SpringBoot事件监听机制

一、监听器模式介绍

将一个监听器(listener)与特定的控件(如按钮等)绑定起来，当发生用户点击等事件(Event)时，调用监听器的处理方法，从而响应用户的动作,就叫做事件/监听器模式。

从上面的语句中，我们可以看出监听器模式有三个要素:

• 事件源

• 事件对象

• 监听器

二、自定义监听器事件

创建天气事件接口和下雨、下雪实现类

public interface WeatherEvent {
    String getWeather();
}

public class RainWeatherEvent implements WeatherEvent {
    @Override
    public String getWeather() {
        return "下雨";
    }
}

public class SnowWeatherEvent implements WeatherEvent {
    @Override
    public String getWeather() {
        return "下雪";
    }
}

创建天气监听实现接口和下雨、下雪监听实现类

public interface WeatherListener {
    void onWeatherEvent(WeatherEvent event);
}

public class RainListener implements WeatherListener {
    @Override
    public void onWeatherEvent(WeatherEvent event) {
        if (event instanceof RainWeatherEvent) {
            System.out.println(event.getWeather());
        }
    }
}

public class SnowListener implements WeatherListener {
    @Override
    public void onWeatherEvent(WeatherEvent event) {
        if (event instanceof SnowWeatherEvent) {
            System.out.println(event.getWeather());
        }
    }
}

创建事件广播器接口和天气事件广播实现类

public interface EventMulticaster {
    void multicastEvent(WeatherEvent event);
    void addListener(WeatherListener listener);
    void removeListener(WeatherListener listener);
}

public class WeatherEventMulticaster implements EventMulticaster {

    private List<WeatherListener> listenerList = new ArrayList<>();

    @Override
    public void multicastEvent(WeatherEvent event) {
        System.out.println("==========开始事件广播==========");
        listenerList.forEach(i -> i.onWeatherEvent(event));
        System.out.println("==========结束事件广播==========");
    }

    @Override
    public void addListener(WeatherListener listener) {
        listenerList.add(listener);
    }

    @Override
    public void removeListener(WeatherListener listener) {
        listenerList.remove(listener);
    }
}

创建启动测试类

public class Start {

    public static void main(String[] args) {
        // 创建广播对象
        EventMulticaster eventMulticaster = new WeatherEventMulticaster();
        // 创建下雨事件监听对象
        RainListener rainListener = new RainListener();
        // 创建下雪事件监听对象
        SnowListener snowListener = new SnowListener();
        
        // 添加下雨、下雪监听事件对象
        eventMulticaster.addListener(rainListener);
        eventMulticaster.addListener(snowListener);
        
        // 广播下雨事件
        eventMulticaster.multicastEvent(new RainWeatherEvent());
        // 广播下雪事件
        eventMulticaster.multicastEvent(new SnowWeatherEvent());
        
        // 移除下雨监听事件对象
        eventMulticaster.removeListener(rainListener);

        // 广播下雨事件
        // 广播下雪事件
        eventMulticaster.multicastEvent(new RainWeatherEvent());
        eventMulticaster.multicastEvent(new SnowWeatherEvent());
    }
}

启动项目，查看控制台输出：

==========开始事件广播==========
下雨
==========结束事件广播==========
==========开始事件广播==========
下雪
==========结束事件广播==========
==========开始事件广播==========
==========结束事件广播==========
==========开始事件广播==========
下雪
==========结束事件广播==========

可以看到当下雨监听器被移除之后，下雨事件就不能被监听处理了。

三、SpringBoot 监听器实现

3.1 监听器

@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
	void onApplicationEvent(E event);
}

• FunctionalInterface 是 jdk8 新增的，表示 ApplicationListener 接口只有一个方法，如果大于一个方法,将报错。
• 接口中有个泛型 <E extends ApplicationEvent>，继承自 ApplicationEvent。代表实现接口时，可以声明对哪些事件（如ApplicationEvent）感兴趣，在触发监听器的时候，对其他事件进行过滤。

3.2 系统广播器

public interface ApplicationEventMulticaster {
	// 添加事件监听器
	void addApplicationListener(ApplicationListener<?> listener);
	// 添加事件监听器
	void addApplicationListenerBean(String listenerBeanName);
	// 移除指定事件监听器
	void removeApplicationListener(ApplicationListener<?> listener);
	// 移除指定事件监听器
	void removeApplicationListenerBean(String listenerBeanName);
	// 移除所有事件监听器
	void removeAllListeners();
	// 事件广播
	void multicastEvent(ApplicationEvent event);
	// 事件广播
	void multicastEvent(ApplicationEvent event, @Nullable ResolvableType eventType);
}

