IoC容器的依赖注入

spring容器默认在启动时就对bean进行实例化，但是可以修改baen的lazy-init属性为true延迟bean的初始化，使用bean时才实例化：

（1）spring中bean的scope默认是singleton，lazy-init为false，容器启动时实例化bean，修改lazy-init为true时，使用bean时实例化。

<bean lazy-init="true"/>   
<beans default-lazy-init="true"></beans>

（2）scope是prototype时，lazy-init不管是true还是false都会在使用的时候才对bean实例化。

参考：

林兴洋的博客：spring中的bean何时被实例化

绝地反击T：Spring的IOC、Spring对象初始化bean时机、Spring容器生命周期

---

从容器中获取bean的过程如下：

//使用FileSystemXmlApplicationContext加载配置文件
ApplicationContext context=new FileSystemXmlApplicationContext("classpath*:/spring/applicationContext.xml");
//从容器中获取bean
Student student= (Stuedent) context.getBean("student");//根据id从配置文件中获取bean

依赖注入的发生是在容器中的BeanDefinition数据已经建立好的前提下进行的，这个getBean方法就是触发依赖注入发生的地方，该方法在BeanFactory中有定义，DefaultListableBeanFactory的基类AbstractBeanFactory对该方法有实现，因此就从AbstractBeanFactory的getBean方法入手，看一下IoC容器依赖注入的过程。

1.AbstractBeanFactory

public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {

    
	//---------------------------------------------------------------------
    // BeanFactory接口的实现类  
    // 这些getBean接口方法最终是通过调用doGetBean来实现的  e
	//---------------------------------------------------------------------

	@Override
	public Object getBean(String name) throws BeansException {
		return doGetBean(name, null, null, false);
	}

	@Override
	public <T> T getBean(String name, @Nullable Class<T> requiredType) throws BeansException {
		return doGetBean(name, requiredType, null, false);
	}

	@Override
	public Object getBean(String name, Object... args) throws BeansException {
		return doGetBean(name, null, args, false);
	}

	public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)
			throws BeansException {

		return doGetBean(name, requiredType, args, false);
	}

    /** 
     * 实际取得Bean的地方，也就是触发依赖注入发生的地方 
     *  
     */  
	@SuppressWarnings("unchecked")
	protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

		final String beanName = transformedBeanName(name);
		Object bean;

	    // 根据name从缓存中获取实例，这里获取的是单例模式的bean  
		Object sharedInstance = getSingleton(beanName);
		if (sharedInstance != null && args == null) {//如果获取不为空
			if (logger.isDebugEnabled()) {
				if (isSingletonCurrentlyInCreation(beanName)) {
					logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
				}
				else {
					logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
				}
			}
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {//如果从缓存中获取失败
		    // 检查是否已经创建
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			/* 检查BeanDefinition是否存在 */
			
			//获取双亲BeanFactory
			BeanFactory parentBeanFactory = getParentBeanFactory();
			//如果双亲BeanFactory不为空，并且当前的双亲BeanFacotry不包含该bean
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				String nameToLookup = originalBeanName(name);
				//沿着双亲链往上查找
				if (parentBeanFactory instanceof AbstractBeanFactory) {
					return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
							nameToLookup, requiredType, args, typeCheckOnly);
				}
				else if (args != null) {
					// 调用双亲带args的getBean方法
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else {
				    // 调用双亲不带args的getBean方法
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
			}

			if (!typeCheckOnly) {
				markBeanAsCreated(beanName);
			}
			//如果双亲容器不包含该bean
			try {
			    //根据Bean的名字取得BeanDefinition  
				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				//获取当前Bean依赖的所有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
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
						if (isDependent(beanName, dep)) {
							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);
						}
					}
				}

				//创建bean的实例  
				if (mbd.isSingleton()) {//单例模式
					sharedInstance = getSingleton(beanName, () -> {
						try {
						    //创建bean的实例
							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);
				}

				else if (mbd.isPrototype()) {//原型模式
					// 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);
				}

				else {
					String scopeName = mbd.getScope();
					final 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，检查类型是否与实例化的bean一致
		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.isDebugEnabled()) {
					logger.debug("Failed to convert bean '" + name + "' to required type '" +
							ClassUtils.getQualifiedName(requiredType) + "'", ex);
				}
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
		}
		return (T) bean;
	}

