@Async注解引发的Spring循环依赖分析

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首先有一点可以确定，spring可以是解决某些循环依赖的，示例：

@Service
public class AService {

    @Autowired
    private BService bService;

    public void testA() {
    }
}

@Service
public class BService {
    @Autowired
    private AService aService;

    public void testB() {
    }
}

上面的例子，AService与BService存在循环依赖，运行也是没有问题的。

现在开启异步(添加@EnableAsync），修改AService：

@Service
public class AService {

    @Autowired
    private BService bService;

    @Async
    public void testA() {
    }
}

再次运行，就有会有如下报错：

ERROR org.springframework.boot.SpringApplication - Application startup failed
org.springframework.beans.factory.BeanCurrentlyInCreationException: 
  Error creating bean with name 'AService': 
  Bean with name 'AService' has been injected into other beans [BService] 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.

要想了解原因，首先得分析下 @Aysnc 注解的Bean的创建代理的时机。@EnableAsync开启时会向容器注入AsyncAnnotationBeanPostProcessor，它是一个BeanPostProcessor，实现了postProcessAfterInitialization，具体的实现在其父类AbstractAdvisingBeanPostProcessor中：

	@Override
	public Object postProcessAfterInitialization(Object bean, String beanName) {
		if (bean instanceof AopInfrastructureBean) {
			// Ignore AOP infrastructure such as scoped proxies.
			return bean;
		}

		if (bean instanceof Advised) {
			Advised advised = (Advised) bean;
			if (!advised.isFrozen() && isEligible(AopUtils.getTargetClass(bean))) {
				// Add our local Advisor to the existing proxy's Advisor chain...
				if (this.beforeExistingAdvisors) {
					advised.addAdvisor(0, this.advisor);
				}
				else {
					advised.addAdvisor(this.advisor);
				}
				return bean;
			}
		}

		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());
		}

		// No async proxy needed.
		return bean;
	}

  
	protected boolean isEligible(Object bean, String beanName) {
		return isEligible(bean.getClass());
	}

	protected boolean isEligible(Class<?> targetClass) {
		Boolean eligible = this.eligibleBeans.get(targetClass);
		if (eligible != null) {
			return eligible;
		}
		// 方法有 @Aysnc 注解，AopUtils.canApply 会判断为 true
		eligible = AopUtils.canApply(this.advisor, targetClass);
		this.eligibleBeans.put(targetClass, eligible);
		return eligible;
	}

对 AService，AopUtils.canApply 返回 true，那么postProcessAfterInitialization就会返回一个proxy对象。

来看看示例异常错误的地方：

	protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
			throws BeanCreationException {

		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		//...略
		if (instanceWrapper == null) {
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		//...略

		// 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) {
			//...略
			// 这里提前暴露
			addSingletonFactory(beanName, new ObjectFactory<Object>() {
				@Override
				public Object getObject() throws BeansException {
					return getEarlyBeanReference(beanName, mbd, bean);
				}
			});
		}

		// Initialize the bean instance.
		Object exposedObject = bean;
		try {
			populateBean(beanName, mbd, instanceWrapper);
			if (exposedObject != null) {
				exposedObject = initializeBean(beanName, exposedObject, mbd);
			}
		}
		catch (Throwable ex) {
			//...略
		}

        // 这里报错了
		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<String> actualDependentBeans = new LinkedHashSet<String>(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.");
					}
				}
			}
		}
  //...略
	}

对AService，因为它支持提前暴露，因此会先加到缓存中addSingletonFactory；之后在populateBean方法中，由于依赖了BService开始初始化BService，BService依赖AService，因此直接调用了getEarlyBeanReference得到了AService，此时BService获取到的是AService的原始类型引用。
之后AService继续执行到了initializeBean方法，拿到了一个代理对象，即exposedObject是一个代理对象，在后面判断的代码就返回了报错：

		if (earlySingletonExposure) {
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
				if (exposedObject == bean) {
					exposedObject = earlySingletonReference;
				}
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {

但是同样是注解，@Transactional却不会跟@Async有同样的问题，原因在于处理@Transactional注解的是InfrastructureAdvisorAutoProxyCreator，它是SmartInstantiationAwareBeanPostProcessor的子类，在getEarlyBeanReference方法中：

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

在SmartInstantiationAwareBeanPostProcessor中，复写了getEarlyBeanReference方法，暴露出去的就是代理对象：

	@Override
	public Object getEarlyBeanReference(Object bean, String beanName) throws BeansException {
		Object cacheKey = getCacheKey(bean.getClass(), beanName);
		if (!this.earlyProxyReferences.contains(cacheKey)) {
			this.earlyProxyReferences.add(cacheKey);
		}
		return wrapIfNecessary(bean, beanName, cacheKey);
	}

之后postProcessAfterInitialization方法中，判断是否代理过，是的话，直接返回：

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

参考：
https://segmentfault/a/1190000021217176
https://cloud.tencent/developer/article/1497689
https://blog.csdn/f641385712/article/details/89430276
https://www.jb51/article/185794.htm

本文标签： 注解AsyncSpring