springboot源码解析(四)：监听器

​ 在看springboot的源码过程中，发现内部使用了大量的监听器，下面来看下监听器的作用。

在springboot的监听器有如下两类：

# Run Listeners
#事件发布运行监听器，是springboot中配置的唯一一个应用运行监听器，作用是通过一个多路广播器，将springboot运行状态的变化，构建成事件，并广播给各个监听器
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener(),\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

​ 当程序开始运行的时候，可以看到启动了一个运行时监听器，并且创建了一个SpringApplicationRunListeners对象，该对象是一个封装工具类，封装了所有的启动监听器：代码如下

class SpringApplicationRunListeners {

    private final Log log;

    //启动类监听器
    private final List<SpringApplicationRunListener> listeners;

    SpringApplicationRunListeners(Log log, Collection<? extends SpringApplicationRunListener> listeners) {
        this.log = log;
        this.listeners = new ArrayList<>(listeners);
    }
//启动上下文事件监听
    void starting() {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.starting();
        }
    }
//environment准备完毕事件监听
    void environmentPrepared(ConfigurableEnvironment environment) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.environmentPrepared(environment);
        }
    }
//spring上下文准备完毕事件监听
    void contextPrepared(ConfigurableApplicationContext context) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.contextPrepared(context);
        }
    }
//上下文配置类加载事件监听
    void contextLoaded(ConfigurableApplicationContext context) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.contextLoaded(context);
        }
    }
//上下文刷新调用事件
    void started(ConfigurableApplicationContext context) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.started(context);
        }
    }
//上下文刷新完成，在run方法执行完之前调用该事件
    void running(ConfigurableApplicationContext context) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.running(context);
        }
    }
//在运行过程中失败调起的事件
    void failed(ConfigurableApplicationContext context, Throwable exception) {
        for (SpringApplicationRunListener listener : this.listeners) {
            callFailedListener(listener, context, exception);
        }
    }

    private void callFailedListener(SpringApplicationRunListener listener, ConfigurableApplicationContext context,
            Throwable exception) {
        try {
            listener.failed(context, exception);
        }
        catch (Throwable ex) {
            if (exception == null) {
                ReflectionUtils.rethrowRuntimeException(ex);
            }
            if (this.log.isDebugEnabled()) {
                this.log.error("Error handling failed", ex);
            }
            else {
                String message = ex.getMessage();
                message = (message != null) ? message : "no error message";
                this.log.warn("Error handling failed (" + message + ")");
            }
        }
    }

}

​ 在启动源码的流程中，我们知道不同的方法会在不同的时间点触发执行，然后广播出不同的事件，进入到EventPublishingRunListener类中

public EventPublishingRunListener(SpringApplication application, String[] args) {
        this.application = application;
        this.args = args;
        this.initialMulticaster = new SimpleApplicationEventMulticaster();
        for (ApplicationListener<?> listener : application.getListeners()) {
            this.initialMulticaster.addApplicationListener(listener);
        }
    }

​ 在当前类的构造方法中默认创建了SimpleApplicationEventMulticaster类，用来完成创建全局的事件发布功能

@Override
    public void starting() {
        this.initialMulticaster.multicastEvent(new ApplicationStartingEvent(this.application, this.args));
    }

    @Override
    public void environmentPrepared(ConfigurableEnvironment environment) {
        this.initialMulticaster
                .multicastEvent(new ApplicationEnvironmentPreparedEvent(this.application, this.args, environment));
    }

    @Override
    public void contextPrepared(ConfigurableApplicationContext context) {
        this.initialMulticaster
                .multicastEvent(new ApplicationContextInitializedEvent(this.application, this.args, context));
    }

    @Override
    public void contextLoaded(ConfigurableApplicationContext context) {
        for (ApplicationListener<?> listener : this.application.getListeners()) {
            if (listener instanceof ApplicationContextAware) {
                ((ApplicationContextAware) listener).setApplicationContext(context);
            }
            context.addApplicationListener(listener);
        }
        this.initialMulticaster.multicastEvent(new ApplicationPreparedEvent(this.application, this.args, context));
    }

    @Override
    public void started(ConfigurableApplicationContext context) {
        context.publishEvent(new ApplicationStartedEvent(this.application, this.args, context));
    }

    @Override
    public void running(ConfigurableApplicationContext context) {
        context.publishEvent(new ApplicationReadyEvent(this.application, this.args, context));
    }

    @Override
    public void failed(ConfigurableApplicationContext context, Throwable exception) {
        ApplicationFailedEvent event = new ApplicationFailedEvent(this.application, this.args, context, exception);
        if (context != null && context.isActive()) {
            // Listeners have been registered to the application context so we should
            // use it at this point if we can
            context.publishEvent(event);
        }
        else {
            // An inactive context may not have a multicaster so we use our multicaster to
            // call all of the context's listeners instead
            if (context instanceof AbstractApplicationContext) {
                for (ApplicationListener<?> listener : ((AbstractApplicationContext) context)
                        .getApplicationListeners()) {
                    this.initialMulticaster.addApplicationListener(listener);
                }
            }
            this.initialMulticaster.setErrorHandler(new LoggingErrorHandler());
            this.initialMulticaster.multicastEvent(event);
        }
    }

