面试必问系列之最强源码分析，带你一步步弄清楚Spring如何解决循环依赖-阿里云开发者社区

写在前面

憋了好几天了，在经历了感冒的悲伤之后，终于大脑变得清晰起来。在《聊聊Spring的IOC以及JVM的类加载》中，之前留下了一个坑，不知道大家还有没有印象。那就是一个考查源码时必然会问到的一个面试题 - -Spring循环依赖是如何解决的。今天，我们就来好好分析下这个话题，我会很细致的讲解。预计，本文会很长很长，希望大家有耐心，去读完，相信你读完之后，对于Spring的IOC部分将有更深的源码理解。采用我的逻辑去把这个问题去解读明白。大致分为以下几部分：

何为循环依赖？
Spring管理bean对象
Spring如何解决循环依赖

何为循环依赖

想要理解这个问题，那么首先呢，需要有基础的知识储备。那就是Spring的IOC。IOC，是控制反转，后来出现更容易的理解 DI，依赖注入。大致上就是，一个A对象内有一个B对象属性，无需A对象显式创建B对象，可以通过Spring容器进行注入B对象到A对象中。这便是依赖注入的含义。

循环依赖的出现，是因为A对象中依赖B对象作为其中属性，B对象中依赖A对象作为其中属性。如下图所示：

举个例子描述：

小明喜欢小红，小红喜欢小黑，小黑喜欢小丽，小丽喜欢小明，如果中间所有人都不放弃喜欢的人，那么每个人都将陷入爱情循环中，无法自拔。

类似于这种依赖关系，在自然界中屡见不鲜，因此，映射到我们java的对象世界中，就必然会存在。

那么，如何处理循环依赖，保证合理的程序运行，是我们需要思考的问题。Spring作为一个技术框架，必然考虑到了循环依赖的问题。

Spring管理Bean对象

理解Spring作为容器管理bean对象之前，我们可以尝试思考下，我们自主，如何实现对于依赖对象的注入

场景描述

现有一个场景，蜜蜂采蜜。蜜蜂是一个对象，蜜蜂采蜜，需要作用于花，因此依赖花的存在。

蜜蜂对象，代码如下：

public class Bee {
   /**
    * 性别
    */
   private String sex;
   /**
    * 年龄
    */
   private Integer age;
   @Autowired
   private Flower flower;
   /**
    * 采蜜
    */
   public void pickingHoney() {
       // 花产蜜
       flower.product();
  }
   public String getSex() {
       return sex;
  }
   public void setSex(String sex) {
       this.sex = sex;
  }
   public Integer getAge() {
       return age;
  }
   public void setAge(Integer age) {
       this.age = age;
  }
}

上述代码，是Spring采用注解注入的方式实现。

那么，试想，如果不使用Spring，我们如何实现呢？

一直接简单粗暴地创建对象（可以实现，但是明显不符合自然界对象定义，因为蜜蜂不能创造出花这个对象）

public class Bee {
   /**
    * 性别
    */
   private String sex;
   /**
    * 年龄
    */
   private Integer age;
   /**
    * 采蜜
    */
   public void pickingHoney() {
       // 花产蜜
       Flower flower = new Flower();
       flower.product();
  }
   public String getSex() {
       return sex;
  }
   public void setSex(String sex) {
       this.sex = sex;
  }
   public Integer getAge() {
       return age;
  }
   public void setAge(Integer age) {
       this.age = age;
  }
}

二反射获取对象

 /**
    * 采蜜
    */
   public void pickingHoney() {
      Class clazz = Class.forName("com.example.demo.test.Flower");
       Flower flower = (Flower)clazz.getDeclaredConstructor().newInstance();
       flower.product();
  }

可见，相比第一种来说，第二种反射获取对象，很优雅的实现了对象的处理。

注：反射作为Java的一个典型技术，非常重要

上述示例代码，通过反射实现，获取类对象，然后创建类实例。且上述采用的类名形式，那么我们就可以通过配置文件读取、自定义注解等多种方式，来实现反射的实现。

有了上述的技术积累，我们可以很开心的完成第一步，反射获取对象，实现对象的注入。

再思考，在每个类对象中，处理反射逻辑，会造成代码的冗余，且会造成，对象的创建频繁，没法保证单一等问题。那么，通过学习设计模式中的单例模式、工厂模式，我们可以发现，如果应用上述设计模式，或许更优雅。就这样，我们慢慢贴近了Spring的实现。

