spring循环依赖策略解析

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 {                        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;}