.postProcessBeanFactory(beanFactory);
    }
}

这里很重要，这个方法里面会调用BeanFactoryPostProcessor，这步执行完以后所有的BeanDefinition都添加到了BeanFactory的beanDefinitionMap中

registerBeanPostProcessors 方法

protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
    //跟进去
    PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
}


//这段代码位置在：org.springframework.context.support.PostProcessorRegistrationDelegate#registerBeanPostProcessors(org.springframework.beans.factory.config.ConfigurableListableBeanFactory, org.springframework.context.support.AbstractApplicationContext)
public static void registerBeanPostProcessors(
    ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
	//获取所有的BeanPostProcessor
    String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

    // Register BeanPostProcessorChecker that logs an info message when
    // a bean is created during BeanPostProcessor instantiation, i.e. when
    // a bean is not eligible for getting processed by all BeanPostProcessors.
    int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
    //添加一个BeanPostProcessor--BeanPostProcessorChecker
    beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

    // Separate between BeanPostProcessors that implement PriorityOrdered,
    // Ordered, and the rest.
    //根据实现的排序接口区别对待
    List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
    List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
    List<String> orderedPostProcessorNames = new ArrayList<>();
    List<String> nonOrderedPostProcessorNames = new ArrayList<>();
    for (String ppName : postProcessorNames) {
        if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            priorityOrderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
            orderedPostProcessorNames.add(ppName);
        }
        else {
            nonOrderedPostProcessorNames.add(ppName);
        }
    }

    // First, register the BeanPostProcessors that implement PriorityOrdered.
    //先添加实现了PriorityOrdered接口的BeanPostProcessor
    sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

    // Next, register the BeanPostProcessors that implement Ordered.
    //再添加实现了Ordered接口的BeanPostProcessor
    List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
    for (String ppName : orderedPostProcessorNames) {
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        orderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    sortPostProcessors(orderedPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, orderedPostProcessors);

    // Now, register all regular BeanPostProcessors.
    //添加普通的BeanPostProcessors
    List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
    for (String ppName : nonOrderedPostProcessorNames) {
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        nonOrderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

    // Finally, re-register all internal BeanPostProcessors.
    //最后重新再添加实现了MergedBeanDefinitionPostProcessor接口的BeanPostProcessors
    sortPostProcessors(internalPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, internalPostProcessors);

    // Re-register post-processor for detecting inner beans as ApplicationListeners,
    // moving it to the end of the processor chain (for picking up proxies etc).
    //添加一个BeanPostProcessor--ApplicationListenerDetector
    beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}


//注册BeanPostProcessor
private static void registerBeanPostProcessors(
    ConfigurableListableBeanFactory beanFactory, List<BeanPostProcessor> postProcessors) {
	
    for (BeanPostProcessor postProcessor : postProcessors) {
        beanFactory.addBeanPostProcessor(postProcessor);
    }
}

这里主要是往beanFactory中注册BeanPostProcessor

initApplicationEventMulticaster 方法

//public static final String APPLICATION_EVENT_MULTICASTER_BEAN_NAME = "applicationEventMulticaster";
protected void initApplicationEventMulticaster() {
    ConfigurableListableBeanFactory beanFactory = getBeanFactory();
    //如果beanFactory中已经包含applicationEventMulticaster对象，直接赋值给applicationEventMulticaster
    //如果不包含，创建一个新的SimpleApplicationEventMulticaster对象，赋值给applicationEventMulticaster并添加到beanFactory
    if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
        this.applicationEventMulticaster =
            beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
        if (logger.isTraceEnabled()) {
            logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
        }
    }
    else {
        this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
        beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
        if (logger.isTraceEnabled()) {
            logger.trace("No '" + APPLICATION_EVENT_MULTICASTER_BEAN_NAME + "' bean, using " +
                         "[" + this.applicationEventMulticaster.getClass().getSimpleName() + "]");
        }
    }
}

这个方法只是为了保证事件广播器存在