ApplicationContext提供事件处理通过ApplicationEvent类和ApplicationListener接口。如果一个bean实现ApplicationListener接口在容器中,每次一个ApplicationEvent被发布到ApplicationContext中,这类bean就会收到这些通知。从本质上讲,这是标准的观察者设计模式。
实现Spring事件机制主要有4个类：
- ApplicationEvent：事件，每个实现类表示一类事件，可携带数据。
- ApplicationListener：事件监听器，用于接收事件处理时间。
- ApplicationEventMulticaster：事件管理者，用于事件监听器的注册和事件的广播。
- ApplicationEventPublisher：事件发布者，委托ApplicationEventMulticaster完成事件发布。
1、Spring中的事件：ApplicationEvent
观察者模式中传播的数据比较抽象，如Observable可发布任意的Object，而事件/监听器模式所发布的内容则有类型限制，在Java中他必须是java.util.EventObject对象，虽然在Java语言层面并无此限制，不过这是行之有年的业界规则，所以无论Java Beans还是Java AWT/Swing都遵照改规则，Spring事件自然也不会例外，所以Spring事件抽象类ApplicationEvent必然扩展EventObject。ApplicationEvent事件就是一个包含了任意对象并含有事件对象创建时间戳的类。
public abstract class ApplicationEvent extends EventObject {
private static final long serialVersionUID = 7099057708183571937L;
private final long timestamp;
public ApplicationEvent(Object source) {
super(source);
this.timestamp = System.currentTimeMillis();
}
public final long getTimestamp() {
return this.timestamp;
}
}
EventObject并不提供默认构造器，它需要外部传递一个名为source的构造器参数用于记录并跟踪事件的来源，如Spring事件ContextRefreshedEvent，其事件源为当前ApplicationContext。
下表描述了Spring提供的标准事件:
| 事件 | 描述 |
|---|---|
| ContextRefreshedEvent | 事件发布在ApplicationContext初始化或刷新时(例如,通过在ConfigurableApplicationContext接口使用refresh()方法)。这里,“初始化”意味着所有bean加载,post-processorbean被检测到并且激活,单例预先实例化,ApplicationContext对象可以使用了。只要上下文没有关闭,可以触发多次刷新,ApplicationContext提供了一种可选择的支持这种“热”刷新。例如,XmlWebApplicationContext支持热刷新,但GenericApplicationContext并非如此。具体是在AbstractApplicationContext的finishRefresh()方法中。 |
| ContextStartedEvent | 事件发布在ApplicationContext开始使用ConfigurableApplicationContext接口start()方法。这里,“开始”意味着所有生命周期bean接收到一个明确的起始信号。通常,这个信号用于明确停止后重新启动,但它也可以用于启动组件没有被配置为自动运行(例如,组件还没有开始初始化)。 |
| ContextStoppedEvent | 事件发布在ApplicationContext停止时通过使用ConfigurableApplicationContext接口上的stop()方法。在这里,“停止”意味着所有生命周期bean接收一个显式的停止信号。停止上下文可以通过重新调用start()方法。 |
| ContextClosedEvent | 事件发布在ApplicationContext关闭时通过关闭ConfigurableApplicationContext接口方法。这里,“封闭”意味着所有单例bean被摧毁。一个封闭的环境达到生命的终结。它不能刷新或重启。 |
| RequestHandledEvent | 一个特定的web事件告诉所有能处理HTTP请求的bean。这个事件是在请求完成后发布的。这个事件只适用于使用Spring的DispatcherServlet的web应用程序。 |
2、Spring事件监听者：ApplicationListener
Java事件监听者必须是EventListener实现类，不过EventListener仅为标签接口，内部并没有提供任何实现方法。
@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
void onApplicationEvent(E event);
}
这么设计是因为他很难适用于所有事件监听场景，如MouseListener，它监听了鼠标点击、鼠标按下及鼠标释放等事件。这么设计的好处是一个实现可以集中处理不同的事件，弊端是未来接口需要增加监听方法时客户端不得不为此变更而适配。
Spring事件监听器实现采取了相反的设计理念，通过限定监听方法数量，仅抽象单一方法onApplicationEvent(ApplicaiotnEvent)，当事件监听器接收到它可以处理的事件，会调用onApplicationEvent()方法。注意到ApplicationListener是泛型参数的这样可以参数化的定制事件。这意味着onApplicationEvent()方法可以保持类型安全,避免任何需要向下类型转换。你可以尽可能多的注册你希望事件侦听器,但是注意,默认情况下,事件监听器同步接收事件。这意味着publishEvent()方法会阻塞直到所有的事件监听器成处理完事件。这种单线程同步方法的一个特点是,当一个监听器接收到一个事件时,它运行在事务上下文的发布者线程上如果事务上下文可用。如果事件的发布需要另一种策略（譬如多线程）需要实现自己的 ApplicationEventMulticaster接口类。
由于泛型参数的限制，泛型化的ApplicationListener无法监听不同类型的ApplicationEvent，如ApplicationListener
public interface SmartApplicationListener extends ApplicationListener<ApplicationEvent>, Ordered {
