[读书笔记]Spring中IOC容器中FileSystemXmlApplicationContext的初始化详解
[读书笔记]FileSystemXmlApplicationContext容器初始化之BeanDefinition的载入和解析

前面我们对IOC容器的初始化过程进行了详细的分析，这个初始化过程完成的主要工作是在IOC容器中建立BeanDefinition数据映射。但是在此过程并没有看到IOC容器对Bean依赖关系进行注入，接下来我们学习这个依赖注入。

假设当前IOC容器已经载入了用户定义的Bean信息，开始分析依赖注入的原理。首先，注意到依赖注入的过程是用户第一次向IOC容器所要bean时触发的，当然也有例外，也就是我们可以在BeanDefinition信息中通过控制lazy-init属性来让容器完成对bean的预实例化(非抽象、单例、lazy-init不为true就会预实例化)。这个预实例化实际上也是一个完成依赖注入的过程，但它是在初始化的过程中完成的。

当用户向IOC容器索要Bean时，会触发BeanFactory的getBean接口。这个接口的实现就是触发依赖注入发生的地方。我们从DefaultListableBeanFactory的基类AbstractBeanFactory去看看getBean的实现。

当项目启动时，在初始化流程中的refresh-finishBeanFactoryInitialization就会对那些组件(比如controller、service等等)实例化并完成依赖注入。

【1】AbstractBeanFactory的doGetBean

该方法是实际取得Bean的方法，也是触发依赖注入发生的地方。

protected <T> T doGetBean(
		String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
		throws BeansException {

	String beanName = transformedBeanName(name);
	Object bean;

//先从缓存中取得Bean，处理那些已经被创建过的单例模式的Bean，对这种Bean的请求不需要重复地创建
// DefaultSingletonBeanRegistry.getSingleton
	// Eagerly check singleton cache for manually registered singletons.
	Object sharedInstance = getSingleton(beanName);
	if (sharedInstance != null && args == null) {
		if (logger.isTraceEnabled()) {
			if (isSingletonCurrentlyInCreation(beanName)) {
				logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
						"' that is not fully initialized yet - a consequence of a circular reference");
			}
			else {
				logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
			}
		}
		//这里完成的是FactoryBean的相关处理，以取得FactoryBean的生产结果
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}

	else {
		// Fail if we're already creating this bean instance:
		// We're assumably within a circular reference.
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}
// 这里对IOC容器中的BeanDefinition是否存在进行检查，检查是否能在当前的BeanFactory中取得需要的Bean。
//如果在当前的工厂中取不到，则到双亲BeanFactory中去取；如果当前的双亲工厂取不到，那就顺着双亲BeanFactory链一直向上查找
		// Check if bean definition exists in this factory.
		BeanFactory parentBeanFactory = getParentBeanFactory();
		if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
			// Not found -> check parent.
			String nameToLookup = originalBeanName(name);
			if (parentBeanFactory instanceof AbstractBeanFactory) {
				return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
						nameToLookup, requiredType, args, typeCheckOnly);
			}
			else if (args != null) {
				// Delegation to parent with explicit args.
				return (T) parentBeanFactory.getBean(nameToLookup, args);
			}
			else if (requiredType != null) {
				// No args -> delegate to standard getBean method.
				return parentBeanFactory.getBean(nameToLookup, requiredType);
			}
			else {
				return (T) parentBeanFactory.getBean(nameToLookup);
			}
		}

		if (!typeCheckOnly) {
			markBeanAsCreated(beanName);
		}

		try {
		// 根据Bean的名字取得BeanDefinition
			RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
			checkMergedBeanDefinition(mbd, beanName, args);

//获取当前Bean的所有依赖Bean，这样会触发getBean的递归调用，直到取到一个没有任何依赖的Bean为止
			// Guarantee initialization of beans that the current bean depends on.
			String[] dependsOn = mbd.getDependsOn();
			if (dependsOn != null) {
				for (String dep : dependsOn) {
					if (isDependent(beanName, dep)) { // 会抛出循环依赖异常
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
					}
					registerDependentBean(dep, beanName);
					try {
						getBean(dep);
					}
					catch (NoSuchBeanDefinitionException ex) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
					}
				}
			}


