{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":" 1. 背景"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲啥突然想到寫這個？起因就是看到了Nacos的"},{"type":"codeinline","content":[{"type":"text","text":"#3757 ISSUE"}]},{"type":"text","text":"，理解錯誤， 以爲是服務啓動，沒有註冊上服務，實際namespace不同，導致服務無法註冊。 但這絲毫不影響再去研究了一波代碼，順便也看到了Nacos是如何利用Spring的事件來進行服務註冊的。分享一波，歡迎大家學習指正！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"2. 娓娓道來 - Spring事件 "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2.1 怎麼發佈事件"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們大家應該都知道，在Spring中，是通過實現"},{"type":"codeinline","content":[{"type":"text","text":"org.springframework.context.ApplicationEventPublisher"}]},{"type":"text","text":"來發佈一個事件。"},{"type":"codeinline","content":[{"type":"text","text":"ApplicationEcentPublisher"}]},{"type":"text","text":"是一個接口，我們來看一下接口中邏輯。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public interface ApplicationEventPublisher {\n\n\t/**\n\t * Notify all matching listeners registered with this\n\t * application of an application event. Events may be framework events\n\t * (such as RequestHandledEvent) or application-specific events.\n\t * @param event the event to publish\n\t * @see org.springframework.web.context.support.RequestHandledEvent\n\t */\n\tdefault void publishEvent(ApplicationEvent event) {\n\t\tpublishEvent((Object) event);\n\t}\n\n\t/**\n\t * Notify all matching listeners registered with this\n\t * application of an event.\n\t * If the specified {@code event} is not an {@link ApplicationEvent},\n\t * it is wrapped in a {@link PayloadApplicationEvent}.\n\t * @param event the event to publish\n\t * @since 4.2\n\t * @see PayloadApplicationEvent\n\t */\n\tvoid publishEvent(Object event);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"接口很簡單，裏面有兩個方法，都是發佈事件，而接口默認實現，是調用"},{"type":"codeinline","content":[{"type":"text","text":"publishEvent(Object event)"}]},{"type":"text","text":"的實現，唯一的區別就是default方法的參數是具體的事件類。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"既然這是個接口，那一定有實現類，那麼我們肯定是要進入實現類，快捷鍵，查看一下具體實現類。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/7e/7e6c97cca5d3f072204e11c1e94543f9.jpeg","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們通過實現類，可以看到，應該是從"},{"type":"codeinline","content":[{"type":"text","text":"AbstractApplicationContext"}]},{"type":"text","text":"進去（其他Context都是實現類，憑感覺和代碼提示，從第一個類進如代碼）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"進去"},{"type":"codeinline","content":[{"type":"text","text":"org.springframework.context.support.AbstractApplicationContext"}]},{"type":"text","text":"，我們直奔主題，查看是如何實現PublishEvent。具體代碼如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"/**\n\t * Publish the given event to all listeners.\n\t * @param event the event to publish (may be an {@link ApplicationEvent}\n\t * or a payload object to be turned into a {@link PayloadApplicationEvent})\n\t * @param eventType the resolved event type, if known\n\t * @since 4.2\n\t */\n\tprotected void publishEvent(Object event, @Nullable ResolvableType eventType) {\n\t\tAssert.notNull(event, \"Event must not be null\");\n\n\t\t// Decorate event as an ApplicationEvent if necessary\n\t\tApplicationEvent applicationEvent;\n\t\tif (event instanceof ApplicationEvent) {\n // 實現的具體的ApplicationEvent, 直接進行轉化即可\n\t\t\tapplicationEvent = (ApplicationEvent) event;\n\t\t}\n\t\telse {\n // 將事件裝飾成PayloadApplicationEvent，\n\t\t\tapplicationEvent = new PayloadApplicationEvent<>(this, event);\n\t\t\tif (eventType == null) {\n\t\t\t\teventType = ((PayloadApplicationEvent>) applicationEvent).getResolvableType();\n\t\t\t}\n\t\t}\n\n\t\t// Multicast right now if possible - or lazily once the multicaster is initialized\n\t\tif (this.earlyApplicationEvents != null) {\n // 廣播還未初始化完成，放入earlyApplicationEvents.\n\t\t\tthis.earlyApplicationEvents.add(applicationEvent);\n\t\t}\n\t\telse {\n // 核心關注點： 使用廣播廣播事件，\n\t\t\tgetApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);\n\t\t}\n\n\t\t// Publish event via parent context as well...