事件及监听并不是SpringBoot的新功能，Spring框架早已提供了完善的事件监听机制，在Spring框架中实现事件监听的流程如下：
自定义事件，继承org.springframework.context.ApplicationEvent抽象类
定义事件监听器，实现org.springframework.context.ApplicationListener接口
在Spring容器中发布事件
实现自定义事件及监听
- 定义事件
//自定义事件
public class ApplicationEventTest extends ApplicationEvent { public ApplicationEventTest(Object source) {
super(source);
} /**
* 事件处理事项
* @param msg
*/
public void printMsg(String msg)
{
System.out.println("监听到事件："+ApplicationEventTest.class);
}
}
- 定义监听器
//自定义事件监听器
//@Component
public class ApplicationListenerTest implements ApplicationListener<ApplicationEventTest> { @Override
public void onApplicationEvent(ApplicationEventTest event) { event.printMsg(null);
}
}
- 在Spring容器中发布事件
public static void main(String[] args) {
SpringApplication application = new SpringApplication(SpringbootdemoApplication.class);
//需要把监听器加入到spring容器中
application.addListeners(new ApplicationListenerTest());
Set<ApplicationListener<?>> listeners = application.getListeners();
ConfigurableApplicationContext context = application.run(args);
//发布事件
context.publishEvent(new ApplicationEventTest(new Object()));
context.close();
}
上面的示例是在SpringBoot应用中简单的测试一下。
实际开发中实现监听还有其他的方式，在Spring框架中提供了两种事件监听的方式：
编程式：通过实现ApplicationListener接口来监听指定类型的事件
注解式：通过在方法上加@EventListener注解的方式监听指定参数类型的事件，写该类需要托管到Spring容器中
在SpringBoot应用中还可以通过配置的方式实现监听：
3. 通过application.properties中配置context.listener.classes属性指定监听器
下面分别分析一下这三种监听方式
编程式实现监听
实现ApplicationListenser接口：
@Component
public class ApplicationListenerTest implements ApplicationListener<ApplicationEventTest> { @Override
public void onApplicationEvent(ApplicationEventTest event) { event.printMsg(null);
}
}
控制台输出测试：
public static void main(String[] args) {
SpringApplication application = new SpringApplication(SpringbootdemoApplication.class);
//需要把监听器加入到spring容器中
//application.addListeners(new ApplicationListenerTest());
//Set<ApplicationListener<?>> listeners = application.getListeners();
ConfigurableApplicationContext context = application.run(args);
//发布事件
context.publishEvent(new ApplicationEventTest(new Object()));
}
那么我们跟踪一下源码，看一下事件是如何发布出去的，又是如何被监听到的呢？
AbstractApplicationContext.java中截取部分代码
protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
Assert.notNull(event, "Event must not be null");
if (logger.isTraceEnabled()) {
logger.trace("Publishing event in " + getDisplayName() + ": " + event);
}
// Decorate event as an ApplicationEvent if necessary
/将object转成ApplicationEvent
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 {
// SimpleApplicationEventMulticaster 获取事件发布器，发布事件
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);
}
}
}
查看一下ApplicationContext类结构图可以发现：应用上下文AbstractApplicationContext实际还是通过继承ApplicationEventPublisher接口，实现了其中的事件发布的方法，使得Spring应用上下文有了发布事件的功能，在AbstractApplicationContext内部通过SimpleApplicationEventMulticaster事件发布类，将具体事件ApplicationEvent发布出去。
那么事件发布出去后又是如何被监听到的呢？下面看一下具Spring中负责处理事件发布类SimpleApplicationEventMulticaster 中multicastEvent方法具体实现过程
SimpleApplicationEventMulticaster.java部分代码，实际尝试将当前事件逐个广播到指定类型的监听器中（listeners已经根据当前事件类型过滤了）
@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
// getApplicationListeners(event, type) 筛选监听器，在context.publish(ApplicationEvent event)中已经将事件传入，getApplicationListeners中将可以根据这个event类型从Spring容器中检索出符合条件的监听器 for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
Executor executor = getTaskExecutor();
if (executor != null) {
executor.execute(() -> invokeListener(listener, event));
}
else {
//尝试逐个向监听器广播
invokeListener(listener, event);
}
}
}
@EventListener注解方式实现
定义注解方法
@Component
public class MyEventHandleTest { /**
* 参数为Object类型时，所有事件都会监听到
* 参数为指定类型事件时，该参数类型事件或者其子事件（子类）都可以接收到
*/
@EventListener
public void event(ApplicationEventTest event){ event.printMsg(null);
} }
实现过程分析：
@EventListener注解主要通过EventListenerMethodProcessor扫描出所有带有@EventListener注解的方法，然后动态构造事件监听器，并将监听器托管到Spring应用上文中。
protected void processBean(
final List<EventListenerFactory> factories, final String beanName, final Class<?> targetType) { if (!this.nonAnnotatedClasses.contains(targetType)) {
Map<Method, EventListener> annotatedMethods = null;
try {
//查找含有@EventListener注解的所有方法
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 = getApplicationContext();
//遍历含有@EventListener注解的方法
for (Method method : annotatedMethods.keySet()) {
for (EventListenerFactory factory : factories) {
if (factory.supportsMethod(method)) {
Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName));
//动态构造相对应的事件监听器
ApplicationListener<?> applicationListener =
factory.createApplicationListener(beanName, targetType, methodToUse);
if (applicationListener instanceof ApplicationListenerMethodAdapter) {
((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
}
//将监听器添加的Spring应用上下文中托管
context.addApplicationListener(applicationListener);
break;
}
}
}
if (logger.isDebugEnabled()) {
logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
beanName + "': " + annotatedMethods);
}
}
}
}
在application.properties中配置context.listener.classes
添加如下配置：
context.listener.classes=com.sl.springbootdemo.Listeners.ApplicationListenerTest
查看一下DelegatingApplicationListener类中实现逻辑：
public class DelegatingApplicationListener
implements ApplicationListener<ApplicationEvent>, Ordered { private static final String PROPERTY_NAME = "context.listener.classes"; private int order = 0;
//Spring framework提供的负责处理发布事件的类，前面说的Spring应用上下文中也是通过这个类发布事件的
private SimpleApplicationEventMulticaster multicaster; @Override
public void onApplicationEvent(ApplicationEvent event) {
if (event instanceof ApplicationEnvironmentPreparedEvent) {
// getListeners内部实现读取context.listener.classes配置的监听器
List<ApplicationListener<ApplicationEvent>> delegates = getListeners(
((ApplicationEnvironmentPreparedEvent) event).getEnvironment());
if (delegates.isEmpty()) {
return;
}
this.multicaster = new SimpleApplicationEventMulticaster();
for (ApplicationListener<ApplicationEvent> listener : delegates) {
this.multicaster.addApplicationListener(listener);
}
}
//发布事件
if (this.multicaster != null) {
this.multicaster.multicastEvent(event);
}
}
Spring-boot-{version}.jar包中提供一个类DelegatingApplicationListener，该类的作用是从application.properties中读取配置context.listener.classes，并将事件广播给这些配置的监听器。通过前面一章对SpringBoot启动流程分析，我们已经了解到SpringBoot启动时会从META-INF/spring.factories中读取key为org.springframework.context.ApplicationListener的所有监听器。DelegatingApplicationListener的功能可以让我们不需要创建META-INF/spring.factories，直接在application.properties中配置即可。