阿里妹导读
本篇不仅仅是介绍Spring循环依赖的原理，而且给出Spring不能支持的循环依赖场景与案例，对其进行详细解析，同时给出解决建议与方案，以后出现此问题可以少走弯路。
背景
1、循环依赖异常信息
  • 应用时间时间久
  • 应用多人同时并行开发
  • 应用保证迭代进度
经常出现启动时出现循环依赖异常
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean withname'taskPunchEvent': Injection ofresource dependencies failed; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean withname'playContentService': Bean withname'playContentService' has been injected into other beans [toVoConvertor] in its rawversionas part of a circular reference, but has eventually been wrapped. This means that said other beans donotuse the finalversionof the bean. This is often the resultofover-eager type matching - considerusing'getBeanNamesOfType'with the 'allowEagerInit' flag turned off, for example.at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessProperties(CommonAnnotationBeanPostProcessor.java:325)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1404)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:592)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:277)at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1255)at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1175)at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:595) ... 40 moreCaused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean withname'playContentService': Bean withname'playContentService' has been injected into other beans [toVoConvertor] in its rawversionas part of a circular reference, but has eventually been wrapped. This means that said other beans donotuse the finalversion of the bean. This is often the resultofover-eager type matching - considerusing'getBeanNamesOfType'with the 'allowEagerInit' flag turned off, for example.at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:622)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:204)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.resolveBeanByName(AbstractAutowireCapableBeanFactory.java:452)at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.autowireResource(CommonAnnotationBeanPostProcessor.java:527)at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.getResource(CommonAnnotationBeanPostProcessor.java:497)at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor$ResourceElement.getResourceToInject(CommonAnnotationBeanPostProcessor.java:637)at org.springframework.beans.factory.annotation.InjectionMetadata$InjectedElement.inject(InjectionMetadata.java:180)at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:90)at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessProperties(CommonAnnotationBeanPostProcessor.java:322) ... 51 more
2、依赖关系
先不关注其他不规范问题，看现象

3、涉及基础知识
  • Spring bean 创建流程
  • Dynamic Proxy 动态代理
  • Spring-AOP 原理
问题

1、什么是循环依赖？

2、为什么会产生循环依赖？

3、循环依赖有哪些场景？

4、Spring如何解决循环依赖的？

5、Spring为什么使用三级缓存？

6、Spring支持AOP循环依赖，为何还存在循环依赖异常？

7、Spring不支持的循环依赖场景及如何解决？

注：Spring启动流程与Bean创建初始化流程如不熟悉，自行补习，篇幅原因此处不做介绍
Spring循环依赖
1、什么是循环依赖
2、核心概念
  • BeanDefinition：spring核心bean的配置信息
  • Spring Bean：spring管理的已经初始化好以后的可使用的实例
    • 首先，通过spring通过扫描各种注解 @Compoent、@Service、@Configuration等等把需要交给spring管理的bean初始化成 BeanDefinition 的列表
    • 然后，根据 BeanDefinition 创建spring bean的实例
  • Java Bean：Java简单通过构造函数创建的对象
    • Spring通过推断构造方法后，通过反射调用构造函数创建的对象
1、什么情况下出现循环依赖
并非使用者手动去getBean才会加载并初始化，而是框架启动时进行加载
Spring创建Bean - #DefaultListableBeanFactory#preInstantiateSingletons@Overridepublicvoid preInstantiateSingletons() throws BeansException {//...... List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) { RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {if (isFactoryBean(beanName)) {//FactoryBean接口处理 ...... }else {//正常Bean的加载入口 getBean(beanName); } } }//......}
4、循环依赖场景
  • 构造器内的循环依赖
    • 注入的好处很明显，如果容器中不存在或者存在多个实现时，可以从容处理。
    • 强依赖，先有鸡还是先有蛋问题暂无解，此依赖方式Spring不支持，除非自身实现代理加延迟注入，这种方式很难解决，除非实现类似于lazy生成代理方式进行解耦来实现注入，Spring没有支持可能因为此种注入场景都可以用其他方式代替且场景极少。
    • 弱依赖，spring 4.3之后增加 ObjectProvider 来处理
//构造器循环依赖示例publicclassStudentA {private StudentB studentB ; publicStudentA(StudentB studentB) {this.studentB = studentB; }}publicclassStudentB {private StudentA studentA ;publicStudentB(StudentA studentA) {this.studentA = studentA; }}
  • setter方式单例，默认方式
  • setter方式原型，prototype

