spring三级缓存介绍

三级缓存解除循环引用原理

A => B => A
一级缓存 singletonObjects
二级缓存 earlySingletonObjects
三级缓存 singletonFactories

1、获取A，从三级缓存中获取，没有获取到
2、构造A，将A置入三级缓存
3、注入属性，构造B
4、获取B，从三级缓存中没有获取到
5、构造B，将B置入三级缓存
6、注入属性，构造A
7、获取A，从三级缓存中获取，一级没有、二级没有、三级存在。此时通过从三级的BeanFactory构造实例对象，放入二级缓存，移除三级缓存
8、创建B实例后续步骤
9、B构造完成，实例放入一级缓存，二级三级缓存移除
10、创建A实例后续步骤
11、A构造完成，实例放入一级缓存，二级三级缓存移除

源码对应

springboot 2.5.6

1、获取A，从三级缓存中获取，没有获取到

// AbstractBeanFactory#doGetBean   256行
protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)throws BeansException {
String beanName = transformedBeanName(name);
Object beanInstance;// 从三个缓存中依次获取，注意方法点进去allowEarlyReference是true
Object sharedInstance = getSingleton(beanName);...

2、构造A，将A置入三级缓存

构造A(创建A实例)

// AbstractAutowireCapableBeanFactory#doCreateBean   582行
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) {
// 这里会通过构造函数创建Bean实例instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//省略代码...

置入缓存

// AbstractAutowireCapableBeanFactory#doCreateBean  613行
//省略代码...
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");}// 这里会把生成beanName的BeanFactory置入三级缓存addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
//省略代码...

3、注入属性，构造B

扫描缓存实例的相关信息

InitDestroyAnnotationBeanPostProcessor会解析缓存类中@PostStruct和@PreDestroy等相关信息
CommonAnnotationBeanPostProcessor会解析缓存类中@Recource、@WebServiceRef、@PostStruct和@PreDestroy等相关信息
AutowiredAnnotationBeanPostProcessor会解析缓存类中@Autowired、@Value、@Inject等相关信息

// AbstractAutowireCapableBeanFactory#doCreateBean 594行
//省略代码...
synchronized (mbd.postProcessingLock) {if (!mbd.postProcessed) {try {// 解析缓存注解相关信息applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);}catch (Throwable ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Post-processing of merged bean definition failed", ex);}mbd.postProcessed = true;}
}
//省略代码...

注入属性

// AbstractAutowireCapableBeanFactory#doCreateBean 619行
//省略代码...
try {// 这里会将注入对应的属性populateBean(beanName, mbd, instanceWrapper);exposedObject = initializeBean(beanName, exposedObject, mbd);
}
//省略代码...// AbstractAutowireCapableBeanFactory#populateBean 1431行
//省略代码...
// 这里会依次遍历 如果是@Resource会在CommonAnnotationBeanPostProcessor中触发注入
// 如果是@Autowired则会在AutowiredAnnotationBeanPostProcessor中触发注入
for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {if (filteredPds == null) {filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}pvsToUse = bp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {return;}}pvs = pvsToUse;
}
//省略代码...

4、获取B，从三级缓存中没有获取到

5、构造B，将B置入三级缓存

6、注入属性，构造A

上面三个其实类似，同前面三步骤差不多

7、获取A，三个缓存中三级缓存命中

 // AbstractBeanFactory#doGetBean   256行  入口
// DefaultSingletonBeanRegistry#getSingleton  150
// 此时，一级缓存和二级缓存里面都没有，三级缓存在第二步里面放进去了，此时能拿到对应的ObjectFactory，
//通过它可以获取对应的实例。然后将实例放入二级缓存，将三级缓存中对应的BeanName移除
protected Object getSingleton(String beanName, boolean allowEarlyReference) {Object singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null && allowEarlyReference) {synchronized (this.singletonObjects) {// Consistent creation of early reference within full singleton locksingletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null) {ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);if (singletonFactory != null) {singletonObject = singletonFactory.getObject();this.earlySingletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);}}}}}}return singletonObject;
}