}

（1）在doGetBean方法中，首先会从缓存中查找bean，如果未找到，则对IoC容器中BeanDefinition是否存在进行检查。检查时会从双亲BeanFactory中查找，如果当前双亲BeanFactory中找不到就顺着双亲BeanFactory链一直向上查找，如果没有双亲BeanFactory，就从当前BeanFactory中取。
（2）获取到bean的BeanDefinition后，会调用ObjectFactory的createBean方法对Bean进行实例化。
（3）实例化之后，会对创建的Bean进行类型检查，检查类型是否与实例化的bean一致，如果正确就返回这个新创建的bean。

2.AbstractAutowireCapableBeanFactory

AbstractAutowireCapableBeanFactory中实现了createBean方法。

public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
		implements AutowireCapableBeanFactory {

//---------------------------------------------------------------------
	// Implementation of relevant AbstractBeanFactory template methods
	//---------------------------------------------------------------------

	/**
	 * AbstractAutowireCapableBeanFactory的核心方法: 创建bean的实例 
	 * @see #doCreateBean
	 */
	@Override
	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

		if (logger.isDebugEnabled()) {
			logger.debug("Creating instance of bean '" + beanName + "'");
		}
		//bean的BeanDefinition对象  
		RootBeanDefinition mbdToUse = mbd;

		//判断需要创建的bean是否可以实例化
		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 {
			//处理bean的后置处理器，返回的是一个proxy代理  
			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 {
		    //创建bean
			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
			if (logger.isDebugEnabled()) {
				logger.debug("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);
		}
	}
    
    /**
     *  创建指定的bean对象 
	 */
	protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
			throws BeanCreationException {

		// BeanWrapper用来持有创建出来的Bean对象 
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
		    //如果是单例模式，清除缓存中同名的bean
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
		    //调用createBeanInstance方法完成bean的实例化
			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 {
					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
				}
				catch (Throwable ex) {
					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
							"Post-processing of merged bean definition failed", ex);
				}
				mbd.postProcessed = true;
			}
		}

		// 判断bean是否是单例模式、允许循环引用和给定的bean是否处于创建状态
		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");
			}
			//将创建的baen放入SingletonFactory
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

		// 初始化bean的实例
		Object exposedObject = bean;
		try {
		    //依赖注入发生的地方
			populateBean(beanName, mbd, instanceWrapper);
			//初始化bean
			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 {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}

		return exposedObject;
	}
	
	/**	
	 * 通过指定的bean name创建一个新的实例，实例化的方法有：工厂方法，构造器注入或者简单的实例化 
	 * @see #instantiateBean
	 */
	protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
		// 确保创建bean可以实例化
		Class<?> beanClass = resolveBeanClass(mbd, beanName);

		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());
		}

		Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
		if (instanceSupplier != null) {
			return obtainFromSupplier(instanceSupplier, beanName);
		}
		//如果工厂方法不为空
		if (mbd.getFactoryMethodName() != null)  {
		    //使用工厂方法对bean实例化
			return instantiateUsingFactoryMethod(beanName, mbd, args);
		}

		// 创建相同的bean时的一个快捷方式
		boolean resolved = false;
		boolean autowireNecessary = false;
		if (args == null) {
			synchronized (mbd.constructorArgumentLock) {
				if (mbd.resolvedConstructorOrFactoryMethod != null) {
					resolved = true;
					autowireNecessary = mbd.constructorArgumentsResolved;
				}
			}
		}
		if (resolved) {
			if (autowireNecessary) {
				return autowireConstructor(beanName, mbd, null, null);
			}
			else {
				return instantiateBean(beanName, mbd);
			}
		}

		// 检测构造函数
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
		if (ctors != null ||
				mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
			//使用带参构造函数对bean实例化
			return autowireConstructor(beanName, mbd, ctors, args);
		}

		//使用无参的构造函数实例化bean
		return instantiateBean(beanName, mbd);
	}
	
	/**
	 * 使用默认的构造函数实例化bean，默认使用CGLIB实例化bean
	 */
	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()返回的是CGLIB策略
						getInstantiationStrategy().instantiate(mbd, beanName, parent),//CGLIB对bean进行实例化  
						getAccessControlContext());
			}
			else {
				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);
		}
	}
	
	protected InstantiationStrategy getInstantiationStrategy() {
		return this.instantiationStrategy;
	}
	//CLIGB
	private InstantiationStrategy instantiationStrategy = new CglibSubclassingInstantiationStrategy();

}

doCreateBean中两个方法：

（1）createBeanInstance方法：

在这个方法中，又会调用instantiateBean和autowireConstructor创建bean的实例。

autowireConstructor中是通过ConstructorResolver的autowireConstructor方法完成bean的实例化的。

instantiateBean中，调用getInstantiationStrategy()返回了CglibSubclassingInstantiationStrategy对象，调用该对象的instantiate方法对bean实例化，从名字上就可以看出是通过CGLIB实例化的。CglibSubclassingInstantiationStrategy是SimpleInstantiationStrategy的子类，实际上先走的SimpleInstantiationStrategy的instantiate方法，在该方法中调用CglibSubclassingInstantiationStrategy的instantiateWithMethodInjection方法完成的。

