Java 圖解Spring啟動時的後置處理器工作流程是怎樣的

AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
// ......
applicationContext.refresh();

public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			// 啟動前的準備工作：記錄啟動時間，活動標記為啟動以及環境屬性變量集合的初始化
			prepareRefresh();
			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
			// Prepare the bean factory for use in this context.
			//還是一些準備工作，添加瞭兩個後置處理器：ApplicationContextAwareProcessor，ApplicationListenerDetector
			//還設置瞭 忽略自動裝配 和 允許自動裝配 的接口
			//對環境，系統環境，系統屬性三個Bean如果不存在某個bean的時候，spring就自動生成singleton bean（Not bd）
			//還設置瞭bean表達式解析器 等
			prepareBeanFactory(beanFactory);
			try {
				// Allows post-processing of the bean factory in context subclasses.
				// 空方法
				postProcessBeanFactory(beanFactory);
				// Invoke factory processors registered as beans in the context.
				//執行自定義的BeanFactoryProcessor和內置的BeanFactoryProcessor
				invokeBeanFactoryPostProcessors(beanFactory);
				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);
				// Initialize message source for this context.
				initMessageSource();
				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();
				// Initialize other special beans in specific context subclasses.
				onRefresh();
				// Check for listener beans and register them.
				registerListeners();
				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);
				// Last step: publish corresponding event.
				finishRefresh();
			}
			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}

	protected void prepareRefresh() {
		// Switch to active.
		this.startupDate = System.currentTimeMillis();
		this.closed.set(false);
		this.active.set(true);
		// Initialize any placeholder property sources in the context environment.
		// 空方法
		initPropertySources();
		// Validate that all properties marked as required are resolvable:
		// see ConfigurablePropertyResolver#setRequiredProperties
		getEnvironment().validateRequiredProperties();
		// Store pre-refresh ApplicationListeners...
		if (this.earlyApplicationListeners == null) {
			this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);
		}
		else {
			// Reset local application listeners to pre-refresh state.
			this.applicationListeners.clear();
			this.applicationListeners.addAll(this.earlyApplicationListeners);
		}
		// Allow for the collection of early ApplicationEvents,
		// to be published once the multicaster is available...
		this.earlyApplicationEvents = new LinkedHashSet<>();
	}

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
		// CAS保證同步
		refreshBeanFactory();
		// 返回beanFactory- DefaultListableBeanFactory.class
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();
		return beanFactory;
	}

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory)  {
    	// Tell the internal bean factory to use the context's class loader etc.
		beanFactory.setBeanClassLoader(getClassLoader());
		//設置bean表達式解析器
		beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
		//屬性編輯器支持
		beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

		// Configure the bean factory with context callbacks.
		//添加一個後置處理器：ApplicationContextAwareProcessor，此後置處理處理器實現瞭BeanPostProcessor接口
		beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
		//以下接口，忽略自動裝配
		beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
   		// .....
    	// BeanFactory interface not registered as resolvable type in a plain factory.
		// MessageSource registered (and found for autowiring) as a bean.
		//以下接口，允許自動裝配,第一個參數是自動裝配的類型，，第二個字段是自動裝配的值
		// 這個接口僅會將註入的參數XXX.class註入為指定的值，但不影響XXX.class創建Bean對象；
		beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
		beanFactory.registerResolvableDependency(ResourceLoader.class, this);
    	// Register default environment beans.
		// 環境，系統環境，系統屬性 因此通常情況下，這三個Bean是沒有bd的
		//如果沒有註冊過bean名稱為XXX，spring就自己創建一個名稱為XXX的singleton bean
		if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
			beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
		}
		if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
			beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
		}
		if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
			beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
		}
}

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
	// getBeanFactoryPostProcessors是spring允許我們手動添加BeanFactoryPostProcessor
	// 即：annotationConfigApplicationContext.addBeanFactoryPostProcessor(XXX);
    // 未手動添加的話，getBeanFactoryPostProcessors()為empty
		PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

		// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
		// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
		if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
			beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
			beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
		}
	}

public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,
		PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {}

public static void invokeBeanFactoryPostProcessors(
			ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

		// Invoke BeanDefinitionRegistryPostProcessors first, if any.
		Set<String> processedBeans = new HashSet<>();

