spring之自定義事件及監聽 原理

上一節說了自定義事件及監聽的實現，這一節說下他的實現原理。

首先他發布事件使用過ApplicationContext.publishEvent()方法來實現,通過追蹤發現publishEvent()方法的實現是在AbstractApplicationContext抽象類中實現。

先捋請這幾個類的具體繼承和接口實現的層級關系。

eclipse下的：

 

然后再看AbstractApplicationContext下的publishEvent方法：

    /**
     * Publish the given event to all listeners.
     * @param event the event to publish (may be an {@link ApplicationEvent}
     * or a payload object to be turned into a {@link PayloadApplicationEvent})
     * @param eventType the resolved event type, if known
     * @since 4.2
     */
    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 {
               // SimpleApplicationEventMulticaster 獲取事件發布器，發布事件 28             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);
            }
        }
    }

getApplicationEventMulticaster()代碼：

/**
     * Return the internal ApplicationEventMulticaster used by the context.
     * @return the internal ApplicationEventMulticaster (never {@code null})
     * @throws IllegalStateException if the context has not been initialized yet
     */
    ApplicationEventMulticaster getApplicationEventMulticaster() throws IllegalStateException {
        if (this.applicationEventMulticaster == null) {
            throw new IllegalStateException("ApplicationEventMulticaster not initialized - " +
                    "call 'refresh' before multicasting events via the context: " + this);
        }
        return this.applicationEventMulticaster;
    }

ApplicationEventMulticaster 的multicastEvent()方法是在SimpleApplicationEventMulticaster類中實現。下面是層級結構圖。

SimpleApplicationEventMulticaster.multicastEvent()代碼

    @Override
    public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
        ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
    // getApplicationListeners(event, type) 篩選監聽器，在context.publish(ApplicationEvent event)中已經將事件傳入，getApplicationListeners中將可以根據這個event類型從Spring容器中檢索出符合條件的監聽器
        Executor executor = getTaskExecutor();
        for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
            if (executor != null) {
                executor.execute(() -> invokeListener(listener, event));
            }
            else {
                invokeListener(listener, event);//嘗試逐個向監聽器廣播
            }
        }
    }

下面是進行debug的結果：

      

在multicastEvent發布完，就直接循環event事件