SimpleInstantiationStrategy

public class SimpleInstantiationStrategy implements InstantiationStrategy {

	private static final ThreadLocal<Method> currentlyInvokedFactoryMethod = new ThreadLocal<>();

	@Nullable
	public static Method getCurrentlyInvokedFactoryMethod() {
		return currentlyInvokedFactoryMethod.get();
	}


	@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) {
		     	//取得指定的构造器或者生成对象的工厂方法对Bean进行实例化
				constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
				if (constructorToUse == null) {
			     	//获取bean的class  
					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);
					}
				}
			}
			//调用BeanUtils的instantiateClass方法通过构造器对bean进行实例化
			return BeanUtils.instantiateClass(constructorToUse);
		}
		else {
			//CglibSubclassingInstantiationStrategy中对该方法有实现
			return instantiateWithMethodInjection(bd, beanName, owner);
		}
	}
	
	protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName,
			BeanFactory owner, @Nullable Constructor<?> ctor, @Nullable Object... args) {

		throw new UnsupportedOperationException("Method Injection not supported in SimpleInstantiationStrategy");
	}

SimpleInstantiationStrategy的instantiateWithMethodInjection只抛出了一个异常，其他的什么也没做。CglibSubclassingInstantiationStrategy对该方法进行了重写。

CglibSubclassingInstantiationStrategy

public class CglibSubclassingInstantiationStrategy extends SimpleInstantiationStrategy {
@Override
	protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
	    // 调用instantiate完成bean的实例化
		return instantiateWithMethodInjection(bd, beanName, owner, null);
	}

	@Override
	protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
			@Nullable Constructor<?> ctor, @Nullable Object... args) {

		// 调用instantiate完成bean的实例化
		return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
	}


	/**
	 * An inner class created for historical reasons to avoid external CGLIB dependency
	 * in Spring versions earlier than 3.2.
	 */
	private static class CglibSubclassCreator {

		private static final Class<?>[] CALLBACK_TYPES = new Class<?>[]
				{NoOp.class, LookupOverrideMethodInterceptor.class, ReplaceOverrideMethodInterceptor.class};

		private final RootBeanDefinition beanDefinition;

		private final BeanFactory owner;

		CglibSubclassCreator(RootBeanDefinition beanDefinition, BeanFactory owner) {
			this.beanDefinition = beanDefinition;
			this.owner = owner;
		}

		/**
		 * 创建实例的地方
		 */
		public Object instantiate(@Nullable Constructor<?> ctor, @Nullable Object... args) {
			Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
			Object instance;
			if (ctor == null) {
				instance = BeanUtils.instantiateClass(subclass);
			}
			else {
				try {
					Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
					instance = enhancedSubclassConstructor.newInstance(args);
				}
				catch (Exception ex) {
					throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
							"Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
				}
			}
			// SPR-10785: set callbacks directly on the instance instead of in the
			// enhanced class (via the Enhancer) in order to avoid memory leaks.
			Factory factory = (Factory) instance;
			factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
					new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
					new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
			return instance;
		}
		
		/**
		 * 根据提供的bean使用CGLIB创建一个增强的子类 
		 */
		private Class<?> createEnhancedSubclass(RootBeanDefinition beanDefinition) {
			Enhancer enhancer = new Enhancer();
			enhancer.setSuperclass(beanDefinition.getBeanClass());
			enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
			if (this.owner instanceof ConfigurableBeanFactory) {
				ClassLoader cl = ((ConfigurableBeanFactory) this.owner).getBeanClassLoader();
				enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(cl));
			}
			enhancer.setCallbackFilter(new MethodOverrideCallbackFilter(beanDefinition));
			enhancer.setCallbackTypes(CALLBACK_TYPES);
			return enhancer.createClass();
		}
	}
}

CglibSubclassingInstantiationStrategy的instantiateWithMethodInjection方法中调用了CglibSubclassCreator的instantiate方法完成了bean的实例化。

