SpringBoot2 | Spring IOC 流程中核心扩展接口的12个扩展点源码分析（十一）-白红宇

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概述

Spring 的核心思想即是容器。整个容器 refresh 时，外部看似风平浪静，内部实则一片汪洋大海。另外整个流程严格遵守开闭原则，内部对修改关闭，对扩展开放。

可以这么理解：把 Spring 容器理解为一个钥匙环，上面挂满了钥匙，每个钥匙理解为一个扩展接口。钥匙的顺序是固定的，可理解为接口的调用顺序固定，对修改关闭。每个钥匙可以用来做不同的事情，可理解为扩展接口的不同实现，对扩展开放。

Spring 提供了各种丰富的扩展接口，本篇主要对 IOC 过程中涉及的扩展接口做个整理。

对应的 UML 如下：

调用顺序如下：

分别来看。

1。BeanDefinitionRegistryPostProcessor.postProcessBeanDefinitionRegistry

BeanDefinitionRegistryPostProcessor接口在读取项目中的beanDefinition之后执行，提供的一个补充扩展接口，用来动态注册beanDefinition。调用点：在PostProcessorRegistrationDelegate中：

if (beanFactory instanceof BeanDefinitionRegistry) {			//......			// Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.			boolean reiterate = true;			while (reiterate) {				reiterate = false;				//获取所有的 BeanDefinitionRegistryPostProcessor 类型的bean				postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);				for (String ppName : postProcessorNames) {					if (!processedBeans.contains(ppName)) {						//通过 getBean 方法进行实例化						currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));						processedBeans.add(ppName);						reiterate = true;					}				}				sortPostProcessors(currentRegistryProcessors, beanFactory);				registryProcessors.addAll(currentRegistryProcessors);				invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);				currentRegistryProcessors.clear();			}		//......		}复制代码

示例：手动注册BeanDefinition：：

@Componentpublic class MyBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {    @Override    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {        System.out.println("postProcessBeanDefinitionRegistry ...");        //手动注册 beanDefinition        registry.registerBeanDefinition("myBeanDefinitionRegistrar",new AnnotatedGenericBeanDefinition(MyBeanDefinitionRegistrar.class));    }}复制代码

2 。BeanFactoryPostProcessor.postProcessBeanFactory

BeanFactoryPostProcessor和BeanPostProcessor 接口比较相似，从字面不难看出，前者多了一个 factory，所以该接口正是beanFactory的扩展接口，使用场景：一般用来在读取所有的beanDefinition信息之后，实例化之前，通过该接口可进一步自行处理，比如修改beanDefinition等。调用点在上面第一个扩展接口之后，也在PostProcessorRegistrationDelegate中：

if (beanFactory instanceof BeanDefinitionRegistry) {		//......		// Do not initialize FactoryBeans here: We need to leave all regular beans		// 获取所有的 BeanFactoryPostProcessor 类型		String[] postProcessorNames =				beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);		// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,		// Ordered, and the rest.		List
    
      priorityOrderedPostProcessors = new ArrayList<>();		List
     
       orderedPostProcessorNames = new ArrayList<>();		List
      
        nonOrderedPostProcessorNames = new ArrayList<>();		for (String ppName : postProcessorNames) {			if (processedBeans.contains(ppName)) {				// skip - already processed in first phase above			}			else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {				priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));			}			else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {				orderedPostProcessorNames.add(ppName);			}			else {				nonOrderedPostProcessorNames.add(ppName);			}		}		// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.		sortPostProcessors(priorityOrderedPostProcessors, beanFactory);		//执行所有的 BeanFactoryPostProcessor 实现逻辑		invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);		//......		}复制代码

示例：动态修改BeanDefinition：

@Componentpublic class MyBeanDefinitionRegistryPostProcessor implements BeanFactoryPostProcessor {    @Override    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {        BeanDefinition myBeanDefinitionRegistrar = beanFactory.getBeanDefinition("myBeanDefinitionRegistrar");        //可以修改 beanDefinition 信息。这里将bean 设置为单例        myBeanDefinitionRegistrar.setScope(BeanDefinition.SCOPE_SINGLETON);    }}复制代码