​ 在进行事件广播的时候，会进入如下方法：

@Override
    public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
        ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
        //获取线程池
        Executor executor = getTaskExecutor();
        //根据事件类型选取需要通知的监听器
        for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
            //如果不为空，则异步执行    
            if (executor != null) {
                executor.execute(() -> invokeListener(listener, event));
            }
            else {
                //如果为空，则同步执行
                invokeListener(listener, event);
            }
        }
    }

​ 在进行事件广播之前，需要将监听器进行过滤，符合类型的留下，不符合类型的过滤掉

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);

        // Quick check for existing entry on ConcurrentHashMap...
        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)))) {
            // Fully synchronized building and caching of a ListenerRetriever
            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 {
            // No ListenerRetriever caching -> no synchronization necessary
            return retrieveApplicationListeners(eventType, sourceType, null);
        }
    }

​ 实际处理判断逻辑的类：

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

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

        // Add programmatically registered listeners, including ones coming
        // from ApplicationListenerDetector (singleton beans and inner beans).
        for (ApplicationListener<?> listener : listeners) {
            if (supportsEvent(listener, eventType, sourceType)) {
                if (retriever != null) {
                    retriever.applicationListeners.add(listener);
                }
                allListeners.add(listener);
            }
        }

        // Add listeners by bean name, potentially overlapping with programmatically
        // registered listeners above - but here potentially with additional metadata.
        if (!listenerBeans.isEmpty()) {
            ConfigurableBeanFactory beanFactory = getBeanFactory();
            for (String listenerBeanName : listenerBeans) {
                try {
                    if (supportsEvent(beanFactory, listenerBeanName, eventType)) {
                        ApplicationListener<?> listener =
                                beanFactory.getBean(listenerBeanName, ApplicationListener.class);
                        if (!allListeners.contains(listener) && supportsEvent(listener, eventType, sourceType)) {
                            if (retriever != null) {
                                if (beanFactory.isSingleton(listenerBeanName)) {
                                    retriever.applicationListeners.add(listener);
                                }
                                else {
                                    retriever.applicationListenerBeans.add(listenerBeanName);
                                }
                            }
                            allListeners.add(listener);
                        }
                    }
                    else {
                        // Remove non-matching listeners that originally came from
                        // ApplicationListenerDetector, possibly ruled out by additional
                        // BeanDefinition metadata (e.g. factory method generics) above.
                        Object listener = beanFactory.getSingleton(listenerBeanName);
                        if (retriever != null) {
                            retriever.applicationListeners.remove(listener);
                        }
                        allListeners.remove(listener);
                    }
                }
                catch (NoSuchBeanDefinitionException ex) {
                    // Singleton listener instance (without backing bean definition) disappeared -
                    // probably in the middle of the destruction phase
                }
            }
        }

        AnnotationAwareOrderComparator.sort(allListeners);
        if (retriever != null && retriever.applicationListenerBeans.isEmpty()) {
            retriever.applicationListeners.clear();
            retriever.applicationListeners.addAll(allListeners);
        }
        return allListeners;
    }

在监听器实例化之前，检查是否符合固定的类型

        protected boolean supportsEvent(
            ApplicationListener<?> listener, ResolvableType eventType, @Nullable Class<?> sourceType) {
        //判断监听器是否是GenericApplicationListener子类，如不是返回一个GenericApplicationListenerAdapter
        GenericApplicationListener smartListener = (listener instanceof GenericApplicationListener ?
                (GenericApplicationListener) listener : new GenericApplicationListenerAdapter(listener));
        return (smartListener.supportsEventType(eventType) && smartListener.supportsSourceType(sourceType));
    }

public interface GenericApplicationListener extends ApplicationListener<ApplicationEvent>, Ordered {
    boolean supportsEventType(ResolvableType eventType);
    default boolean supportsSourceType(@Nullable Class<?> sourceType) {
        return true;
    }
    @Override
    default int getOrder() {
        return LOWEST_PRECEDENCE;
    }

}

此时可以看到GenericApplicationListener类，该类是spring提供的用于重写匹配监听器事件的接口，如果需要判断的监听器是GenericApplicationListener的子类，说明类型匹配方法已被重现，就调用子类的匹配方法，如果不是，提供一个默认的适配器来匹配GenericApplicationListenerAdapter

    public boolean supportsEventType(ResolvableType eventType) {
        if (this.delegate instanceof SmartApplicationListener) {
            Class<? extends ApplicationEvent> eventClass = (Class<? extends ApplicationEvent>) eventType.resolve();
            return (eventClass != null && ((SmartApplicationListener) this.delegate).supportsEventType(eventClass));
        }
        else {
            return (this.declaredEventType == null || this.declaredEventType.isAssignableFrom(eventType));
        }
    }

可以看到该类最终调用的是declaredEventType.isAssignableFrom(eventType)方法，也就是说，如果我们没有重写监听器匹配方法，那么发布的事件 event 会被监听 event以及监听event的父类的监听器监听到。