一起来读Spring源码吧（四）循环依赖踩坑笔记

摘要：从创建的的起点开始首先会在缓存中查找，这里一共有三级缓存，保存初始化完成的单例实例，保存提前曝光的单例实例，保存单例的工厂函数对象后面两级都是为了解决循环依赖设置的，具体查找逻辑在后续其他情况下调用会说明。

在开发过程中，遇到需要把方法调用改为异步的情况，本来以为简单得加个@Asyn在方法上就行了，没想到项目启动的时候报了如下的错误：

Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: 
Error creating bean with name "customerServiceImpl": 
Bean with name "customerServiceImpl" has been injected into other beans [customerServiceImpl,followServiceImpl,cupidService] 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.

看了下好像报的是循环依赖的错误，但是Spring单例是支持循环依赖的，当时一脸懵逼。
拿着报错去百度了下，说是多个动态代理导致的循环依赖报错，也找到了报错的地点，但是还是不明白为什么会这样，所以打算深入源码探个究竟，顺便回顾下Bean的获取流程和循环依赖的内容。

用SpringBoot新建一个demo项目，因为原项目是有定义切面的，这里也定义一个切面：

@Aspect
@Component
public class TestAspect {

    @Pointcut("execution(public * com.example.demo.service.CyclicDependencyService.sameClassMethod(..))")
    private void testPointcut() {}

    @AfterReturning("testPointcut()")
    public void after(JoinPoint point) {
        System.out.println("在" + point.getSignature() + "之后干点事情");
    }

}

然后新建一个注入自己的Service构成循环依赖，然后提供一个方法满足切点要求，并且加上@Async注解：

@Service
public class CyclicDependencyService {

    @Autowired
    private CyclicDependencyService cyclicDependencyService;

    public void test() {
        System.out.println("调用同类方法");
        cyclicDependencyService.sameClassMethod();
    }

    @Async
    public void sameClassMethod() {
        System.out.println("循环依赖中的异步方法");
        System.out.println("方法线程：" + Thread.currentThread().getName());
    }

}

还有别忘了给Application启动类加上@EnableAsync和@EnableAspectJAutoProxy：

@EnableAsync
@EnableAspectJAutoProxy
@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

最后打好断点，开始debug。

从Bean创建的的起点--AbstractBeanFactory#getBean开始

// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);

首先会在缓存中查找，DefaultSingletonBeanRegistry#getSingleton(String beanName, boolean allowEarlyReference)：

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
   Object singletonObject = this.singletonObjects.get(beanName);
   if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
      synchronized (this.singletonObjects) {
         singletonObject = this.earlySingletonObjects.get(beanName);
         if (singletonObject == null && allowEarlyReference) {
            ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
            if (singletonFactory != null) {
               singletonObject = singletonFactory.getObject();
               this.earlySingletonObjects.put(beanName, singletonObject);
               this.singletonFactories.remove(beanName);
            }
         }
      }
   }
   return singletonObject;
}

这里一共有三级缓存：

singletonObjects，保存初始化完成的单例bean实例；

earlySingletonObjects，保存提前曝光的单例bean实例；

singletonFactories，保存单例bean的工厂函数对象；

后面两级都是为了解决循环依赖设置的，具体查找逻辑在后续其他情况下调用会说明。

缓存中找不到，就要创建单例：

sharedInstance = getSingleton(beanName, () -> {
   try {
      return createBean(beanName, mbd, args);
   }
   catch (BeansException ex) {
      // Explicitly remove instance from singleton cache: It might have been put there
      // eagerly by the creation process, to allow for circular reference resolution.
      // Also remove any beans that received a temporary reference to the bean.
      destroySingleton(beanName);
      throw ex;
   }
});

调用DefaultSingletonBeanRegistry#getSingleton(String beanName, ObjectFactory singletonFactory):

public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
    ...
    beforeSingletonCreation(beanName);
    ...
    singletonObject = singletonFactory.getObject();
    ...
    afterSingletonCreation(beanName);
    ...
    addSingleton(beanName, singletonObject);
    ...
}

