Spring源码解析三：IOC容器的依赖注入

一、Bean对象创建的时机

依赖注入是在Bean对象创建的时候完成的，那么第一个问题来了，Bean对象什么时候创建？

Bean对象的创建是在getBean方法被调用的时候发生的，而在Spring中有两个场景会触发getBean方法被调用。

1、单例模式并且是非延迟加载的对象，会在IOC容器初始化的时候被创建且初始化。

2、非单例模式或者是延迟加载的对象，是应用第一次向容器索要该Bean对象的时候被创建且初始化。

虽然入口场景不同，但是Bean对象创建的过程是一样的，都是调用AbstractBeanFactory中getBean方法。

我们来重点看下DefaultListableBeanFactory的preInstantiateSingletons 的源码，就能对前面提到的规则有一个很好的理解。

 public void preInstantiateSingletons() throws BeansException {
         if (this.logger.isInfoEnabled()) {
             this.logger.info("Pre-instantiating singletons in " + this);
         }

         synchronized (this.beanDefinitionMap) {
             for (String beanName : this.beanDefinitionNames) {
                 RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
                 if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
                     if (isFactoryBean(beanName)) {
                         final FactoryBean factory = (FactoryBean) getBean(FACTORY_BEAN_PREFIX + beanName);
                         boolean isEagerInit;
                         if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                             isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
                                 public Boolean run() {
                                     return ((SmartFactoryBean) factory).isEagerInit();
                                 }
                             }, getAccessControlContext());
                         }
                         else {
                             isEagerInit = factory instanceof SmartFactoryBean && ((SmartFactoryBean) factory).isEagerInit();
                         }
                         if (isEagerInit) {
                             getBean(beanName);
                         }
                     }
                     else {
                         getBean(beanName);
                     }
                 }
             }
         }
     }

这里验证了第一个规则，容器初始化时，会对单例并且是非延迟加载的对象进行实例化。

二、依赖注入的源码分析

这里以DefaultListableBeanFactory的基类AbstractBeanFactory中的getBean()方法来进行介绍。

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

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

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

         // 先尝试从缓存中获取bean，对于那些单例模式的Bean，不需要重复创建。
         Object sharedInstance = getSingleton(beanName);
         if (sharedInstance != null && args == null) {
             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 + "'");
                 }
             }
　　　　　　　　　//这里处理的是FactoryBean相关的处理，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);
             }

             //对IOC容器中BeanDefinition是否存在进行检查，如果在当前Bean工厂中找不到需要的Bean，则到双亲BeanFactory中去查找，依次类推
             BeanFactory parentBeanFactory = getParentBeanFactory();
             if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                 // Not found -> check parent.
                 String nameToLookup = originalBeanName(name);
                 if (args != null) {
                     // Delegation to parent with explicit args.
                     return (T) parentBeanFactory.getBean(nameToLookup, args);
                 }
                 else {
                     // No args -> delegate to standard getBean method.
                     return parentBeanFactory.getBean(nameToLookup, requiredType);
                 }
             }

             if (!typeCheckOnly) {
                 markBeanAsCreated(beanName);
             }
             //根据BeanName取得BeanDefinition
             final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
             checkMergedBeanDefinition(mbd, beanName, args);

             // 获取当前Bean依赖的Bean，这里可能会触发getBean方法的递归调用，直到没有任何以来的Bean为止。
             String[] dependsOn = mbd.getDependsOn();
             if (dependsOn != null) {
                 for (String dependsOnBean : dependsOn) {
                     getBean(dependsOnBean);
                     registerDependentBean(dependsOnBean, beanName);
                 }
             }

             // Create bean instance.
             if (mbd.isSingleton()) {
                 sharedInstance = getSingleton(beanName, new ObjectFactory() {
                     public Object getObject() throws BeansException {
                         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);
             }

             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);
             }

             else {
                 String scopeName = mbd.getScope();
                 final Scope scope = this.scopes.get(scopeName);
                 if (scope == null) {
                     throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
                 }
                 try {
                     Object scopedInstance = scope.get(beanName, new ObjectFactory() {
                         public Object getObject() throws BeansException {
                             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);
                 }
             }
         }

         // 对新创建的Bean进行类型检查，如果没有问题就返回这个Bean，这个Bean此时已经包含了依赖关系的Bean
         if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
             throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
         }
         return (T) bean;
     }

接下来看看Bean对象通过createBean方法是如何被创建的，在类AbstractAutowireCapableBeanFactory中createBean()调用doCreateBean()

 protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
         // Instantiate the bean.
         BeanWrapper instanceWrapper = null;
           //如果是单例模式，现将缓存中的同名Bean删除
         if (mbd.isSingleton()) {
             instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
         }
         if (instanceWrapper == null) {
               //这里是创建Bean的地方
             instanceWrapper = createBeanInstance(beanName, mbd, args);
         }
         final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
         Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);