（2）populateBean方法

该方法的作用是设置bean之间的依赖关系，完成整个依赖注入的过程。

回到AbstractAutowireCapableBeanFactory的populateBean方法：

   /**
 * Populate the bean instance in the given BeanWrapper with the property values
 * from the bean definition.
 * @param beanName bean的name
 * @param mbd bean的BeanDefinition
 * @param bw bean的实例
 */
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
	if (bw == null) {//如果bean的实例为空
		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;
		}
	}


	boolean continueWithPropertyPopulation = true;
    // 处理初始化前置处理器
	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;
	}
	//获取BeanDefinition中的property值
	PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
	//如果是byName或者byType注入
	if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
			mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
		MutablePropertyValues newPvs = new MutablePropertyValues(pvs);

		// 根据bean的名字注入
		if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
			autowireByName(beanName, mbd, bw, newPvs);
		}

		// 根据类型注入
		if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
			autowireByType(beanName, mbd, bw, newPvs);
		}

		pvs = newPvs;
	}

	boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
	boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);

	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;
					pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
					if (pvs == null) {
						return;
					}
				}
			}
		}
		if (needsDepCheck) {
			checkDependencies(beanName, mbd, filteredPds, pvs);
		}
	}

	if (pvs != null) {
	    //对属性进行注入
		applyPropertyValues(beanName, mbd, bw, pvs);
	}
}

/**
 * 具体的对属性进行解析然后注入的过程  
 */
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
	if (pvs.isEmpty()) {//如果PropertyValues为空
		return;
	}

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

	MutablePropertyValues mpvs = null;
	List<PropertyValue> original;

	if (pvs instanceof MutablePropertyValues) {//如果是MutablePropertyValues
		mpvs = (MutablePropertyValues) pvs;
		if (mpvs.isConverted()) {
			// 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());
	}

	TypeConverter converter = getCustomTypeConverter();
	if (converter == null) {
		converter = bw;
	}
	BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);

	// Create a deep copy, resolving any references for values.
	List<PropertyValue> deepCopy = new ArrayList<>(original.size());
	boolean resolveNecessary = false;
	for (PropertyValue pv : original) {
		if (pv.isConverted()) {
			deepCopy.add(pv);
		}
		else {
			String propertyName = pv.getName();
			Object originalValue = pv.getValue();
			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();
	}


	try {
	    // 设置属性值，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);
	}
}

BeanWrapper是一个接口，setPropertyValues方法是在 AbstractNestablePropertyAccessor中实现的，BeanWrapperImpl又是AbstractNestablePropertyAccessor的子类，因此可以通过BeanWrapperImpl调用setPropertyValues方法完成属性值的设置。

public abstract class AbstractNestablePropertyAccessor extends AbstractPropertyAccessor {
    
    /* 
     * 设置property value 
     */
    @Override
	public void setPropertyValue(String propertyName, @Nullable Object value) throws BeansException {
		AbstractNestablePropertyAccessor nestedPa;
		try {
			nestedPa = getPropertyAccessorForPropertyPath(propertyName);
		}
		catch (NotReadablePropertyException ex) {
			throw new NotWritablePropertyException(getRootClass(), this.nestedPath + propertyName,
					"Nested property in path '" + propertyName + "' does not exist", ex);
		}
		PropertyTokenHolder tokens = getPropertyNameTokens(getFinalPath(nestedPa, propertyName));
		nestedPa.setPropertyValue(tokens, new PropertyValue(propertyName, value));
	}

    /* 
     * 设置property value 
     */
	@Override
	public void setPropertyValue(PropertyValue pv) throws BeansException {
		PropertyTokenHolder tokens = (PropertyTokenHolder) pv.resolvedTokens;
		if (tokens == null) {
			String propertyName = pv.getName();
			AbstractNestablePropertyAccessor nestedPa;
			try {
				nestedPa = getPropertyAccessorForPropertyPath(propertyName);
			}
			catch (NotReadablePropertyException ex) {
				throw new NotWritablePropertyException(getRootClass(), this.nestedPath + propertyName,
						"Nested property in path '" + propertyName + "' does not exist", ex);
			}
			tokens = getPropertyNameTokens(getFinalPath(nestedPa, propertyName));
			if (nestedPa == this) {
				pv.getOriginalPropertyValue().resolvedTokens = tokens;
			}
			nestedPa.setPropertyValue(tokens, pv);
		}
		else {
			setPropertyValue(tokens, pv);
		}
	}

	protected void setPropertyValue(PropertyTokenHolder tokens, PropertyValue pv) throws BeansException {
		if (tokens.keys != null) {
			processKeyedProperty(tokens, pv);
		}
		else {
			processLocalProperty(tokens, pv);
		}
	}
	