//这里通过调用createBean方法创建Singleton bean的实例
			// Create bean instance.
			if (mbd.isSingleton()) {
				sharedInstance = getSingleton(beanName, () -> {
					try {
						return createBean(beanName, mbd, args);
					}
					catch (BeansException ex) {
						// Explicitly remove instance from singleton cache: It might have been put there
						// eagerly by the creation process, to allow for circular reference resolution.
						// Also remove any beans that received a temporary reference to the bean.
						destroySingleton(beanName);
						throw ex;
					}
				});
				bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}
// 这里是创建prototype bean的地方
			else if (mbd.isPrototype()) {
				// It's a prototype -> create a new instance.
				Object prototypeInstance = null;
				try {
					beforePrototypeCreation(beanName);
					prototypeInstance = createBean(beanName, mbd, args);
				}
				finally {
					afterPrototypeCreation(beanName);
				}
				bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
			}
// 创建 Scope Bean
			else {
				String scopeName = mbd.getScope();
				if (!StringUtils.hasLength(scopeName)) {
					throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
				}
				Scope scope = this.scopes.get(scopeName);
				if (scope == null) {
					throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
				}
				try {
					Object scopedInstance = scope.get(beanName, () -> {
						beforePrototypeCreation(beanName);
						try {
							return createBean(beanName, mbd, args);
						}
						finally {
							afterPrototypeCreation(beanName);
						}
					});
					bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
				}
				catch (IllegalStateException ex) {
					throw new BeanCreationException(beanName,
							"Scope '" + scopeName + "' is not active for the current thread; consider " +
							"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
							ex);
				}
			}
		}
		catch (BeansException ex) {
			cleanupAfterBeanCreationFailure(beanName);
			throw ex;
		}
	}

// 对创建的Bean进行类型检查，如果没有问题就返回这个新创建的bean，这个Bean已经是包含了依赖关系的bean
	// Check if required type matches the type of the actual bean instance.
	if (requiredType != null && !requiredType.isInstance(bean)) {
		try {
			T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
			if (convertedBean == null) {
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
			return convertedBean;
		}
		catch (TypeMismatchException ex) {
			if (logger.isTraceEnabled()) {
				logger.trace("Failed to convert bean '" + name + "' to required type '" +
						ClassUtils.getQualifiedName(requiredType) + "'", ex);
			}
			throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
		}
	}
	return (T) bean;
}

重点来说，getBean是依赖注入的起点，之后会调用createBean。在这个过程中，Bean对象会依据BeanDefinition定义的要求生成，在AbstractAutowireCapableBeanFactory中实现了这个createBean，createBean不但生成了需要的Bean，还对Bean初始化进行处理，比如实现了在BeanDefinition中的init-method属性定义，Bean后置处理器等。

其中getObjectForBeanInstance方法将会返回当前bean实例或者工厂bean生产的bean（如果当前bean是一个FactoryBean的话）。

【2】AbstractAutowireCapableBeanFactory.createBean

该方法是AbstractAutowireCapableBeanFactory类的核心方法，其创建了一个Bean实例，为Bean实例属性赋值，触发Bean后置处理器。在AbstractBeanFactory的doGetBean会调用createBean方法。

@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
		throws BeanCreationException {

	if (logger.isTraceEnabled()) {
		logger.trace("Creating instance of bean '" + beanName + "'");
	}
	RootBeanDefinition mbdToUse = mbd;

	// Make sure bean class is actually resolved at this point, and
	// clone the bean definition in case of a dynamically resolved Class
	// which cannot be stored in the shared merged bean definition.
	// 这里判断需要创建的Bean是否可以实例化，这个类是否可以通过类装载器来载入
	Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
	if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
		mbdToUse = new RootBeanDefinition(mbd);
		mbdToUse.setBeanClass(resolvedClass);
	}

	// Prepare method overrides.
	try {
		mbdToUse.prepareMethodOverrides();
	}
	catch (BeanDefinitionValidationException ex) {
		throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
				beanName, "Validation of method overrides failed", ex);
	}

	try {
		// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
		// 如果Bean配置了postProcessor，那么这里返回的是一个proxy
		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);
	}

	try {
	// 核心方法，创建Bean
		Object beanInstance = doCreateBean(beanName, mbdToUse, args);
		if (logger.isTraceEnabled()) {
			logger.trace("Finished creating instance of bean '" + beanName + "'");
		}
		return beanInstance;
	}
	catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
		// A previously detected exception with proper bean creation context already,
		// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
		throw ex;
	}
	catch (Throwable ex) {
		throw new BeanCreationException(
				mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
	}
}

我们继续看上面的核心方法doCreateBean(beanName, mbdToUse, args)。

① doCreateBean

AbstractAutowireCapableBeanFactory.doCreateBean源码如下所示：

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
		throws BeanCreationException {

	// Instantiate the bean. 这个BeanWrapper是用来持有创建出来的Bean对象的
	BeanWrapper instanceWrapper = null;
	// 如果是Singleton，先把缓存中的同名Bean清除
	if (mbd.isSingleton()) {
		instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
	}
// 这里是创建Bean的地方，由createBeanInstance来完成
	if (instanceWrapper == null) {
		instanceWrapper = createBeanInstance(beanName, mbd, args);
	}
	final Object bean = instanceWrapper.getWrappedInstance();
	Class<?> beanType = instanceWrapper.getWrappedClass();
	if (beanType != NullBean.class) {
		mbd.resolvedTargetType = beanType;//设置BeanDefinition的BeanTYPE
	}

