关于Spring的事件监听机制，你知道多少？

谁践踏了优雅 2024-03-22 11:23 55阅读 0赞

### 前言 ###

前一节，我们了解了Spring 提供的 `Aware`接口，我们可以通过这个实现这个接口的一些类获取到我们需要的东西。具体内容见前一节。 Spring 也提供了一种单机的事件机制。可以通过发送、监听，来实现一些异步操作。 使用这种 `类似 MQ` 的事件机制，我们可以通过 这个事件机制来完成一些自己的业务操作。在我们使用spring提供的事件机制时，我们只需要关注自己的事件，和自己的事件处理器。所有的事件都会继承 `ApplicationEvent`，然后再完成对于自己时间的监听器，监听器都 实现 `ApplicationListener`接口，实现 `onApplicationEvent`方法，来处理监听到的事件。 下面是事件注册、监听的整体过程。主要就是 在spring容器加载的时候，把所有的监听器统一注册到`SimpleApplicationEventMulticaster`事件分发器中，然后统一处理。

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### 实现 ###

`ApplicationEvent` 事件基类, `EventObject`是java提供的一个类

public 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);
        }
    }
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`ApplicationContextEvent`系统事件

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(ApplicationContext source) {
            super(source);
        }
    
    
        public final ApplicationContext getApplicationContext() {
            return (ApplicationContext) getSource();
        }
    }
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`ApplicationEventPublisher` 事件发布器，统一发布事件。

public interface ApplicationEventPublisher {
    
        /**
         * 发布事件
         * @param event 事件
         */
        void publishEvent(ApplicationEvent event);
    
    }
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`ApplicationEventMulticaster`事件管理、分发器。`ApplicationEventMulticaster`统一定义公共行为， `AbstractApplicationEventMulticaster`抽象类处理公共逻辑。`SimpleApplicationEventMulticaster`默认分发器，只需要执行具体的分发逻辑。`supportEvent`方法检查事件是否需要被处理。

public interface ApplicationEventMulticaster {
    
        /**
         * Add a listener to be notified of all events.
         * @param listener the listener to add
         */
        void addApplicationListener(ApplicationListener listener);
    
        /**
         * Remove a listener from the notification list.
         * @param listener the listener to remove
         */
        void removeApplicationListener(ApplicationListener listener);
    
        /**
         * Multicast the given application event to appropriate listeners.
         * @param event the event to multicast
         */
        void multicastEvent(ApplicationEvent event);
    
    }
    
    public abstract class AbstractApplicationEventMulticaster implements ApplicationEventMulticaster, BeanFactoryAware {
    
        private final Set<ApplicationListener> applicationListeners = new LinkedHashSet<>();
    
        private BeanFactory beanFactory;
    
        @Override
        public void addApplicationListener(ApplicationListener listener) {
            this.applicationListeners.add((ApplicationListener) listener);
        }
    
        @Override
        public void removeApplicationListener(ApplicationListener listener) {
            this.applicationListeners.remove(listener);
        }
    
        @Override
        public void setBeanFactory(BeanFactory beanFactory) {
            this.beanFactory = beanFactory;
        }
    
        protected Collection<ApplicationListener<?>> getApplicationListeners(ApplicationEvent event) throws BeansException {
            LinkedList<ApplicationListener<?>> allListeners = new LinkedList<>();
            for (ApplicationListener<?> listener : applicationListeners) {
                if (supportEvent(listener, event)) {
                    allListeners.add(listener);
                }
            }
            return allListeners;
        }
    
        /**
         * 监听器是否对该事件感兴趣
         */
        protected boolean supportEvent(ApplicationListener listener, ApplicationEvent event) throws BeansException {
            // 检查事件是否需要关注
            Class<? extends ApplicationListener> clazz = listener.getClass();
            Class<?> targetClass = ClassUtils.isCglibProxyClass(clazz) ? clazz.getSuperclass() : clazz;
            Type genericInterface = targetClass.getGenericInterfaces()[0];
            Type typeArgument = ((ParameterizedType) genericInterface).getActualTypeArguments()[0];
            String className = typeArgument.getTypeName();
            Class<?> eventClassName;
            try {
                eventClassName = Class.forName(className);
            } catch (ClassNotFoundException e) {
                throw new BeansException("wrong event class name: " + className);
            }
            // 判定此 eventClassName 对象所表示的类或接口与指定的 event.getClass() 参数所表示的类或接口是否相同，或是否是其超类或超接口。
            // isAssignableFrom是用来判断子类和父类的关系的，或者接口的实现类和接口的关系的，默认所有的类的终极父类都是Object。如果A.isAssignableFrom(B)结果是true，证明B可以转换成为A,也就是A可以由B转换而来。
            return eventClassName.isAssignableFrom(event.getClass());
        }
    }
    
