Spring循环依赖的原理解析

1、什么是循环依赖？

​ 我们使用Spring的时候，在一个对象中注入另一个对象，但是另外的一个对象中也包含该对象。如图：

在Student中包含了teacher的一个属性；

在Teacher中包含有student的属性。这样就形成了一个循环依赖。

2、代码描述

xml配置文件

testCycle.java

private static void testCycle(){ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("cycle.xml");Teacher teacher = applicationContext.getBean(Teacher.class);System.out.println(teacher);Student student = applicationContext.getBean(Student.class);System.out.println(student);}public static void main(String[] args) {testCycle();}

Student.java

public class Student {private Teacher teacher;public Teacher getTeacher() {return teacher;}public void setTeacher(Teacher teacher) {this.teacher = teacher;}
}

Teacher.java

public class Teacher {private Student student;public Student getStudent() {return student;}public void setStudent(Student student) {this.student = student;}
}

3、 测试结果

此处输出的teacher中包含有student对象，student对象中也包含有teacher对象，且包含的对象都是不为null的。

4、为什么能够循环依赖解释

先给出一张图

在Spring创建bean的时候肯定也是一个一个去创建的。首先肯定会先去走一个（Teacher/Student）生命周期。这里以Teacher为例，当Spring去getBean(teacher)的时候，首先会去容器中获取，获取不到就会去创建teacher，当teacher创建完成后，会给teacher的属性（student）赋值，实际上容器中没有student对象，这时候也会去创建student对象，当student创建的时候会去给student中的teacher属性赋值，teacher之前已经创建过了，此时去getBean(teacher)是能够拿到的(注意：此时的teacher中student属性并没有赋值)，这样student就创建完成了，那么就会回到teacher的student属性赋值的步骤，此时student已经创建是可以用getBean()拿到的，这样teacher对象就创建完毕了。然后回到第一步去创建student对象，这里student对象在创建teacher的时候就已经创建，可以直接使用getBean()获取到。给student中的属性赋值的时候也是一样，能够直接获取到teacher。自此循环依赖就已经结束了。

5、疑问

1. 当我在给Teacher属性student的赋值的时候是怎么去getBean()的？
2. 当给student中属性teacher赋值的时候getBean()为什么能够取到teacher？
3. 为什么获取到的teacher属性是为完成注入的？

6、源码解释

整体的方法线

先看看源码：

getBean()->doGetBean()

getBean() -> doGetBean() 实际上是 doGetBean 在去获取bean对象

public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)throws BeansException {return doGetBean(name, requiredType, args, false);
}/*** Return an instance, which may be shared or independent, of the specified bean.* 返回指定 bean 的一个实例，该实例可以是共享的，也可以是独立的。*/
@SuppressWarnings("unchecked")
protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)throws BeansException {// 转换beanName，FactoryBean的情况下beanName为&beanName，这里就是去掉&符号String beanName = transformedBeanName(name);Object beanInstance;// Eagerly check singleton cache for manually registered singletons.// 急切检查单例缓存从手动创建的单例中，获取bean判断是否存在当前beanName的beanObject sharedInstance = getSingleton(beanName);if (sharedInstance != null && args == null) {if (logger.isTraceEnabled()) {if (isSingletonCurrentlyInCreation(beanName)) {... 省略代码...}beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);}else {// Fail if we're already creating this bean instance:// We're assumably within a circular reference.// 没有获取到，如果已经创建bean的实例，我们在一个循环引用中。当前的bean是否为正在创建中if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// Check if bean definition exists in this factory.// 检查该工厂中是否存在bean的定义BeanFactory parentBeanFactory = getParentBeanFactory();... 省略代码...if (!typeCheckOnly) {// 标记bean已经创建，正在创建markBeanAsCreated(beanName);}StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate").tag("beanName", name);try {if (requiredType != null) {beanCreation.tag("beanType", requiredType::toString);}RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);// Guarantee initialization of beans that the current bean depends on.// 保证当前的bean所依赖的bean已经初始化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);}}}// Create bean instance.// 创建bean的实例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.// 从单例缓存中删除实例，它可能已经在这里// 通过创建过程-允许循环引用解析// 删除接收到任何对bean引用的临时beandestroySingleton(beanName);throw ex;}});beanInstance = 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);}beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}... 省略代码...finally {beanCreation.end();}}return adaptBeanInstance(name, beanInstance, requiredType);
}