    对于“prototype”作用域Bean，Spring容器不进行缓存，因此无法提前暴露一个创建中的Bean。
  • field属性循环依赖

    最常用，此场景是通过反射注入，以下为@Autowire 注入代码，@Resource省略

    AutowiredAnnotationBeanPostProcessor#postProcessProperties
@Overridepublic PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) { InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);try {//属性注入 metadata.inject(bean, beanName, pvs); }catch (BeanCreationException ex) {throw ex; }catch (Throwable ex) {thrownew BeanCreationException(beanName, "Injection of autowired dependencies failed", ex); }return pvs;}
5、三级缓存解决循环依赖

(1)、一级缓存

DefaultSingletonBeanRegistry
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
  • 最基础的单例缓存
  • 限制 bean 在 beanFactory 中只存一份，即实现 singleton scope

(2)、二级缓存

二级缓存(未初始化未填充属性提前暴露的Bean)
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
  • 看名字应该就能猜到，缓存earlySingletonBean，与三级缓存配合使用的
  • 需要注意：
    • 在没有AOP场景时是可以的，每次earlySingletonObjects.get()换成去三级缓存取就可以，存在问题
    • 存在AOP场景时
    • 因此，让使用者去做重复性判断是不可控的，很容易出现问题，于是引入了第二级缓存，当调用三级缓存里的对象工厂的getObject方法之后，getEarlyBeanReference 就会把返回值放入二级缓存，删除三级缓存，后续其他依赖该对象的Bean获取的都是同一个earlyBean，保证singleton原则。
    • 每次都调用 getEarlyBeanReference，即使返回对象都一致，也浪费不必要时间
    • 如果使用者在 getEarlyBeanReference 时直接 new XXX()，则对象又不一致，无法保证 singleton，所以需要使用者熟悉这块原理，并且自身维护，并且暴露内部实现细节
    • 每次都调用 getEarlyBeanReference 返回代理对象都不一致，无法保证 singleton
    • 如果没有此缓存，可不可以解决循环依赖问题？