//确定此监听器是否实际支持给定的事件类型。
boolean supportsEventType(Class<? extends ApplicationEvent> eventType);
default boolean supportsSourceType(@Nullable Class<?> sourceType) {
return true;
}
@Override
default int getOrder() {
return LOWEST_PRECEDENCE;
}
}
该接口通过supportsXXXXType方法过滤需要监听的ApplicationEvent类型和事件源类型，从而达到监听不同类型的ApplicationEvent的目的。
SmartApplicationListener是Spring3.0引入的，在4.2又引入了一个GenericApplicationListener可用来代替SmartApplicationListener，它的supportsEventType方法的参数是一个ResolvableType，更加全面的支持泛型化。
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;
}
}
3、Spring事件监听器注册/事件广播：ApplicationEventMulticaster
ApplicationEventMulticaster接口方法分为三类，注册事件监听器、移除事件监听器、发布事件。
public interface ApplicationEventMulticaster {
void addApplicationListener(ApplicationListener<?> listener);
void addApplicationListenerBean(String listenerBeanName);
void removeApplicationListener(ApplicationListener<?> listener);
void removeApplicationListenerBean(String listenerBeanName);
void removeAllListeners();
void multicastEvent(ApplicationEvent event);
void multicastEvent(ApplicationEvent event, @Nullable ResolvableType eventType);
}
执行AbstractApplicationContext.initApplicationEventMulticaster() 方法时会实例化一个bean name为applicationEventMulticaster的SimpleApplicationEventMulticaster，它的父类实现了前5个方法依靠一个内部类ListenerRetriever维护了一个Set<ApplicationListener<?>>，本质事件监听器的注册或移除就是对这个Set的添加和移除操作。
public abstract class AbstractApplicationEventMulticaster
implements ApplicationEventMulticaster, BeanClassLoaderAware, BeanFactoryAware {
//包含所有的事件监听器
private final ListenerRetriever defaultRetriever = new ListenerRetriever(false);
//根据事件类型对事件监听器做了分组同时起到缓存的作用
final Map<ListenerCacheKey, ListenerRetriever> retrieverCache = new ConcurrentHashMap<>(64);
@Override
public void addApplicationListener(ApplicationListener<?> listener) {
synchronized (this.retrievalMutex) {
// 如果已经注册，则显式删除代理的目标，以避免对同一个侦听器进行双重调用。
Object singletonTarget = AopProxyUtils.getSingletonTarget(listener);
if (singletonTarget instanceof ApplicationListener) {
this.defaultRetriever.applicationListeners.remove(singletonTarget);
}
this.defaultRetriever.applicationListeners.add(listener);
this.retrieverCache.clear();
}
}
@Override
public void removeApplicationListener(ApplicationListener<?> listener) {
synchronized (this.retrievalMutex) {
this.defaultRetriever.applicationListeners.remove(listener);
this.retrieverCache.clear();
}
}
private class ListenerRetriever {
public final Set<ApplicationListener<?>> applicationListeners = new LinkedHashSet<>();
public final Set<String> applicationListenerBeans = new LinkedHashSet<>();
private final boolean preFiltered;
public ListenerRetriever(boolean preFiltered) {
this.preFiltered = preFiltered;
}
public Collection<ApplicationListener<?>> getApplicationListeners() {
List<ApplicationListener<?>> allListeners = new ArrayList<>(
this.applicationListeners.size() + this.applicationListenerBeans.size());
allListeners.addAll(this.applicationListeners);
if (!this.applicationListenerBeans.isEmpty()) {
BeanFactory beanFactory = getBeanFactory();
