Spring监听器机制

Spring监听器机制：

产生Spring事件的时候会默认使用SimpleApplicationEventMulticaster的multicastEvent来广播事件，遍历所有监听器：并使用监听器中的onApplicationEvent方法来进行事件的处理

广播事件：把一个事件发送给所有的监听器，让每一个监听器都对该事件进行处理

对发出的每一个事件都会遍历所有的监听器，然后使用监听器中的onApplicationEvent方法来进行事件的处理

 

下面是其中一个监听器的例子

listeners.starting();
//产生一个事件，并广播给所有的监听器
public void starting() {
        this.initialMulticaster.multicastEvent(new ApplicationStartingEvent(this.application, this.args));
    }

public void multicastEvent(ApplicationEvent event) {
        this.multicastEvent(event, this.resolveDefaultEventType(event));
    }

public void multicastEvent(ApplicationEvent event, @Nullable ResolvableType eventType) {
        ResolvableType type = eventType != null ? eventType : this.resolveDefaultEventType(event);
        Executor executor = this.getTaskExecutor();
        Iterator var5 = this.getApplicationListeners(event, type).iterator();

        while(var5.hasNext()) {
            ApplicationListener<?> listener = (ApplicationListener)var5.next();
            if (executor != null) {
                executor.execute(() -> {
                    this.invokeListener(listener, event);
                });
            } else {
                this.invokeListener(listener, event);
            }
        }

    }

protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
        ErrorHandler errorHandler = this.getErrorHandler();
        if (errorHandler != null) {
            try {
                this.doInvokeListener(listener, event);
            } catch (Throwable var5) {
                errorHandler.handleError(var5);
            }
        } else {
            this.doInvokeListener(listener, event);
        }

    }

private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
        try {
            listener.onApplicationEvent(event);//开始处理事件
        } catch (ClassCastException var6) {
            String msg = var6.getMessage();
            if (msg != null && !this.matchesClassCastMessage(msg, event.getClass())) {
                throw var6;
            }

            Log logger = LogFactory.getLog(this.getClass());
            if (logger.isTraceEnabled()) {
                logger.trace("Non-matching event type for listener: " + listener, var6);
            }
        }

    }

//处理事件
public void onApplicationEvent(SpringApplicationEvent event) {
        if (!Boolean.getBoolean("spring.backgroundpreinitializer.ignore") && event instanceof ApplicationStartingEvent && this.multipleProcessors() && preinitializationStarted.compareAndSet(false, true)) {
            this.performPreinitialization();
        }

        if ((event instanceof ApplicationReadyEvent || event instanceof ApplicationFailedEvent) && preinitializationStarted.get()) {
            try {
                preinitializationComplete.await();
            } catch (InterruptedException var3) {
                Thread.currentThread().interrupt();
            }
        }

    }