(3)、三级缓存

三级缓存(Bean创建时提供代理机会的Bean工厂缓存)
privatefinal Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
  • 所以二级缓存和三级缓存是组合，不要拆成两个独立的东西去理解
  • 基于这种设计，没有发生循环依赖的bean就是正常的创建流程
  • 相互引用的bean 会触发链路中最初结点放入三级缓存内容，调用 getEarlyBeanReference 返回相应对象
6、Spring为何不使用一级、二级缓存解决循环依赖
循环依赖产生在Bean创建时
protectedObject doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) { BeanWrapper instanceWrapper = null;if (instanceWrapper == null) {//创建Bean instanceWrapper = createBeanInstance(beanName, mbd, args); } .....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)); }//填充Bean依赖与Bean的初始化Object exposedObject = bean;try {//填充依赖的bean实例 populateBean(beanName, mbd, instanceWrapper);//初始化---注意！注意！注意！此方法中可能调用 BeanPostProcessor//的applyBeanPostProcessorsAfterInitialization时可能会返回代理对象，如果代理途径与创建时代理方式不同则也会产生不同代理对象//从而产生循环依赖中对象不一致情况 exposedObject = initializeBean(beanName, exposedObject, mbd); }//如果存在循环依赖，则保证最开始创建的Bean需要是循环依赖 getEarlyBeanReference触发生成的bean//因为getEarlyBeanReference 可能返回的是代理类，因为singleton必须全局唯一if (earlySingletonExposure) {Object earlySingletonReference = getSingleton(beanName, false);//只有真正存在循环依赖时，才会触发 getEarlyBeanReference调用产生EarlyBean//未存在循环依赖，则getEarlyBeanReference不触发，earlySingletonReference为null，返回exposedObject即可if (earlySingletonReference != null) {if (exposedObject == bean) { exposedObject = earlySingletonReference; }elseif (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) { ......if (!actualDependentBeans.isEmpty()) {thrownew 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 " +"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example."); } } } }return exposedObject;}
三级缓存获取Bean
protected Object getSingleton(String beanName, boolean allowEarlyReference) {//一级缓存(单例池)获取Bean Object singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { synchronized (this.singletonObjects) {//二级缓存获取(提前暴露不完全)Bean singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null && allowEarlyReference) { ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);if (singletonFactory != null) {//三级缓存Bean的创建工厂获取bean(可提前被代理) singletonObject = singletonFactory.getObject();this.earlySingletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName); } } } }return singletonObject;}
SmartInstantiationAwareBeanPostProcessor重点 -> APC之父
//提供提前创建并返回代理的工厂singletonFactory.getObject()执行的是个回调//addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));protectedObject getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {Object exposedObject = bean;if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {for (BeanPostProcessor bp : getBeanPostProcessors()) {//getEarlyBeanReference是SmartInstantiationAwareBeanPostProcessor接口定义方法，//此方法很关键(构造函数推断也在此定义)if (bp instanceof SmartInstantiationAwareBeanPostProcessor) { SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp; exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName); } } }return exposedObject;}
7、Spring支持动态代理循环依赖，为何还会出循环依赖异常？

(1)、相互依赖的Bean只有需要AOP或者动态代理时才有可能出现循环依赖异常

  • 正常情况原始Spring Bean无论怎样相互依赖都没有问题，Spring完全可以处理这种场景
  • 绝大多数存在AOP场景也都是支持的，Spring支持的
  • 只有相互依赖场景下某些Bean需要被动态代理时偶尔会出现循环依赖异常问题，以下解释异常场景：
通俗解释(省略很多细节)：A -> B -> C -> A
  1. Spring 启动开始创建 A，doCreateBean()中对A进行属性填充populateBean()时需要发现依赖B对象，此时A还没有进行初始化，把A原始对象包装成SingletonFactory 放入三级缓存。
  2. A依赖B，因此doCreateBean()会创建B，并对B进行属性填空populateBean()时需要发现依赖C对象。
  3. C依赖A，因此doCreateBean()会创建C，并对C进行属性填空populateBean()时需要发现依赖A对象。

    3.1. 此时去一级缓存获取A，因为A前边并没有填充与初始化完成，因此在一级缓存中不存在；

    3.2. 去二级缓存取A，因为A前边并没有填充与初始化完成，因此在二级缓存中不存在；

    3.3. 去三级缓存取A，第一步中把A封装成SingletonFactory放入三级缓存的，因此三级缓存中可以获取到A的对象

    3.3.1. 此时获取的A如果有必要会对A进行动态代理，返回代理对象；

    3.3.2. 否则不需要代理则返回未填充、未初始化的原始对象A；
    3.4. 获取到A对象，注入到C中，接着初始化C，返回C对象；
  4. C对象返回，注入到B中，接着初始化B，返回B对象；
  5. B对象返回，注入到A中，接着初始化A，问题就在这儿：