         // Allow post-processors to modify the merged bean definition.
         synchronized (mbd.postProcessingLock) {
             if (!mbd.postProcessed) {
                 applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
                 mbd.postProcessed = true;
             }
         }

         // Eagerly cache singletons to be able to resolve circular references
         // even when triggered by lifecycle interfaces like BeanFactoryAware.
         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");
             }
             addSingletonFactory(beanName, new ObjectFactory() {
                 public Object getObject() throws BeansException {
                     return getEarlyBeanReference(beanName, mbd, bean);
                 }
             });
         }

         // 这里是对Bean初始化，依赖注入就是在这里完成的，exposedObject会作为Bean依赖注入完成后的对象返回
         Object exposedObject = bean;
         try {
             populateBean(beanName, mbd, instanceWrapper);
             if (exposedObject != null) {
                 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<String>(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;
     }

代码片段中标记的两处，一个是Bean对象的生成；另一个Bean的初始化，即依赖注入。下面分别来这两个方法的实现：

1、createBeanInstance

 protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
         // Make sure bean class is actually resolved at this point.
         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());
         }

         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;
                 }
             }
         }
         if (resolved) {
             if (autowireNecessary) {
                 return autowireConstructor(beanName, mbd, null, null);
             }
             else {
                 return instantiateBean(beanName, mbd);
             }
         }

         // Need to determine the constructor...
         Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
         if (ctors != null ||
                 mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
                 mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
             return autowireConstructor(beanName, mbd, ctors, args);
         }

         // No special handling: simply use no-arg constructor.
         return instantiateBean(beanName, mbd);
     }

这里实例化Bean有三种方式：工厂方法、构造函数以及默认的实例化策略CGLB。前两种方式我们都很熟悉，这个CGLB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AOP中也是用CGLB对JAVA的字节码进行增强。

CGLB创建对象的过程在CglibSubclassingInstantiationStrategy中：

 public Object instantiate(Constructor ctor, Object[] args) {
             Enhancer enhancer = new Enhancer();
             enhancer.setSuperclass(this.beanDefinition.getBeanClass());
             enhancer.setCallbackFilter(new CallbackFilterImpl());
             enhancer.setCallbacks(new Callback[] {
                     NoOp.INSTANCE,
                     new LookupOverrideMethodInterceptor(),
                     new ReplaceOverrideMethodInterceptor()
             });

             return (ctor == null) ?
                     enhancer.create() :
                     enhancer.create(ctor.getParameterTypes(), args);
         }

2、populateBean

 protected void populateBean(String beanName, AbstractBeanDefinition mbd, BeanWrapper bw) {
         PropertyValues pvs = mbd.getPropertyValues();

         if (bw == null) {
             if (!pvs.isEmpty()) {
                 throw new BeanCreationException(
                         mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
             }
             else {
                 // Skip property population phase for null instance.
                 return;
             }
         }

         // 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.
         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;
         }
           //开始进行依赖注入过程，先处理autoWire的注入
         if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
                 mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
             MutablePropertyValues newPvs = new MutablePropertyValues(pvs);

             // Add property values based on autowire by name if applicable.
             if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
                 autowireByName(beanName, mbd, bw, newPvs);
             }

             // Add property values based on autowire by type if applicable.
             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) {
             PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw);
             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);
             }
         }
         //对属性进行注入
         applyPropertyValues(beanName, mbd, bw, pvs);
     }

对属性进行注入：

 protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
         if (pvs == null || pvs.isEmpty()) {
             return;
         }

         MutablePropertyValues mpvs = null;
         List<PropertyValue> original;

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

         if (pvs instanceof 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<PropertyValue>(original.size());
         boolean resolveNecessary = false;
         for (PropertyValue pv : original) {
             if (pv.isConverted()) {
                 deepCopy.add(pv);
             }
             else {
                 String propertyName = pv.getName();
                 Object originalValue = pv.getValue();
                   //对BeanDefinition进行解析
                 Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
　　　　　　　　　　　//解析后生成一个副本
                 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();
         }

         // 真正依赖注入的地方，实现类为WrapperImpl
         try {
             bw.setPropertyValues(new MutablePropertyValues(deepCopy));
         }
         catch (BeansException ex) {
             throw new BeanCreationException(
                     mbd.getResourceDescription(), beanName, "Error setting property values", ex);
         }
     }

