探析Spring容器内部事件发布
其实在 JDK 中已经提供相应的自定义事件发布功能的基础类:
java.util.EventObject类 ：自定义事件类型java.util.EventListener接口：事件的监听器
首先了解几个概念：
Spring 事件类结构
1. 事件类
事件类也就是定义发送的内容，比如可以通过继承ApplicationContextEvent来自定义一个特定事件类。
1.1 ApplicationEvent类
首先是继承 EventObject的ApplicationEvent，通过source来指定事件源：
public abstract class ApplicationEvent extends EventObject { /** * Constructs a prototypical Event. * * @param source The object on which the Event initially occurred. * @throws IllegalArgumentException if source is null. */ public ApplicationEvent(Object source) { super(source); } }
1.2 ApplicationContextEvent类
是主要的容器事件，它有容器启动、刷新、停止以及关闭各种事件的子类。
public class ApplicationContextEvent extends ApplicationEvent { /** * Constructs a prototypical Event. * * @param source The object on which the Event initially occurred. * @throws IllegalArgumentException if source is null. */ public ApplicationContextEvent(Object source) { super(source); } /** * Get the ApplicationContext that the event was raised for. */ public final ApplicationContext getApplicationContext() { return (ApplicationContext) getSource(); } } public class ContextClosedEvent extends ApplicationContextEvent{ /** * Constructs a prototypical Event. * * @param source The object on which the Event initially occurred. * @throws IllegalArgumentException if source is null. */ public ContextClosedEvent(Object source) { super(source); } } public class ContextRefreshedEvent extends ApplicationContextEvent{ /** * Constructs a prototypical Event. * * @param source The object on which the Event initially occurred. * @throws IllegalArgumentException if source is null. */ public ContextRefreshedEvent(Object source) { super(source); } }
我们可以通过继承该类来实现，特定的事件类型需求，比如要实现一个邮件发送事件。只需要继承ApplicationContextEvent即可：
public class MailSendEvent extends ApplicationContextEvent { private String msg; public MailSendEvent(Object source, String msg) { super(source); this.msg = msg; } public String getMsg() { return msg; } public void setMsg(String msg) { this.msg = msg; } }
同时ApplicationContextEvent也有特定的几个子类，来表示容器启动、刷新、停止以及关闭事件：
2.事件监听器
事件监听器接口中，只定义了一个方法：onApplicationEvent(E event)该方法接收ApplicationEvent事件对象，在该方法中编写事件的响应处理逻辑。
public interface ApplicationListener extends EventListener { /** * 接收ApplicationEvent 事件对象 * 在该方法中编写事件的响应处理逻辑 * @param event */ void onApplicationEvent(E event); }
我们同样也可以实现该接口来实现特定的事件监听器功能，比如邮件发送的监听器：
public class MailSenderListener implements ApplicationListener { @Override public void onApplicationEvent(MailSendEvent event) { System.out.println("邮件发送器的 resource:" + event.getSource() + "邮件发送器的 msg:" + event.getMsg()); } }
3.事件广播器
事件广播器负责将事件通知监听器注册表中的事件监听器，然后再由事件监听器分别对事件进行响应。Spring中定义了如下接口：
public interface ApplicationEventMulticaster { /** * 添加事件监听器 * @param listener */ void addApplicationListener(ApplicationListener listener); /** * 移除事件监听器 * @param listener */ void removeApplicationListener(ApplicationListener listener); /** * 广播事件 * @param event */ void multicastEvent(ApplicationEvent event); }
及其简单实现类SimpleApplicationEventMulticaster：
public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster{ public SimpleApplicationEventMulticaster(BeanFactory beanFactory) { setBeanFactory(beanFactory); } /**unchecked 表示告诉编译器忽略指定的警告，不用再编译完成后出现警告信息*/ @SuppressWarnings("unchecked") @Override public void multicastEvent(ApplicationEvent event) { for (ApplicationListener applicationListener : getApplicationListeners(event)) { applicationListener.onApplicationEvent(event); } } }
4.事件发布者
它本身作为事件源，会在合适的时点，将相应事件发布给对应的事件监听器：
public interface ApplicationEventPublisher { /** * 通知监听者并发布事件 * @param event */ void publishEvent(ApplicationEvent event); }
在Spring容器事件中，ApplicationContext接口定义继承了ApplicationEventPublisher接口，所以实际上AbstractApplicationContext在事件中承担了事件发布者的角色。
但是在实际上具体实现事件的发布和事件监听器注册方面，将功能转接给ApplicationEventMulticaster接口，最终具体实现则放在AbstractApplicationEventMulticaster的实现类中：
Spring 事件类的应用
那么在Spring中，事件类到底是如何运行的呢？首先我们会在xml配置文件中配置相应的ApplicationListener类型的监听器，因此在容器启动后，这些类型的bean会被ApplicationContext容器所识别，它们负责监听容器内发布的对应的ApplicationEvent类型的事件。
在AbstractApplicationContext的refresh()方法中可以看到自动注册的内容：
public void refresh() throws BeansException { // 6. 初始化事件发布者 initApplicationEventMulticaster(); // 7. 注册事件监听器 registerListeners(); // 9. 发布容器刷新完成事件 finishRefresh(); } private void initApplicationEventMulticaster() { ConfigurableListableBeanFactory beanFactory = getBeanFactory(); applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory); beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, applicationEventMulticaster); } private void registerListeners() { Collection applicationListeners = getBeansOfType(ApplicationListener.class).values(); for (ApplicationListener listener : applicationListeners) { applicationEventMulticaster.addApplicationListener(listener); } } private void finishRefresh() { publishEvent(new ContextRefreshedEvent(this)); } public void publishEvent(ApplicationEvent event) { applicationEventMulticaster.multicastEvent(event); }
