结论：当A、B对象之间相互依赖，A早于B实例化，并且A对象使用了@Async注解或由BeanPostProcessor后置处理器返回了代理对象时，会产生以下报错：

org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Bean with name 'a' has been injected into other beans [b] 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 'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example. at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:585) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483) at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197) at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761) at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867) at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543) at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:84)

在springboot中，以上报错被捕捉，抛出的异常是：

The dependencies of some of the beans in the application context form a cycle

原因：我们知道，spring三级缓存一定程度上解决了循环依赖问题。A对象在实例化之后，属性赋值【opulateBean(beanName, mbd, instanceWrapper)】执行之前，将ObjectFactory添加至三级缓存中，从而使得在B对象实例化后的属性赋值过程中，能从三级缓存拿到ObjectFactory，调用getObject()方法拿到A的引用，B由此能顺利完成初始化并加入到IOC容器。此时A对象完成属性赋值之后，将会执行初始化【initializeBean(beanName, exposedObject, mbd)方法】，重点是@Async注解的处理正是在这地方完成的，其对应的后置处理器AsyncAnnotationBeanPostProcessor，在postProcessAfterInitialization方法中将返回代理对象，此代理对象与B中持有的A对象引用不同，导致了以上报错。

有以下解决办法：

    1）在A类上加@Lazy，保证A对象实例化晚于B对象；

    2）在A类上使用@DependsOn注解，保证A对象实例化晚于B对象(注意如果AB两个类相互DependsOn，也会形成循环依赖)

    3）设置 AbstractAutowireCapableBeanFactory.setAllowRawInjectionDespiteWrapping(true)，不会报错但会导致B持有的A引用不是最终的代理对象;

    4）不使用@Async注解，通过自定义异步工具类发起异步线程；