	// Allow post-processors to modify the merged bean definition.
	// 允许后置处理器修改BeanDefinition
	synchronized (mbd.postProcessingLock) {
		if (!mbd.postProcessed) {
			try {
			// 尝试调用MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition
				applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
			}
			catch (Throwable ex) {
				throw new BeanCreationException(mbd.getResourceDescription(), beanName,
						"Post-processing of merged bean definition failed", ex);
			}
			mbd.postProcessed = true;//标明已经进行过bean的后置处理
		}
	}

	// Eagerly cache singletons to be able to resolve circular references
	// even when triggered by lifecycle interfaces like BeanFactoryAware.
	//急切地缓存单例，以便能够解析循环引用，即使是由BeanFactoryAware等生命周期接口触发
	boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
			isSingletonCurrentlyInCreation(beanName));
	if (earlySingletonExposure) {
		if (logger.isTraceEnabled()) {
			logger.trace("Eagerly caching bean '" + beanName +
					"' to allow for resolving potential circular references");
		}
		addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
	}

	// Initialize the bean instance.
	//这里是对Bean的初始化，依赖注入往往发生在这里，这个exposedObject 在初始化处理完以后会返回作为依赖注入完成以后的Bean
	Object exposedObject = bean;
	try {
		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);
		}
	}

	if (earlySingletonExposure) {
		Object earlySingletonReference = getSingleton(beanName, false);
		if (earlySingletonReference != null) {
			if (exposedObject == bean) {
				exposedObject = earlySingletonReference;
			}
			else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
				String[] dependentBeans = getDependentBeans(beanName);
				Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
				for (String dependentBean : dependentBeans) {
					if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
						actualDependentBeans.add(dependentBean);
					}
				}
				if (!actualDependentBeans.isEmpty()) {
					throw new BeanCurrentlyInCreationException(beanName,
							"Bean with name '" + beanName + "' has been injected into other beans [" +
							StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
							"] in its raw version as part of a circular reference, but has eventually been " +
							"wrapped. This means that said other beans do not use the final version of the " +
							"bean. This is often the result of over-eager type matching - consider using " +
							"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
				}
			}
		}
	}

	// Register bean as disposable.
	try {
	// 尝试作为DisposableBean 放入 private final Map<String, Object> disposableBeans = new LinkedHashMap<>();
		registerDisposableBeanIfNecessary(beanName, bean, mbd);
	}
	catch (BeanDefinitionValidationException ex) {
		throw new BeanCreationException(
				mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
	}

	return exposedObject;
}

这里有三个核心方法：createBeanInstance、populateBean以及initializeBean。

在createBeanInstance中生成了Bean所包含的Java对象，这个对象的生成有很多种不同的方式。可以通过工厂方法生成，也可以通过容器的autowire特性生成，这些生成方式都是由相关的BeanDefinition来指定的。

② createBeanInstance

AbstractAutowireCapableBeanFactory.createBeanInstance，这里完成实例对象的创建。

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
	// Make sure bean class is actually resolved at this point.
	// 确认需要创建的Bean实例的类可以实例化
	Class<?> beanClass = resolveBeanClass(mbd, beanName);

// 访问权限校验
	if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
		throw new BeanCreationException(mbd.getResourceDescription(), beanName,
				"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
	}

// 从给定的Supplier 获取BeanName
	Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
	if (instanceSupplier != null) {
		return obtainFromSupplier(instanceSupplier, beanName);
	}
// 使用工厂方法进行实例化
	if (mbd.getFactoryMethodName() != null) {
		return instantiateUsingFactoryMethod(beanName, mbd, args);
	}

	// Shortcut when re-creating the same bean...
	boolean resolved = false;
	boolean autowireNecessary = false;
	if (args == null) {
		synchronized (mbd.constructorArgumentLock) {
			if (mbd.resolvedConstructorOrFactoryMethod != null) {
				resolved = true;
				autowireNecessary = mbd.constructorArgumentsResolved;
			}
		}
	}
//是否曾经解析过/实例化过这个Bean
	if (resolved) {
		if (autowireNecessary) { 
			return autowireConstructor(beanName, mbd, null, null);
		}
		else {//使用默认的构造函数进行实例化
			return instantiateBean(beanName, mbd);
		}
	}

// AUTOWIRE_CONSTRUCTOR  表示根据构造函数进行依赖注入 
//自动连接可以满足的最贪婪的构造函数（包括解析适当的构造函数）
	// Candidate constructors for autowiring?
	Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
	if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
			mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
		return autowireConstructor(beanName, mbd, ctors, args);
	}