3。InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation

Instantiation 实例化的意思，和Initialization初始化比较相似，容易混淆。 postProcessBeforeInstantiation用来获取 bean，如果获取到，则不再执行对应 bean的初始化之前流程，直接执行后面要讲的postProcessAfterInitialization方法。调用点在AbstractAutowireCapableBeanFactory中：

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)			throws BeanCreationException {		//......		try {			//执行实例化之前的方法			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);			if (bean != null) {				return bean;			}		}		catch (Throwable ex) {			throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,					"BeanPostProcessor before instantiation of bean failed", ex);		}		//......	}复制代码

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {		Object bean = null;		if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {			// Make sure bean class is actually resolved at this point.			if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {				Class
     targetType = determineTargetType(beanName, mbd);				if (targetType != null) {					//开始执行 postProcessBeforeInstantiation 方法					bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);					//如果获得结果不为空，则直接执行实例化之后的扩展接口。结束 bean 实例化流程。					if (bean != null) {						bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);					}				}			}			mbd.beforeInstantiationResolved = (bean != null);		}		return bean;	}复制代码

如果postProcessBeforeInstantiation获得结果不为空，则结束 bean 实例化流程。

4。SmartInstantiationAwareBeanPostProcessor.determineCandidateConstructors

该扩展点决定判断合适的 bean 构造方法。具体可参考AutowiredAnnotationBeanPostProcessor实现类，针对以下使用场景：在MyComponent1中通过构造方法注入MyComponent2:

@Autowired    public MyComponent(MyComponent2 component2){        System.out.println("myComponent init...");    }复制代码

这里会判断选择出合适的构造方法，并实例化需要的参数 bean。调用点在AbstractAutowireCapableBeanFactory中：

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {		//......		//获取合适的构造方法，如果为空，则走默认的构造方法。		Constructor
    [] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);		if (ctors != null ||				mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {		   //如果发现有构造方法引用了依赖注入注解，比如：@AutoWired,则调用autowireConstructor方法进行注入			return autowireConstructor(beanName, mbd, ctors, args);		}		// No special handling: simply use no-arg constructor.		return instantiateBean(beanName, mbd);	}复制代码

5。MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition

该接口用来合并BeanDefinition，也是对BeanDefinition处理一种扩展接口。最常用的使用场景：AutowiredAnnotationBeanPostProcessor实现类中，通过该接口解析当前 bean 中所有指定注解类型的属性：

this.autowiredAnnotationTypes.add(Autowired.class);	    this.autowiredAnnotationTypes.add(Value.class);复制代码

默认解析上两种注解的属性，将其描述信息合并到当前对象的beanDefinition中，在后面属性填充populateBean的过程中，会取出这些对象，进行注入。调用点在AbstractAutowireCapableBeanFactory中：

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)			throws BeanCreationException {		// Instantiate the bean.		BeanWrapper instanceWrapper = null;		if (mbd.isSingleton()) {			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);		}		//实例化 bean		if (instanceWrapper == null) {			instanceWrapper = createBeanInstance(beanName, mbd, args);		}		final Object bean = instanceWrapper.getWrappedInstance();		Class
     beanType = instanceWrapper.getWrappedClass();		if (beanType != NullBean.class) {			mbd.resolvedTargetType = beanType;		}		// Allow post-processors to modify the merged bean definition.		synchronized (mbd.postProcessingLock) {			if (!mbd.postProcessed) {				try {				    //执行 postProcessMergedBeanDefinition 逻辑					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);				}				catch (Throwable ex) {					throw new BeanCreationException(mbd.getResourceDescription(), beanName,							"Post-processing of merged bean definition failed", ex);				}				mbd.postProcessed = true;			}		}		//......		return exposedObject;	}复制代码

6。InstantiationAwareBeanPostProcessor.postProcessAfterInstantiation

实例化之后调用的方法，在AbstractAutowireCapableBeanFactory.populateBean()填充方法中会触发。该方法默认返回为true，如果返回false，则中断populateBean方法，即不再执行属性注入的过程。实际项目中，该扩展方法使用不多。

