是这样的。一个实习朋友问我关于循环依赖的问题。我跟他讲了Spring内部的三级缓存原理，确定Spring已经解决了这个问题。然后他抛出一道题给我，说错了。好朋友，感觉是想让我查bug你们看吗？@ComponentpublicclassA{privatefinalBb;publicA(finalBb){this.b=b;}publicvoidprint(){System.out.println("ina");}}@ComponentpublicclassB{privatefinalAa;publicB(finalAa){this.a=a;}publicvoidprint(){System.out.println("inb");}}Causedby:org.springframework.beans.factory.UnsatisfiedDependencyException:Errorcreatingbeanwithname'a'definedinfile[C:\soft\code\common\MongodbDataTest\dbDataTest\target\test-classes\com\db\model\A.class]:Unsatisfieddependencyexpressedthroughconstructorparameter0;nestedexceptionisorg.springframework.beans.factory.UnsatisfiedDependencyException:Errorcreatingbeanwithname'b'定义文件[C:\soft\code\common\MongodbDataTest\dbDataTest\target\test-classes\com\db\model\B.class]:Unsatisfieddependencyexpressedthroughconstructorparameter0;嵌套异常isorg.springframework.beans.factory.BeanCurrentlyInCreationException:Errorcreatingbeanwithname'a':Requestedbeaniscurrentlyincreation:Isthereanunresolvablecircularreference?atorg.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:800)atorg.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:229)atorg.springframework.beans.factory。support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1354)atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1204)atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:564）atorg.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean（AbstractAutowireCapableBeanFactory.java:524）atorg.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0（AbstractBeanFactory.java:335）atorg.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton（DefaultSingletonBeanRegistry.java:234）atorg.springframework.beans.factory.support.AbstractBeanFactory.doGetBean（AbstractBeanFactory.java:333）atorg.springframework.beans.factory.support.AbstractBeanFactory.getBean（Abstract.javaBean:Factory）208）atorg.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons（DefaultListableBeanFactory.java:944）atorg.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization（AbstractApplicationContext.java:918）atorg.springframework.context.support.AbstractApplicationContext。刷新（AbstractApplicationContext.java:583）atorg.springframework.boot.SpringApplication.refresh（SpringApplication.java:782）atorg.springframework.boot.SpringApplication.refresh（SpringApplication.java:774）atorg.springframework.boot.SpringApplication.refreshContext（SpringApplication.java:439)atorg.springframework.boot.SpringApplication.run(SpringApplication.java:339)atorg.springframework.boot.test.context.SpringBootContextLoader.loadContext(SpringBootContextLoader.java:123)atorg.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoadermeworkDelegate.java:99)测试上下文。cache.DefaultCacheAwareContextLoaderDelegate.loadContext（DefaultCacheAwareContextLoaderDelegate.java:124）...68moreCausedby：org.springframework.beans.factory.UnsatisfiedDependencyException：Errorcreatingbeanwithname'b'defined\TdbestData\Tdbtarget\test-classes\com\db\model\B。类]：通过构造函数参数0表达的不满意依赖关系；嵌套异常是org.springframework.beans.factory.BeanCurrentlyInCreationException：创建名称为“a”的bean时出错：请求的bean当前正在通过IsthereanunTMresolvercircuence创建吗？难道spring对这种循环依赖真的束手无策？其实不然，spring也有大杀器@Lazy...只是让我们对刚才的两个类做一个小小的修改：为什么lazy可以解决这个问题呢？逆向@Lazy注解，从源码可以看出，我们其实是为@Lazy的依赖返回了一个代理类（以下简称LazyProxy），而不是通过getBean获取目标bean注入，实际获取bean逻辑，它被封装到一个TargetSource类的getTarget方法中，而这个TargetSource类最终是用来生成LazyProxy的，那么我们是不是可以推测LazyProxy应该持有这个TargetSource对象。从我们懒加载的语义来看，就是在实际使用bean的时候（调用bean的某个方法时）注入/初始化属性。所以对于当前的例子来说，就是说在创建B的时候，并没有调用getBean("a")来获取构造函数的参数，而是直接生成了一个LazyProxy作为B构造函数的参数，而B之后，当真正调用A的方法，会调用TargetSource中的getTarget获取A实例，即会调用getBean("a")。这个时候A已经实例化了，所以不会有循环依赖的问题了。
