spring之自定义事件及监听 原理

上一节说了自定义事件及监听的实现，这一节说下他的实现原理。

首先他发布事件使用过ApplicationContext.publishEvent()方法来实现,通过追踪发现publishEvent()方法的实现是在AbstractApplicationContext抽象类中实现。

先捋请这几个类的具体继承和接口实现的层级关系。

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 获取事件发布器，发布事件
            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()代码：

/**
     * 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类中实现。下面是层级结构图。

SimpleApplicationEventMulticaster.multicastEvent()代码

    @Override
    public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
        ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
    // getApplicationListeners(event, type) 筛选监听器，在context.publish(ApplicationEvent event)中已经将事件传入，getApplicationListeners中将可以根据这个event类型从Spring容器中检索出符合条件的监听器
        Executor executor = getTaskExecutor();
        for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
            if (executor != null) {
                executor.execute(() -> invokeListener(listener, event));
            }
            else {
                invokeListener(listener, event);//尝试逐个向监听器广播
            }
        }
    }

下面是进行debug的结果：

      

在multicastEvent发布完，就直接循环event事件