    5.1. 如接下来初始化A无需被代理

    5.1.1. exposedObject返回是A原始对象，此时与C中被注入A都是原始Bean，完美；
    5.2. 如接下来初始化A需要被代理：

    5.2.1. APC根据缓存检查之前创建A时是否被代理过，如已被代理，直接返回原始对象，与A原始一致，完美；

    5.2.2. 但是，如此时A初始化过程中有独特的其他BeanPostProcessor，对A的代理方式有单独处理，则被代理后的proxy2与原始Bean、被注入到C中的A的Proxy均不再一致，抛出异常；
  6. 总结重点：

    6.1. 最终原因就是提前暴露的已经注入到C中的A(无论是否被代理)与后来经过初始化后被代理的A(proxy2)不再是同一个Bean；

    6.2. 因为Spring管理Bean默认是Singleton的，现在出现了两个bean，默认情况下无法决断，因此就抛出了异常。

(2)、各别注解使用不当

  • @Respository 
    • 处理器 PersistenceExceptionTranslationPostProcessor#postProcessAfterInitialization
    • 被 @Respository注解的类在Spring启动初始化时存在循环依赖链路中，如果此时Spring中开启了AOP，则必抛出循环依赖异常
    • 所以DAO层使用时，最好不要引入外部业务逻辑，业务逻辑可以提取到Manager、Service层等中，保持DAO纯净
    • 案例分析：见第四节
  • @Asyn
    • 处理器 AsyncAnnotationBeanPostProcessor#postProcessAfterInitialization
    • 被 @Asyn注解的类在Spring启动初始化时存在循环依赖链路中，如果此时Spring中开启了AOP，则必抛出循环依赖异常
  • 以上等注解的类使用不当都比较容易出现循环依赖，这两个注解同一个父类，造成循环依赖原理一样 

    AbstractAdvisingBeanPostProcessor#postProcessAfterInitialization

(3)、存在多个AutoProxyCreator(APC)，出现多层代理

spring默认保证一个容器中只能有一个Aop的APC，如过手动添加或者自定义会出现多个APC情况
  • InfrastructureAdvisorAutoProxyCreator
  • AspectJAwareAdvisorAutoProxyCreator
  • AnnotationAwareAspectJAutoProxyCreator
三者有就按照优先级覆盖，否则就注册一个，因此始终就只会有一个APC
AopConfigUtils
static { APC_PRIORITY_LIST.add(InfrastructureAdvisorAutoProxyCreator.class); APC_PRIORITY_LIST.add(AspectJAwareAdvisorAutoProxyCreator.class); APC_PRIORITY_LIST.add(AnnotationAwareAspectJAutoProxyCreator.class);}privatestatic BeanDefinition registerOrEscalateApcAsRequired( Class<?> cls, BeanDefinitionRegistry registry, @Nullable Object source) {if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) { BeanDefinition apcDefinition = registry.getBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME);if (!cls.getName().equals(apcDefinition.getBeanClassName())) {//因为三个APC存在能力父子关系，按照指定注册的APC自动调整优先级，从而保证只存在一个APC//如未指定APC，则默认为InfrastructureAdvisorAutoProxyCreatorint currentPriority = findPriorityForClass(apcDefinition.getBeanClassName());int requiredPriority = findPriorityForClass(cls);if (currentPriority < requiredPriority) { apcDefinition.setBeanClassName(cls.getName()); } }returnnull; } RootBeanDefinition beanDefinition = new RootBeanDefinition(cls); beanDefinition.setSource(source); beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE); beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE); registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);return beanDefinition;}
存在多个APC时，如存在循环依赖，此时触发之前放入三级缓存逻辑
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
从而触发多个APC的 getEarlyBeanReference
protectedObject getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {Object exposedObject = bean;if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {//此时如存在多个APC,则依次执行 getEarlyBeanReference 返回多层代理对象for (SmartInstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().smartInstantiationAware) { exposedObject = bp.getEarlyBeanReference(exposedObject, beanName); } }return exposedObject;}