创建前后分别做了这几件事：

前，beanName放入singletonsCurrentlyInCreation，表示单例正在创建中

后，从singletonsCurrentlyInCreation中移除beanName

后，将创建好的bean放入singletonObjects，移除在singletonFactories和earlySingletonObjects的对象

创建单例调用getSingleton时传入的工厂函数对象的getObject方法，实际上就是createBean方法，主要逻辑在AbstractAutowireCapableBeanFactory#doCreateBean中：

...
instanceWrapper = createBeanInstance(beanName, mbd, args);
final Object bean = instanceWrapper.getWrappedInstance();
...
// 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.isTraceEnabled()) {
      logger.trace("Eagerly caching bean "" + beanName +
            "" to allow for resolving potential circular references");
   }
   addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
   populateBean(beanName, mbd, instanceWrapper);
   exposedObject = initializeBean(beanName, exposedObject, mbd);
}
...
if (earlySingletonExposure) {
   Object earlySingletonReference = getSingleton(beanName, false);
   if (earlySingletonReference != null) {
      if (exposedObject == bean) {
         exposedObject = earlySingletonReference;
      }
      else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
         String[] dependentBeans = getDependentBeans(beanName);
         Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
         for (String dependentBean : dependentBeans) {
            if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
               actualDependentBeans.add(dependentBean);
            }
         }
         if (!actualDependentBeans.isEmpty()) {
            throw new BeanCurrentlyInCreationException(beanName,
                  "Bean with name "" + beanName + "" has been injected into other beans [" +
                  StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                  "] 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.");
         }
      }
   }
}

可以看到报错就是在这个方法里抛出的，那么这个方法就是重点中的重点。

首先实例化单例，instantiate，只是实例化获取对象引用，还没有注入依赖。我debug时记录的bean对象是CyclicDependencyService@4509；

然后判断bean是否需要提前暴露，需要满足三个条件：1、是单例；2、支持循环依赖；3、bean正在创建中，也就是到前面提到的singletonsCurrentlyInCreation中能查找到，全满足的话就会调用DefaultSingletonBeanRegistry#addSingletonFactory把beanName和单例工厂函数对象（匿名实现调用AbstractAutowireCapableBeanFactory#getEarlyBeanReference方法）放入singletonFactories；

接着就是注入依赖，填充属性，具体怎么注入这里就不展开了，最后会为属性cyclicDependencyService调用DefaultSingletonBeanRegistry.getSingleton(beanName, true)，注意这里和最开始的那次调用不一样，isSingletonCurrentlyInCreation为true，就会在singletonFactories中找到bean的单例工厂函数对象，也就是在上一步提前暴露时放入的，然后调用它的匿名实现AbstractAutowireCapableBeanFactory#getEarlyBeanReference：

protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
   Object exposedObject = bean;
   if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      for (BeanPostProcessor bp : getBeanPostProcessors()) {
         if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
            SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
            exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
         }
      }
   }
   return exposedObject;
}

方法逻辑就是挨个调用实现了SmartInstantiationAwareBeanPostProcessor接口的后置处理器（以下简称BBP）的getEarlyBeanReference方法。一个一个debug下来，其他都是原样返回bean，只有AnnotationAwareAspectJAutoProxyCreator会把原bean（CyclicDependencyService@4509）存在earlyProxyReferences，然后将bean的代理返回（debug时记录的返回对象是CyclicDependencyService$$EnhancerBySpringCGLIB$$6ed9e2db@4740）并放入earlySingletonObjects，再赋给属性cyclicDependencyService。

public Object getEarlyBeanReference(Object bean, String beanName) {
   Object cacheKey = getCacheKey(bean.getClass(), beanName);
   this.earlyProxyReferences.put(cacheKey, bean);
   return wrapIfNecessary(bean, beanName, cacheKey);
}

属性填充完成后就是调用初始化方法AbstractAutowireCapableBeanFactory#initializeBean：

...
invokeAwareMethods(beanName, bean);
...
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
...
invokeInitMethods(beanName, wrappedBean, mbd);
...
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
...