8、创建B实例后续步骤

@Async循环引用触发点

这里属性注入完后，剩下bean增强、循环依赖校验、注册DisposableBean

9、B构造完成，实例放入一级缓存，二级三级缓存移除

// DefaultSingletonBeanRegistry#getSingleton  259行
//省略代码...
// 这在执行链路的上面几层。这里判断是否是新的单例，如果是则置入缓存(一级缓存)中
if (newSingleton) {addSingleton(beanName, singletonObject);
}
//省略代码...protected void addSingleton(String beanName, Object singletonObject) {
// 放入一级缓存，从二级缓存和三级缓存中移除synchronized (this.singletonObjects) {this.singletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);this.earlySingletonObjects.remove(beanName);this.registeredSingletons.add(beanName);}
}

10、创建A实例后续步骤

11、A构造完成，实例放入一级缓存，二级三级缓存移除

逻辑基本同上

@Async导致三级缓存没有解除循环引用

复现

测试类

@Service
@Slf4j
public class AService {static {log.info("AService init===>");}@Resourceprivate BService bService;public String a2bService(){return bService.get();}@Async("async-2")public void exec(){log.info("aService exec");}}

@Service
@Slf4j
public class BService {static {log.info("BService init===>");}@Autowiredprivate AService aService;public String get(){return "BService 666";}public void exec(){log.info("BService exec");}}

描述

这里AService 和BService在同一个目录下，也没有被在别的类中注入(如果有一个类把两者都注入了，且Bservice先注入，此时如果三个类中该类最先注入，就会导致Bservice先初始化)。在这种情况下，默认就会Aservice先初始化，然后在Bservice初始化，如此，才能复现问题。
如果想要调整类加载顺序，可以通过@DependsOn(value = "AService")或者@Lazy

备注

别开启懒加载，不然启动不会报错。
spring.main.lazy-initialization=true

启动测试

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 'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:649)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542)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.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:145)at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:754)at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:434)at org.springframework.boot.SpringApplication.run(SpringApplication.java:338)at com.yichen.casetest.CaseTestApplication.main(CaseTestApplication.java:47)

原因分析

在bean实例后续步骤中，会对bean增强

// AbstractAutowireCapableBeanFactory#doCreateBean  620行
try {populateBean(beanName, mbd, instanceWrapper);// 初始化bean，对Bean增强// 1、invokeAwareMethods// 2、applyBeanPostProcessorsBeforeInitialization// 3、invokeInitMethods// 4、applyBeanPostProcessorsAfterInitializationexposedObject = initializeBean(beanName, exposedObject, mbd);
}

在applyBeanPostProcessorsAfterInitialization中，由于使用了异步线程池(用了@EnableAsync)，使得AsyncAnnotationBeanPostProcessor注入了spring容器，它会为原有的bean实例创建CGLIB代理，使得最初的bean和实例化后暴露出去的bean不是同一个，没有通过循环引用校验，抛出了异常。

// AsyncAnnotationBeanPostProcessor的子类
//AbstractAdvisingBeanPostProcessor#postProcessAfterInitialization  86行
// 如果类中有@Async会创建代理
if (isEligible(bean, beanName)) {ProxyFactory proxyFactory = prepareProxyFactory(bean, beanName);if (!proxyFactory.isProxyTargetClass()) {evaluateProxyInterfaces(bean.getClass(), proxyFactory);}proxyFactory.addAdvisor(this.advisor);customizeProxyFactory(proxyFactory);// Use original ClassLoader if bean class not locally loaded in overriding class loaderClassLoader classLoader = getProxyClassLoader();if (classLoader instanceof SmartClassLoader && classLoader != bean.getClass().getClassLoader()) {classLoader = ((SmartClassLoader) classLoader).getOriginalClassLoader();}return proxyFactory.getProxy(classLoader);
}

// AbstractAutowireCapableBeanFactory#doCreateBean  632行
// 循环引用检测
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<>(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.");}}}
}

解决方法

1、不用异步线程池，自己实现线程池
2、通过@lazy、@DependsOn，调整类加载顺序，如果让Bservice先加载就不会出错
3、梳理业务逻辑，调整技术实现，让AService和Bservice不循环引用

参考

Using @Async in a circular dependency will throw a BeanCurrentlyInCreationException