最终proxy2会被注入到依赖的Bean中，即例如：A-proxy2 注入到 B中
存在多个多层代理情况，getEarlyBeanReference 没有问题，但是执行到初始化时
@OverridepublicObject postProcessAfterInitialization(@NullableObject bean, String beanName) {if (bean != null) {//注意这个Bean可是原始对象，每个APC都缓存自身代理过的类，但是存在多个APC时，后续的APC缓存的确是代理类的代理//即如第二个APC是BeanNameAutoProxyCreator，其缓存的可是 proxy1的class，原始类在此APC是没被代理过的，//因此此时会对原始类进行二次代理，产生Proxy3Object cacheKey = getCacheKey(bean.getClass(), beanName);if (this.earlyProxyReferences.remove(cacheKey) != bean) {return wrapIfNecessary(bean, beanName, cacheKey); } }return bean;}//视线返回本次循环依赖最初实例化的结点:A->B->C->A，则此处为A的创建流程//此时A 通过 getEarlyBeanReference生成A ->proxy2注入到C中，//C直接实例创建不会触发getEarlyBeanReference，注入到B中//B直接实例创建不会触发getEarlyBeanReference，注入到A中//A依赖处理完毕，继续初始化 initializeBean流程 -> postProcessAfterInitialization，返回 proxy3if (earlySingletonExposure) {//此时获取到的代理类是 proxy2，即已经注入到依赖类C中的代理，因此不为nullObject earlySingletonReference = getSingleton(beanName, false);if (earlySingletonReference != null) {//多APC时,exposedObject 在之前initializeBean -> postProcessAfterInitialization作用下返回proxy3//proxy3 != bean 不一致，违反了singletion原则，因此会抛出循环依赖异常if (exposedObject == bean) { exposedObject = earlySingletonReference; }elseif (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) { ......if (!actualDependentBeans.isEmpty()) {thrownew 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."); } } }}
8、正常AOP代理为何没问
SmartInstantiationAwareBeanPostProcessor
@OverridepublicObject getEarlyBeanReference(Object bean, String beanName) {Object cacheKey = getCacheKey(bean.getClass(), beanName);this.earlyProxyReferences.put(cacheKey, bean);return wrapIfNecessary(bean, beanName, cacheKey);}//提前通过singletonFactory.getObject()创建的代理缓存起来以后，这里如果再次判断需要代理，//缓存中存在已被代理则直接返回原始bean，无需再次代理，后续直接获取earlySingletonReference，//因此前后代理出来的对象是一致的@OverridepublicObject postProcessAfterInitialization(@NullableObject 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;}
解决方案
1、无需代理场景使用原始对象
  • 原始对象相互注入没有问题，检查不许要生成代理的类
2、@lazy解耦
  • 原理是发现有@lazy注解的依赖为其生成代理类，依赖代理类，从而实现了解耦
  • @Lazy 用来标识类是否需要延迟加载；
  • @Lazy 可以作用在类上、方法上、构造器上、方法参数上、成员变量中；
  • @Lazy 作用于类上时，通常与 @Component 及其衍生注解配合使用；
  • @Lazy 注解作用于方法上时，通常与 @Bean 注解配合使用；
DefaultListableBeanFactory#resolveDependency
publicObject resolveDependency(DependencyDescriptor descriptor, @NullableString requestingBeanName,@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException { descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());if (Optional.class == descriptor.getDependencyType()) {return createOptionalDependency(descriptor, requestingBeanName); } ......else {//处理@lazyObject result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary( descriptor, requestingBeanName);if (result == null) { result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter); }return result; }}ContextAnnotationAutowireCandidateResolver#getLazyResolutionProxyIfNecessarypublicObject getLazyResolutionProxyIfNecessary(DependencyDescriptor descriptor, @NullableString beanName) {return (isLazy(descriptor) ? buildLazyResolutionProxy(descriptor, beanName) : null);}ContextAnnotationAutowireCandidateResolver#isLazy//是否为@lazy，如果为@lazy则创建依赖代理protectedboolean isLazy(DependencyDescriptor descriptor) {for (Annotation ann : descriptor.getAnnotations()) { Lazy lazy = AnnotationUtils.getAnnotation(ann, Lazy.class);if (lazy != null && lazy.value()) {returntrue; } } .......}
3、抽取公共逻辑
  • 业务层面重构，不再相互依赖而是依赖公共模块，并且各个对外业务与内部接口拆分
案例(可直接运行)
1、@Repository案例分析