for (String listenerBeanName : this.applicationListenerBeans) {
try {
ApplicationListener<?> listener = beanFactory.getBean(listenerBeanName, ApplicationListener.class);
if (this.preFiltered || !allListeners.contains(listener)) {
allListeners.add(listener);
}
}
catch (NoSuchBeanDefinitionException ex) {
// Singleton listener instance (without backing bean definition) disappeared -
// probably in the middle of the destruction phase
}
}
}
if (!this.preFiltered || !this.applicationListenerBeans.isEmpty()) {
AnnotationAwareOrderComparator.sort(allListeners);
}
return allListeners;
}
}
}
接口后两个方法由子类实现，可以看到SimpleApplicationEventMulticaster拥有一个Executor和ErrorHandler，分表表示监听器的调用线程池（如果不想使用单线程同步处理则可以设置一个线程池）和监听器处理事件失败的处理者(如果设置了的话)否则抛异常。
public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {
@Nullable
private Executor taskExecutor;
@Nullable
private ErrorHandler errorHandler;
public SimpleApplicationEventMulticaster() {
}
public SimpleApplicationEventMulticaster(BeanFactory beanFactory) {
setBeanFactory(beanFactory);
}
public void setTaskExecutor(@Nullable Executor taskExecutor) {
this.taskExecutor = taskExecutor;
}
public void setErrorHandler(@Nullable ErrorHandler errorHandler) {
this.errorHandler = errorHandler;
}
@Override
public void multicastEvent(ApplicationEvent event) {
//广播事件，可以自动分析出ApplicationEvent是那种事件类型
multicastEvent(event, resolveDefaultEventType(event));
}
@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
//如果eventType是null，会使用ResolvableType.forInstance(event)返回一个ResolvableType
//这个方法内部会先判断event如果是ResolvableTypeProvider则调用getResolvableType()
ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
//调用父类方法getApplicationListeners只取得能处理此类事件的时间监听器，依次处理
for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
Executor executor = getTaskExecutor();
if (executor != null) {
executor.execute(() -> invokeListener(listener, event));
}
else {
invokeListener(listener, event);
}
}
}
private ResolvableType resolveDefaultEventType(ApplicationEvent event) {
return ResolvableType.forInstance(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 {
doInvokeListener(listener, event);
}
}
@SuppressWarnings({"unchecked", "rawtypes"})
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.isDebugEnabled()) {
logger.debug("Non-matching event type for listener: " + listener, ex);
}
}
else {
throw ex;
}
}
}
private boolean matchesClassCastMessage(String classCastMessage, Class<?> eventClass) {
// On Java 8, the message starts with the class name: "java.lang.String cannot be cast..."
if (classCastMessage.startsWith(eventClass.getName())) {
return true;
}
// On Java 11, the message starts with "class ..." a.k.a. Class.toString()
if (classCastMessage.startsWith(eventClass.toString())) {
return true;
}
// On Java 9, the message used to contain the module name: "java.base/java.lang.String cannot be cast..."
int moduleSeparatorIndex = classCastMessage.indexOf('/');
if (moduleSeparatorIndex != -1 && classCastMessage.startsWith(eventClass.getName(), moduleSeparatorIndex + 1)) {
return true;
}
// Assuming an unrelated class cast failure...
