Spring中如何解决循环依赖，简单举个下面这样例子

• A类中注入了B类对象
• B类中注入了A类对象

 如果是通过构造函数注入，那么在初始化时的时候会报循环注入。A初始化时B可能尚未初始化。

反之，如果是通过get set这种注入或者@Autowired 注入，并且是singelton（默认）而非prototype则不会报错。对于setter注入造成的依赖是通过Spring容器提前暴露刚完成构造器注入但未完成其他步骤（如setter注入）的bean来完成的，而且只能解决单例作用域的bean循环依赖。spring通过提前暴露一个单例工厂方法，从而使其他bean能引用到该bean。

protected void beforeSingletonCreation(String beanName) {          if (!this.inCreationCheckExclusions.containsKey(beanName)                  && this.singletonsCurrentlyInCreation.put(beanName, Boolean.TRUE) != null) {              throw new BeanCurrentlyInCreationException(beanName);          }      }

1.从源码中可以看出，A实例化时会去singletonsCurrentlyInCreation写入A，A中依赖B因此，需要实例化B,B中依赖A，因此需要实例A，而A之前已经在Increation，因此会报错。

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {        // Instantiate the bean.        //.....        // 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);                }            });        }        // ......    }protected Object getSingleton(String beanName, boolean allowEarlyReference) {                Object singletonObject = this.singletonObjects.get(beanName);        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {                        synchronized (singletonObject) {                singletonObject = this.earlySingletonObjects.get(beanName);                                 if (singletonObject == null && allowEarlyReference) {                                         ObjectFactory objectFactory = this.singletonFactories.get(beanName);                    if (objectFactory != null) {                        singletonObject = objectFactory.getObject();                        this.earlySingletonObjects.put(beanName, singletonObject);                        this.singletonFactories.remove(beanName);                    }                }            }        }        return (singletonObject != NULL_OBJECT) ? singletonObject : null;    }

2.采用set方法时，Spring略有不同，A实例化时会将一个 A的实例引用写入objectFactory中。执行setB时去实例化B，B的setA调用getSingleton(A,true),因为A 实例还在创建中，会接着往下执行，最终通过objectFactory返回A的实例引用，之后删除factory，缓存A的引用。