import org.junit.Test;import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;import org.springframework.context.annotation.AnnotationConfigApplicationContext;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.ComponentScan;import org.springframework.context.annotation.Configuration;import org.springframework.context.annotation.EnableAspectJAutoProxy;import org.springframework.core.env.Environment;import org.springframework.dao.annotation.PersistenceExceptionTranslationPostProcessor;import org.springframework.stereotype.Component;import org.springframework.stereotype.Repository;import javax.annotation.Resource;/** * @author: Superizer */@ComponentpublicclassMainSpringCircularDependencyTester{@Testpublic void springCircularDependencyTest() { AnnotationConfigApplicationContext ac = new AnnotationConfigApplicationContext(SpringCircularDependencyConfig.class); X x = ac.getBean(X.class); System.out.println("Spring bean X =" + x.getClass().getName()); x.display(); Y y = ac.getBean(Y.class); System.out.println("Spring bean Y =" + y.getClass().getName()); y.display(); Z z = ac.getBean(Z.class); System.out.println("Spring bean Z =" + z.getClass().getName()); z.display(); System.out.println("******************Main********************"); }@Configuration@ComponentScan("com.myself.demo.spring.v5.circular.dependency")// @EnableAspectJAutoProxy@ConditionalOnClass(PersistenceExceptionTranslationPostProcessor.class) static classSpringCircularDependencyConfig{@Bean@ConditionalOnMissingBean@ConditionalOnProperty(prefix = "spring.dao.exceptiontranslation", name = "enabled", matchIfMissing = true)public static PersistenceExceptionTranslationPostProcessor persistenceExceptionTranslationPostProcessor(Environment environment) { PersistenceExceptionTranslationPostProcessor postProcessor = new PersistenceExceptionTranslationPostProcessor(); boolean proxyTargetClass = environment.getProperty("spring.aop.proxy-target-class", Boolean.class, Boolean.TRUE); postProcessor.setProxyTargetClass(proxyTargetClass);return postProcessor; } }abstract static classA{publicabstract A injectSources();publicabstract A self();public void display(){ System.out.println("injectSources:" + injectSources().getClass().getName()); System.out.println("*******************************************************"); } }//X、Y、Z 只要循环依赖中第一个类X有注解@Repository，就会出现循环依赖异常//执行X的singletonFactory.getObject()返回的原对象，但是后边初始化时//执行到PersistenceExceptionTranslationPostProcessor时单独创建代理逻辑返回的是代理类//exposedObject = initializeBean(beanName, exposedObject, mbd);@Repository// @Component static classXextendsA{@Resourceprivate Y y;@Overridepublic Y injectSources() {return y; }@Overridepublic X self() {returnthis; } }@Component// @Repository static classYextendsA{@Resourceprivate Z z;@Overridepublic Z injectSources() {return z; }@Overridepublic Y self() {returnthis; } }@Component// @Repository static classZextendsA{@Resourceprivate X x;@Overridepublic X injectSources() {return x; }@Overridepublic Z self() {returnthis; } }}
2、多AutoProxyCreator场景
