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1前言:
之前我们有分析过Spring是怎么解决循环引用的问题,主要思路就是三级缓存；Spring在加载beanA的时候会先调用默认的空构造函数(在没有指定构造函数实例化的前提下)得到一个空的实例引用对象，这个时候没有设置任何值，但是Spring会用缓存把它给提前暴露出来，让其他依赖beanA的bean可以持有它提前暴露的引用；比如 a 依赖b ，b依赖a，并且他们都是通过默认方法实例化，那么简单流程是这样的：
- ioc实例化a,a提前暴露自己的，然后填充属性值，在填充属性值的时候发现有个对象b,这个时候去容器里面取到b的引用，发现b还没有被创建，那么就走实例化b的流程；
- 实例化b；流程跟a一样；但是不同的是b填充属性的时候，发现有引用a的实例，这个时候a已经提前暴露了自己了，所以b可以直接在容器里面拿到a的引用；那么b就实例化并且也初始化完成了;
- 拿到b了之后，a就可以持有b的引用 ，整个流程就走完了;
具体详细一点可以看这篇文章Spring-bean的循环依赖以及解决方式
Spring不能解决“A的构造方法中依赖了B的实例对象，同时B依赖了A的实例对象”这类问题
这篇文章我想从源码的角度来分析一下整个流程；并且分析一下Spring为什么不能解决“A的构造方法中依赖了B的实例对象，同时B依赖了A的实例对象”这类问题
2例子
首先创建两个bean类;CirculationA有个属性circulationB，并且有个构造函数给circulationB赋值；
public class CirculationA { private CirculationB circulationB; public CirculationA(CirculationB circulationB) { this.circulationB = circulationB; } }
CirculationB有个属性circulationA，然后set方法
public class CirculationB { private CirculationA circulationA; public CirculationA getCirculationA() { return circulationA; } public void setCirculationA(CirculationA circulationA) { this.circulationA = circulationA; } }
SpringContextConfig.xmlcirculationa 用给定的构造函数实例化; circulationb 就用默认的实例化方法(默认的空构造函数)
<bean id="circulationa" class="src.bean.CirculationA"> <constructor-arg name="circulationB" ref="circulationb"/> </bean> <bean id="circulationb" class="src.bean.CirculationB" > <property name="circulationA" ref="circulationa"/> </bean>
好，例子准完毕，上面的例子是 circulationa的构造函数里面有circulationb；然后circulationb属性里面有circulationa;启动容器！结果如下:
警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circulationa' defined in class path resource [config.xml]: Cannot resolve reference to bean 'circulationb' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circulationb' defined in class path resource [config.xml]: Cannot resolve reference to bean 'circulationa' while setting bean property 'circulationA'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circulationa': Requested bean is currently in creation: Is there an unresolvable circular reference? Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circulationa' defined in class path resource [config.xml]: Cannot resolve reference to bean 'circulationb' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circulationb' defined in class path resource [config.xml]: Cannot resolve reference to bean 'circulationa' while setting bean property 'circulationA'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circulationa': Requested bean is currently in creation: Is there an unresolvable circular reference? Disconnected from the target VM, address: '127.0.0.1:64128', transport: 'socket' at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359) at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108) at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:648) at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:145) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1193) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1095) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:513) 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.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139) at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83) at StartIOCUseDefaultListAbleBeanFactory.main(StartIOCUseDefaultListAbleBeanFactory.java:30) Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circulationb' defined in class path resource [config.xml]: Cannot resolve reference to bean 'circulationa' while setting bean property 'circulationA'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circulationa': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359) at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1531) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1276) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553) 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.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351) ... 17 more Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circulationa': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:347) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223) 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.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351) ... 27 more
报错了,Spring它解决不了这种情况Ok,源码走起来： 为了节省篇幅我只贴重要代码 第一步,加载circulationaAbstractBeanFactory