    public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster{
        @Override
        public void multicastEvent(ApplicationEvent event) {
            try {
                // 实现事件处理
                Collection<ApplicationListener<?>> listeners = getApplicationListeners(event);
                for (ApplicationListener listener : listeners) {
                    listener.onApplicationEvent(event);
                }
            }catch (Exception e){
                // 执行失败
            }
        }
    }
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`AbstractApplicationContext`容器刷新，完成 事件分发器的注册、事件监听器的注册。

public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext {
    
        public static final String APPLICATION_EVENT_MULTICASTER_BEAN_NAME = "applicationEventMulticaster";
    
        private ApplicationEventMulticaster applicationEventMulticaster;
    
    	/**
         * 模板方法，控制定义spring上下文的刷新与创建
         * @throws BeansException
         */
        @Override
        public void refresh() throws BeansException {
            // 1. 创建beanFactory，加载beanDefinition
            refreshBeanFactory();
            // 2. 获取 beanFactory
            ConfigurableListableBeanFactory beanFactory = getBeanFactory();
            // 3. 添加 ApplicationContextAwareProcessor，让继承自 ApplicationContextAware 的 Bean 对象都能感知所属的 ApplicationContext
            beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
            // 4. bean 实例化之前 执行 beanFactoryPostProcessor
            invokeBeanFactoryPostProcessors(beanFactory);
            // 5. beanPostProcessor 完成 bean 实例化后的注册操作
            registerBeanPostProcessors(beanFactory);
            // 6. 提前实例化 单例bean 对象
            beanFactory.preInstantiateSingletons();
            // 初始化事件发布者
            initApplicationEventMulticaster();
            // 注册监听事件
            registerListeners();
            // 完成刷新
            finishRefresh();
        }
    
        private void finishRefresh() {
            publishEvent(new ContextRefreshedEvent(this));
        }
    
        private void registerListeners() {
            ConfigurableListableBeanFactory beanFactory = getBeanFactory();
            Collection<ApplicationListener> listeners = beanFactory.getBeansOfType(ApplicationListener.class).values();
            for (ApplicationListener listener : listeners) {
                applicationEventMulticaster.addApplicationListener(listener);
            }
        }
    
        private void initApplicationEventMulticaster() {
            ConfigurableListableBeanFactory beanFactory = getBeanFactory();
            applicationEventMulticaster = new SimpleApplicationEventMulticaster();
            beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, applicationEventMulticaster);
        }
    
    
        @Override
        public void publishEvent(ApplicationEvent event) {
            applicationEventMulticaster.multicastEvent(event);
        }
    
    }
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### 测试 ###

定义事件、定义事件监听器。

/**
     * @author huangle
     * @date 2023/3/14 16:21
     */
    public class ConsumerEvent extends ApplicationContextEvent {
    
        private String msgId;
    
        private String msgBody;
    
        public ConsumerEvent(ApplicationContext source, String msgId, String msgBody) {
            super(source);
            this.msgId = msgId;
            this.msgBody = msgBody;
        }
    }
    
    
    public class ConsumerEventLister implements ApplicationListener<ConsumerEvent> {
        @Override
        public void onApplicationEvent(ConsumerEvent consumerEvent) {
            System.out.println(this.getClass()+"接受到消息:"+consumerEvent.getMsgId()+"-"+consumerEvent.getMsgBody());
        }
    }
    
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注入事件监听器

<?xml version="1.0" encoding="UTF-8"?>
    <beans>
    	<bean id="consumerEventLister" class="cn.anoxia.springframework.beans.factory.support.event.ConsumerEventLister"/>
        <bean id="applicationRefershEventListener" class="cn.anoxia.springframework.beans.factory.support.event.ApplicationRefershEventListener"/>
        <bean id="applicationCloseEventListener" class="cn.anoxia.springframework.beans.factory.support.event.ApplicationCloseEventListener"/>
    </beans>
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发布事件

@Test
        public void testEvent() throws BeansException{
            ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath:spring.xml");
            applicationContext.publishEvent(new ConsumerEvent(applicationContext, UUID.randomUUID().toString(),"消息发送成功！"));
            applicationContext.registerShutdownHook();
        }
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![format_png 1][]

[format_png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/22/be5674f2ac2a4cdfb2c18f0c843dcd25.png
[format_png 1]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/22/f5172aecb6bb424fb776233c32d54646.png