Spring Boot 自定义事件及监听

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：

  1. 自定义事件，继承org.springframework.context.ApplicationEvent抽象类

  2. 定义事件监听器，实现org.springframework.context.ApplicationListener接口

  3. 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<ApplicationEventTest> {
 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：

  1. 编程式：通过实现ApplicationListener接口来监听指定类型的事件

  2. 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 @Component
2 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

@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。

 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。

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原文地址：https://www.cnblogs.com/ashleyboy/p/9566579.html