springboot学习----事件监听

起源

Spring的Application拥有发布事件并且注册事件监听器的能力，拥有一套完整的事件发布和监听机制。在Java中，通过java.util. EventObject来描述事件，通过java.util. EventListener来描述事件监听器，在众多的框架和组件中，建立一套事件机制通常是基于这两个接口来进行扩展。

概念

• ApplicationEvent就是Spring的事件接口
• ApplicationListener就是Spring的事件监听器接口，所有的监听器都实现该接口
• ApplicationEventPublisher是Spring的事件发布接口，ApplicationContext实现了该接口
• ApplicationEventMulticaster就是Spring事件机制中的事件广播器，默认实现SimpleApplicationEventMulticaster

在Spring中通常是ApplicationContext本身担任监听器注册表的角色，在其子类AbstractApplicationContext中就聚合了事件广播器ApplicationEventMulticaster和事件监听器ApplicationListnener，并且提供注册监听器的addApplicationListnener方法。

执行流程

当一个事件源产生事件时，它通过事件发布器ApplicationEventPublisher发布事件，然后事件广播器ApplicationEventMulticaster会去事件注册表ApplicationContext中找到事件监听器ApplicationListnener，并且逐个执行监听器的onApplicationEvent方法，从而完成事件监听器的逻辑。

默认启动事件

• ApplicationStartedEvent
• ApplicationEnvironmentPreparedEvent
• ApplicationPreparedEvent
• ApplicationFailedEvent

源码片段

AbstractApplicationContext实现了ApplicationEventPublisher接口方法,通过ApplicationEventMulticaster广播事件

public void publishEvent(ApplicationEvent event) {        Assert.notNull(event, "Event must not be null");        if(this.logger.isTraceEnabled()) {            this.logger.trace("Publishing event in " + this.getDisplayName() + ": " + event);        }        this.getApplicationEventMulticaster().multicastEvent(event);        if(this.parent != null) {            this.parent.publishEvent(event);        }    }

spring boot 系统事件分发

EventPublishingRunListener中通过initialMulticaster广播事件

public ConfigurableApplicationContext run(String... args) {        StopWatch stopWatch = new StopWatch();        stopWatch.start();        ConfigurableApplicationContext context = null;        Object analyzers = null;        this.configureHeadlessProperty();        //获取RunListeners 通过SpringFactoriesLoader获取spring.factories配置        //默认实现类EventPublishingRunListener        SpringApplicationRunListeners listeners = this.getRunListeners(args);        //发布ApplicationStartingEvent事件          listeners.starting();        try {            DefaultApplicationArguments ex = new DefaultApplicationArguments(args);            //发布ApplicationEnvironmentPreparedEvent事件            ConfigurableEnvironment environment = this.prepareEnvironment(listeners, ex);            Banner printedBanner = this.printBanner(environment);            context = this.createApplicationContext();            new FailureAnalyzers(context);            //发布ApplicationPreparedEvent事件            this.prepareContext(context, environment, listeners, ex, printedBanner);             this.refreshContext(context);            this.afterRefresh(context, ex);            //发布ApplicationReadyEvent事件            listeners.finished(context, (Throwable)null);            stopWatch.stop();            if(this.logStartupInfo) {                (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);            }            return context;        } catch (Throwable var9) {            this.handleRunFailure(context, listeners, (FailureAnalyzers)analyzers, var9);            throw new IllegalStateException(var9);        }    }

EventPublishingRunListener 以starting为例，发布ApplicationStartedEvent启动事件

 public void starting() {        this.initialMulticaster.multicastEvent(new ApplicationStartedEvent(this.application, this.args));    }

SimpleApplicationEventMulticaster

 public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {        ResolvableType type = eventType != null?eventType:this.resolveDefaultEventType(event);        //根据事件类型获取事件对应的ApplicationListener        Iterator var4 = this.getApplicationListeners(event, type).iterator();        while(var4.hasNext()) {            final ApplicationListener listener = (ApplicationListener)var4.next();            //如果配置了线程池，则异步执行listener的onApplicationEvent方法            Executor executor = this.getTaskExecutor();            if(executor != null) {                executor.execute(new Runnable() {                    public void run() {                        SimpleApplicationEventMulticaster.this.invokeListener(listener, event);                    }                });            } else {                this.invokeListener(listener, event);            }        }    }//执行监听中onApplicationEvent方法protected void invokeListener(ApplicationListener listener, ApplicationEvent event) {        ErrorHandler errorHandler = this.getErrorHandler();        if(errorHandler != null) {            try {                listener.onApplicationEvent(event);            } catch (Throwable var7) {                errorHandler.handleError(var7);            }        } else {            try {                listener.onApplicationEvent(event);            } catch (ClassCastException var8) {                String msg = var8.getMessage();                if(msg != null && !msg.startsWith(event.getClass().getName())) {                    throw var8;                }                Log logger = LogFactory.getLog(this.getClass());                if(logger.isDebugEnabled()) {                    logger.debug("Non-matching event type for listener: " + listener, var8);                }            }        }    }

参考

http://liuxiamai.iteye.com/blog/2322197
http://blog.csdn.net/nution/article/details/72765813
http://blog.csdn.net/chszs/article/details/49097919