// 使用默认的构造函数进行实例化 -确定用于默认构造的首选构造函数（如果有）。如有必要，构造函数参数将自动连接。
// 默认为null
	// Preferred constructors for default construction?
	ctors = mbd.getPreferredConstructors();
	if (ctors != null) {
		return autowireConstructor(beanName, mbd, ctors, null);
	}

// 简单使用无参构造函数进行实例化
	// No special handling: simply use no-arg constructor.
	return instantiateBean(beanName, mbd);
}

我们接下来看一下其instantiateBean方法。

protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
	try {
		Object beanInstance;
		final BeanFactory parent = this;
		if (System.getSecurityManager() != null) {
			beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
					getInstantiationStrategy().instantiate(mbd, beanName, parent),
					getAccessControlContext());
		}
		else {
		// 使用默认的实例化策略对Bean进行实例化，默认的实例化策略是CglibSubclassingInstantiationStrategy
		// 也就是使用CGLIB来对Bean进行实例化
			beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
		}
		BeanWrapper bw = new BeanWrapperImpl(beanInstance);
		initBeanWrapper(bw);
		return bw;
	}
	catch (Throwable ex) {
		throw new BeanCreationException(
				mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
	}
}

这里使用CGLIB对Bean进行实例化。CGLIB是一个常用的字节码生成器的类库，它提供了一系列的API来提供生成和转换Java的字节码的功能。在Spring AOP中也使用CGLIB对Java的字节码进行增强。在IOC容器中，要了解怎样使用CGLIB来生成Bean对象，需要看一下SimpleInstantiationStrategy其是CglibSubclassingInstantiationStrategy的父类。这个SimpleInstantiationStrategy是Spring用来生成Bean对象的默认类，它提供了两种实例化Java对象的方法：一种是通过BeanUtils，它使用了JVM的反射功能；另外一种则就是前面提到的的CGLIB来生成。

@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
	// Don't override the class with CGLIB if no overrides.
	if (!bd.hasMethodOverrides()) {
		Constructor<?> constructorToUse;
		synchronized (bd.constructorArgumentLock) {
		//这里取得指定的构造器或者生成对象的工厂方法来对Bean进行实例化
			constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
			if (constructorToUse == null) {
				final Class<?> clazz = bd.getBeanClass();
				if (clazz.isInterface()) {
					throw new BeanInstantiationException(clazz, "Specified class is an interface");
				}
				try {
					if (System.getSecurityManager() != null) {
						constructorToUse = AccessController.doPrivileged(
								(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
					}
					else {
						constructorToUse = clazz.getDeclaredConstructor();
					}
					bd.resolvedConstructorOrFactoryMethod = constructorToUse;
				}
				catch (Throwable ex) {
					throw new BeanInstantiationException(clazz, "No default constructor found", ex);
				}
			}
		}
		// 通过BeanUtils进行实例化，这个BeanUtils的实例化通过Constructor来实例化Bean
		// 在BeanUtils中可以看到具体的调用ctor.newInstance(args)
		return BeanUtils.instantiateClass(constructorToUse);
	}
	else {
		// Must generate CGLIB subclass.
		// 使用CGLIB来实例化对象
		return instantiateWithMethodInjection(bd, beanName, owner);
	}
}

到这里Bean对应已经实例化了，此时还只是一个基础模型。还需要Spring对Bean对象进行处理，把其依赖关系设置好，属性赋值等等完成整个依赖注入的过程。依赖关系处理的依据就是已经解析得到的BeanDefinition，而这个过程就是前面的populateBean(beanName, mbd, instanceWrapper)。

③ populateBean

用bean定义中的属性值填充给定BeanWrapper中的bean实例，也就是属性赋值过程。

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
	if (bw == null) {
		if (mbd.hasPropertyValues()) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
		}
		else {
			// Skip property population phase for null instance.
			return;
		}
	}

// 给InstantiationAwareBeanPostProcessors 机会，(在实例化后，设置属性值前 )修改Bean的状态
	// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
	// state of the bean before properties are set. This can be used, for example,
	// to support styles of field injection.
	if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
		for (BeanPostProcessor bp : getBeanPostProcessors()) {
			if (bp instanceof InstantiationAwareBeanPostProcessor) {
				InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
				if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
					return;
				}
			}
		}
	}
// 取得BeanDefinition中设置的property值
	PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);