此处传入进来的 beanName 为teacher

doGetBean()->createBean()

分开看

// Create bean instance.
// 创建bean的实例
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.// 从单例缓存中删除实例，它可能已经在这里// 通过创建过程-允许循环引用解析// 删除接收到任何对bean引用的临时beandestroySingleton(beanName);throw ex;}});beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}

重要的点在这里当没有获取到bean的时候就会去调用 createBean 方法，创建bean，最终其实是走的 doCreateBean 方法取创建bean

createBean()->doCreateBean()

这里就到了上面方法线的第四部

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {// Instantiate the bean.// BeanWrapper：持有创建出来的BeanBeanWrapper instanceWrapper = null;if (mbd.isSingleton()) {instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) {/*** 创建bean的实例* 实例化但是并未初始化，就是没有给bean的属性复制*/instanceWrapper = createBeanInstance(beanName, mbd, args);}Object bean = instanceWrapper.getWrappedInstance();Class<?> beanType = instanceWrapper.getWrappedClass();if (beanType != NullBean.class) {mbd.resolvedTargetType = beanType;}// Allow post-processors to modify the merged bean definition.// 允许增强器修改合并的bean definitionsynchronized (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;}}// Eagerly cache singletons to be able to resolve circular references// even when triggered by lifecycle interfaces like BeanFactoryAware.// 缓存单例的bean能够解析循环引用// 即使生命周期接口触发像BeanFactoryAware，// 判断当前的bean是否需要提前曝光（加入singletonFactories缓存）：bean是单例的&允许循环依赖&bean正在创建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");}// 将bean添加到singletonFactories  也就是说的三级缓存，但是这个地方的属性是没有赋值的addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));}// 到这里，bean就已经实例化完成，并且将bean放入到了singletonFactories缓存中// Initialize the bean instance.// 初始化bean的实例Object exposedObject = bean;try {/*** 填充bean，填充Bean的属性*/populateBean(beanName, mbd, instanceWrapper);/*** 去执行*     BeanPostProcessor的postProcessBeforeInitialization方法* */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 " +"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");}}}}// Register bean as disposable.// 将bean注册为一次性的try {registerDisposableBeanIfNecessary(beanName, bean, mbd);}catch (BeanDefinitionValidationException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);}return exposedObject;
}

分开解释 doCreateBean 方法

// Instantiate the bean.// BeanWrapper：持有创建出来的BeanBeanWrapper instanceWrapper = null;if (mbd.isSingleton()) {instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) {/*** 创建bean的实例* 实例化但是并未初始化，就是没有给bean的属性复制*/instanceWrapper = createBeanInstance(beanName, mbd, args);}

初始化 bean ，这个地方开始调用 createBeanInstance 方法创建一个bean的实例

// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
// 缓存单例的bean能够解析循环引用
// 即使生命周期接口触发像BeanFactoryAware，
// 判断当前的bean是否需要提前曝光（加入singletonFactories缓存）：bean是单例的&允许循环依赖&bean正在创建
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");}// 将bean添加到singletonFactories  也就是说的三级缓存，但是这个地方的属性是没有赋值的addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}

记住 addSingletonFactory() 方法，这是循环依赖的核心

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(singletonFactory, "Singleton factory must not be null");synchronized (this.singletonObjects) {if (!this.singletonObjects.containsKey(beanName)) {// 将beanName，singletonFactory放入到单例工厂的缓存【beanName-singletonFactory】this.singletonFactories.put(beanName, singletonFactory);// 从早起的单例对象缓存中移除【beanName-bean实例】this.earlySingletonObjects.remove(beanName);// 将beanName添加到已经注册的实例中this.registeredSingletons.add(beanName);}}
}

此处存入的 singletonFactory 是一个lambda表达式， ObjectFactory 是一个函数接口，当执行 getObject 方法的时候会去调用存入的 getEarlyBeanReference(beanName, mbd, bean)

doCreateBean() -> createBeanInstance()

这里也没什么好说的就是通过反射去创建Teacher对象

createBeanInstance() -> populateBean()

