怎么在spring中解决循环依赖问题

怎么在spring中解决循环依赖问题？针对这个问题，这篇文章详细介绍了相对应的分析和解答，希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。

setter singleton循环依赖

使用

SingleSetterBeanA依赖SingleSetterBeanB，SingleSetterBeanB依赖SingleSetterBeanA。

@Datapublic class SingleSetterBeanA {@Autowiredprivate SingleSetterBeanB singleSetterBeanB;}

@Datapublic class SingleSetterBeanB {@Autowiredprivate SingleSetterBeanA singleSetterBeanA;}

源码分析

spring是通过三级缓存来解决循环依赖的，那么三级缓存是怎么工作的呢？

三级缓存对应org.springframework.beans.factory.support.DefaultSingletonBeanReGIStry类的三个属性：

private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256); // 一级缓存private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); // 二级缓存private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16); // 三级缓存

对于setter注入造成的依赖是通过Spring容器提前暴露刚完成实例化但未完成初始化的bean来完成的，而且只能解决单例作用域的bean循环依赖。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，关键源码如下所示：

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean

// 处理循环依赖，实例化后放入三级缓存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));}

bean实例化后放入三级缓存中：

org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#addSingletonFactory

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(singletonFactory, "Singleton factory must not be null");synchronized (this.singletonObjects) {if (!this.singletonObjects.containsKey(beanName)) {this.singletonFactories.put(beanName, singletonFactory); // 三级缓存this.earlySingletonObjects.remove(beanName);this.registeredSingletons.add(beanName);}}}

放入三级缓存中的是ObjectFactory类型的lambda表达式：

org.springframework.beans.factory.support.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;// 使用AbstractAutoProxyCreator#getEarlyBeanReference创建代理对象exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);}}}return exposedObject;}

构造器参数循环依赖

通过构造器注入构成的循环依赖，此依赖是无法解决的，只能抛出BeanCurrentlyInCreationException异常表示循环依赖。

使用

@Datapublic class SingleConstrutorBeanA {public SingleConstrutorBeanA(SingleConstrutorBeanB singleConstrutorBeanB) {}}

@Datapublic class SingleConstrutorBeanB {public SingleConstrutorBeanB(SingleConstrutorBeanA singleConstrutorBeanA) {}}

上面的代码运行时会抛出如下异常：

... ...
Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'singleConstrutorBeanB': Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'singleConstrutorBeanA': Requested bean is currently in creation: Is there an unresolvable circular reference?
 at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:805)
 at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:228)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1403)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1245)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:579)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:538)
 at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:329)
 at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
 at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:323)
 at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
 at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1321)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1240)
 at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:892)
 at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:796)
 ... 76 more
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'singleConstrutorBeanA': Requested bean is currently in creation: Is there an unresolvable circular reference?
 at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:355)
 at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:227)
 at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:323)
 at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
 at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1321)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1240)
 at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:892)
 at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:796)
 ... 90 more

源码分析

Spring容器会将每一个正在创建的Bean标识符放在一个“当前创建Bean池”中，Bean标识符在创建过程中将一直保持在这个池中，因此如果在创建Bean过程中发现自己已经在“当前创建Bean池”里时将抛出BeanCurrentlyInCreationException异常表示循环依赖；而对于创建完毕的Bean将从“当前创建Bean池”中清除掉。

protected void beforeSingletonCreation(String beanName) {if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}}

@Lazy打破循环依赖

在上面的例子中只需要在SingleConstrutorBeanA或者SingleConstrutorBeanB的构造方法上面加上@Lazy注解，就会发现不会抛出异常了，这又是为什么呢？

下面假设在SingleConstrutorBeanA的构造方法上面加了@Lazy注解，在构造B时，发现参数A时被@Lazy注解修饰时，那么就不会调用getBean来获取对象，而是创建了一个代理对象，所以不会构成真正的循环依赖，不会抛出BeanCurrentlyInCreationException异常。

Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(descriptor, requestingBeanName);if (result == null) {// 调用beanFactory.getBean(beanName)从容器中获取依赖对象result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);}return result;

setter prototype循环依赖

对于prototype作用域bean，Spring容器无法完成依赖注入，因为Spring容器不进行缓存"prototype"作用域的bean，因此无法提前暴露一个创建中的bean。

使用

@Data@Component@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)public class PrototypeBeanA {@Autowiredprivate PrototypeBeanB prototypeBeanB;}

@Data@Component@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)public class PrototypeBeanB {@Autowiredprivate PrototypeBeanA prototypeBeanA;}

@Testpublic void test3() {AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();applicationContext.register(PrototypeBeanA.class);applicationContext.register(PrototypeBeanB.class);applicationContext.refresh();applicationContext.getBean(PrototypeBeanA.class); // 此时必须要获取Spring管理的实例，因为现在scope="prototype" 只有请求获取的时候才会实例化对象}

运行结果如下：

Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'prototypeBeanA': Requested bean is currently in creation: Is there an unresolvable circular reference?
 at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:269)
 at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
 at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1322)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1240)
 at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:668)
 ... 89 more

源码分析

org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean

... ...// 判断是否在当前创建Bean池中if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}... ...

异常就是在上面的代码中抛出来的，那么beanName是什么时候添加至当前创建Bean池中的呢？

org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean

else if (mbd.isPrototype()) {// It's a prototype -> create a new instance.// prototype类型的bean的实例化Object prototypeInstance = null;try {beforePrototypeCreation(beanName);prototypeInstance = createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}

org.springframework.beans.factory.support.AbstractBeanFactory#beforePrototypeCreation

protected void beforePrototypeCreation(String beanName) {// ThreadLocalObject curVal = this.prototypesCurrentlyInCreation.get();if (curVal == null) {this.prototypesCurrentlyInCreation.set(beanName);}else if (curVal instanceof String) {Set<String> beanNameSet = new HashSet<>(2);beanNameSet.add((String) curVal);beanNameSet.add(beanName);this.prototypesCurrentlyInCreation.set(beanNameSet);}else {Set<String> beanNameSet = (Set<String>) curVal;beanNameSet.add(beanName);}}

其根本原因就是Spring容器不会对prototype类型的bean进行缓存，因此无法提前利用三级缓存暴露一个代理对象。

循环依赖开关

可以通过allowCircularReferences来禁止循环依赖，这样的话，singleton bean的setter循环依赖也会报错。

applicationContext.setAllowCircularReferences(false);

二级缓存可行？

理论上二级缓存时可行的，只需要将三级缓存中BeanFactory创建的对象提前放入二级缓存中，这样三级缓存就可以移除了。

那么spring中为什么还要使用三级缓存呢？如果要使用二级缓存解决循环依赖，意味着所有Bean在实例化后就要完成aop代理，这样违背了Spring设计的原则，Spring在设计之初就是通过AnnotationAwareAspectJAutoProxyCreator这个后置处理器来在Bean生命周期的最后一步来完成AOP代理，而不是在实例化后就立马进行AOP代理。

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