// 获取autowire标识， AUTOWIRE_BY_NAME  or AUTOWIRE_BY_TYPE
// 开始进行依赖注入的过程，先处理autowire的注入
// 如果resolvedAutowireMode为 0 ，则不会触发if逻辑
	int resolvedAutowireMode = mbd.getResolvedAutowireMode();
	if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
		MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
		// Add property values based on autowire by name if applicable.
		if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
			autowireByName(beanName, mbd, bw, newPvs);
		}
		// Add property values based on autowire by type if applicable.
		if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
			autowireByType(beanName, mbd, bw, newPvs);
		}
		pvs = newPvs;
	}
// 当前factory是否有InstantiationAwareBeanPostProcessor
	boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
// 是否需要依赖检查
	boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);

	PropertyDescriptor[] filteredPds = null;

//如果有InstantiationAwareBeanPostProcessor--AutowiredAnnotationBeanPostProcessor就是其实现
	if (hasInstAwareBpps) {
		if (pvs == null) {
			pvs = mbd.getPropertyValues();
		}
		for (BeanPostProcessor bp : getBeanPostProcessors()) {
		// 尝试调用每一个InstantiationAwareBeanPostProcessor.postProcessProperties
		// postProcessProperties 使用属性值之前提供一个机会/窗口
			if (bp instanceof InstantiationAwareBeanPostProcessor) {
				InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
				PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
				if (pvsToUse == null) {
					if (filteredPds == null) {
						filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
					}
					pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
					if (pvsToUse == null) {
						return;
					}
				}
				pvs = pvsToUse;
			}
		}
	}

	// 依赖检查
	if (needsDepCheck) {
		if (filteredPds == null) {
			filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
		}
		checkDependencies(beanName, mbd, filteredPds, pvs);
	}
// 对属性进行注入
	if (pvs != null) {
		applyPropertyValues(beanName, mbd, bw, pvs);
	}
}

这里需要注意，AutowiredAnnotationBeanPostProcessor是InstantiationAwareBeanPostProcessorAdapter的实现。如果resolvedAutowireMode为0，那么我们@Autowired注解的依赖解析就发生在这里。执行其postProcessProperties方法时会触发其inject方法，该方法会解析autowire注入依赖的。

④ applyPropertyValues

AbstractAutowireCapableBeanFactory的applyPropertyValues方法。应用给定的属性值，解析对该bean工厂中其他bean的任何运行时引用。必须使用深度复制，因此我们不会永久修改此属性。

protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
// 为空直接返回
	if (pvs.isEmpty()) {
		return;
	}

	if (System.getSecurityManager() != null && bw instanceof BeanWrapperImpl) {
		((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
	}

	MutablePropertyValues mpvs = null;
	List<PropertyValue> original;

// 检测是否为MutablePropertyValues类型
	if (pvs instanceof MutablePropertyValues) {
		mpvs = (MutablePropertyValues) pvs;
		if (mpvs.isConverted()) {//判断值是否被转换过
			// Shortcut: use the pre-converted values as-is.
			try {
				bw.setPropertyValues(mpvs);
				return;
			}
			catch (BeansException ex) {
				throw new BeanCreationException(
						mbd.getResourceDescription(), beanName, "Error setting property values", ex);
			}
		}
		original = mpvs.getPropertyValueList();
	}
	else {
		original = Arrays.asList(pvs.getPropertyValues());
	}


//类型转换器
	TypeConverter converter = getCustomTypeConverter();
	if (converter == null) {
		converter = bw;
	}
//对BeanDefinition的解析是在这个valueResolver 中完成的
	BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);

	// Create a deep copy, resolving any references for values.
	//这里为解析值创建一个副本，副本的数据将会被注入到Bean中
	List<PropertyValue> deepCopy = new ArrayList<>(original.size());
	boolean resolveNecessary = false;
//下面这一部分是向deepCopy放入一个个PV，如果是未转化/解析过的，则进行解析
	for (PropertyValue pv : original) {
		if (pv.isConverted()) {
			deepCopy.add(pv);
		}
		else {
			String propertyName = pv.getName();
			Object originalValue = pv.getValue();
			if (originalValue == AutowiredPropertyMarker.INSTANCE) {
				Method writeMethod = bw.getPropertyDescriptor(propertyName).getWriteMethod();
				if (writeMethod == null) {
					throw new IllegalArgumentException("Autowire marker for property without write method: " + pv);
				}
				originalValue = new DependencyDescriptor(new MethodParameter(writeMethod, 0), true);
			}
			Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
			Object convertedValue = resolvedValue;
			boolean convertible = bw.isWritableProperty(propertyName) &&
					!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
			if (convertible) {
				convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
			}
			// Possibly store converted value in merged bean definition,
			// in order to avoid re-conversion for every created bean instance.
			if (resolvedValue == originalValue) {
				if (convertible) {
					pv.setConvertedValue(convertedValue);
				}
				deepCopy.add(pv);
			}
			else if (convertible && originalValue instanceof TypedStringValue &&
					!((TypedStringValue) originalValue).isDynamic() &&
					!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
				pv.setConvertedValue(convertedValue);
				deepCopy.add(pv);
			}
			else {
				resolveNecessary = true;
				deepCopy.add(new PropertyValue(pv, convertedValue));
			}
		}
	}
	if (mpvs != null && !resolveNecessary) {
		mpvs.setConverted();
	}