这里就是开始给创建的Teacher属性student赋值了

/*** Populate the bean instance in the given BeanWrapper with the property values* from the bean definition.* @param beanName the name of the bean* @param mbd the bean definition for the bean* @param bw the BeanWrapper with bean instance* 允许属性值填充给BeanWrapper中的Bean实例*/@SuppressWarnings("deprecation")  // for postProcessPropertyValues  后处理属性值protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {... 省略代码 ...// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the 给所有InstantiationAwareBeanPostProcessors有修改的机会// state of the bean before properties are set. This can be used, for example,// to support styles of field injection.// 设置属性之前bean的状态，例如// 支持字段注入if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {return;}}}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. 通过名称自动注入参数的值if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {autowireByName(beanName, mbd, bw, newPvs);}// Add property values based on autowire by type if applicable. 通过类型注入参数的值if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {autowireByType(beanName, mbd, bw, newPvs);}pvs = newPvs;}/*** 有没有实例化的AwareBeanPostProcessor*/boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();/*** 是否需要深度检查*/boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);if (hasInstAwareBpps) {if (pvs == null) {pvs = mbd.getPropertyValues();}for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {return;}pvs = pvsToUse;}}if (needsDepCheck) {PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);checkDependencies(beanName, mbd, filteredPds, pvs);}if (pvs != null) {// 应用给定的属性值，解决任何在这个bean工厂运行时它bean的引用。必须使用深copy。所以不会永久的修改此属性applyPropertyValues(beanName, mbd, bw, pvs);}}

分开解析

if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {return;}}}

InstantiationAwareBeanPostProcessor 的方法 postProcessAfterInstantiation ,该方法的返回值是boolean，如果返回true,则什么都不干,如果返回false,那么此类则不会进行自动装配(属性填充)，这里就是可以让我们通过 postprocessor 的方式控制某些bean不用属性填充。这里很明显如果我们没做特殊处理，这里最里面的if的return是不会被执行到的。

if (pvs != null) {// 应用给定的属性值，解决任何在这个bean工厂运行时它bean的引用。必须使用深copy。所以不会永久的修改此属性applyPropertyValues(beanName, mbd, bw, pvs);}

这里就是给Teacher的student属性赋值的

/*** Apply the given property values, resolving any runtime references* to other beans in this bean factory. Must use deep copy, so we* don't permanently modify this property.* @param beanName the bean name passed for better exception information* @param mbd the merged bean definition* @param bw the BeanWrapper wrapping the target object* @param pvs the new property values*  应用给定的属性值，解析对此 bean 工厂中其他 bean 的任何运行时引用。必须使用深拷贝，所以我们不会永久修改这个属性*/
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {if (pvs.isEmpty()) {// 如果pvs没有propertyValues，直接结束return;}MutablePropertyValues mpvs = null;List<PropertyValue> original;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. 创建一个深copy，解析任何引用值List<PropertyValue> deepCopy = new ArrayList<>(original.size());boolean resolveNecessary = false;for (PropertyValue pv : original) {if (pv.isConverted()) {deepCopy.add(pv);}else {// 获取属性的名称String propertyName = pv.getName();// 获取属性的值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);}// 解析属性值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 {bw.setPropertyValues(new MutablePropertyValues(deepCopy));}catch (BeansException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Error setting property values", ex);}
}

解析属性值

Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);

此处会直接走到 resolveReference 方法中去

/*** Resolve a reference to another bean in the factory.解析对另一个bean的引用*/
@Nullable
private Object resolveReference(Object argName, RuntimeBeanReference ref) {try {// 用来存放实例化出来的beanObject bean;// 获取bean的类型Class<?> beanType = ref.getBeanType();if (ref.isToParent()) {BeanFactory parent = this.beanFactory.getParentBeanFactory();if (parent == null) {... 省略代码 ...}else {String resolvedName;if (beanType != null) {... 省略代码... }else {resolvedName = String.valueOf(doEvaluate(ref.getBeanName()));// 获取resolvedName的bean对象bean = this.beanFactory.getBean(resolvedName);}// 注册依赖的beanthis.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);}
}

方法会走到这里去getBean() 之前的getBean还没走完是不是有走到getBean()，从这里开始就是套娃。

resolvedName = String.valueOf(doEvaluate(ref.getBeanName()));// 获取resolvedName的bean对象bean = this.beanFactory.getBean(resolvedName);}// 注册依赖的bean

到此处就会去寻找Student的实例，就会走一遍之前的方法，但是走到 pupolate() 方法的时候给student的teacher属性赋值，会去容器中获取一个teacher，还记得之前存在singletonFactories中的teacher吗？这里获取的时候就会直接拿到之前的存储的teacher。下面看一看