return false;
}
}
SimpleApplicationEventMulticaster在进行广播时，调用父类方法AbstractApplicationEventMulticaster.getApplicationListeners(ApplicationEvent, ResolvableType)返回一组能够处理当前事件类型的事件监听器，依次去调用事件监听器的onApplicationEvent方法，默认是同步调用，也可以通过setTaskExecutor方法传入一个Executor来达到异步处理的方式。
下面重点看一下AbstractApplicationEventMulticaster#getApplicationListeners(ApplicationEvent, ResolvableType)
protected Collection<ApplicationListener<?>> getApplicationListeners(
ApplicationEvent event, ResolvableType eventType) {
Object source = event.getSource();
Class<?> sourceType = (source != null ? source.getClass() : null);
//根据事件类型和事件源类型分组做缓存
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);
}
}
上面代码主要检索缓存中是否存在，实际上为给定事件和源类型检索应用程序监听器的是retrieveApplicationListeners方法。
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) {
//GenericApplicationListener、SmartApplicationListener通过supportsXXX方法
//普通ApplicationListener通过具体事件类型过滤
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;
}
4、Spring事件发布：ApplicationEventPublisher
@FunctionalInterface
public interface ApplicationEventPublisher {
default void publishEvent(ApplicationEvent event) {
publishEvent((Object) event);
}
void publishEvent(Object event);
}
ApplicationEventPublisher仅存在两个发布ApplicationEvent的重载方法publishEvent，并无关联ApplicationListener的操作方法。不过ApplicationEventPublisher接口被ApplicationContext扩展，因此无论使用哪种ApplicationContext实例均具备发布ApplicationEvent的能力。同时ApplicationContext的子接口ConfigurableApplicationContext提供了添加ApplicationListener实例的关联方法addApplicationListener(ApplicationListener)。
@Override
public void addApplicationListener(ApplicationListener<?> listener) {
Assert.notNull(listener, "ApplicationListener must not be null");
if (this.applicationEventMulticaster != null) {
this.applicationEventMulticaster.addApplicationListener(listener);
}
this.applicationListeners.add(listener);
}
可以看到通过ConfigurableApplicationContext的addApplicationListener方法注册事件监听器也会注册到内部成员变量applicationEventMulticaster中。
既然ApplicationContext对象是ApplicationEventPublisher实例，则不难发现AbstractApplicationContext完全实现了ApplicationEventPublisher接口：
@Override
public void publishEvent(ApplicationEvent event) {
publishEvent(event, null);
}
@Override
public void publishEvent(Object event) {
publishEvent(event, null);
}
protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
Assert.notNull(event, "Event must not be null");
// Decorate event as an ApplicationEvent if necessary
ApplicationEvent applicationEvent;
if (event instanceof ApplicationEvent) {
applicationEvent = (ApplicationEvent) event;
}
else {
applicationEvent = new PayloadApplicationEvent<>(this, event);
if (eventType == null) {
eventType = ((PayloadApplicationEvent) applicationEvent).getResolvableType();
}
}
// Multicast right now if possible - or lazily once the multicaster is initialized
if (this.earlyApplicationEvents != null) {
this.earlyApplicationEvents.add(applicationEvent);
}
else {
getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
}
// Publish event via parent context as well...
if (this.parent != null) {
if (this.parent instanceof AbstractApplicationContext) {
((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
}
else {
this.parent.publishEvent(event);
}
}
}
明显地在publishEvent(Object,ResolvableType)方法中显示地调用了getApplicationEventMulticaster().multicastEvent(applicationEvent,eventType)，而applicationEventMulticaster属性又由initApplicationEventMulticaster()方法初始化。
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 {
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() + "]");
}
}
}
在applicationEventMulticaster初始化中，首先判断Spring应用上下文是否存在名为applicationEventMulticaster且类型为ApplicationEventMulticaster的bean，如果不存在则将其构造为SimpleApplicationEventMulticaster对象注册到Spring容器中。
5、Spring注解驱动事件监听：@EventListener
@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface EventListener {
@AliasFor("classes")
Class<?>[] value() default {};
@AliasFor("value")
Class<?>[] classes() default {};
String condition() default "";
}
condition的值为SpEL，代表true才可被处理。下表列出了项目可用的上下文,这样您就可以使用这些条件事件处理:
| 名称 | 所在位置 | 描述 | 例子 |
|---|---|---|---|
| Event | rootobject | 实际的ApplicationEvent. | #root.event |
| Argumentsarray | rootobject | 执行目标方法的参数数组 | #root.args[0] |
| Argumentname | evaluationcontext | 任何方法参数的名称。出于某种原因,如果名字不是可用的(例如,因为没有调试信息),参数名称也可以使用这种形式 #a<#arg>，#arg代表参数索引(从0开始)。 | #blEvent or #a0 (也可使用 #p0 或 #p<#arg>符号作为一个别名) |
@EventListener(condition = "#blEvent.content == 'my-event'")
public void processBlackListEvent(BlackListEvent blEvent) {
// notify appropriate parties via notificationAddress...