Spring框架就是应用了各种设计模式，同时采用反射技术，来实现对象的创建、初始化等操作的。

Spring管理bean对象

Bean对象定义方式
Bean对象的定义，正如我们掌握的，在Spring中，可以通过几种方式完成：

XML文件定义

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
   <bean id="a" class="com.example.demo.test.A">
       <property name="b" ref="b"></property>
   </bean>
   <bean id="b" class="com.example.demo.test.B">
       <property name="a" ref="a"></property>
   </bean>
</beans>

如代码展示，可以直接通过xml文件，管理bean对象，实现bean对象的定义，以及属性的注入

注解形式可以通过各类注解，比如@Component @Controller @Service 等等

Bean对象读取
在Spring管理下，对于Bean对象的读取，会形成一个抽象层，BeanDefinitionReader，然后形成BeanDefinition，完成对于Bean对象描述的定义
BeanFactory
Bean对象的创建，由创建工程BeanFactory完成。BeanFactory会完成BeanDefinition的转换，然后构建bean对象，完成bean对象的实例化、初始化。
工厂，通过反射技术，根据BeanDefinition创建Bean对象。
FactoryBean
FactoryBean是特殊的BeanFactory
BeanFactoryPostPocessor
BeanFactory的后置处理器，完成BeanFactory创建后的一系列补充
BeanPostPocessor
Bean对象初始化前后的处理

流程描述：

Spring读取XML文件等形式对于Bean对象的定义，构建BeanDefinition
Spring的BeanFactory，读取BeanDefinition，完成Bean对象的读取，通过反射技术进行实例化
BeanFactoryPostPocessor作为BeanFactory的后置处理器，完成补充处理
BeanPostPocessor - before 完成对于bean对象初始化前的对象处理
BeanPostPocessor - after完成对于bean对象初始化后的对象处理
context.getBean 进行bean对象的使用

通过以上的分析，我们掌握了，Spring对于bean对象的读取、创建实例化、初始化的过程。了解了这些，我们才能去看循环依赖的事情。如若不然，个人觉得理解循环依赖问题，就比较困难。

Spring如何解决循环依赖

当我们能够明确了上述学习之后，我想，我们可以进入到源码的世界中去分析了。

查看解决循环依赖，也就是了解Spring是如何对Bean对象进行实例化、初始化的。

知识点：

对象的创建分为两部分，一部分为实例化，此时，在堆内存中分配了内存空间，但是未对其属性进行赋值，另一部分为初始化，完成对于实例化后对象的属性的赋值操作。因此类对象在内存空间中，两种状态，实例化状态（实例化完成、未初始化），完全状态（实例化完成、初始化完成）

一切的开始

以XML文件为例，代码如下，进入debug模式，走起，跟上队伍！

public class SpringTestApplication {
   public static void main(String[] args) {
       ApplicationContext ac = new ClassPathXmlApplicationContext("bean.xml");
  }
}

第一站

进入到，上下文的处理构造器中，看代码，找重点。

public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
           // 此处是我们的第一站
refresh();
}
}

refresh方法，在AbstractApplicationContext内。代码内部，做了点什么事情呢，可以详细的看看代码注释。

public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 完成准备，设置时间啊等等，我们无需太关心
prepareRefresh();
// 无需多关心
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// bean factory 创建前的准备阶段，对应我们上述图的流程
prepareBeanFactory(beanFactory);
try {
               //BeanFactory的处理过程，跟我们的对象创建关系不大 开始
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
               //BeanFactory的处理过程，跟我们的对象创建关系不大 完成
// Instantiate all remaining (non-lazy-init) singletons.
               //实例化，所有的非懒加载的单例bean对象，可见第二站入口来了，开启
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}

第二站，真正的入口

看这个前，我们有必要了解一个类对象，其中提供了三个缓存对象

DefaultSingletonBeanRegistry

/** Cache of singleton objects: bean name to bean instance. */
// 一级缓存
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
// 三级缓存
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name to bean instance. */
// 二级缓存
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

