Spring Boot 自定义事件及监听
本文内容纲要：
- 实现自定义事件及监听- 编程式实现监听
- @EventListener注解方式实现
- 在application.properties中配置context.listener.classes
事件及监听并不是SpringBoot" title="SpringBoot">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
1 //自定义事件
2 public class ApplicationEventTest extends ApplicationEvent {
3
4 public ApplicationEventTest(Object source) {
5 super(source);
6 }
7
8 /**
9 * 事件处理事项
10 * @param msg
11 */
12 public void printMsg(String msg)
13 {
14 System.out.println("监听到事件："+ApplicationEventTest.class);
15 }
16 }
定义监听器
1 //自定义事件监听器
2 //@Component
3 public class ApplicationListenerTest implements ApplicationListener {
4
5 @Override
6 public void onApplicationEvent(ApplicationEventTest event) {
7
8 event.printMsg(null);
9 }
10 }
在Spring容器中发布事件
1 public static void main(String[] args) {
2
3 SpringApplication application = new SpringApplication(SpringbootdemoApplication.class);
4 //需要把监听器加入到spring容器中
5 application.addListeners(new ApplicationListenerTest());
6 Set<ApplicationListener<?>> listeners = application.getListeners();
7 ConfigurableApplicationContext context = application.run(args);
8 //发布事件
9 context.publishEvent(new ApplicationEventTest(new Object()));
10
11 context.close();
12 }
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：
1 @Component2 public class ApplicationListenerTest implements ApplicationListener<ApplicationEventTest> {
3
4 @Override
5 public void onApplicationEvent(ApplicationEventTest event) {
6
7 event.printMsg(null);
8 }
9 }
控制台输出测试：
1 public static void main(String[] args) { 2
3 SpringApplication application = new SpringApplication(SpringbootdemoApplication.class);
4 //需要把监听器加入到spring容器中
5 //application.addListeners(new ApplicationListenerTest());
6 //Set<ApplicationListener<?>> listeners = application.getListeners();
7
8 ConfigurableApplicationContext context = application.run(args);
9 //发布事件
10 context.publishEvent(new ApplicationEventTest(new Object()));
11 }
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
1 protected void publishEvent(Object event, @Nullable ResolvableType eventType) { 2 Assert.notNull(event, "Event must not be null");
3 if (logger.isTraceEnabled()) {
4 logger.trace("Publishing event in " + getDisplayName() + ": " + event);
5 }
6
7 // Decorate event as an ApplicationEvent if necessary
8 /将object转成ApplicationEvent
9 ApplicationEvent applicationEvent;
10 if (event instanceof ApplicationEvent) {
11 applicationEvent = (ApplicationEvent) event;
12 }
13 else {
14 applicationEvent = new PayloadApplicationEvent<>(this, event);
15 if (eventType == null) {
16 eventType = ((PayloadApplicationEvent) applicationEvent).getResolvableType();
17 }
18 }
19
20 // Multicast right now if possible - or lazily once the multicaster is initialized
22 if (this.earlyApplicationEvents != null) {
23 this.earlyApplicationEvents.add(applicationEvent);
24 }
25 else {
26 // SimpleApplicationEventMulticaster 获取事件发布器，发布事件
27 getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
28 }
29
30 // Publish event via parent context as well...
31 if (this.parent != null) {
32 if (this.parent instanceof AbstractApplicationContext) {
33 ((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
34 }
35 else {
36 this.parent.publishEvent(event);
37 }
38 }
39 }
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）
1 @Override 2 public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
3 ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
4 // 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
5
6 for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
7 Executor executor = getTaskExecutor();
8 if (executor != null) {
9 executor.execute(() -> invokeListener(listener, event));
10 }
11 else {
12 //尝试逐个向监听器广播
13 invokeListener(listener, event);
14 }
15 }
16 }
@EventListener注解方式实现
定义注解方法
@Componentpublic 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。
1 protected void processBean( 2 final List<EventListenerFactory> factories, final String beanName, final Class<?> targetType) {
3
4 if (!this.nonAnnotatedClasses.contains(targetType)) {
5 Map<Method, EventListener> annotatedMethods = null;
6 try {
7 //查找含有@EventListener注解的所有方法
8 annotatedMethods = MethodIntrospector.selectMethods(targetType,
9 (MethodIntrospector.MetadataLookup<EventListener>) method ->
10 AnnotatedElementUtils.findMergedAnnotation(method, EventListener.class));
11 }
12 catch (Throwable ex) {
13 // An unresolvable type in a method signature, probably from a lazy bean - let's ignore it.
14 if (logger.isDebugEnabled()) {
15 logger.debug("Could not resolve methods for bean with name '" + beanName + "'", ex);
16 }
17 }
18 if (CollectionUtils.isEmpty(annotatedMethods)) {
19 this.nonAnnotatedClasses.add(targetType);
20 if (logger.isTraceEnabled()) {
21 logger.trace("No @EventListener annotations found on bean class: " + targetType.getName());
22 }
23 }
24 else {
25 // Non-empty set of methods
26 ConfigurableApplicationContext context = getApplicationContext();
27 //遍历含有@EventListener注解的方法
28 for (Method method : annotatedMethods.keySet()) {
29 for (EventListenerFactory factory : factories) {
30 if (factory.supportsMethod(method)) {
31 Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName));
32 //动态构造相对应的事件监听器
33 ApplicationListener<?> applicationListener =
34 factory.createApplicationListener(beanName, targetType, methodToUse);
35 if (applicationListener instanceof ApplicationListenerMethodAdapter) {
36 ((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
37 }
38 //将监听器添加的Spring应用上下文中托管
39 context.addApplicationListener(applicationListener);
40 break;
41 }
42 }
43 }
44 if (logger.isDebugEnabled()) {
45 logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
46 beanName + "': " + annotatedMethods);
47 }
48 }
49 }
50 }
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：
1 public class DelegatingApplicationListener 2 implements ApplicationListener<ApplicationEvent>, Ordered {
3
4 private static final String PROPERTY_NAME = "context.listener.classes";
5
6 private int order = 0;
7 //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
8 private SimpleApplicationEventMulticaster multicaster;
9
10 @Override
11 public void onApplicationEvent(ApplicationEvent event) {
12 if (event instanceof ApplicationEnvironmentPreparedEvent) {
13 // getListeners内部实现读取context.listener.classes配置的监听器
14 List<ApplicationListener<ApplicationEvent>> delegates = getListeners(
15 ((ApplicationEnvironmentPreparedEvent) event).getEnvironment());
16 if (delegates.isEmpty()) {
17 return;
18 }
19 this.multicaster = new SimpleApplicationEventMulticaster();
20 for (ApplicationListener<ApplicationEvent> listener : delegates) {
21 this.multicaster.addApplicationListener(listener);
22 }
23 }
24 //发布事件
25 if (this.multicaster != null) {
26 this.multicaster.multicastEvent(event);
27 }
28 }
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。
本文内容总结：实现自定义事件及监听，编程式实现监听，@EventListener注解方式实现，在application.properties中配置context.listener.classes，
原文链接：https://www.cnblogs.com/ashleyboy/p/9566579.html
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