Spring事件监听器ApplicationListener源码详解
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ApplicationEvent以及Listener是Spring为我们提供的一个事件监听、订阅的实现，内部实现原理是观察者设计模式，设计初衷也是为了系统业务逻辑之间的解耦，提高可扩展性以及可维护性。事件发布者并不需要考虑谁去监听，监听具体的实现内容是什么，发布者的工作只是为了发布事件而已。
Spring提供的内置事件：
- ContextRefreshedEvent：容器刷新事件
- ContextStartedEvent：容器启动事件
- ContextStoppedEvent：容器停止事件
- ContextClosedEvent：容器关闭事件
使用方式
监听容器的刷新事件
自定义一个ApplicationListener，指定监听的事件类型ContextRefreshedEvent：
package com.morris.spring.listener;
import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextRefreshedEvent;
public class ContextRefreshedListener implements ApplicationListener<ContextRefreshedEvent> {
@Override
public void onApplicationEvent(ContextRefreshedEvent event) {
System.out.println("context refresh");
}
}注入到容器中：
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
applicationContext.register(ContextRefreshedListener.class);
applicationContext.refresh(); applicationContext.refresh()内部会发送容器刷新的事件。
自定义事件
自定义的事件需要继承ApplicationEvent：
package com.morris.spring.event;
import org.springframework.context.ApplicationEvent;
public class CustomEvent extends ApplicationEvent {
public CustomEvent(Object source) {
super(source);
}
}监听的时候使用ApplicationEvent的子类CustomEvent：
package com.morris.spring.listener;
import com.morris.spring.event.CustomEvent;
import org.springframework.context.ApplicationListener;
public class CustomEventListener implements ApplicationListener<CustomEvent> {
@Override
public void onApplicationEvent(CustomEvent event) {
System.out.println("custom event: " + event.getSource());
}
}可以使用AnnotationConfigApplicationContext发布事件：
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
applicationContext.register(CustomEventListener.class);
applicationContext.refresh();
applicationContext.publishEvent(new CustomEvent("custom event"));可以向bean中注入一个ApplicationEventPublisher来发布事件：
package com.morris.spring.service;
import com.morris.spring.event.CustomEvent;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationEventPublisher;
public class CustomEventService {
@Autowired
private ApplicationEventPublisher applicationEventPublisher;
public void publishEvent() {
applicationEventPublisher.publishEvent(new CustomEvent("自定义事件"));
}
}可以通过实现ApplicationEventPublisherAware接口注入ApplicationEventPublisher来发布事件：
package com.morris.spring.service;
import com.morris.spring.event.CustomEvent;
import org.springframework.context.ApplicationEventPublisher;
import org.springframework.context.ApplicationEventPublisherAware;
public class CustomEventService2 implements ApplicationEventPublisherAware {
private ApplicationEventPublisher applicationEventPublisher;
public void publishEvent() {
applicationEventPublisher.publishEvent(new CustomEvent("自定义事件"));
}
@Override
public void setApplicationEventPublisher(ApplicationEventPublisher applicationEventPublisher) {
this.applicationEventPublisher = applicationEventPublisher;
}
}由于ApplicationContext实现了ApplicationEventPublisher接口，也可以直接注入ApplicationContext来发布事件。
使用@EventListener监听事件
在监听事件时，由于类需要实现ApplicationListener接口，对代码有很大的侵入性，可以使用@EventListener注解随时随地监听事件，这样一个Service中可以监听多个事件：
package com.morris.spring.listener;
import com.morris.spring.event.CustomEvent;
import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.context.event.EventListener;
public class CustomEventListener2 {
@EventListener
public void listenContextRefreshedEvent(ContextRefreshedEvent event) {
System.out.println("context refresh");
}
@EventListener
public void listenCustomEvent(CustomEvent event) {
System.out.println("custom event: " + event.getSource());
}
}还可以在@EventListener注解上指定监听的事件类型：
package com.morris.spring.listener;
import com.morris.spring.event.CustomEvent;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.context.event.EventListener;
public class CustomEventListener3 {
@EventListener({ContextRefreshedEvent.class, CustomEvent.class})
public void listenEvent(ApplicationEvent event) {
System.out.println(event);
}
}异步发送消息
spring消息的发送默认都是同步的，如果要异步发送消息，首先要在配置类上开启异步功能@EnableAsync：
package com.morris.spring.config;
import org.springframework.context.annotation.Configuration;
import org.springframework.scheduling.annotation.EnableAsync;
@Configuration
@EnableAsync // 开启异步
public class EventListenerConfig {
}在监听的方法上加上@Async：
package com.morris.spring.listener;
import com.morris.spring.event.CustomEvent;
import lombok.extern.slf4j.Slf4j;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.context.event.EventListener;
import org.springframework.scheduling.annotation.Async;
@Slf4j
public class AsyncCustomEventListener {
@EventListener({ContextRefreshedEvent.class, CustomEvent.class})
@Async // 异步
public void listenEvent(ApplicationEvent event) {
log.info("receive event: {}", event);
}
}也可以自定义执行异步消息的线程池（默认就是SimpleAsyncTaskExecutor）：
@Bean
public TaskExecutor executor() {
return new SimpleAsyncTaskExecutor("eventListen-");
}异步消息只是借用spring的异步执行机制，在方法上加上@Async注解，方法都会异步执行。
ApplicationListener原理分析
发布消息的入口
发布消息入口： org.springframework.context.support.AbstractApplicationContext#publishEvent(org.springframework.context.ApplicationEvent)
public void publishEvent(ApplicationEvent event) {
publishEvent(event, null);
}
protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
...