省略之前创建个逻辑，直接到赋值的操作

到这里就开始去获取teacher对象了，看一下 getSingleton() 方法是怎么拿的；

protected Object getSingleton(String beanName, boolean allowEarlyReference) {// Quick check for existing instance without full singleton lock// 从单例对象缓存（singletonObjects--一级缓存）中获取bean对象Object singletonObject = this.singletonObjects.get(beanName);// 如果单例对象中没有找到，并且改bean正在创建中if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {// 从早期单例对象缓存中获取单例对象（之所以成为早期单例对象，是因为earlySingletonObjects里面// 的对象都是通过提前曝光的ObjectFactory创建出来的。还没有进行属性填充等操作）singletonObject = this.earlySingletonObjects.get(beanName);// 早期单例对象缓存（二级缓存）中也没有并且允许创建早期单例对象if (singletonObject == null && allowEarlyReference) {// 如果为空，则锁定全局变量进行处理synchronized (this.singletonObjects) {// Consistent creation of early reference within full singleton lock//在完整的单例锁中一致地创建早期引用singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null) {// 当某些方法需要提前初始化的时候则会调用addSingletonFactory方法将对应的objectFactory初始化策略储存在singletonFactories中ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);if (singletonFactory != null) {// 如果存在单例对象工厂，则使用该工厂创建一个单例对象singletonObject = singletonFactory.getObject();// 创建的单例对象放如早期单例对象缓存中this.earlySingletonObjects.put(beanName, singletonObject);// 移除对应的单例对象工厂this.singletonFactories.remove(beanName);}}}}}}return singletonObject;
}

这里拿到了之前存入 singletonFactories Map中的lambda表达式，调用 getObject() 方法去执行 getEarlyBeanReference 方法

/*** Obtain a reference for early access to the specified bean,* typically for the purpose of resolving a circular reference.* @param beanName the name of the bean (for error handling purposes)* @param mbd the merged bean definition for the bean* @param bean the raw bean instance* @return the object to expose as bean reference** 获得对指定bean的早期访问的引用 通常用于解析循环依赖*/
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {// 默认最终公开的对象是bean，通过createBeanInstance创建出来的普通对象Object exposedObject = bean;// mbd的synthetic属性：设置bean定义是否是synthetic的，一般是指只有AOP相关的pointCut配置或者advice配置才会将synthetic设置为true// 如果mbd不是synthetic且此工厂拥有InstantiationAwareBeanPostProcessorif (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {// 遍历工厂的所有后置处理器，并获取smartInstantiationAware-ArrayListfor (SmartInstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().smartInstantiationAware) {// 让exposedObject对象经过每一个smartInstantiationAwareBeanPostProcessor报装exposedObject = bp.getEarlyBeanReference(exposedObject, beanName);}}// 返回最终经过层次报装后的对象return exposedObject;
}

这个方法没有什么好解释的，注释很明确的表明了方法的作用

拿到teacher之后就给Student中的teacher属性赋值

resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));

就此Student对像创建完毕，会将创建完成的Student对象放入

try {// 去容器中获取bean对象singletonObject = singletonFactory.getObject();newSingleton = true;
}
catch (IllegalStateException ex) {// Has the singleton object implicitly appeared in the meantime ->// if yes, proceed with it since the exception indicates that state.singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {throw ex;}
}
catch (BeanCreationException ex) {if (recordSuppressedExceptions) {for (Exception suppressedException : this.suppressedExceptions) {ex.addRelatedCause(suppressedException);}}throw ex;
}
finally {if (recordSuppressedExceptions) {this.suppressedExceptions = null;}afterSingletonCreation(beanName);
}
if (newSingleton) {// 将beanName和singletonObject的映射关系添加到该工厂的单例缓存中addSingleton(beanName, singletonObject);
}

addSingleton(beanName, singletonObject); 放入缓存中

至此会返回去给Teacher中的student属性赋值。至此一次的循环依赖就完成了。Spring还回去创建Student对象，但是这次容器中存在直接取出来就可以了。

疑问解答

为什么最后还要去创建一次Student对象，因为开始创建Student对象是因为创建Teacher对象的时候需要使用Student得实例，所以去创建了一次，但是最后一次去创建Student对象的时候不会真的创建，直接从缓存singletonObjects中就能去获取到。