3.3 系统事件

事件名	作用
ApplicationStartingEvent	框架启动事件
ApplicationEnvironmentPreparedEvent	环境准备完毕事件
ApplicationContextInitializedEvent	上下文初始化
ApplicationPreparedEvent	上下文创建完毕，但是Bean还没有加载完毕
ApplicationStartedEvent	bean 实例化完成，但是未调用 Runners接口
ApplicationReadyEvent	调用 Runners 接口完毕
ApplicationFailedEvent	启动失败事件

系统事件发生顺序:

3.4 监听器注册

在 SpringApplication 初始化的时候就进行了监听器注册

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
	......
	setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
	......
}

3.5 监听器事件触发机制

以 starting 事件为例

public ConfigurableApplicationContext run(String... args) {
	......
	SpringApplicationRunListeners listeners = getRunListeners(args);
	listeners.starting();
	......
}

进入 starting 方法,里面是遍历所有的 SpringApplicationRunListeners:

void starting() {
	for (SpringApplicationRunListener listener : this.listeners) {
		listener.starting();
	}
}

SpringApplicationRunListeners 接口定义如下,可以看到申明了多个事件：

public interface SpringApplicationRunListener {
    
	default void starting() {}

	default void environmentPrepared(ConfigurableEnvironment environment) {}

	default void contextPrepared(ConfigurableApplicationContext context) {}

	default void contextLoaded(ConfigurableApplicationContext context) {}

	default void started(ConfigurableApplicationContext context) {}

	default void running(ConfigurableApplicationContext context) {}

	default void failed(ConfigurableApplicationContext context, Throwable exception){}

}

看下默认的实现类 EventPublishingRunListener：

public class EventPublishingRunListener{
    ......
    private final SimpleApplicationEventMulticaster initialMulticaster;
    @Override
	public void starting() {
        // 调用广播器来发送事件
		this.initialMulticaster.multicastEvent(new ApplicationStartingEvent(this.application, this.args));
	}
    ......
}    

进入 multicastEvent 方法

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

resolveDefaultEventType 是对 event 的包装，不需要理会。

private ResolvableType resolveDefaultEventType(ApplicationEvent event) {
	return ResolvableType.forInstance(event);
}

回到 multicastEvent 方法

@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
	ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
    // 获得线程池
	Executor executor = getTaskExecutor();
    // getApplicationListeners --> 获得对当前event感兴趣的监听器列表
	for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
		if (executor != null) {
			executor.execute(() -> invokeListener(listener, event));
		}
		else {
			invokeListener(listener, event);
		}
	}
}

获得对当前 event 感兴趣的监听器列表:

protected Collection<ApplicationListener<?>> getApplicationListeners(
		ApplicationEvent event, ResolvableType eventType) {
	// 获取事件来源，这里的 source 就是 SpringApplication
	Object source = event.getSource();
    
	Class<?> sourceType = (source != null ? source.getClass() : null);
	ListenerCacheKey cacheKey = new ListenerCacheKey(eventType, sourceType);

	// 从缓存中获取结果
	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)))) {
		// 完全同步构建和缓存ListenerRetriever
		synchronized (this.retrievalMutex) {
			retriever = this.retrieverCache.get(cacheKey);
			if (retriever != null) {
				return retriever.getApplicationListeners();
			}
			retriever = new ListenerRetriever(true);
            // 检索感兴趣的监听器
			Collection<ApplicationListener<?>> listeners =
					retrieveApplicationListeners(eventType, sourceType, retriever);
			// 存放到缓存中
            this.retrieverCache.put(cacheKey, retriever);
			return listeners;
		}
	}
	else {
		// 无需 ListenerRetriever 缓存->无需同步
		return retrieveApplicationListeners(eventType, sourceType, null);
	}
}

检索感兴趣的监听器实现：

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

	List<ApplicationListener<?>> allListeners = new ArrayList<>();
	Set<ApplicationListener<?>> listeners;
	Set<String> listenerBeans;
     // 获取默认事件监听器
	synchronized (this.retrievalMutex) {     
        // 这些监听器定义在 spring.factories 文件中
		listeners = new LinkedHashSet<>(this.defaultRetriever.applicationListeners);
		listenerBeans = new LinkedHashSet<>(this.defaultRetriever.applicationListenerBeans);
	}

	for (ApplicationListener<?> listener : listeners) {
        // 只有对当前 eventType 感兴趣的 listerer 才会添加到监听器列表中
		if (supportsEvent(listener, eventType, sourceType)) {
			if (retriever != null) {
				retriever.applicationListeners.add(listener);
			}
			allListeners.add(listener);
		}
	}

	// 通过 bean 名称获取监听器列表
	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 {
					Object listener = beanFactory.getSingleton(listenerBeanName);
					if (retriever != null) {
						retriever.applicationListeners.remove(listener);
					}
					allListeners.remove(listener);
				}
			}
			catch (NoSuchBeanDefinitionException ex) {
			}
		}
	}
	// 对监听器列表进行排序
	AnnotationAwareOrderComparator.sort(allListeners);
	if (retriever != null && retriever.applicationListenerBeans.isEmpty()) {
		retriever.applicationListeners.clear();
		retriever.applicationListeners.addAll(allListeners);
	}
	return allListeners;
}