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {		// ......		boolean continueWithPropertyPopulation = true;		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {			for (BeanPostProcessor bp : getBeanPostProcessors()) {				if (bp instanceof InstantiationAwareBeanPostProcessor) {					InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;					if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {						continueWithPropertyPopulation = false;						break;					}				}			}		}		if (!continueWithPropertyPopulation) {			return;		}		//......	}复制代码

7。SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference

getEarlyBeanReference方法只要有在 Spring 发生循环依赖时调用。首先，当bean 创建时，为了防止后续有循环依赖，会提前暴露回调方法，用于 bean 实例化的后置处理。getEarlyBeanReference方法就是在提前暴露的回调方法中触发。

具体调用点在DefaultSingletonBeanRegistry：

@Nullable	protected Object getSingleton(String beanName, boolean allowEarlyReference) {		Object singletonObject = this.singletonObjects.get(beanName);		//如果 bean 还未实例化，并且正在创建中。		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {			synchronized (this.singletonObjects) {			//判断是否已经提前提前暴露了bean 引用。				singletonObject = this.earlySingletonObjects.get(beanName);				//如果运行循环依赖				if (singletonObject == null && allowEarlyReference) {					ObjectFactory
     singletonFactory = this.singletonFactories.get(beanName);					if (singletonFactory != null) {					//则调用 getObject() 方法						singletonObject = singletonFactory.getObject();						this.earlySingletonObjects.put(beanName, singletonObject);						this.singletonFactories.remove(beanName);					}				}			}		}		return singletonObject;	}复制代码

在getObject()中调用getEarlyBeanReference方法完成 bean的初始化流程。

protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {		Object exposedObject = bean;		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {			for (BeanPostProcessor bp : getBeanPostProcessors()) {				if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {					SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;					exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);				}			}		}		return exposedObject;	}复制代码

大致的流程：

当A实例化之后，Spring IOC会对A依赖的属性进行填充，此时如果发现A依赖了B，会去实例化B。同样在填充B的属性时，如果B也引用了A，就发生了循环依赖。因为A还未创建完成，还未注入Spring中。

Spring的做法是通过对创建中的缓存一个回调函数，类似于一个埋点操作，如果后续填充属性阶段，发生了循环依赖，则通过触发该回调函数来结束该bean的初始化。

当对A实例化时，会提前暴露一个回调方法 ObjectFactory（Spring5中改为了函数式接口）放入缓存。当B引用A，发现A还未实例化结束，就会通过缓存中的回调方法结束A的初始化流程，然后注入B。然后继续A的填充属性流程，将B注入A，然后结束循环依赖。

boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&				isSingletonCurrentlyInCreation(beanName));		//......		if (earlySingletonExposure) {			if (logger.isDebugEnabled()) {				logger.debug("Eagerly caching bean '" + beanName +						"' to allow for resolving potential circular references");			}			//添加回调方法，循环依赖时会回调			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));		}		try {		//填充属性，也就是发生循环依赖的地方			populateBean(beanName, mbd, instanceWrapper);			exposedObject = initializeBean(beanName, exposedObject, mbd);		}		catch (Throwable ex) {			if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {				throw (BeanCreationException) ex;			}			else {				throw new BeanCreationException(						mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);			}		}		//......复制代码

8。InstantiationAwareBeanPostProcessor.postProcessPropertyValues

该方法用于属性注入，在 bean 初始化阶段属性填充时触发。@Autowired，@Resource 等注解原理基于此方法实现。具体调用点在AbstractAutowireCapableBeanFactory中populateBean方法：

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {		//......	if (hasInstAwareBpps || needsDepCheck) {			if (pvs == null) {				pvs = mbd.getPropertyValues();			}			PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);			if (hasInstAwareBpps) {				for (BeanPostProcessor bp : getBeanPostProcessors()) {					if (bp instanceof InstantiationAwareBeanPostProcessor) {						InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;						//获取该方法的所有实现类						pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);						if (pvs == null) {							return;						}					}				}			}			if (needsDepCheck) {				checkDependencies(beanName, mbd, filteredPds, pvs);			}		}		//......	}复制代码