// 发布消息
getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
}
... ...
}然后调用SimpleApplicationEventMulticaster来进行广播消息：
// org.springframework.context.event.SimpleApplicationEventMulticaster#multicastEvent(org.springframework.context.ApplicationEvent, org.springframework.core.ResolvableType)
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
// 如果有线程池，将会异步执行
Executor executor = getTaskExecutor();
for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
if (executor != null) {
executor.execute(() -> invokeListener(listener, event));
}
else {
invokeListener(listener, event);
}
}
}
protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
ErrorHandler errorHandler = getErrorHandler();
if (errorHandler != null) {
try {
doInvokeListener(listener, event);
}
catch (Throwable err) {
errorHandler.handleError(err);
}
}
else {
// ApplicationListener.调用onApplicationEvent
doInvokeListener(listener, event);
}
}
private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
try {
listener.onApplicationEvent(event);
}
catch (ClassCastException ex) {
String msg = ex.getMessage();
if (msg == null || matchesClassCastMessage(msg, event.getClass())) {
// Possibly a lambda-defined listener which we could not resolve the generic event type for
// -> let's suppress the exception and just log a debug message.
Log logger = LogFactory.getLog(getClass());
if (logger.isTraceEnabled()) {
logger.trace("Non-matching event type for listener: " + listener, ex);
}
}
else {
throw ex;
}
}
}何时注入SimpleApplicationEventMulticaster
从上面的源码可以发现spring是通过SimpleApplicationEventMulticaster事件多播器来发布消息的，那么这个类是何时注入的呢？容器refresh()时。
org.springframework.context.support.AbstractApplicationContext#initApplicationEventMulticaster
protected void initApplicationEventMulticaster() {
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
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 {
// 直接new，然后放入到spring一级缓存中
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() + "]");
}
}
}何时注入ApplicationListener
spring在发布消息时，会从SimpleApplicationEventMulticaster中拿出所有的ApplicationListener，那么这些ApplicationListener何时被注入的呢？容器refresh()时。
org.springframework.context.support.AbstractApplicationContext#registerListeners
protected void registerListeners() {
// Register statically specified listeners first.
for (ApplicationListener<?> listener : getApplicationListeners()) {
getApplicationEventMulticaster().addApplicationListener(listener);
}
// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let post-processors apply to them!
String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
for (String listenerBeanName : listenerBeanNames) {
// 添加到SimpleApplicationEventMulticaster中
getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
}
// Publish early application events now that we finally have a multicaster...
Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
this.earlyApplicationEvents = null;
if (!CollectionUtils.isEmpty(earlyEventsToProcess)) {
for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
getApplicationEventMulticaster().multicastEvent(earlyEvent);
}
}
}@EventListener的原理
@EventListener注解的功能是通过EventListenerMethodProcessor来实现的，EventListenerMethodProcessor这个类在AnnotationConfigApplicationContext的构造方法中被注入。
EventListenerMethodProcessor主要实现了两个接口：SmartInitializingSingleton和BeanFactoryPostProcessor。
先来看看BeanFactoryPostProcessor的postProcessBeanFactory()，这个方法主要是保存beanFactory和eventListenerFactories，后面的方法将会使用到：
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
// 保存beanFactory
this.beanFactory = beanFactory;
Map<String, EventListenerFactory> beans = beanFactory.getBeansOfType(EventListenerFactory.class, false, false);
List<EventListenerFactory> factories = new ArrayList<>(beans.values());
AnnotationAwareOrderComparator.sort(factories);
// 保存eventListenerFactories
this.eventListenerFactories = factories;
}再来看看SmartInitializingSingleton的afterSingletonsInstantiated()方法，这个方法会在所有的bean初始化完后执行。
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)) {
// 目标类的类型
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 {
// 处理目标对象
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) &&
AnnotationUtils.isCandidateClass(targetType, EventListener.class) &&
!isSpringContainerClass(targetType)) {
Map<Method, EventListener> annotatedMethods = null;
try {
// 获得类中所有的带有@EventListener注解的方法
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));
// 使用factory创建一个ApplicationListener
ApplicationListener<?> applicationListener =
factory.createApplicationListener(beanName, targetType, methodToUse);
if (applicationListener instanceof ApplicationListenerMethodAdapter) {
((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
}
// 将ApplicationListener添加到Spring容器中
context.addApplicationListener(applicationListener);
break;
}
}
}
if (logger.isDebugEnabled()) {
logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
beanName + "': " + annotatedMethods);
}
}
}
}SpringBoot中的事件
SpringBoot在启动时会按以下顺序发送消息：
- ApplicationStartingEvent：在运行开始时发送 ，但在进行任何处理之前（侦听器和初始化程序的注册除外）发送
- ApplicationEnvironmentPreparedEvent：当被发送Environment到中已知的上下文中使用，但是在创建上下文之前
- ApplicationContextInitializedEvent：在ApplicationContext准备好且已调用ApplicationContextInitializers之后但任何bean定义未加载之前发送
- ApplicationPreparedEvent：在刷新开始之前但在加载bean定义之后发送
- ApplicationStartedEvent：上下文已被刷新后发送，但是任何应用程序和命令行都被调用前
- ApplicationReadyEvent：在所有的命令行应用启动后发送此事件，可以处理请求
- ApplicationFailedEvent：在启动时异常发送
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