import org.aopalliance.intercept.MethodInterceptor;import org.aopalliance.intercept.MethodInvocation;import org.junit.Test;import org.springframework.aop.ClassFilter;import org.springframework.aop.MethodMatcher;import org.springframework.aop.Pointcut;import org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator;import org.springframework.aop.support.AbstractExpressionPointcut;import org.springframework.aop.support.DefaultPointcutAdvisor;import org.springframework.context.annotation.AnnotationConfigApplicationContext;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.ComponentScan;import org.springframework.context.annotation.Configuration;import org.springframework.context.annotation.EnableAspectJAutoProxy;import org.springframework.stereotype.Component;import javax.annotation.Resource;import java.util.Arrays;/** * @author: Superizer * Copyright (C) 2021 * All rights reserved */@ComponentpublicclassMainSpringCircularDependencyV2Tester{@Testpublic void circularDependencyV2Tester() { AnnotationConfigApplicationContext ac = new AnnotationConfigApplicationContext(SpringCircularDependencyConfig.class); A a = ac.getBean(A.class); System.out.println("Spring bean A =" + a.getClass().getName()); a.display(); B y = ac.getBean(B.class); System.out.println("Spring bean B =" + y.getClass().getName()); y.display(); C z = ac.getBean(C.class); System.out.println("Spring bean C =" + z.getClass().getName()); z.display(); System.out.println("******************Main********************"); }@Configuration@ComponentScan("com.myself.demo.spring.v5.circular.dependency.v2")@EnableAspectJAutoProxy static classSpringCircularDependencyConfig{@Beanpublic DefaultPointcutAdvisor defaultPointcutAdvisor() { DefaultPointcutAdvisor advisor = new DefaultPointcutAdvisor(); Pointcut pointcut = new AbstractExpressionPointcut() {@Overridepublic ClassFilter getClassFilter() {return (tmp) -> { String name = tmp.getName();if(name.equals(A.class.getName())) {returntrue; }returnfalse; }; }@Overridepublic MethodMatcher getMethodMatcher() {return MethodMatcher.TRUE; } }; advisor.setPointcut(pointcut); advisor.setAdvice(new SpringAopAroundMethod()); advisor.setOrder(0);return advisor; }@Beanpublic BeanNameAutoProxyCreator beanNameAutoProxyCreator() { BeanNameAutoProxyCreator apc = new BeanNameAutoProxyCreator(); apc.setBeanNames("a"); apc.setOrder(-1); apc.setProxyTargetClass(true);return apc; } }abstract static classG{publicabstract G injectSources();publicabstract G self();public void display(){ System.out.println("injectSources:" + injectSources().getClass().getName()); System.out.println("*******************************************************"); } }@Component(value = "a") static classAextendsG{@Resourceprivate B b;@Overridepublic B injectSources() {return b; }@Overridepublic A self() {returnthis; } }@Component static classBextendsG{@Resourceprivate C c;@Overridepublic C injectSources() {return c; }@Overridepublic B self() {returnthis; } }@Component static classCextendsG{@Resourceprivate A a;@Overridepublic A injectSources() {return a; }@Overridepublic C self() {returnthis; } } static classSpringAopAroundMethodimplementsMethodInterceptor{@Overridepublic Object invoke(MethodInvocation methodInvocation) throws Throwable { System.out.println("Aop Before method!");try { Object result = methodInvocation.proceed(); System.out.println("Aop after method!");return result; } catch (IllegalArgumentException e) { System.out.println("Aop throw exception!");throw e; } } }}
总结
出现循环依赖其实反映代码结构设计上的问题，理论上应当将循环依赖进行分层，抽取公共部分，然后由各个功能类再去依赖公共部分。
但是在复杂代码中，各个service、manager类互相调用太多，总会一不小心出现一些类之间的循环依赖的问题。可有时候我们又发现在用Spring进行依赖注入时，虽然Bean之间有循环依赖，但是代码本身却大概率能很正常的work，似乎也没有任何bug。
很多敏感的同学心里肯定有些犯嘀咕，循环依赖这种触犯因果律的事情怎么能发生呢？没错，这一切其实都并不是那么理所当然。Spring已经为我们背负了太多，但绝不是偷懒的借口，还是应该规范设计，规范代码，尽量做到从根本上避免这种循环依赖的发生。
Spring流程图
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