public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory { protected <T> T doGetBean( final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException { if (mbd.isSingleton()) { sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() { @Override public Object getObject() throws BeansException { try { return createBean(beanName, mbd, args); } } }); } } public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) { //在创建之前把beanName加入到正在创建中的属性中singletonsCurrentlyInCreation； //但是这个是一个set，如果之前已经加进去了，再进去就抛异常BeanCurrentlyInCreationException //Requested bean is currently in creation: Is there an unresolvable circular reference?")提示可能存在循环引用 beforeSingletonCreation(beanName); } protected void beforeSingletonCreation(String beanName) { if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } } @Override protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException { Object beanInstance = doCreateBean(beanName, mbdToUse, args); } protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) throws BeanCreationException { instanceWrapper = createBeanInstance(beanName, mbd, args); //....... // Initialize the bean instance. Object exposedObject = bean; try { populateBean(beanName, mbd, instanceWrapper); if (exposedObject != null) { exposedObject = initializeBean(beanName, exposedObject, mbd); } } } protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) { Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName); //因为circulationa是有构造函数的，所以使用autowireConstructor if (ctors != null || mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR || mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) { return autowireConstructor(beanName, mbd, ctors, args); } } //最终执行 public BeanWrapper autowireConstructor(final String beanName, final RootBeanDefinition mbd, Constructor<?>[] chosenCtors, final Object[] explicitArgs) { //解析构造函数参数值 minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues); //...... //选择对应的策略来实例化对象；这里是生成正在的实例了。 //但是在这之前，构造参数要拿到 beanInstance = this.beanFactory.getInstantiationStrategy().instantiate( mbd, beanName, this.beanFactory, constructorToUse, argsToUse); } }
省略....最终调用BeanDefinitionValueResolver
/** * Resolve a reference to another bean in the factory. */ private Object resolveReference(Object argName, RuntimeBeanReference ref) { try { String refName = ref.getBeanName(); refName = String.valueOf(doEvaluate(refName)); if (ref.isToParent()) { if (this.beanFactory.getParentBeanFactory() == null) { throw new BeanCreationException( this.beanDefinition.getResourceDescription(), this.beanName, "Can't resolve reference to bean '" + refName + "' in parent factory: no parent factory available"); } //!!!这里,要先去查找refName的实例 return this.beanFactory.getParentBeanFactory().getBean(refName); } else { Object bean = this.beanFactory.getBean(refName); this.beanFactory.registerDependentBean(refName, this.beanName); return bean; } } catch (BeansException ex) { throw new BeanCreationException( this.beanDefinition.getResourceDescription(), this.beanName, "Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex); } }
跟着上面的顺序我们整理一下;
- 启动容器,加载circulationa,因为是构造函数生成，所以要先解析构造函数的属性，这时候发现有引用circulationb，那么通过getBean(circulationb)先拿到circulationb的实例；
- 如果拿到了，则生成circulationa的实例对象返回;但是这个时候代码执行circulationb的加载过程了；
然后我们分析一下circulationb加载 circulationb跟circulationa差不多 加载circulationb,把它加入到正在创建的属性中
protected void beforeSingletonCreation(String beanName) { if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } }
然后用默认的方式创建实例; circulationa 是rautowireConstructor(beanName, mbd, ctors, args)创建的；这个方法需要先拿到构造函数的值;所以执行了调用getBean(circulationb)
circulationa是调用了instantiateBean；这个方法不需要提前知道属性；它用默认的构造函数生成实例；这时候的实例是没有设置任何属性的；
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) { beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent); }
不过生成了实例之后，在doCreateBean方法中有一个populateBean；这个方法就是专门填充属性值的，因为circulationb有circulationa的属性; 所以会去容器里面取circulationa的引用；但是circulationa这个时候还没有成功创建实例啊；因为它还一直在等circulationb创建成功之后返回给它引用呢，返回了circulationa才能创建实例啊；
这个时候circulationb没有拿到circulationa，那么又会去调用getBean(circulationa); 大家想一想如果这样下去就没完没了了啊; 所以Spring就抛出异常了那么在哪里抛出异常呢？在第二次调用getBean(circulationa)的时候会走到下面
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } }
因为circulationa之前加进来过一次啊，而且没有创建成功是不会删除的啊；现在又add一次，因为this.singletonsCurrentlyInCreation是一个set；已经存在的再次add会返回false;那么这段代码就会抛出异常了;
Error creating bean with name 'circulationa': Requested bean is currently in creation: Is there an unresolvable circular reference?
情况就是这样,只要是用构造函数创建一个实例，并且构造函数里包含的值存在循环引用，那么spring就会抛出异常; 所以如果有循环引用的情况请避免使用构造函数的方式