}
@EventListener必须标记在Spring托管bean的public方法中，并且支持单一类型事件监听，除此之外单个@EventListener方法也支持多种事件类型的监听。
当@EventListener方法监听一个或多个ApplicationEvent时，其参数可为零到一个ApplicationEvent。
| 监听类型 | 访问性 | 顺序控制 | 返回类型 | 参数数量 | 参数类型 | 泛型事件 |
|---|---|---|---|---|---|---|
| 监听类型 | 访问性 | 顺序控制 | 返回类型 | 参数数量 | 参数类型 | 泛型事件 |
| @EventListener同步方法 | public | @Order | 任意 | 0或1 | 事件类型或泛型参数类型 | 支持 |
| @EventListener异步方法 | public | @Order | 非原生类型 | 0或1 | 事件类型或泛型参数类型 | 支持 |
| 实现ApplicationListener | public | @Order或Ordered | void | 1 | 事件类型 | 不支持 |
6、事件监听器的注册
在AbstractApplicationContext.prepareBeanFactory()方法执行的时候会添加一个BeanPostProcessor——ApplicationListenerDetector，它实现了MergedBeanDefinitionPostProcessor接口，在postProcessAfterInitialization()方法执行的时候检测已注册到容器中的bean如果是ApplicationListener，就通过applicationContext添加。
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) {
if (bean instanceof ApplicationListener) {
// potentially not detected as a listener by getBeanNamesForType retrieval
Boolean flag = this.singletonNames.get(beanName);
if (Boolean.TRUE.equals(flag)) {
// singleton bean (top-level or inner): register on the fly
this.applicationContext.addApplicationListener((ApplicationListener<?>) bean);
}
else if (Boolean.FALSE.equals(flag)) {
if (logger.isWarnEnabled() && !this.applicationContext.containsBean(beanName)) {
// inner bean with other scope - can't reliably process events
logger.warn("Inner bean '" + beanName + "' implements ApplicationListener interface " +
"but is not reachable for event multicasting by its containing ApplicationContext " +
"because it does not have singleton scope. Only top-level listener beans are allowed " +
"to be of non-singleton scope.");
}
this.singletonNames.remove(beanName);
}
}
return bean;
}
自Spring 4.2起,事件机制明显改善,提供了一个基于注解的模型以及发布任意事件的能力。基于注解的Spring启动形式会在AnnotationConfigUtils#registerAnnotationConfigProcessors(BeanDefinitionRegistry,Object)方法内部注册一个bean name为org.springframework.context.event.internalEventListenerProcessor的EventListenerMethodProcessor。这个bean实现了SmartInitializingSingleton接口，在接口方法afterSingletonsInstantiated()调用时会检测容器内所有的bean，如果发现有@EventListener方法将使用容器内EventListenerFactory将此@EventListener方法包装一个ApplicationListener放入容器中。
@Override
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) {
//如果一个类被代理，那么处理它的代理类即可
if (!ScopedProxyUtils.isScopedTarget(beanName)) {
Class<?> type = null;
try {
//代理模式需要取得目标类，然后取得目标类上@EventListener方法
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 {
//解析@EventListener方法的类，注册事件监听器
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) {
//缓存防止重复注册
//只解析用户定义的类
if (!this.nonAnnotatedClasses.contains(targetType) && !isSpringContainerClass(targetType)) {
Map<Method, EventListener> annotatedMethods = null;
try {
//所有@EventListener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) {
//使用Spring容器中第一个能解析此@EventListener方法的EventListenerFactory
if (factory.supportsMethod(method)) {
Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName));
//使用EventListenerFactory将@EventListener解析为一个ApplicationListener后加入容器
ApplicationListener<?> applicationListener =
factory.createApplicationListener(beanName, targetType, methodToUse);