	// Set our (possibly massaged) deep copy.
	try {
	// 这里是依赖注入发生的地方，会在BeanWrapperImpl中完成
		bw.setPropertyValues(new MutablePropertyValues(deepCopy));
	}
	catch (BeansException ex) {
		throw new BeanCreationException(
				mbd.getResourceDescription(), beanName, "Error setting property values", ex);
	}
}

这里我们看下value解析的过程。

⑤ resolveValueIfNecessary

BeanDefinitionValueResolver的resolveValueIfNecessary方法如下所示，其包含了所有对注入类型的处理。

给定一个PropertyValue，返回一个值，必要时解析对工厂中其他bean的任何引用。该值可以是：

• 一个BeanDefinition，它导致创建相应的新bean实例。这种“内部bean”的单例标志和名称总是被忽略：内部bean是匿名原型。
• 必须解析的RuntimeBeanReference。
• ManagedList。这是一个特殊集合，可能包含RuntimeBeanReferences或需要解析的集合。
• ManagedSet。还可能包含需要解析的RuntimeBeanReferences或集合。
• ManagedMap。在这种情况下，该值可能是需要解析的RuntimeBeanReference或集合。
• 一个普通的对象或 null，在这种情况下，它是单独存在的。

public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {
	// We must check each value to see whether it requires a runtime reference
	// to another bean to be resolved.
	// RuntimeBeanReference是在对BeanDefinition进行解析时生成的数据对象
	if (value instanceof RuntimeBeanReference) { //判断RuntimeBeanReference
		RuntimeBeanReference ref = (RuntimeBeanReference) value;
		return resolveReference(argName, ref);
	}
	else if (value instanceof RuntimeBeanNameReference) { //判断RuntimeBeanNameReference
		String refName = ((RuntimeBeanNameReference) value).getBeanName();
		refName = String.valueOf(doEvaluate(refName));
		if (!this.beanFactory.containsBean(refName)) {
			throw new BeanDefinitionStoreException(
					"Invalid bean name '" + refName + "' in bean reference for " + argName);
		}
		return refName;
	}
	// 判断BeanDefinitionHolder
	else if (value instanceof BeanDefinitionHolder) {
		// Resolve BeanDefinitionHolder: contains BeanDefinition with name and aliases.
		BeanDefinitionHolder bdHolder = (BeanDefinitionHolder) value;
		return resolveInnerBean(argName, bdHolder.getBeanName(), bdHolder.getBeanDefinition());
	}
	// 判断BeanDefinition
	else if (value instanceof BeanDefinition) {
		// Resolve plain BeanDefinition, without contained name: use dummy name.
		BeanDefinition bd = (BeanDefinition) value;
		String innerBeanName = "(inner bean)" + BeanFactoryUtils.GENERATED_BEAN_NAME_SEPARATOR +
				ObjectUtils.getIdentityHexString(bd);
		return resolveInnerBean(argName, innerBeanName, bd);
	}
	// 判断DependencyDescriptor
	else if (value instanceof DependencyDescriptor) {
		Set<String> autowiredBeanNames = new LinkedHashSet<>(4);
		Object result = this.beanFactory.resolveDependency(
				(DependencyDescriptor) value, this.beanName, autowiredBeanNames, this.typeConverter);
		for (String autowiredBeanName : autowiredBeanNames) {
			if (this.beanFactory.containsBean(autowiredBeanName)) {
				this.beanFactory.registerDependentBean(autowiredBeanName, this.beanName);
			}
		}
		return result;
	}
	// 判断ManagedArray
	else if (value instanceof ManagedArray) {
		// May need to resolve contained runtime references.
		ManagedArray array = (ManagedArray) value;
		Class<?> elementType = array.resolvedElementType;
		if (elementType == null) {
			String elementTypeName = array.getElementTypeName();
			if (StringUtils.hasText(elementTypeName)) {
				try {
					elementType = ClassUtils.forName(elementTypeName, this.beanFactory.getBeanClassLoader());
					array.resolvedElementType = elementType;
				}
				catch (Throwable ex) {
					// Improve the message by showing the context.
					throw new BeanCreationException(
							this.beanDefinition.getResourceDescription(), this.beanName,
							"Error resolving array type for " + argName, ex);
				}
			}
			else {
				elementType = Object.class;
			}
		}
		return resolveManagedArray(argName, (List<?>) value, elementType);
	}
	// 判断ManagedList
	else if (value instanceof ManagedList) {
		// May need to resolve contained runtime references.
		return resolveManagedList(argName, (List<?>) value);
	}
	// 判断ManagedSet
	else if (value instanceof ManagedSet) {
		// May need to resolve contained runtime references.
		return resolveManagedSet(argName, (Set<?>) value);
	}
	// 判断 ManagedMap
	else if (value instanceof ManagedMap) {
		// May need to resolve contained runtime references.
		return resolveManagedMap(argName, (Map<?, ?>) value);
	}
	// 判断 ManagedProperties
	else if (value instanceof ManagedProperties) {
		Properties original = (Properties) value;
		Properties copy = new Properties();
		original.forEach((propKey, propValue) -> {
			if (propKey instanceof TypedStringValue) {
				propKey = evaluate((TypedStringValue) propKey);
			}
			if (propValue instanceof TypedStringValue) {
				propValue = evaluate((TypedStringValue) propValue);
			}
			if (propKey == null || propValue == null) {
				throw new BeanCreationException(
						this.beanDefinition.getResourceDescription(), this.beanName,
						"Error converting Properties key/value pair for " + argName + ": resolved to null");
			}
			copy.put(propKey, propValue);
		});
		return copy;
	}
	// 判断TypedStringValue
	else if (value instanceof TypedStringValue) {
		// Convert value to target type here.
		TypedStringValue typedStringValue = (TypedStringValue) value;
		Object valueObject = evaluate(typedStringValue);
		try {
			Class<?> resolvedTargetType = resolveTargetType(typedStringValue);
			if (resolvedTargetType != null) {
				return this.typeConverter.convertIfNecessary(valueObject, resolvedTargetType);
			}
			else {
				return valueObject;
			}
		}
		catch (Throwable ex) {
			// Improve the message by showing the context.
			throw new BeanCreationException(
					this.beanDefinition.getResourceDescription(), this.beanName,
					"Error converting typed String value for " + argName, ex);
		}
	}
	// 是否为null
	else if (value instanceof NullBean) {
		return null;
	}
	// Evaluate the given value as an expression, if necessary
	else {
		return evaluate(value);
	}
}