在类BeanDefinitionValueResolver中对BeanDefinition进行解析

 public Object resolveValueIfNecessary(Object argName, Object value) {
         // We must check each value to see whether it requires a runtime reference
         // to another bean to be resolved.
         if (value instanceof RuntimeBeanReference) {
             RuntimeBeanReference ref = (RuntimeBeanReference) value;
             return resolveReference(argName, ref);
         }
         else if (value instanceof RuntimeBeanNameReference) {
             String refName = ((RuntimeBeanNameReference) value).getBeanName();
             refName = String.valueOf(evaluate(refName));
             if (!this.beanFactory.containsBean(refName)) {
                 throw new BeanDefinitionStoreException(
                         "Invalid bean name '" + refName + "' in bean reference for " + argName);
             }
             return refName;
         }
         else if (value instanceof BeanDefinitionHolder) {
             // Resolve BeanDefinitionHolder: contains BeanDefinition with name and aliases.
             BeanDefinitionHolder bdHolder = (BeanDefinitionHolder) value;
             return resolveInnerBean(argName, bdHolder.getBeanName(), bdHolder.getBeanDefinition());
         }
         else if (value instanceof BeanDefinition) {
             // Resolve plain BeanDefinition, without contained name: use dummy name.
             BeanDefinition bd = (BeanDefinition) value;
             return resolveInnerBean(argName, "(inner bean)", bd);
         }
         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);
         }
         else if (value instanceof ManagedList) {
             // May need to resolve contained runtime references.
             return resolveManagedList(argName, (List<?>) value);
         }
         else if (value instanceof ManagedSet) {
             // May need to resolve contained runtime references.
             return resolveManagedSet(argName, (Set<?>) value);
         }
         else if (value instanceof ManagedMap) {
             // May need to resolve contained runtime references.
             return resolveManagedMap(argName, (Map<?, ?>) value);
         }
         else if (value instanceof ManagedProperties) {
             Properties original = (Properties) value;
             Properties copy = new Properties();
             for (Map.Entry propEntry : original.entrySet()) {
                 Object propKey = propEntry.getKey();
                 Object propValue = propEntry.getValue();
                 if (propKey instanceof TypedStringValue) {
                     propKey = evaluate((TypedStringValue) propKey);
                 }
                 if (propValue instanceof TypedStringValue) {
                     propValue = evaluate((TypedStringValue) propValue);
                 }
                 copy.put(propKey, propValue);
             }
             return copy;
         }
         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);
             }
         }
         else {
             return evaluate(value);
         }
     }

这里可以看到不同类型的属性，常见的list、map、array等，我们最关注的还是依赖的Bean是如何解析的，请看第6行：

 private Object resolveReference(Object argName, RuntimeBeanReference ref) {
         try {
             String refName = ref.getBeanName();
             refName = String.valueOf(evaluate(refName));
               //如果引用的Bean在双亲容器中，就从双亲容器中查找
             if (ref.isToParent()) {
                 if (this.beanFactory.getParentBeanFactory() == null) {
                     throw new BeanCreationException(
                             this.beanDefinition.getResourceDescription(), this.beanName,
                             "Can't resolve reference to bean '" + refName +
                             "' in parent factory: no parent factory available");
                 }
                 return this.beanFactory.getParentBeanFactory().getBean(refName);
             }
               //如果在当前容器中获取Bean，会触发一个getBean的过程，如果依赖注入没有发生，也会相应触发该过程
             else {
                 Object bean = this.beanFactory.getBean(refName);
                 this.beanFactory.registerDependentBean(refName, this.beanName);
                 return bean;
             }
         }
         catch (BeansException ex) {
             throw new BeanCreationException(
                     this.beanDefinition.getResourceDescription(), this.beanName,
                     "Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
         }
     }

到这里已经将依赖注入所需要的数据全都准备OK，现在要做的就是将这些属性设置对应的Bean中。而属性设置发生在类BeanWrapperImpl中的setPropertyValue方法中

 public void setPropertyValue(PropertyValue pv) throws BeansException {
         PropertyTokenHolder tokens = (PropertyTokenHolder) pv.resolvedTokens;
         if (tokens == null) {
             String propertyName = pv.getName();
             BeanWrapperImpl nestedBw;
             try {
                 nestedBw = getBeanWrapperForPropertyPath(propertyName);
             }
             catch (NotReadablePropertyException ex) {
                 throw new NotWritablePropertyException(getRootClass(), this.nestedPath + propertyName,
                         "Nested property in path '" + propertyName + "' does not exist", ex);
             }
             tokens = getPropertyNameTokens(getFinalPath(nestedBw, propertyName));
             if (nestedBw == this) {
                 pv.getOriginalPropertyValue().resolvedTokens = tokens;
             }
             nestedBw.setPropertyValue(tokens, pv);
         }
         else {
             setPropertyValue(tokens, pv);
         }
     }

属性设置的具体位置在第17行，有兴趣的可以进去看下。。。。。。。。

到此，我们已经完成了找到水源、将水装入木桶、对桶里的水进行处理，如消毒、煮沸。那么现在可以使用IOC容器中存在的Bean对象了。