最近在项目中使用@Async注解在方法上引起了循环依赖报错。

代码类似如下：

package com.morris.spring.entity.circular;import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.scheduling.annotation.Async;
import org.springframework.scheduling.annotation.EnableAsync;@EnableAsync
public class SingleFieldBeanD {@Autowiredprivate SingleFieldBeanD singleFieldBeanD;@Asyncpublic void test() {System.out.println("test");}
}

报错内容如下：

org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'singleFieldBeanD': Bean with name 'singleFieldBeanD' has been injected into other beans [singleFieldBeanD] 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.at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:655)
... ...

其实当我们使用Spring的时候，默认是会解决循环依赖，如果不加@Async注解，程序就能正常运行，一旦加上了@Async注解方法后，处理循环依赖就失效了，就会抛出异常。

循环依赖错误原因分析

源码分析，查看异常抛出时的方法：
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {// Instantiate the bean.BeanWrapper instanceWrapper = null;if (mbd.isSingleton()) {instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) {// 实例化instanceWrapper = createBeanInstance(beanName, mbd, args);}Object bean = instanceWrapper.getWrappedInstance();Class<?> beanType = instanceWrapper.getWrappedClass();if (beanType != NullBean.class) {mbd.resolvedTargetType = beanType;}// Allow post-processors to modify the merged bean definition.synchronized (mbd.postProcessingLock) {if (!mbd.postProcessed) {try {/*** 使用下面的BeanPostProcessor收集带有注解的方法，方便下面初始化属性时调用* AutowiredAnnotationBeanPostProcessor @Autowired @Value* CommonAnnotationBeanPostProcessor @Resource @PostConstruct @PreDestroy*/applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);}catch (Throwable ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Post-processing of merged bean definition failed", ex);}mbd.postProcessed = true;}}// Eagerly cache singletons to be able to resolve circular references// even when triggered by lifecycle interfaces like BeanFactoryAware.// 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");}// 放入三级缓存addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));}// Initialize the bean instance.Object exposedObject = bean;try {// 初始化bean，为bean赋予属性populateBean(beanName, mbd, instanceWrapper);// 调用初始化方法exposedObject = initializeBean(beanName, exposedObject, mbd);}catch (Throwable ex) {if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {throw (BeanCreationException) ex;}else {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);}}if (earlySingletonExposure) {// 从一级和二级缓存中获取Object earlySingletonReference = getSingleton(beanName, false);if (earlySingletonReference != null) {// 不为空说明产生了循环依赖// bean是通过构造方法创建的// 而exposedObject是initializeBean方法返回，这里有可能产生aop代理后返回if (exposedObject == bean) {// 相等说明没有产生aop代理exposedObject = earlySingletonReference;}else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {// 产生了aop代理，且有对象依赖他String[] dependentBeans = getDependentBeans(beanName);Set<String> 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 " +"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");}}}}// Register bean as disposable.try {registerDisposableBeanIfNecessary(beanName, bean, mbd);}catch (BeanDefinitionValidationException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);}return exposedObject;
}

举个例子，假设类A和类B通过属性形成了循环依赖：

创建A：使用构造方法构造A，将A放入三级缓存，给A的属性B复赋值
创建B：使用构造方法构造B，将B放入三级缓存，给B的属性A复赋值
此时因为A支持循环依赖，所以会从三级缓存中获取A填充到B的属性中，此时B的属性A持有的是A的原始对象的引用
B完成其他属性的赋值及初始化方法的调用
继续完成了A的属性的赋值（此时已持有B的实例的引用），继续执行初始化方法initializeBean(…)，在此处会解析@Aysnc注解，从而生成一个代理对象，所以最终exposedObject是一个代理对象（而非原始对象）最终加入到容器里
最后B引用的属性A是个原始对象，而此处准备return的实例A竟然是个代理对象，也就是说B引用的并非是最终对象（不是最终放进容器里的对象），最后抛出异常。

循环依赖问题的解决方案

allowRawInjectionDespiteWrapping设置为true

从源码中可以发现有一个allowRawInjectionDespiteWrapping变量来控制是否进入else if代码段，最后抛出异常，此字段默认为false，如果将此字段设置为true，就不会进入else if代码段，从而就不会抛出异常了。

