Spring的事件监听机制

这里写自定义目录标题

[1. 概述（重点）](#1. 概述（重点）)
[2. ApplicationEventMulticaster](#2. ApplicationEventMulticaster)
- [2.1 SimpleApplicationEventMulticaster](#2.1 SimpleApplicationEventMulticaster)
- [2.2 AbstractApplicationEventMulticaster](#2.2 AbstractApplicationEventMulticaster)
[3. ApplicationListener](#3. ApplicationListener)
- [3.1 注册监听器](#3.1 注册监听器)
- [3.2 自定义](#3.2 自定义)
[4. SpringApplicationRunListeners](#4. SpringApplicationRunListeners)

1. 概述（重点）

事件监听机制是观察者模式的一种，Spring的事件监听机制有几个重要的顶层接口，分别是：

ApplicationEventMulticaster 主要看 AbstractApplicationEventMulticaster
ApplicationListener
ApplicationEvent

它们三哥们的关系可以用下面的图来概括

AbstractApplicationEventMulticaster维护了源类型和事件类型作为KEY 跟监听器的关系，在下面 [2.1 SimpleApplicationEventMulticaster](#2.1 SimpleApplicationEventMulticaster)有代码的实现解读。

2. ApplicationEventMulticaster

AbstractApplicationEventMulticaster <<Interface>> ApplicationEventMulticaster <<Interface>> Aware <<Interface>> BeanClassLoaderAware <<Interface>> BeanFactoryAware SimpleApplicationEventMulticaster

目前就只有一个实现类SimpleApplicationEventMulticaster和一个抽象类AbstractApplicationEventMulticaster。

2.1 SimpleApplicationEventMulticaster

发布和广播事件，主要注意一下getApplicationListeners方法就行了，会根据事件类型和源类型找监听器。

java 复制代码

@Override
public void multicastEvent(ApplicationEvent event) {
    multicastEvent(event, resolveDefaultEventType(event));
}

@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
    ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
    Executor executor = getTaskExecutor();
    for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
        if (executor != null) {
            executor.execute(() -> invokeListener(listener, event));
        }
        else {
            invokeListener(listener, event);
        }
    }
}

2.2 AbstractApplicationEventMulticaster

getApplicationListeners方法就是根据Class<?>、ResolvableType两个对象作为一个缓存key，这个key与一堆监听器`ApplicationListener`做映射，也就是说通过Class<?>、ResolvableType两个对象可以找到监听器，在用法上可以从SimpleApplicationEventMuticaster的 multicastEvent 方法去看。

看看getApplicationListeners方法的代码实现

java 复制代码

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);
    CachedListenerRetriever newRetriever = null;

    // 这个cacheKey就是ApplicationEvent,ResolvableType两个对象。
    // 根据cacheKey找监听器，找不到就把cacheKey作为key，创建一个什么都么有的CachedListenerRetriever对象作为值缓存到retrieverCache中。
    // 这里有一些细节我没搞明白，putIfAbsent总是返回null。
    CachedListenerRetriever existingRetriever = this.retrieverCache.get(cacheKey);
    if (existingRetriever == null) {
        if (this.beanClassLoader == null ||
            (ClassUtils.isCacheSafe(event.getClass(), this.beanClassLoader) &&
            (sourceType == null || ClassUtils.isCacheSafe(sourceType, this.beanClassLoader)))) {
            newRetriever = new CachedListenerRetriever();
            existingRetriever = this.retrieverCache.putIfAbsent(cacheKey, newRetriever);
            if (existingRetriever != null) {
                newRetriever = null;  // no need to populate it in retrieveApplicationListeners
            }
        }
    }

    if (existingRetriever != null) {
        Collection<ApplicationListener<?>> result = existingRetriever.getApplicationListeners();
        if (result != null) {
            return result;
        }
    }

    // 假设上面cacheKey对应的值没有监听器，那这个方面就找 给定事件类型和源类型 的监听器。
    return retrieveApplicationListeners(eventType, sourceType, newRetriever);
}

// 从默认的 defaultRetriever 和 我们注入的bean 中找监听器。
private Collection<ApplicationListener<?>> retrieveApplicationListeners(
        ResolvableType eventType, @Nullable Class<?> sourceType, @Nullable CachedListenerRetriever retriever) {

    List<ApplicationListener<?>> allListeners = new ArrayList<>();
    Set<ApplicationListener<?>> filteredListeners = (retriever != null ? new LinkedHashSet<>() : null);
    Set<String> filteredListenerBeans = (retriever != null ? new LinkedHashSet<>() : null);