if (applicationListener instanceof ApplicationListenerMethodAdapter) {
((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
}
context.addApplicationListener(applicationListener);
break;
}
}
}
if (logger.isDebugEnabled()) {
logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
beanName + "': " + annotatedMethods);
}
}
}
}
上面的代码核心部分就是processBean()方法，内部使用容器第一个可能解析当前@EventListener的EventListenerFactory创建一个ApplicationListener用来处理@EventListener指定的类型事件。下面就是看看Spring容器都有哪些EventListenerFactory，和EventListenerFactory是如何创建ApplicationListener的？
EventListenerFactory的注册和EventListenerMethodProcessor一样在AnnotationConfigBeanDefinitionParser.parse(Element, ParserContext)方法内，bean name为org.springframework.context.event.internalEventListenerFactory，是DefaultEventListenerFactory。
public class DefaultEventListenerFactory implements EventListenerFactory, Ordered {
private int order = LOWEST_PRECEDENCE;
public void setOrder(int order) {
this.order = order;
}
@Override
public int getOrder() {
return this.order;
}
public boolean supportsMethod(Method method) {
return true;
}
@Override
public ApplicationListener<?> createApplicationListener(String beanName, Class<?> type, Method method) {
return new ApplicationListenerMethodAdapter(beanName, type, method);
}
}
DefaultEventListenerFactory支持所有的方法成为ApplicationListener，下面看createApplicationListener()方法返回的ApplicationListenerMethodAdapter是如何处理@EventListener指定的事件的。
public ApplicationListenerMethodAdapter(String beanName, Class<?> targetClass, Method method) {
this.beanName = beanName;
this.method = BridgeMethodResolver.findBridgedMethod(method);
this.targetMethod = (!Proxy.isProxyClass(targetClass) ?
AopUtils.getMostSpecificMethod(method, targetClass) : this.method);
this.methodKey = new AnnotatedElementKey(this.targetMethod, targetClass);
EventListener ann = AnnotatedElementUtils.findMergedAnnotation(this.targetMethod, EventListener.class);
this.declaredEventTypes = resolveDeclaredEventTypes(method, ann);
this.condition = (ann != null ? ann.condition() : null);
this.order = resolveOrder(method);
}
declaredEventTypes是指事件监听器能够支持的事件类型。
private List<ResolvableType> resolveDeclaredEventTypes(Method method, @Nullable EventListener ann) {
//注意方法参数最多只能有一个
int count = method.getParameterCount();
if (count > 1) {
throw new IllegalStateException(
"Maximum one parameter is allowed for event listener method: " + method);
}
if (ann != null) {
Class<?>[] classes = ann.classes();
if (classes.length > 0) {
List<ResolvableType> types = new ArrayList<>(classes.length);
for (Class<?> eventType : classes) {
types.add(ResolvableType.forClass(eventType));
}
return types;
}
}
if (count == 0) {
throw new IllegalStateException(
"Event parameter is mandatory for event listener method: " + method);
}
return Collections.singletonList(ResolvableType.forMethodParameter(method, 0));
}
@Override
public boolean supportsEventType(ResolvableType eventType) {
for (ResolvableType declaredEventType : this.declaredEventTypes) {
if (declaredEventType.isAssignableFrom(eventType)) {
return true;
}