记住，对应的对象名，后续在源码中会依次出现。

回到第二站，查看相关源码，直接查看注释中中文标记点，继续追踪

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
// Register a default embedded value resolver if no bean post-processor
// (such as a PropertyPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
}
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null);
// Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
       // 实例化，所有的非懒加载的单例bean对象
beanFactory.preInstantiateSingletons();
}

DefaultListableBeanFactory

@Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
       // 根据BeanDefinition 获取bean定义信息
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
           // 首先 A对象 此处非抽象类 单例 非懒加载才能进入，A对象符合条件，进入
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
                   //A对象 进入
getBean(beanName);
}
}
}
// 省略
}

AbstractBeanFactory

@Override
public Object getBean(String name) throws BeansException {
       // Spring源码内 do开头，都是真正做事情的方法
return doGetBean(name, null, null, false);
}

第三站创建bean

AbstractBeanFactory

查看中文代码注释部分，快速定位。按照代码执行序号，查看逻辑。

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;
    // Eagerly check singleton cache for manually registered singletons.
      // 1. 获取单例bean对象 第四站入口
    Object sharedInstance = getSingleton(beanName);
      // 根据第四站代码分析，此时A对象为空，返回空，将继续执行
    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 + "'");
        }
      }
      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);
      }
      // 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 {
        final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
        checkMergedBeanDefinition(mbd, beanName, args);
        // 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)) {
              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.
                // 3. 创建对象
        if (mbd.isSingleton()) {
                    // 进入方法
          sharedInstance = getSingleton(beanName, () -> {
            try {
                            // 5. 创建对象 实现A 对象实例
              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()) {
          // 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;
      }
    }
    // 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());
      }
    }
      // 返回
    return (T) bean;
  }

第四站

DefaultSingletonBeanRegistry

@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
       // 2.
       // 一级缓存中查找，是否存在bean A对象，此时为空
Object singletonObject = this.singletonObjects.get(beanName);
       // 判断一级缓存为空并且是否在创建中，我们在知识点中提及，类对象两种状态，实例化、完全状态，此时均未发生，因此不会进入判断
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
       // 直接返回 空对象
return singletonObject;
}

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
       // 4. 执行对象处理
synchronized (this.singletonObjects) {
           // 一级缓存中获取A 对象，无
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
               //进入判断
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
                   // 获取对象 ObjectFactory<?> 通过匿名内部类执行，匿名内部类 ，获取到A实例化对象，此时A 为实例化状态，创建中
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) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}

执行匿名内部类创建A 对象

AbstractAutowireCapableBeanFactory

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
           //实例A创建
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;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
       // 此处 A对象已创建，会进入判断
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");
}
           //三级缓存，放入实例化对象A 此时A 未完全状态 缓存内状态，见代码下图示
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
           // 6. 填充bean
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 " +
"'getBeanNamesForType' 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;
}

缓存内部状态

填充bean属性内容

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
// 省略
if (pvs != null) {
           //处理属性内容
applyPropertyValues(beanName, mbd, bw, pvs);
}
}

protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
// 省略代码
// 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 {
               // B对象
String propertyName = pv.getName();
               // 值为 null
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);
}
               // 7. 解决B对象
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
Object convertedValue = resolvedValue;
//省略代码
}

BeanDefinitionValueResolver

@Nullable
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.
if (value instanceof RuntimeBeanReference) {
RuntimeBeanReference ref = (RuntimeBeanReference) value;
           //B对象作为A对象属性值，类型为RuntimeBeanReference 进入判断，执行方法resolveReference
return resolveReference(argName, ref);
}
// 省略代码

@Nullable
private Object resolveReference(Object argName, RuntimeBeanReference ref) {
try {
Object bean;
Class<?> beanType = ref.getBeanType();
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 {
                   // A对象依赖B对象，获取B对象
bean = parent.getBean(String.valueOf(doEvaluate(ref.getBeanName())));
}
}
//省略代码

开始B对象的获取

第五站依赖对象 B生成

重新回到

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;
// Eagerly check singleton cache for manually registered singletons.
  //进入获取
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 + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
   //省略代码