设置方法如下：

AbstractAutowireCapableBeanFactory beanFactory = (AbstractAutowireCapableBeanFactory) applicationContext.getBeanFactory();
beanFactory.setAllowRawInjectionDespiteWrapping(true);

属性上面添加@Lazy注解

在属性上面添加@Lazy注解就能打破循环依赖，就不会抛出异常了。

代码如下：

package com.morris.spring.entity.circular;import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Lazy;
import org.springframework.scheduling.annotation.Async;
import org.springframework.scheduling.annotation.EnableAsync;@EnableAsync
public class SingleFieldBeanD {@Autowired@Lazyprivate SingleFieldBeanD singleFieldBeanD;@Asyncpublic void test() {System.out.println("test");}
}

为什么加上@Lazy就不会抛出异常了呢？因为此时注入的是代理对象，而不是原始对象，不构成循环依赖。

且看源码：

在对属性进行注入时会执行下面的代码：
org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.ResourceElement#getResourceToInject

protected Object getResourceToInject(Object target, @Nullable String requestingBeanName) {// buildLazyResourceProxy里面还有调用了getResource方法从Spring容器中获取beanreturn (this.lazyLookup ? buildLazyResourceProxy(this, requestingBeanName) :getResource(this, requestingBeanName));
}

lazyLookup什么时候为true呢？属性上面有@Lazy注解，lazyLookup=true

private class ResourceElement extends LookupElement {private final boolean lazyLookup;public ResourceElement(Member member, AnnotatedElement ae, @Nullable PropertyDescriptor pd) {super(member, pd);Resource resource = ae.getAnnotation(Resource.class);String resourceName = resource.name();Class<?> resourceType = resource.type();this.isDefaultName = !StringUtils.hasLength(resourceName);if (this.isDefaultName) {resourceName = this.member.getName();if (this.member instanceof Method && resourceName.startsWith("set") && resourceName.length() > 3) {resourceName = Introspector.decapitalize(resourceName.substring(3));}}else if (embeddedValueResolver != null) {resourceName = embeddedValueResolver.resolveStringValue(resourceName);}if (Object.class != resourceType) {checkResourceType(resourceType);}else {// No resource type specified... check field/method.resourceType = getResourceType();}this.name = (resourceName != null ? resourceName : "");this.lookupType = resourceType;String lookupValue = resource.lookup();this.mappedName = (StringUtils.hasLength(lookupValue) ? lookupValue : resource.mappedName());// 有@Lazy注解，lazyLookup=trueLazy lazy = ae.getAnnotation(Lazy.class);this.lazyLookup = (lazy != null && lazy.value());}
... ...

属性上面如果有@Lazy注解，lazyLookup=true，就会创建代理：

protected Object buildLazyResourceProxy(final LookupElement element, final @Nullable String requestingBeanName) {TargetSource ts = new TargetSource() {@Overridepublic Class<?> getTargetClass() {return element.lookupType;}@Overridepublic boolean isStatic() {return false;}@Overridepublic Object getTarget() {// 从Spring容器中获得beanreturn getResource(element, requestingBeanName);}@Overridepublic void releaseTarget(Object target) {}};// 创建Aop代理ProxyFactory pf = new ProxyFactory();pf.setTargetSource(ts);if (element.lookupType.isInterface()) {pf.addInterface(element.lookupType);}ClassLoader classLoader = (this.beanFactory instanceof ConfigurableBeanFactory ?((ConfigurableBeanFactory) this.beanFactory).getBeanClassLoader() : null);return pf.getProxy(classLoader);
}

从ApplicationContext获取对象

在方法具体使用对象时再去Spring容器中获取对象，这样拿到的就是代理对象：

package com.morris.spring.entity.circular;import org.springframework.beans.BeansException;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.annotation.Lazy;
import org.springframework.scheduling.annotation.Async;
import org.springframework.scheduling.annotation.EnableAsync;@EnableAsync
public class SingleFieldBeanD implements ApplicationContextAware {private ApplicationContext applicationContext;@Overridepublic void setApplicationContext(ApplicationContext applicationContext) throws BeansException {this.applicationContext = applicationContext;}public SingleFieldBeanD getSingleFieldBeanD() {return applicationContext.getBean(SingleFieldBeanD.class);}@Asyncpublic void test() {System.out.println("test");}}

重构方法

重新建class，把@Async的方法放在新的类中，从根本上消除循环依赖。

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