Spring源碼深度解析（十）——Spring內置後置處理器哪來的

今天在看源碼的時候，有些後置處理器莫名其妙就加了進來，因此我就看看它是從哪來的？

Spring內部會加載的選項如下

/**
	 * Register all relevant annotation post processors in the given registry.
	 * @param registry the registry to operate on
	 * @param source the configuration source element (already extracted)
	 * that this registration was triggered from. May be {@code null}.
	 * @return a Set of BeanDefinitionHolders, containing all bean definitions
	 * that have actually been registered by this call
	 */
	public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
			BeanDefinitionRegistry registry, @Nullable Object source) {

		//得到DefaultListableBeanFactory工廠
		DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
		if (beanFactory != null) {
			//判斷有沒有排序相關的註解
			if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
				//(1)AnnotationAwareOrderComparator主要解析@Priority和@Order等排序註解
				beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
			}
			if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
				//(2)提供處理延遲加載功能ContextAnnotationAutowireCandidateResolver
				beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
			}
		}

		//BeanDefinitionHolder是存的beanName，alias和BeanDefinition的一個對應關係的數據結構
		Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(8);

		if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			//(3)internalConfigurationAnnotationProcessor，處理配置類
			RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
			def.setSource(source);
			//間接調用BDR的registryBeanDefinition方法將這個BD註冊進Map去
			beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		// 註冊AutowiredAnnotationProcessor
		if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			//（4）internalAutowiredAnnotationProcessor  處理@Autowired
			RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		// Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
		if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			// （5）internalCommonAnnotationProcessor   在bean實例化那塊有用到
			RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		// Check for JPA support, and if present add the PersistenceAnnotationBeanPostProcessor.
		if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			// （6）internalPersistenceAnnotationProcessor  如果存在jpa支持就註冊
			RootBeanDefinition def = new RootBeanDefinition();
			try {
				def.setBeanClass(ClassUtils.forName(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME,
						AnnotationConfigUtils.class.getClassLoader()));
			}
			catch (ClassNotFoundException ex) {
				throw new IllegalStateException(
						"Cannot load optional framework class: " + PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, ex);
			}
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		if (!registry.containsBeanDefinition(EVENT_LISTENER_PROCESSOR_BEAN_NAME)) {
			//（7）internalEventListenerProcessor   事件監聽後置處理器
			RootBeanDefinition def = new RootBeanDefinition(EventListenerMethodProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_PROCESSOR_BEAN_NAME));
		}

		if (!registry.containsBeanDefinition(EVENT_LISTENER_FACTORY_BEAN_NAME)) {
			//（8）internalEventListenerFactory  事件監聽工廠
			RootBeanDefinition def = new RootBeanDefinition(DefaultEventListenerFactory.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_FACTORY_BEAN_NAME));
		}

		return beanDefs;
	}

可以看到在單純的Spring會默認去註冊五個Bean

就是如下的五個

1. internalConfigurationAnnotationProcessor
2. internalAutowiredAnnotationProcessor
3. internalCommonAnnotationProcessor
4. internalEventListenerProcessor
5. internalEventListenerFactory

最後跟蹤發現，它的註冊是在AnnotationConfigApplicationContext中的 new AnnotatedBeanDefinitionReader(this)進行的

public AnnotationConfigApplicationContext() {
		/**
		 * 實例化reader
		 * AnnotatedBeanDefinition  被註解的Bean
		 * 讀取那些被加註解了的Bean   會默認去註冊那些Spring的內部類
		 */
		this.reader = new AnnotatedBeanDefinitionReader(this);

		/**
		 * 實例化一個scanner
		 * 能夠掃描類，包，並轉換爲bd
		 * 這個scan存在的意義就是手動掃描，平時用的@ComponentScan並沒有用這個scanner而是又重新new了一個ClassPathBeanDefinitionScanner
		 */
		this.scanner = new ClassPathBeanDefinitionScanner(this);
	}

調用它的構造方法

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {
		this(registry, getOrCreateEnvironment(registry));
	}

	public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
		Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
		Assert.notNull(environment, "Environment must not be null");
		this.registry = registry;
		this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
		//AnnotatedBeanDefinitionReader將具體任務交給AnnotationConfigUtils.registerAnnotationConfigProcessors
		// 註冊Spring內部Bean
		AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
	}

從而調用AnnotationConfigUtils的registerAnnotationConfigProcessors

	/**
	 * Register all relevant annotation post processors in the given registry.
	 * @param registry the registry to operate on
	 *
	 * 在給定註冊表中註冊所有相關的註解類的處理器，就是Spring內部的8個內部bean，如果是原生Spring會是5個
	 *
	 */

	public static void registerAnnotationConfigProcessors(BeanDefinitionRegistry registry) {
		registerAnnotationConfigProcessors(registry, null);
	}

接下來的代碼就是最上面的代碼了！