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

    // 从defaultRetriever中找监听器
    // 这里的supportsEvent有点看不懂，不过只要理解通过事件类型和源类型判断监听器是否符合 事件要求就得了。
    for (ApplicationListener<?> listener : listeners) {
        if (supportsEvent(listener, eventType, sourceType)) {
            if (retriever != null) {
                filteredListeners.add(listener);
            }
            allListeners.add(listener);
        }
    }

    // 根据bean名字去找监听器
    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)) {
                                filteredListeners.add(listener);
                            }
                            else {
                                filteredListenerBeans.add(listenerBeanName);
                            }
                        }
                        allListeners.add(listener);
                    }
                }
                else {
                    Object listener = beanFactory.getSingleton(listenerBeanName);
                    if (retriever != null) {
                        filteredListeners.remove(listener);
                    }
                    allListeners.remove(listener);
                }
                }
            catch (NoSuchBeanDefinitionException ex) {
            }
        }
    }

    // 把找到的监听器都放到CachedListenerRetriever中。
    AnnotationAwareOrderComparator.sort(allListeners);
    if (retriever != null) {
        if (filteredListenerBeans.isEmpty()) {
            retriever.applicationListeners = new LinkedHashSet<>(allListeners);
            retriever.applicationListenerBeans = filteredListenerBeans;
        }
        else {
            retriever.applicationListeners = filteredListeners;
            retriever.applicationListenerBeans = filteredListenerBeans;
        }
    }
    return allListeners;
}

3. ApplicationListener

主要是了解如何注册和自定义监听器就行了

3.1 注册监听器

通过META-INF/spring.factories注册监听器。

java 复制代码

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    this.resourceLoader = resourceLoader;
    Assert.notNull(primarySources, "PrimarySources must not be null");
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    this.bootstrapRegistryInitializers = new ArrayList<>(
        getSpringFactoriesInstances(BootstrapRegistryInitializer.class));
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    // 这里就是注册ApplicationListener的代码了，通过META-INF/spring.factories注册。
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    this.mainApplicationClass = deduceMainApplicationClass();
}

3.2 自定义

java 复制代码

public class MyApplicationPreparedListener implements ApplicationListener<ApplicationPreparedEvent> {
    @Override
    public void onApplicationEvent(ApplicationPreparedEvent event) {
        System.out.println("开始填充上下文。。。。");
    }
}

MATE-INF/spring.factories

txt 复制代码

org.springframework.context.ApplicationListener=sample.config.MyApplicationPreparedListener

4. SpringApplicationRunListeners

SpringApplicationRunListeners管理了多个EventPublishingRunListener，EventPublishingRunListener里面包含了事件监听器模型#1.概述（重点）中描述的部分。

java 复制代码

class SpringApplicationRunListeners {

    private final Log log;

    private final List<SpringApplicationRunListener> listeners;

    private final ApplicationStartup applicationStartup;

    ...
}

public class EventPublishingRunListener implements SpringApplicationRunListener, Ordered {

    private final SpringApplication application;

    private final String[] args;

    private final SimpleApplicationEventMulticaster initialMulticaster;

    ...
}

EventPublishingRunListener中定义了Spring整个启动过程中会触发的事件，下面对触发位置进行大概的描述，更具体的内容还有待研究。

starting
在SpringApplication#run，事件ApplicationStartingEvent
environmentPrepared
在SpringApplication#prepareEnvironment，事件ApplicationEnvironmentPreparedEvent
contextPrepared
在SpringApplication#prepareContext，事件ApplicationContextInitializedEvent
contextLoaded
在SpringApplication#prepareContext，事件ApplicationPreparedEvent
started
在SpringApplication#run，事件ApplicationStartedEvent
ready
在SpringApplication#run，事件ApplicationReadyEvent
failed
在SpringApplication#handleRunFailure，事件ApplicationFailedEvent

我们也可以利用上面的事件，自己创建一个监听器，然后放到spring.factories，比如现在注册一个ApplicationPreparedEvent的监听器。

java 复制代码

public class MyApplicationPreparedListener implements ApplicationListener<ApplicationPreparedEvent> {
    @Override
    public void onApplicationEvent(ApplicationPreparedEvent event) {
        System.out.println("开始填充上下文。。。。");
    }
}

MATE-INF/spring.factories

txt 复制代码

org.springframework.context.ApplicationListener=sample.config.MyApplicationPreparedListener