初始化主要分为这几步：

如果bean实现了BeanNameAware、BeanClassLoaderAware或BeanFactoryAware，把相应的资源放入bean；

顺序执行BBP的postProcessBeforeInitialization方法；

如果实现了InitializingBean就执行afterPropertiesSet方法，然后执行自己的init-method；

顺序执行BBP的postProcessAfterInitialization。

debug的时候发现是第4步改变了bean，先执行AnnotationAwareAspectJAutoProxyCreator#postProcessAfterInitialization：

public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
   if (bean != null) {
      Object cacheKey = getCacheKey(bean.getClass(), beanName);
      if (this.earlyProxyReferences.remove(cacheKey) != bean) {
         return wrapIfNecessary(bean, beanName, cacheKey);
      }
   }
   return bean;
}

这里会获取并移除之前存在earlyProxyReferences的bean（CyclicDependencyService@4509），因为和当前bean是同一个对象，所以什么都没做直接返回。随后会执行AsyncAnnotationBeanPostProcessor#postProcessAfterInitialization：

if (isEligible(bean, beanName)) {
   ProxyFactory proxyFactory = prepareProxyFactory(bean, beanName);
   if (!proxyFactory.isProxyTargetClass()) {
      evaluateProxyInterfaces(bean.getClass(), proxyFactory);
   }
   proxyFactory.addAdvisor(this.advisor);
   customizeProxyFactory(proxyFactory);
   return proxyFactory.getProxy(getProxyClassLoader());
}

先判断bean是否有需要代理，因为CyclicDependencyService有方法带有@Async注解就需要代理，返回代理对象是CyclicDependencyService$$EnhancerBySpringCGLIB$$e66d8f6e@5273。

返回的代理对象赋值给AbstractAutowireCapableBeanFactory#doCreateBean方法内的exposedObject，接下来就到了检查循环依赖的地方了：

if (earlySingletonExposure) {
   Object earlySingletonReference = getSingleton(beanName, false);
   if (earlySingletonReference != null) {
      if (exposedObject == bean) {
         exposedObject = earlySingletonReference;
      }
      else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
         String[] dependentBeans = getDependentBeans(beanName);
         Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
         for (String dependentBean : dependentBeans) {
            if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
               actualDependentBeans.add(dependentBean);
            }
         }
         if (!actualDependentBeans.isEmpty()) {
            throw new BeanCurrentlyInCreationException(beanName,
                  "Bean with name "" + beanName + "" has been injected into other beans [" +
                  StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                  "] 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.");
         }
      }
   }
}

首先从earlySingletonObjects里拿到前面属性填充时放入的bean代理（CyclicDependencyService$$EnhancerBySpringCGLIB$$6ed9e2db@4740），不为空的话就比较bean和exposedObject，分别是CyclicDependencyService@4509和CyclicDependencyService$$EnhancerBySpringCGLIB$$e66d8f6e@5273，很明显不是同一个对象，然后会判断allowRawInjectionDespiteWrapping属性和是否有依赖的bean，然后判断这些bean是否是真实依赖的，一旦存在真实依赖的bean，就会抛出BeanCurrentlyInCreationException。

总结下Spring解决循环依赖的思路：在创建bean时，对于满足提前曝光条件的单例，会把该单例的工厂函数对象放入三级缓存中的singletonFactories中；然后在填充属性时，如果存在循环依赖，必然会尝试获取该单例，也就是执行之前放入的工厂函数的匿名实现，这时候拿到的有可能是原bean对象，也有可能是被某些BBP处理过返回的代理对象，会放入三级缓存中的earlySingletonObjects中；接着bean开始初始化，结果返回的有可能是原bean对象，也有可能是代理对象；最后对于满足提前曝光的单例，如果真的有提前曝光的动作，就会去检查初始化后的bean对象是不是原bean对象是同一个对象，只有不是的情况下才可能抛出异常。重点就在于存在循环依赖的情况下，初始化过的bean对象是不是跟原bean是同一个对象。

从以上的debug过程可以看出，是AsyncAnnotationBeanPostProcessor这个BBP在初始化过程中改变了bean，使得结果bean和原bean不是一个对象，而AnnotationAwareAspectJAutoProxyCreator则是在填充属性获取提前曝光的对象时把原始bean缓存起来，返回代理的bean。然后在初始化时执行它的postProcessAfterInitialization方法时如果传入的bean是之前缓存的原始bean，就直接返回，不进行代理。如果其他BBP也都没有改变bean的话，初始化过后的bean就是跟原始bean是同一个对象，这时就会把提前曝光的对象（代理过的）作为最终生成的bean。