获取感兴趣的事件判断逻辑

protected boolean supportsEvent(
		ApplicationListener<?> listener, ResolvableType eventType, @Nullable Class<?> sourceType) {
	// 必须是 GenericApplicationListener 监听器类型,如果不是需要进行转换
	GenericApplicationListener smartListener = (listener instanceof GenericApplicationListener ?
			(GenericApplicationListener) listener : new GenericApplicationListenerAdapter(listener));
	return (smartListener.supportsEventType(eventType) && smartListener.supportsSourceType(sourceType));
}

构建为 GenericApplicationListenerAdapter

public GenericApplicationListenerAdapter(ApplicationListener<?> delegate) {
	Assert.notNull(delegate, "Delegate listener must not be null");
	this.delegate = (ApplicationListener<ApplicationEvent>) delegate;
	this.declaredEventType = resolveDeclaredEventType(this.delegate);
}

@Nullable
private static ResolvableType resolveDeclaredEventType(ApplicationListener<ApplicationEvent> listener) {
	ResolvableType declaredEventType = resolveDeclaredEventType(listener.getClass());
	if (declaredEventType == null || declaredEventType.isAssignableFrom(ApplicationEvent.class)) {
		Class<?> targetClass = AopUtils.getTargetClass(listener);
		if (targetClass != listener.getClass()) {
			declaredEventType = resolveDeclaredEventType(targetClass);
		}
	}
	return declaredEventType;
}

@Nullable
static ResolvableType resolveDeclaredEventType(Class<?> listenerType) {
	ResolvableType eventType = eventTypeCache.get(listenerType);
	if (eventType == null) {
		eventType = ResolvableType.forClass(listenerType).as(ApplicationListener.class).getGeneric();
		eventTypeCache.put(listenerType, eventType);
	}
	return (eventType != ResolvableType.NONE ? eventType : null);
}

进入 GenericApplicationListenerAdapter 类 supportsEventType 和 supportsSourceType 方法

@Override
@SuppressWarnings("unchecked")
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));
	}
}	

@Override
public boolean supportsSourceType(@Nullable Class<?> sourceType) {
	return !(this.delegate instanceof SmartApplicationListener) ||
			((SmartApplicationListener) this.delegate).supportsSourceType(sourceType);
}

我们回到 invokeListener 方法的实现上来：

protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
	ErrorHandler errorHandler = getErrorHandler();
	if (errorHandler != null) {
		try {
			doInvokeListener(listener, event);
		}
		catch (Throwable err) {
			errorHandler.handleError(err);
		}
	}
	else {
		doInvokeListener(listener, event);
	}
}

private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
	try {
        // 发送事件
		listener.onApplicationEvent(event);
	} catch (ClassCastException ex) {
		String msg = ex.getMessage();
		if (msg == null || matchesClassCastMessage(msg, event.getClass())) {
			Log logger = LogFactory.getLog(getClass());
			if (logger.isTraceEnabled()) {
				logger.trace("Non-matching event type for listener: " + listener, ex);
			}
		}
		else {
			throw ex;
		}
	}
}

四、自定义监听器

自定义监听器事件也有三种方式，我们依次来实现下。首先我们先创建三个监听器：

@Order(1)
public class FirstListner implements ApplicationListener<ApplicationStartingEvent> {
    @Override
    public void onApplicationEvent(ApplicationStartingEvent event) {
        System.out.println("run firstListner");
    }
}

public class SecondListner implements ApplicationListener<ApplicationStartingEvent> {
    @Override
    public void onApplicationEvent(ApplicationStartingEvent event) {
        System.out.println("run SecondListner");
    }
}

public class ThirdListner implements ApplicationListener<ApplicationStartingEvent> {
    @Override
    public void onApplicationEvent(ApplicationStartingEvent event) {
        System.out.println("run ThirdListner");
    }
}

4.1 在 application.properties 配置文件中配置

context.listner.classes=com.learn.springboot.listener.FirstListner

4.2 在 META-INF/spring.factories 文件中配置

org.springframework.context.ApplicationListener=com.learn.springboot.listener.SecondListner

4.3 在代码中配置

public static void main(String[] args) {
    // SpringApplication.run(SpringbootApplication.class, args);
    SpringApplication springApplication = new SpringApplication(SpringbootApplication.class);
    springApplication.addListeners(new ThirdListner());
    springApplication.run();
}

启动项目，观察控制台输出：

run firstListner 
run SecondListner
run ThirdListner
.   ____          _            __ _ _
 /\ / ___'_ __ _ _(_)_ __  __ _    
( ( )\___ | '_ | '_| | '_ / _` |    
 \/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.5.RELEASE)