上述方法会获取所有postProcessPropertyValues的实现方法。例如：在AutowiredAnnotationBeanPostProcessor中实现方式如下，也就是依赖注入的实现代码：

@Override	public PropertyValues postProcessPropertyValues(			PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {		InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);		try {			metadata.inject(bean, beanName, pvs);		}		catch (BeanCreationException ex) {			throw ex;		}		catch (Throwable ex) {			throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);		}		return pvs;	}复制代码

9。ApplicationContextAwareProcessor.invokeAwareInterfaces

该扩展点用于执行各种驱动接口。在 bean实例化之后，属性填充之后，通过扩展接口，执行如下驱动接口：

private void invokeAwareInterfaces(Object bean) {		if (bean instanceof Aware) {			if (bean instanceof EnvironmentAware) {				((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());			}			if (bean instanceof EmbeddedValueResolverAware) {				((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);			}			if (bean instanceof ResourceLoaderAware) {				((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);			}			if (bean instanceof ApplicationEventPublisherAware) {				((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);			}			if (bean instanceof MessageSourceAware) {				((MessageSourceAware) bean).setMessageSource(this.applicationContext);			}			if (bean instanceof ApplicationContextAware) {				((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);			}		}	}复制代码

所以，只需要实现以上6种驱动接口，就可以获得对应的容器相关的变量。这些变量在实际项目中是比较常用的了。

使用方式：

@Componentpublic class MyComponent implements ApplicationContextAware, InitializingBean, BeanClassLoaderAware ,ResourceLoaderAware,EnvironmentAware {    @Override    public void afterPropertiesSet() throws Exception {        System.out.println("afterPropertiesSet init...");    }    @Override    public void setBeanClassLoader(ClassLoader classLoader) {        System.out.println("setBeanClassLoader init...");    }    @Override    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {        System.out.println("setApplicationContext init...");    }    @Override    public void setEnvironment(Environment environment) {        System.out.println("setEnvironment init...");    }    @Override    public void setResourceLoader(ResourceLoader resourceLoader) {        System.out.println("setResourceLoader init...");    }}复制代码

10。BeanFactoryPostProcessor.postProcessBeforeInitialization

BeanFactoryPostProcessor中的两个扩展接口是 Spring IOC过程中最后两个扩展接口。其中postProcessBeforeInitialization用于在 bean 实例化之后，afterPropertiesSet方法之前执行的前置接口。用于对 bean 进行一些属性设置，上面的设置驱动的方法invokeAwareInterfaces便是实现了此接口。调用点如下：

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {		if (System.getSecurityManager() != null) {			AccessController.doPrivileged((PrivilegedAction

11。InitializingBean.afterPropertiesSet

用于bean实例化之后，设置属性的方法。上面已经提到，在invokeInitMethods方法中会触发该方法调用：

protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)			throws Throwable {		boolean isInitializingBean = (bean instanceof InitializingBean);		if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {			if (logger.isDebugEnabled()) {				logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");			}			if (System.getSecurityManager() != null) {				try {				//执行afterPropertiesSet					AccessController.doPrivileged((PrivilegedExceptionAction

12。BeanFactoryPostProcessor.postProcessAfterInitialization

该方法Spring IOC过程中最后一个常用的扩展点，用于 bean 初始化之后的后置处理。IOC 流程执行到此处，一个完整的 bean 已经创建结束，可在此处对 bean 进行包装或者代理。Spring AOP 原理便是基于此扩展点实现，实现方式在AbstractAutoProxyCreator中：

public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) throws BeansException {		if (bean != null) {			Object cacheKey = getCacheKey(bean.getClass(), beanName);			if (!this.earlyProxyReferences.contains(cacheKey)) {				return wrapIfNecessary(bean, beanName, cacheKey);			}		}		return bean;	}复制代码

有兴趣的可以移步 Spring AOP相关的文章：

具体使用方式已上传至 github：

总结

我们使用 Spring 或者 SpringBoot 时，通过 Spring 预留的以上扩展接口，可以方便的实现对 Spring IOC 过程中的逻辑做一些扩展和增强。通 Servlet 规范一样，可以理解为面向接口编程。