if (PayloadApplicationEvent.class.isAssignableFrom(eventType.toClass())) {
ResolvableType payloadType = eventType.as(PayloadApplicationEvent.class).getGeneric();
if (declaredEventType.isAssignableFrom(payloadType)) {
return true;
}
}
}
return eventType.hasUnresolvableGenerics();
}
处理事件的大致逻辑为，事件作为@EventListener方法的参数(如果是PayloadApplicationEvent则使用payload作为参数)，然后执行该方法，如果有返回值则继续讲返回值作为事件发布。注意返回值是数组或集合会将集合内所有的元素单独发布。
@Override
public void onApplicationEvent(ApplicationEvent event) {
processEvent(event);
}
public void processEvent(ApplicationEvent event) {
Object[] args = resolveArguments(event);
if (shouldHandle(event, args)) {
Object result = doInvoke(args);
if (result != null) {
handleResult(result);
}
else {
logger.trace("No result object given - no result to handle");
}
}
}
private boolean shouldHandle(ApplicationEvent event, @Nullable Object[] args) {
if (args == null) {
return false;
}
String condition = getCondition();
if (StringUtils.hasText(condition)) {
Assert.notNull(this.evaluator, "EventExpressionEvaluator must not be null");
return this.evaluator.condition(
condition, event, this.targetMethod, this.methodKey, args, this.applicationContext);
}
return true;
}
protected void handleResult(Object result) {
if (result.getClass().isArray()) {
Object[] events = ObjectUtils.toObjectArray(result);
for (Object event : events) {
publishEvent(event);
}
}
else if (result instanceof Collection<?>) {
Collection<?> events = (Collection<?>) result;
for (Object event : events) {
publishEvent(event);
}
}
else {
publishEvent(result);
}
}
由shouldHandle()方法可知，@EventListener的condition属性决定着事件可否被处理。
7、总结Spring事件/监听机制
总结Spring事件
Spring事件的API为ApplicationEvent、继承于Java规约的抽象类java.util.EventObject，并需要显示地调用父类的构造器传递事件源参数。Spring Framework内建事件有五种，包括ContextRefreshedEvent、ContextStartedEvent、ContextStoppedEvent、ContextClosedEvent和RequestHandledEvent。其中前四种为Spring应用上下文事件，均继承于抽象类ApplicationContextEvent，以ApplicationConext作为事件源。
除了以上Spring内置事件，Spring允许应用自定义ApplicationEvent，并且能够实现自定义反省Spring事件、不过这类事件需要实现ResolvableTypeProvider接口，通过该接口暴露其泛型元信息，搭建与Spring事件/监听机制的桥梁。从广义上来看，无论Spring Boot事件还是Spring Cloud事件均属于ApplicationEvent的扩展，是特定领域的自定义Spring事件，如Spring Boot事件基类SpringApplicationEvent和Spring Cloud Bus基类RemoteApplicationEvent。
总结Spring事件监听手段
关于Spring事件监听手段，Spring层面提供了两种途径，其一是面向接口编程的ApplicationListener其二是注解驱动编程的@EventListener方法。两种途径均能监听一种或多种事件，并且支持泛型事件。从实现层面分析，@EventListener方法属于Bean方法与ApplicationListener接口的适配，即ApplicationListenerMethodAdapter。两者的差异在于，@EventListener方法必须依赖于Spring应用上下文，而ApplicationListener对象尽管默认业余ApplicationContext关联，但实际上他是ApplicationEventMulticaster实例的组成元素。
总结Spring事件广播器
Spring事件广播在Spring Framework中有两类API表达方式，一是ApplicationEventPublisher.publishEvent方法，二是ApplicationEventMulticaster.multicastEvent。默认情况下Spring Framework仅提供一种实现，前者由AbstractApplicationContext实现，后者由SimpleApplicationEventMulticaster实现。其中SimpleApplicationEventMulticaster不但是AbstractApplicationContext实现ApplicationEventPublisher接口语义的基础，而且也为AbstractApplicationContext提供关联ApplicationListener的实现存储。
综上所述，SimpleApplicationEventMulticaster是ApplicationEvent、ApplicationListener和ConfigurableApplicationContext之间连接的纽带。