所以在ApplicationContext容器启动时，会自动注册EventListener类型的 Bean，一旦检测到有ApplicationContextEvent类型的事件发布，将通知这些注册到容器的EventListener
应用实例
下面将构建一个发送邮件的Spring事件实例：
1. 邮件发送事件MailSendEvent
public class MailSendEvent extends ApplicationContextEvent { private String msg; public MailSendEvent(Object source, String msg) { super(source); this.msg = msg; } public String getMsg() { return msg; } }
2.邮件发送事件监听器MailSendListener(邮件发送事件)、ContextRefreshedEventListener(容器刷新事件) 和 ContextClosedEventListener(容器关闭事件)
public class MailSenderListener implements ApplicationListener { @Override public void onApplicationEvent(MailSendEvent event) { System.out.println("邮件发送器的 resource:" + event.getSource() + "邮件发送器的 msg:" + event.getMsg()); } }
public class ContextClosedEventListener implements ApplicationListener { @Override public void onApplicationEvent(ContextClosedEvent event) { System.out.println("关闭事件：" + this.getClass().getName()); } }
public class ContextRefreshedEventListener implements ApplicationListener { @Override public void onApplicationEvent(ContextRefreshedEvent event) { System.out.println("刷新/打开事件：" + this.getClass().getName()); } }
这时，将监听器们注入xml文件中：
3.邮件发送事件发布者
事件发布者ApplicationEventPublisher，因为前面提到，applicationContext继承了ApplicationEventPublisher，而applicationContext将事件发布功能委托给了ApplicationEventMulticaster，容器在启动开始就会检查是否存在名称为applicationEventMulticaster的 ApplicationEventMulticaster对象实例，如果有就使用提供的实现，没有则默认初始化一个SimpleApplicationEventMulticaster作为将会使用的ApplicationEventMulticaster
/** * @description: 实现了事件监听器的管理功能 * @author: wjw * @date: 2022/7/9 */ public abstract class AbstractApplicationEventMulticaster implements ApplicationEventMulticaster, BeanFactoryAware { public final Set applicationListeners = new LinkedHashSet(); private BeanFactory beanFactory; @Override public void addApplicationListener(ApplicationListener listener) { applicationListeners.add((ApplicationListener) listener); } @Override public void removeApplicationListener(ApplicationListener listener) { applicationListeners.remove(listener); } @Override public void setBeanFactory(BeanFactory beanFactory) throws BeansException { this.beanFactory = beanFactory; } /** * 获得监听器 * @param event * @return */ protected Collection getApplicationListeners(ApplicationEvent event) { LinkedList allListeners = new LinkedList(); for (ApplicationListener listener : allListeners) { if (supportsEvent(listener, event)) { allListeners.add(listener); } } return allListeners; } protected boolean supportsEvent(ApplicationListener applicationListener, ApplicationEvent event) { Class targetClass = ClassUtils.isCglibProxyClass(listenerClass) ? listenerClass.getSuperclass() : listenerClass; Type genericInterface = targetClass.getGenericInterfaces()[0]; Type actualTypeArgument = ((ParameterizedType) genericInterface).getActualTypeArguments()[0]; String className = actualTypeArgument.getTypeName(); Class eventClassName; try { eventClassName = Class.forName(className); } catch (ClassNotFoundException e) { throw new BeansException("wrong event class name: " + className); } return eventClassName.isAssignableFrom(event.getClass()); } }
public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster{ public SimpleApplicationEventMulticaster(BeanFactory beanFactory) { setBeanFactory(beanFactory); } /**unchecked 表示告诉编译器忽略指定的警告，不用再编译完成后出现警告信息*/ @SuppressWarnings("unchecked") @Override public void multicastEvent(ApplicationEvent event) { for (ApplicationListener applicationListener : getApplicationListeners(event)) { applicationListener.onApplicationEvent(event); } } }
4.测试验证
public void test_event() { ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath:spring.xml"); applicationContext.publishEvent(new CustomEvent(applicationContext, 110L, "test！")); System.out.println("-----------------------------------------------------------------"); applicationContext.publishEvent(new MailSendEvent(applicationContext, "邮件发送测试")); applicationContext.registerShutdownHook(); }
刷新/打开事件：cn.ethan.springframework.test.event.ContextRefreshedEventListener$$EnhancerByCGLIB$$2e5c458 ----------------------------------------------------------------- 邮件发送器的 resource:cn.ethan.springframework.context.support.ClassPathXmlApplicationContext@5f2050f6邮件发送器的 msg:邮件发送测试 关闭事件：cn.ethan.springframework.test.event.ContextClosedEventListener$$EnhancerByCGLIB$$fbc2c978