		// 如果不是BeanDefinitionRegistry 則直接執行beanFactoryPostProcessors
		// 剛啟動時傳入的beanFactory是DefaultListableBeanFactory，他是實現瞭BeanDefinitionRegistry 因此會走這裡
		if (beanFactory instanceof BeanDefinitionRegistry) {
			BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
			// bf後置處理器集合（手動添加與bdr後置處理器集合【下面的那個集合】）：因為bdrp屬於bfp
			List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
			// bdr後置處理器集合（手動添加與spring自己的）
			List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();

			// 循環傳進來的beanFactoryPostProcessors，剛啟動時未手動增加的情況下beanFactoryPostProcessors肯定沒有數據
			// 因為beanFactoryPostProcessors是獲得手動添加的，而不是spring掃描的
			// 隻有手動調用annotationConfigApplicationContext.addBeanFactoryPostProcessor(XXX)才會有數據
			// 執行手動添加的beanFactoryPostProcessors， 如果是BeanDefinitionRegistryPostProcessor，則執行其postProcessBeanDefinitionRegistry再加到list中
			for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
				if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
					BeanDefinitionRegistryPostProcessor registryProcessor =
							(BeanDefinitionRegistryPostProcessor) postProcessor;
					registryProcessor.postProcessBeanDefinitionRegistry(registry);
					registryProcessors.add(registryProcessor);
				}
				else {
					regularPostProcessors.add(postProcessor);
				}
			}
			//一個臨時變量，用來裝載BeanDefinitionRegistryPostProcessor為瞭排序
			//BeanDefinitionRegistry繼承瞭PostProcessorBeanFactoryPostProcessor
			List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();

			// 獲得實現BeanDefinitionRegistryPostProcessor接口的類
			// 就是ConfigurationClassPostProcessor（Spring自己添加的-在reader()時增加的）
			String[] postProcessorNames =
					beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			for (String ppName : postProcessorNames) {
				if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
					//獲得ConfigurationClassPostProcessor類，並且放到currentRegistryProcessors
					//ConfigurationClassPostProcessor是很重要的一個類，它實現瞭BeanDefinitionRegistryPostProcessor接口
					//BeanDefinitionRegistryPostProcessor接口又實現瞭BeanFactoryPostProcessor接口
					//ConfigurationClassPostProcessor是極其重要的類
					//裡面執行瞭掃描Bean，Import，ImportResouce等各種操作
					//用來處理配置類（有兩種情況 一種是傳統意義上的配置類，一種是普通的bean）的各種邏輯
					currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
					//把name放到processedBeans，後續會根據這個集合來判斷處理器是否已經被執行過瞭
					processedBeans.add(ppName);
				}
			}
			//處理排序
			sortPostProcessors(currentRegistryProcessors, beanFactory);
			//合並Processors，為什麼要合並，因為registryProcessors是裝載BeanDefinitionRegistryPostProcessor的
			//一開始的時候，spring隻會執行BeanDefinitionRegistryPostProcessor獨有的方法
			//而不會執行BeanDefinitionRegistryPostProcessor父類的方法，即BeanFactoryProcessor的方法
			//所以這裡需要把處理器放入一個集合中，後續統一執行父類的方法
			registryProcessors.addAll(currentRegistryProcessors);
			//可以理解為執行ConfigurationClassPostProcessor的postProcessBeanDefinitionRegistry方法
			//Spring熱插播的體現，像ConfigurationClassPostProcessor就相當於一個組件，Spring很多事情就是交給組件去管理
			//將spring提供的RegistryProcessors(就是這個ConfigurationClassPostProcessor)執行其postProcessBeanDefinitionRegistry
			invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
			//清空臨時變量
			currentRegistryProcessors.clear();
			// 再次根據BeanDefinitionRegistryPostProcessor獲得BeanName，看這個BeanName是否已經被執行過瞭，有沒有實現Ordered接口
			// 如果沒有被執行過，也實現瞭Ordered接口的話，把對象推送到currentRegistryProcessors，名稱推送到processedBeans
			// 如果沒有實現Ordered接口的話，這裡不把數據加到currentRegistryProcessors，processedBeans中，後續再做處理
			// 這裡才可以獲得我們定義的實現瞭BeanDefinitionRegistryPostProcessor的Bean
			postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			for (String ppName : postProcessorNames) {
				if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
					currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
					processedBeans.add(ppName);
				}
			}
			//處理排序
			sortPostProcessors(currentRegistryProcessors, beanFactory);
			//合並Processors
			registryProcessors.addAll(currentRegistryProcessors);
			//執行有Ordered的BeanDefinitionRegistryPostProcessor
			invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
			//清空臨時變量
			currentRegistryProcessors.clear();

			// Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
			// 下面的代碼就是執行沒有實現PriorityOrdered接口也沒有Ordered的BeanDefinitionRegistryPostProcessor
			boolean reiterate = true;
			while (reiterate) {
				reiterate = false;
				postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
				for (String ppName : postProcessorNames) {
					if (!processedBeans.contains(ppName)) {
						currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
						processedBeans.add(ppName);
						reiterate = true;
					}
				}
				sortPostProcessors(currentRegistryProcessors, beanFactory);
				registryProcessors.addAll(currentRegistryProcessors);
				invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
				currentRegistryProcessors.clear();
			}

			// Now, invoke the postProcessBeanFactory callback of all processors handled so far.
			//registryProcessors集合裝載BeanDefinitionRegistryPostProcessor
			//上面的代碼是執行bfr後置處理器子類獨有的方法，這裡需要再把bfr後置處理器父類的方法也執行一次
			invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
			//regularPostProcessors裝載BeanFactoryPostProcessor，執行BeanFactoryPostProcessor的方法
			//但是regularPostProcessors一般情況下，是不會有數據的，隻有在外面手動添加BeanFactoryPostProcessor，才會有數據
			invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
		}

		else {
			// Invoke factory processors registered with the context instance.
			// 若bfp沒有繼承bdrp則直接執行手動增加bf後置處理器的後置處理器
			invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
		}

		// Do not initialize FactoryBeans here: We need to leave all regular beans
		// uninitialized to let the bean factory post-processors apply to them!
		// 找到BeanFactoryPostProcessor實現類的BeanName數組
		// 處理Spring自己的bf後置處理器
		String[] postProcessorNames =
				beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

		// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
		// Ordered, and the rest.
		// PriorityOrdered的bf後置處理器集合
		List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
		// Ordered的bf後置處理器集合
		List<String> orderedPostProcessorNames = new ArrayList<>();
		// 無PriorityOrdered無Ordered的bf後置處理器集合
		List<String> nonOrderedPostProcessorNames = new ArrayList<>();
		//循環BeanName數組
		for (String ppName : postProcessorNames) {
			//如果這個Bean被執行過瞭，跳過
			if (processedBeans.contains(ppName)) {
				// skip - already processed in first phase above
			}
			//如果實現瞭PriorityOrdered接口，加入到priorityOrderedPostProcessors
			else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
				priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
			}
			//如果實現瞭Ordered接口，加入到orderedPostProcessorNames
			else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
				orderedPostProcessorNames.add(ppName);
			}
			//如果既沒有實現PriorityOrdered，也沒有實現Ordered。加入到nonOrderedPostProcessorNames
			else {
				nonOrderedPostProcessorNames.add(ppName);
			}
		}

		// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
		//排序處理priorityOrderedPostProcessors，即實現瞭PriorityOrdered接口的BeanFactoryPostProcessor
		sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
		//執行priorityOrderedPostProcessors
		invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

		// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
		//執行實現瞭Ordered接口的BeanFactoryPostProcessor
		List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>();
		for (String postProcessorName : orderedPostProcessorNames) {
			orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
		}
		sortPostProcessors(orderedPostProcessors, beanFactory);
		invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

		// Finally, invoke all other BeanFactoryPostProcessors.
		// 執行既沒有實現PriorityOrdered接口，也沒有實現Ordered接口的BeanFactoryPostProcessor
		List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
		for (String postProcessorName : nonOrderedPostProcessorNames) {
			nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
		}
		invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

		// Clear cached merged bean definitions since the post-processors might have
		// modified the original metadata, e.g. replacing placeholders in values...
		// 清除瞭allBeanNamesByType&singletonBeanNamesByType()
		// 清除緩存中的bd，因為後處理器可能有修改瞭原始元數據，例如替換值中的占位符
		beanFactory.clearMetadataCache();
	}