@Async引发的循环依赖问题

这个以前分析过了，但是再看的时候感觉写的太乱了(流水账)，这个是精简版本。

bug复现

先上bug，众所周知，spring通过实例化和属性注入分开解决了循环依赖，理论上也不会有问题。但是意外就那么来了，经过排查是@Async导致，报错如下所示。

java.lang.IllegalStateException: Failed to load ApplicationContext

	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:132)
	at org.springframework.test.context.support.DefaultTestContext.getApplicationContext(DefaultTestContext.java:124)
	at org.springframework.test.context.web.ServletTestExecutionListener.setUpRequestContextIfNecessary(ServletTestExecutionListener.java:190)
	at org.springframework.test.context.web.ServletTestExecutionListener.prepareTestInstance(ServletTestExecutionListener.java:132)
	at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:244)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.createTest(SpringJUnit4ClassRunner.java:227)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner$1.runReflectiveCall(SpringJUnit4ClassRunner.java:289)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.methodBlock(SpringJUnit4ClassRunner.java:291)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:246)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
	at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'testA': Bean with name 'testA' has been injected into other beans [testB] 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:631)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:944)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:918)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:583)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:769)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:761)
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:426)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:326)
	at org.springframework.boot.test.context.SpringBootContextLoader.loadContext(SpringBootContextLoader.java:123)
	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:99)
	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:124)
	... 25 more

源码分析

引发原因

简单的说：BeanA为了解决循环依赖，实例化后注入给了BeanB，BeanB实例化，属性注入完后，轮到BeanA属性注入，注入完后的引用和注入给BeanB的引用不一致了，封装了一层代理，spring就报错了

Aop为没有

不走循环依赖的Aop分析

抛开源码不看，async如果有问题，理论上aop也有问题，都是返回代理，你说对吧。但是，如果aop没有问题，aop对循环依赖做了特殊处理，这里，先分析下正常aop代理，懂哥可以跳过

如果没有循环依赖，那么也是通过AbstractAutoProxyCreator::applyBeanPostProcessorsAfterInitialization来创建的

走依循环依赖的Aop分析

如果遇到依赖循环，那么就会在实例化的时候就创建代理了，这里还有一个重要的就是，他会把依赖循环中创建代理的bean给缓存起来，图中已经标注了，后续执行到applyBeanPostProcessorsAfterInitialization的时候也不会再创建一遍代理，而是跳过

再回到最上层，简单的分析下

• 属性注入完成
• 引用地址是一样的
• 代理对象引用在缓存中

通过一个简单的if判断，换下指针，就可以给spring容器交差了。

至此aop分析完成

@Async分析

翻阅了AsyncAnnotationBeanPostProcessor没有实现SmartInstantiationAwareBeanPostProcessor

创建代理对象都是在applyBeanPostProcessorsAfterInitialization中创建的

总结

原理总结

AsyncAnnotationBeanPostProcessor未做特殊处理

解决方案

网上解决方案很多

• @Lazy
• 不使用@Async，自己用线程池实现
• 解决循环依赖