\n // 父容器的監聽器進行廣播\n\t\tif (this.parent != null) {\n\t\t\tif (this.parent instanceof AbstractApplicationContext) {\n\t\t\t\t((AbstractApplicationContext) this.parent).publishEvent(event, eventType);\n\t\t\t}\n\t\t\telse {\n\t\t\t\tthis.parent.publishEvent(event);\n\t\t\t}\n\t\t}\n\t}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在這段代碼中，我們得到了兩點有用的信息，分別是："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"發佈事件的整體流程。（封裝事件，廣播事件）"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"發佈的時間通過"},{"type":"codeinline","content":[{"type":"text","text":"getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);"}]},{"type":"text","text":"廣播事件"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通過代碼可以知道，我們接下來的關注點是"},{"type":"codeinline","content":[{"type":"text","text":"getApplicationEventMulticaster()"}]},{"type":"text","text":"，如何通過"},{"type":"codeinline","content":[{"type":"text","text":"multicastEvent()"}]},{"type":"text","text":"方法將事件廣播出去呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2.2 什麼是ApplicationEventMulticaster"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"進入到核心關注點代碼，"},{"type":"codeinline","content":[{"type":"text","text":"getApplicationEventMulticaster()"}]},{"type":"text","text":"是什麼？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"ApplicationEventMulticaster getApplicationEventMulticaster() throws IllegalStateException {\n // ...略... 沒什麼重要信息\n return this.applicationEventMulticaster;\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"代碼很簡單，沒什麼邏輯，但是讓我們明白了"},{"type":"codeinline","content":[{"type":"text","text":"getApplicationEventMulticaster()"}]},{"type":"text","text":"獲取到的對象是"},{"type":"codeinline","content":[{"type":"text","text":"ApplicationEventMulticaster"}]},{"type":"text","text":"，那我們接下來搞懂這個東西（"},{"type":"codeinline","content":[{"type":"text","text":"ApplicationEventMulticaster"}]},{"type":"text","text":"）是如何初始化？理清楚後可以弄清楚邏輯了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"輕輕鬆鬆翻到"},{"type":"codeinline","content":[{"type":"text","text":"ApplicationEventMulticaster"}]},{"type":"text","text":"初始化的代碼（給自己鼓個掌👏），代碼如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"/**\n * Initialize the ApplicationEventMulticaster.\n * Uses SimpleApplicationEventMulticaster if none defined in the context.\n * @see org.springframework.context.event.SimpleApplicationEventMulticaster\n */\nprotected void initApplicationEventMulticaster() {\n ConfigurableListableBeanFactory beanFactory = getBeanFactory();\n // 判斷容器中是否已經創建ApplicationEventMulticaster\n if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {\n this.applicationEventMulticaster =\n beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);\n }\n else {\n // 初始化默認ApplicationEventMulticaster\n this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);\n beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"代碼很簡單，就是判斷容器中是否已經包含了"},{"type":"codeinline","content":[{"type":"text","text":"ApplicationEventMulticaster"}]},{"type":"text","text":"（可以理解爲是否自己實現了"},{"type":"codeinline","content":[{"type":"text","text":"ApplicationEventMulticaster"}]},{"type":"text","text":"），如果沒實現，則採用Spring的默認實現"},{"type":"codeinline","content":[{"type":"text","text":"SimpleApplicationEventMulticaster"}]},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"到目前爲止，我們已經知道的信息是："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"SimpleApplicationEventMulticaster"}]},{"type":"text","text":"是"},{"type":"codeinline","content":[{"type":"text","text":"ApplicationEventMulticaster"}]},{"type":"text","text":"的默認實現"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"搞到了這個有用的信息，那麼我們去搞懂"},{"type":"codeinline","content":[{"type":"text","text":"SimpleApplicationEventMulticaster"}]},{"type":"text","text":"是什麼就可以知道Spring的廣播流程了"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"2.2.1 SimpleApplicationEventMulticaster 是什麼？如何廣播事件？