类似A对象的处理，此处忽略

最终到了，处理B对象属性A对象，上述描述6 。此时，缓存情况如下：

进一步补充属性，即完成B对象中属性A对象的获取。

重新进入doGetBean ，进入 getSingleton

@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
       // 此时一级缓存中获取A对象，同样 A对象为空，此时一级缓存中不存在
Object singletonObject = this.singletonObjects.get(beanName);
       // A对象状态为 实例化 因此，在创建中，进入判断
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
               // 二级缓存中获取 A对象 ，A对象为空，进入判断
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
                   //三级缓存中获取 A对象，可以读取到值
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
                       // 将A 对象放入 二级缓存
this.earlySingletonObjects.put(beanName, singletonObject);
                       // 将A 对象从三级缓存中删除
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}

此时，缓存情况如下：

处理完B对象的属性A对象后，B对象此时变为初始化完成、实例化完成，成为完全态

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
                   //
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) {
                   // 8. A对象属性对象 B对象完成初始化后执行
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}

protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
           // 一级缓存放入 B对象 完全态
this.singletonObjects.put(beanName, singletonObject);
           // 三级缓存删除 B对象
this.singletonFactories.remove(beanName);
           // 二级缓存删除 B对象
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}

此时，缓存情况如下：

此时A对象中属性B对象，完成初始化，A对象会执行以下代码

protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
           // 一级缓存放入 A对象 完全态
this.singletonObjects.put(beanName, singletonObject);
           // 三级缓存删除 A对象
this.singletonFactories.remove(beanName);
           // 二级缓存删除 A对象
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}

此时，缓存情况如下：

第六站遍历Bean对象 B

@Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
       // 根据BeanDefinition 获取bean定义信息
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
           // 首先 A对象 此处非抽象类 单例 非懒加载才能进入，A对象符合条件，进入
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
                   //B对象 进入
getBean(beanName);
}
}
}
// 省略
}

故事结尾

最后直接返回，因为完全态在一级缓存中都存在了。

@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
       // 从一级缓存中获取B对象，获取得到，直接返回
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}

终于结束了，带着大家走了一遍源码，不知道你能理解多少。

从以上源码，我们可以看到，Spring采用三级缓存，来实现循环依赖的解决。

常见的面试问题

循环依赖必须使用三级缓存么
其实，从我们解读看来，只需要把对象的两个状态处理好，就能处理好循环依赖的问题，因此实际上来说，不必要使用三级缓存对于一般的bean对象来讲。
为何Spring解决三级缓存要使用三级缓存，而不是二级缓存
为何Spring采用三级缓存，其实主要是因为Bean对象不仅仅是基础的一般对象，还会有一些AOP后期增强对象，通过Cglib或者JDK动态代理手段，织入的对象。二级缓存，只能满足一般Bean对象的处理，无法处理更多的对象。
再看三级缓存

/** Cache of singleton objects: bean name to bean instance. */
// 一级缓存
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
// 三级缓存 返回值类型 泛型为 ObjectFactory<?> 能够处理更多的事项
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name to bean instance. */
// 二级缓存
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

以上就比较明确，为何使用三级而不是二级了

总结

看源码就是需要细致认真的解读，因此需要耐心。希望读着也能有耐心看下去。如果你可以，我相信，你对于循环依赖，不会有什么难点。

面试必问系列之最强源码分析，带你一步步弄清楚Spring如何解决循环依赖

写在前面

何为循环依赖

Spring管理Bean对象

场景描述

Spring管理bean对象

Spring如何解决循环依赖

一切的开始

第一站

第二站，真正的入口

第三站创建bean

第四站

第五站依赖对象 B生成

第六站遍历Bean对象 B

故事结尾

常见的面试问题

总结

热门文章

最新文章

相关课程

相关电子书

相关实验场景

面试必问系列之最强源码分析，带你一步步弄清楚Spring如何解决循环依赖

写在前面

何为循环依赖

Spring管理Bean对象

场景描述

Spring管理bean对象

Spring如何解决循环依赖

一切的开始

第一站

第二站，真正的入口

第三站 创建bean

第四站

第五站 依赖对象 B生成

第六站 遍历Bean对象 B

故事结尾

常见的面试问题

总结

热门文章

最新文章

相关课程

相关电子书

相关实验场景

第三站创建bean

第五站依赖对象 B生成

第六站遍历Bean对象 B