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;
  
  //尝试获取单例对象，因为spring大部分的bean都是单例的，所以这里先尝试能否获取。
  registered singletons.
  Object sharedInstance = getSingleton(beanName);
  //单例存在的情况下，那么beanName返回的肯定是单例类，但是这里还需要判断是不是FactoryBean
  if (sharedInstance != null && args == null) {
    ...
    //FactoryBean应该返回getObject()对象
    bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
  }
  
  else {
    //走到这里，有可能beanName是单例模式，但之前并没有实例化，或者是Prototype类型。
    //首先判断不是循环依赖，这里的循环依赖指的是Prototype类型
    if (isPrototypeCurrentlyInCreation(beanName)) {
      throw new BeanCurrentlyInCreationException(beanName);
    }
  
  
    try {
      final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
      // 如果是单例，则创建单例模式
      if (mbd.isSingleton()) {
        // !!!这里是解决单例循环依赖的关键，后面再分析
        sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
          @Override
          public Object getObject() throws BeansException {
            try {
              return createBean(beanName, mbd, args);
            }
            catch (BeansException ex) {
              throw ex;
            }
          }
        });
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
      }
  
      else if (mbd.isPrototype()) {
        // 原型模式，则创建一个新对象.
        Object prototypeInstance = null;
        try {
          /*这里是Prototype循环依赖的问题，会记录在map中beanName，
          如果在解决当前Bean的依赖过程中还依赖当前Bean，
          则说明了出现了循环依赖
          */
              beforePrototypeCreation(beanName);
              prototypeInstance = createBean(beanName, mbd, args);
        }
        finally {
          //对应beforePrototypeCreation()，从map中移除
              afterPrototypeCreation(beanName);
        }
        bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
      }
      ...
    }
  }
  
  ...
  return (T) bean;
}

sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
  @Override
  public Object getObject() throws BeansException {
    try {
      return createBean(beanName, mbd, args);
    }
    catch (BeansException ex) {
      throw ex;
    }
  }
});

/** 缓存创建的单例对象: bean名字 --> bean对象 */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(256);

/** 缓存单例的factory,就是ObjectFactory这个东西，: bean name --> ObjectFactory */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<String, ObjectFactory<?>>(16);

/** 也是缓存创建的单例对象，功能和singletonObjects不一样，
在bean构造成功之后，属性初始化之前会把对象放入到这里，
主要是用于解决属性注入的循环引用: bean name --> bean instance 
*/
private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16);

/** 记录在创建单例对象中循环依赖的问题，还记得Prototype中又记录创建过程中依赖的map吗？
在Prototype中只要出现了循环依赖就抛出异常，而在单例中会尝试解决 */
private final Set<String> singletonsCurrentlyInCreation =
    Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>(16));

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {

  synchronized (this.singletonObjects) {
    //尝试在singletonObjects中获取
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null) {
      //不存在则创建
      //把当前beanName加入到singletonsCurrentlyInCreation中
      beforeSingletonCreation(beanName);
      try {
        
        singletonObject = singletonFactory.getObject();
      }
      ...
      finally {
        ...
        //从singletonsCurrentlyInCreation中删除beanName
        afterSingletonCreation(beanName);
      }
    }
    return (singletonObject != NULL_OBJECT ? singletonObject : null);
  }
}

@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
  ...
  Object beanInstance = doCreateBean(beanName, mbdToUse, args);
  ...
}

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

  // 实例化bean
  BeanWrapper instanceWrapper = null;
  if (mbd.isSingleton()) {
    instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
  }
  if (instanceWrapper == null) {
    //如果没实例化则创建新的BeanWrapper
    //如果是通过构造器注入，这里是一个关键点
    /*
    因为在A初始化的时候发现构造函数依赖B，就会去实例化B，
    然后B也会运行到这段逻辑，构造函数中发现依赖A，
    这个时候就会抛出循环依赖的异常
    */
        instanceWrapper = createBeanInstance(beanName, mbd, args);
  }
  

  //如果当前是单例，并且allowCircularReferences为true(默认就是true，除非我们不希望Spring帮我们解决)
  boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
      isSingletonCurrentlyInCreation(beanName));
  if (earlySingletonExposure) {
    /*
    ！！！这里很重要，把构造成功，但属性还没注入的
    的bean加到singletonFactory中，这样再解决A的依赖
    过程中如果依赖A，就把这个半成品返回回去。
    */
    addSingletonFactory(beanName, new ObjectFactory<Object>() {
      @Override
      public Object getObject() throws BeansException {
        return getEarlyBeanReference(beanName, mbd, bean);
      }
    });
  }

  
  Object exposedObject = bean;
  try {
    //自动注入属性
    populateBean(beanName, mbd, instanceWrapper);
    if (exposedObject != null) {
      exposedObject = initializeBean(beanName, exposedObject, mbd);
    }
  }
  ...

  return exposedObject;
}

@Component
public class BeanA {
  private BeanB beanB;

  @Autowired
  public BeanA(BeanB beanB) {
    this.beanB = beanB;
  }
}

@Component
public class BeanB {
  @Autowired
  private BeanA beanA;
}