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"帶着思考與疑惑，我們進入"},{"type":"codeinline","content":[{"type":"text","text":"SimpleApplicationEventMulticaster"}]},{"type":"text","text":"，就看看"},{"type":"codeinline","content":[{"type":"text","text":"multicastEvent()"}]},{"type":"text","text":"方法， 因爲上面代碼中有寫到，發送事件是這樣操作的："},{"type":"codeinline","content":[{"type":"text","text":"getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"方法如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {\n // 解析事件類型。（不知道沒關係，先通過名字猜，然後再去驗證自己的猜想）\n ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));\n // 遍歷所有的監聽者，\n for (final ApplicationListener> listener : getApplicationListeners(event, type)) {\n Executor executor = getTaskExecutor();\n if (executor != null) {\n // 有線程池，則用線程池\n executor.execute(() -> invokeListener(listener, event));\n }\n else {\n // 沒有線程池則直接調用\n invokeListener(listener, event);\n }\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"代碼看到這， 我們已經有了大概的瞭解，但大家應該還有兩個疑問，分別是："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"getApplicationListeners(event, type)"}]},{"type":"text","text":"是什麼？（這個很重要，包含著爲什麼特定的Listener能接收到特定的事件）"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"invokeListener(listener, event);"}]},{"type":"text","text":"裏面的邏輯是什麼？"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在這兩個問題中，第一個問題我們暫時先放下，因爲這暫時不涉及事件調用流程，後邊說。我們就先說一下第二個以爲。"},{"type":"codeinline","content":[{"type":"text","text":"invokeListener(listener, event);"}]},{"type":"text","text":"邏輯是什麼。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們帶着疑惑，接着進入代碼："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {\n try {\n // 監聽器處理事件\n listener.onApplicationEvent(event);\n }\n catch (ClassCastException ex) {\n ...略...\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"到這，知道了。邏輯蠻簡單的，就是調用Listener處理事件的邏輯。我們腦海中的有個大概流程圖了，如下圖所示："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/97/97000d74acb55db696dc562e0d9c430e.jpeg","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通過"},{"type":"codeinline","content":[{"type":"text","text":"AbstractApplicationContext實現類"}]},{"type":"text","text":"發佈事件，在通過"},{"type":"codeinline","content":[{"type":"text","text":"SimpleApplicationEventMulticaster"}]},{"type":"text","text":"將事件廣播到所有Listener，就完成了整個過程的調用。但是，我們還有其中二塊沒有搞清楚，分別是："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"allListener是怎麼來的呢？ "}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"爲什麼能根據事件類型來決定調用哪些Listener？"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2.3 Listener的祕密"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"代碼疑問，回到代碼。從"},{"type":"codeinline","content":[{"type":"text","text":"multicastEvent()"}]},{"type":"text","text":"方法中的"},{"type":"codeinline","content":[{"type":"text","text":"getApplicationListeners(event, type)"}]},{"type":"text","text":"進入，發現進入到了"},{"type":"codeinline","content":[{"type":"text","text":"AbstractApplicationEventMulticaster"}]},{"type":"text","text":"代碼中，發現了Listener的增減和刪除邏輯（爲什麼先看這兩個？類的代碼不多，可以先隨便瞅瞅，不要太侷限）。增加代碼邏輯如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"// 通過實現類添加listener\npublic void addApplicationListener(ApplicationListener> listener) {\n synchronized (this.retrievalMutex) {\n // Explicitly remove target for a proxy, if registered already,\n // in order to avoid double invocations of the same listener.\n Object singletonTarget = AopProxyUtils.getSingletonTarget(listener);\n if (singletonTarget instanceof ApplicationListener) {\n this.defaultRetriever.applicationListeners.remove(singletonTarget);\n }\n this.defaultRetriever.applicationListeners.add(listener);\n this.retrieverCache.clear();\n }\n}\n// 通過beanName添加listener\npublic void addApplicationListenerBean(String listenerBeanName) {\n synchronized (this.retrievalMutex) {\n this.defaultRetriever.applicationListenerBeans.add(listenerBeanName);\n this.retrieverCache.clear();\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"從參數可以看出，兩個方法的差一點，就不細說。但是我們好奇啊，代碼中的"},{"type":"codeinline","content":[{"type":"text","text":"defaultRetriever"}]},{"type":"text","text":"是什麼，有屬性？爲什麼listener保存在這個裏面？還是帶着疑惑，進入代碼，核心代碼如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public Collection