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  • 十一、Spring之事件监听

    Spring之事件监听

    ApplicationListener

    ApplicationListener是Spring事件机制的一部分,与抽象类ApplicationEvent类配合来完成ApplicationContext的事件机制。

    如果容器中存在ApplicationListener的Bean,当ApplicationContext调用publishEvent方法时,对应的Bean会被触发。这一过程是典型的观察者模式的实现。

    源码

    @FunctionalInterface
    public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
    
        /**
         * Handle an application event.
         * @param event the event to respond to
         */
        void onApplicationEvent(E event);
    
    }
    

    ContextRefreshedEvent事件的监听

    以Spring的内置事件ContextRefreshedEvent为例,当ApplicationContext被初始化或刷新时,会触发ContextRefreshedEvent事件,下面我们就实现一个ApplicationListener来监听此事件的发生。

    @Component // 需对该类进行Bean的实例化
    public class LearnListener implements ApplicationListener<ContextRefreshedEvent> {
       @Override
       public void onApplicationEvent(ContextRefreshedEvent event) {
          // 打印容器中出事Bean的数量
          System.out.println("监听器获得容器中初始化Bean数量:" + event.getApplicationContext().getBeanDefinitionCount());
       }
    }
    

    事件发布

    在容器创建完成后,在finishRefresh()方法中发布了一个事件——ContextRefreshedEvent

    我们来具体看一下这个事件是如何发布的

    protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
        Assert.notNull(event, "Event must not be null");
    
        // Decorate event as an ApplicationEvent if necessary
        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 {
            //获取事件的派发器
            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);
            }
        }
    }
    

    派发事件:getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);

    这里的执行invokeListener主要是来回调listener的接口方法

    以上就是spring中事件发布的流程。

    事件派发器

    在事件发布的过程中,有一步是获取事件的派发器,那么事件派发器是在哪里创建的呢?

    实际上在容器初始化时,执行了initApplicationEventMulticaster()这个方法,来为容器初始化事件派发器。

    protected void initApplicationEventMulticaster() {
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        //先来判断容器中有没有applicationEventMulticaster
        if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
            this.applicationEventMulticaster =
                beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
            }
        }
        else {
            //如果没有则创建一个派发器
            this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
            beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
            if (logger.isTraceEnabled()) {
                logger.trace("No '" + APPLICATION_EVENT_MULTICASTER_BEAN_NAME + "' bean, using " +
                             "[" + this.applicationEventMulticaster.getClass().getSimpleName() + "]");
            }
        }
    }
    

    APPLICATION_EVENT_MULTICASTER_BEAN_NAME:

    public static final String APPLICATION_EVENT_MULTICASTER_BEAN_NAME = "applicationEventMulticaster";
    

    监听器从哪里来

    refresh()方法中执行了registerListeners()来给容器中注册监听器

    protected void registerListeners() {
        // Register statically specified listeners first.
        for (ApplicationListener<?> listener : getApplicationListeners()) {
            getApplicationEventMulticaster().addApplicationListener(listener);
        }
    
        // Do not initialize FactoryBeans here: We need to leave all regular beans
        // uninitialized to let post-processors apply to them!
        //根据类型获取所有的监听器的Bean名称
        String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
        for (String listenerBeanName : listenerBeanNames) {
           //将监听器加入到派发器当中
           getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
        }
    
        // Publish early application events now that we finally have a multicaster...
        Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
        this.earlyApplicationEvents = null;
        if (earlyEventsToProcess != null) {
            for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
                getApplicationEventMulticaster().multicastEvent(earlyEvent);
            }
        }
    }
    

    @EventListener

    除了实现ApplicationListener接口来完成事件监听以外,@EventListener这个注解也同样可以监听事件的发生

    只需要将@EventListener标注在方法上面:

    @EventListener(classes = {ApplicationEvent.class})
    public void listen(ApplicationEvent applicationEvent){
        System.out.println("监听到:"+applicationEvent);
    }
    
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  • 原文地址:https://www.cnblogs.com/lee0527/p/11829305.html
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