⑥ resolveReference

对RuntimeBeanReference类型的注入解析在resolveReference方法中。

@Nullable
private Object resolveReference(Object argName, RuntimeBeanReference ref) {
	try {
		Object bean;
		Class<?> beanType = ref.getBeanType();
		// 如果ref是在双亲IOC容器中，那就到双亲IOC容器中去获取
		if (ref.isToParent()) {
			BeanFactory parent = this.beanFactory.getParentBeanFactory();
			if (parent == null) {
				throw new BeanCreationException(
						this.beanDefinition.getResourceDescription(), this.beanName,
						"Cannot resolve reference to bean " + ref +
								" in parent factory: no parent factory available");
			}
			if (beanType != null) {
				bean = parent.getBean(beanType);
			}
			else {
				bean = parent.getBean(String.valueOf(doEvaluate(ref.getBeanName())));
			}
		}
		// 在当前IOC容器中取得Bean，这里会触发一个getBean的过程
		// 如果依赖注入还没有发生，这里会触发相应的依赖注入的发生
		else {
			String resolvedName;
			if (beanType != null) { //根据type
				NamedBeanHolder<?> namedBean = this.beanFactory.resolveNamedBean(beanType);
				bean = namedBean.getBeanInstance();
				resolvedName = namedBean.getBeanName();
			}
			else { // 根据name
				resolvedName = String.valueOf(doEvaluate(ref.getBeanName()));
				bean = this.beanFactory.getBean(resolvedName);
			}
			// 这里很重要，会将当前bean名称与依赖bean放入map中
			this.beanFactory.registerDependentBean(resolvedName, this.beanName);
		}
		if (bean instanceof NullBean) {
			bean = null;
		}
		return bean;
	}
	catch (BeansException ex) {
		throw new BeanCreationException(
				this.beanDefinition.getResourceDescription(), this.beanName,
				"Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
	}
}

在完成这个解析过程后，已经为依赖注入准备好了条件。这是真正把Bean对象设置到它所依赖的另一个Bean的属性中去的地方其中处理的属性是各种各样的。

依赖注入的发生是在BeanWrapper的setPropertyValues中实现的，具体的完成却是在AbstractNestablePropertyAccessor中实现的。然后到这里就完成了对各种Bean属性的依赖注入过程。

在Bean的创建和对象依赖注入的过程中，需要依据BeanDefinition中的信息来递归地完成依赖注入。这些递归都是以getBean为入口的。一个递归是在上下文体系中查找需要的Bean和创建Bean的递归调用；另外一个递归是在依赖注入时，通过递归调用容器的getBean方法，得到当前Bean的依赖Bean，同时也触发对依赖bean的创建和注入。在对Bean的属性进行依赖注入时，解析的过程也是一个递归的过程。