	private void processKeyedProperty(PropertyTokenHolder tokens, PropertyValue pv) {
		Object propValue = getPropertyHoldingValue(tokens);
		PropertyHandler ph = getLocalPropertyHandler(tokens.actualName);
		if (ph == null) {
			throw new InvalidPropertyException(
					getRootClass(), this.nestedPath + tokens.actualName, "No property handler found");
		}
		Assert.state(tokens.keys != null, "No token keys");
		String lastKey = tokens.keys[tokens.keys.length - 1];

		if (propValue.getClass().isArray()) {//如果属性是Array类型
			Class<?> requiredType = propValue.getClass().getComponentType();
			int arrayIndex = Integer.parseInt(lastKey);
			Object oldValue = null;
			try {
				if (isExtractOldValueForEditor() && arrayIndex < Array.getLength(propValue)) {
					oldValue = Array.get(propValue, arrayIndex);
				}
				Object convertedValue = convertIfNecessary(tokens.canonicalName, oldValue, pv.getValue(),
						requiredType, ph.nested(tokens.keys.length));
				int length = Array.getLength(propValue);
				if (arrayIndex >= length && arrayIndex < this.autoGrowCollectionLimit) {
					Class<?> componentType = propValue.getClass().getComponentType();
					Object newArray = Array.newInstance(componentType, arrayIndex + 1);
					System.arraycopy(propValue, 0, newArray, 0, length);
					setPropertyValue(tokens.actualName, newArray);
					propValue = getPropertyValue(tokens.actualName);
				}
				Array.set(propValue, arrayIndex, convertedValue);
			}
			catch (IndexOutOfBoundsException ex) {
				throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName,
						"Invalid array index in property path '" + tokens.canonicalName + "'", ex);
			}
		}

		else if (propValue instanceof List) {//如果bean的属性是list类型
			Class<?> requiredType = ph.getCollectionType(tokens.keys.length);
			List<Object> list = (List<Object>) propValue;
			int index = Integer.parseInt(lastKey);
			Object oldValue = null;
			if (isExtractOldValueForEditor() && index < list.size()) {
				oldValue = list.get(index);
			}
			Object convertedValue = convertIfNecessary(tokens.canonicalName, oldValue, pv.getValue(),
					requiredType, ph.nested(tokens.keys.length));
			int size = list.size();
			if (index >= size && index < this.autoGrowCollectionLimit) {
				for (int i = size; i < index; i++) {
					try {
						list.add(null);
					}
					catch (NullPointerException ex) {
						throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName,
								"Cannot set element with index " + index + " in List of size " +
								size + ", accessed using property path '" + tokens.canonicalName +
								"': List does not support filling up gaps with null elements");
					}
				}
				list.add(convertedValue);
			}
			else {
				try {
					list.set(index, convertedValue);
				}
				catch (IndexOutOfBoundsException ex) {
					throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName,
							"Invalid list index in property path '" + tokens.canonicalName + "'", ex);
				}
			}
		}

		else if (propValue instanceof Map) {//如果属性是map类型
			Class<?> mapKeyType = ph.getMapKeyType(tokens.keys.length);
			Class<?> mapValueType = ph.getMapValueType(tokens.keys.length);
			Map<Object, Object> map = (Map<Object, Object>) propValue;
			// IMPORTANT: Do not pass full property name in here - property editors
			// must not kick in for map keys but rather only for map values.
			TypeDescriptor typeDescriptor = TypeDescriptor.valueOf(mapKeyType);
			Object convertedMapKey = convertIfNecessary(null, null, lastKey, mapKeyType, typeDescriptor);
			Object oldValue = null;
			if (isExtractOldValueForEditor()) {
				oldValue = map.get(convertedMapKey);
			}
			// Pass full property name and old value in here, since we want full
			// conversion ability for map values.
			Object convertedMapValue = convertIfNecessary(tokens.canonicalName, oldValue, pv.getValue(),
					mapValueType, ph.nested(tokens.keys.length));
			map.put(convertedMapKey, convertedMapValue);
		}

		else {
			throw new InvalidPropertyException(getRootClass(), this.nestedPath + tokens.canonicalName,
					"Property referenced in indexed property path '" + tokens.canonicalName +
					"' is neither an array nor a List nor a Map; returned value was [" + propValue + "]");
		}
	}

}

在Bean的创建和对象依赖注入的过程中，需要依据BeanDefinition中的信息递归的完成依赖注入，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，同时也触发对依赖Bean的创建和注入。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，一层一层的完成Bean的创建和注入，直到最后完成当然Bean的创建。

在Bean创建和依赖注入完成以后，在IoC容器中建立起一系列依靠依赖关系联系起来的Bean。

来自：spring技术内幕:深入解析spring架构与设计原理

Spring版本:5.0.5