这样，根据依赖关系，一层一层地完成Bean的创建和注入，直到最后完成当前Bean的创建。有了这个顶层Bean的创建和对它的属性依赖注入的完成，意味着和当前Bean相关的整个依赖链的注入也完成了。

⑦ initializeBean

关于该方法的详细信息参考博文：Spring中Bean实例化过程中的initializeBean方法

【4】自动依赖装配的实现

从autowiring使用上可以知道，这个autowiring属性在对Bean属性进行依赖注入时起作用。而这是在populateBean中实现的。也就是说，对属性autowiring的处理是populateBean的一部分。

PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);

int resolvedAutowireMode = mbd.getResolvedAutowireMode();
if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
	MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
	// Add property values based on autowire by name if applicable.
	if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
		autowireByName(beanName, mbd, bw, newPvs);
	}
	// Add property values based on autowire by type if applicable.
	if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
		autowireByType(beanName, mbd, bw, newPvs);
	}
	pvs = newPvs;
}

• autowireByName 根据name从factory找到合适的bean
• autowireByType 根据type从factory找到合适的bean

上面部分是更新PropertyValues pvs 的一个环节。以autowireByName 来说，它首先需要得到当前Bean的属性名，这些属性名已经在BeanWrapperImpl和BeanDefinition中封装好了，然后是对这一系列属性进行匹配的过程。在匹配的过程中，因为已经有了属性的名字，所以可以直接使用属性名作为Bean的名字向容器索取Bean，这个getBean会触发当前Bean的依赖Bean的依赖注入，从而得到属性对应的依赖Bean。在执行完这个getBean后，把这个依赖Bean注入到当前Bean的属性中去，这样就完成了通过这个依赖属性名自动完成依赖注入的过程。

protected void autowireByName(
		String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {

	String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
	for (String propertyName : propertyNames) {
		if (containsBean(propertyName)) {
			Object bean = getBean(propertyName);
			pvs.add(propertyName, bean);
			registerDependentBean(propertyName, beanName);
			if (logger.isTraceEnabled()) {
				logger.trace("Added autowiring by name from bean name '" + beanName +
						"' via property '" + propertyName + "' to bean named '" + propertyName + "'");
			}
		}
		else {
			if (logger.isTraceEnabled()) {
				logger.trace("Not autowiring property '" + propertyName + "' of bean '" + beanName +
						"' by name: no matching bean found");
			}
		}
	}
}

【5】Bean依赖的检查

在调用applyPropertyValues前，如果配置了依赖检查，则会调用checkDependencies方法。在这个方法中，会对Bean的Dependencies进行检查，如果发现不满足要求，就会抛出异常通知应用。

// AbstractAutowireCapableBeanFactory#checkDependencies
protected void checkDependencies(
		String beanName, AbstractBeanDefinition mbd, PropertyDescriptor[] pds, @Nullable PropertyValues pvs)
		throws UnsatisfiedDependencyException {

	int dependencyCheck = mbd.getDependencyCheck();
	for (PropertyDescriptor pd : pds) {
		if (pd.getWriteMethod() != null && (pvs == null || !pvs.contains(pd.getName()))) {
			boolean isSimple = BeanUtils.isSimpleProperty(pd.getPropertyType());
			boolean unsatisfied = (dependencyCheck == AbstractBeanDefinition.DEPENDENCY_CHECK_ALL) ||
					(isSimple && dependencyCheck == AbstractBeanDefinition.DEPENDENCY_CHECK_SIMPLE) ||
					(!isSimple && dependencyCheck == AbstractBeanDefinition.DEPENDENCY_CHECK_OBJECTS);
			if (unsatisfied) {
				throw new UnsatisfiedDependencyException(mbd.getResourceDescription(), beanName, pd.getName(),
						"Set this property value or disable dependency checking for this bean.");
			}
		}
	}
}

也就是说，在spring IOC容器中，设计了一个依赖检查特性，通过它spring可以帮助应用检查是否所有的属性都已经被正确设置。在具体使用的时候，应用只需要在Bean定义中设置dependency-check属性来指定依赖检查模式接口。如下所示，这里可以将属性设置为none、simple、object、all四种模式，默认的模式是none

//no dependency check at all.
public static final int DEPENDENCY_CHECK_NONE = 0;

// dependency checking for object references.
public static final int DEPENDENCY_CHECK_OBJECTS = 1;

// dependency checking for "simple" properties.
public static final int DEPENDENCY_CHECK_SIMPLE = 2;

//dependency checking for all properties
public static final int DEPENDENCY_CHECK_ALL = 3;