文章目录

【Java】Spring框架是如何解决Bean创建过程中的循环依赖问题的
- 引言
- 三级缓存数据结构简介
- 源码阅读
- - 如何启动调试？
  - - 场景1：通过xml配置文件来构造循环引用
    - 场景2：通过注解+自动装配属性来构造循环引用：以`@Controller`为例
  - 脉络
  - 场景1的执行过程
  - `@Controller`+自动装配属性和普通Bean解决循环依赖过程的相同点与不同点
- 一些扩展结论
- 总结
- 参考资料

【Java】Spring框架是如何解决Bean创建过程中的循环依赖问题的

引言

本文主要梳理了Spring框架Bean创建过程中应对循环依赖问题的相关源码。我在手写super-mini-webpack的时候也介绍过解决循环依赖的算法：Map+记忆化搜索。可以猜测这段源码也实现了这个算法，所以在看这段源码的时候，我们可以先找到递归点，再去分析调用栈涉及的那些函数，顺便找出其用到的Map数据结构。

作者：hans774882968以及hans774882968以及hans774882968

本文所用工程

本文CSDN

本文juejin

本文52pojie：https://www.52pojie.cn/thread-1814835-1-1.html

三级缓存数据结构简介

三级缓存数据结构定义和操作三级缓存的函数都位于：spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java

	/** Cache of singleton objects: bean name to bean instance. */private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);/** Cache of singleton factories: bean name to ObjectFactory. */private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);/** Cache of early singleton objects: bean name to bean instance. */private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

缓存先找到第一级，第一级没有才查第二级，依此类推。singletonObjects, earlySingletonObjects, singletonFactories分别是1到3级。第一级缓存是bean名到成品bean的映射；第二级缓存是bean名到半成品bean的映射；第三级缓存是bean名到函数式接口的映射，作用为延迟调用函数。

@FunctionalInterface
public interface ObjectFactory<T> {/*** Return an instance (possibly shared or independent)* of the object managed by this factory.* @return the resulting instance* @throws BeansException in case of creation errors*/T getObject() throws BeansException;}

第三级缓存的Value里的函数的调用方式就是调用.getObject()。

为什么需要2级缓存？因为要体现一个分层的思想，半成品bean原则上是不能暴露到外部的。TODO：外部是指？

为什么需要3级缓存？如果所有bean都没有代理对象就不需要第3级缓存。TODO：补充说明。

源码阅读

如何启动调试？

新建一个普通的SpringBoot项目。然后可以尝试通过xml配置文件和注解等方式构造循环引用。后文称为场景1和场景2。

场景1：通过xml配置文件来构造循环引用

入口src\main\java\com\hans\bean_dependency_cycle\hans\HansApplication.java：

package com.hans.bean_dependency_cycle.hans;import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;@SpringBootApplication
public class HansApplication {public static void main(String[] args) {ApplicationContext ac = new ClassPathXmlApplicationContext("cycle.xml");A beanA = ac.getBean(A.class);System.out.println(beanA);System.out.println(beanA.getB());B beanB = ac.getBean(B.class);System.out.println(beanB);System.out.println(beanB.getA());}}

src\main\resources\cycle.xml：

<?xml version="1.0" encoding="UTF-8"?>
<beansxmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:context="http://www.springframework.org/schema/context"xmlns:mvc="http://www.springframework.org/schema/mvc"xmlns:p="http://www.springframework.org/schema/p"xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsdhttp://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsdhttp://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd"
><bean id="a" class="com.hans.bean_dependency_cycle.hans.A"><property name="b" ref="b"></property></bean><bean id="b" class="com.hans.bean_dependency_cycle.hans.B"><property name="a" ref="a"></property></bean>
</beans>

A.java和B.java：

// A 和 B 不能都使用 lombok，否则无法打印。这里选择了 B 不使用 lombok
package com.hans.bean_dependency_cycle.hans;import lombok.Data;@Data
public class A {private B b;public String toString(A o) {return o.getClass().getName() + "@" +Integer.toHexString(System.identityHashCode(o));}}package com.hans.bean_dependency_cycle.hans;public class B {private A a;public A getA() {return this.a;}public void setA(A a) {this.a = a;}}

期望输出：

A(b=com.hans.bean_dependency_cycle.hans.B@750e2b97)
com.hans.bean_dependency_cycle.hans.B@750e2b97
com.hans.bean_dependency_cycle.hans.B@750e2b97
A(b=com.hans.bean_dependency_cycle.hans.B@750e2b97)

场景1不需要配置spring.main.allow-circular-references为true也能得到期望输出，TODO：原因未知。

场景2：通过注解+自动装配属性来构造循环引用：以`@Controller`为例

入口src\main\java\com\hans\bean_dependency_cycle\hans\AnotherEntry.java：

package com.hans.bean_dependency_cycle.hans;import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
import org.springframework.context.ConfigurableApplicationContext;@SpringBootApplication(exclude = { DataSourceAutoConfiguration.class })
public class AnotherEntry {public static void main(String[] args) {ConfigurableApplicationContext cac = SpringApplication.run(AnotherEntry.class, args);ControllerA beanA = cac.getBean(ControllerA.class);System.out.println(beanA);System.out.println(beanA.getCb());ControllerB beanB = cac.getBean(ControllerB.class);System.out.println(beanB);System.out.println(beanB.getCa());}
}

根据参考链接1，接下来一定要记得修改application.yml：

spring:main:allow-circular-references: true

否则会报错：

***************************
APPLICATION FAILED TO START
***************************Description:The dependencies of some of the beans in the application context form a cycle:┌─────┐
|  controllerA (field private com.hans.bean_dependency_cycle.hans.ControllerB com.hansn_dependency_cycle.hans.ControllerA.cb)
↑     ↓
|  controllerB (field private com.hans.bean_dependency_cycle.hans.ControllerA com.hansn_dependency_cycle.hans.ControllerB.ca)
└─────┘Action:Relying upon circular references is discouraged and they are prohibited by default. Up your application to remove the dependency cycle between beans. As a last resort, it me possible to break the cycle automatically by setting spring.main.allow-circular-refees to true.

两个普通的Controller：

// src\main\java\com\hans\bean_dependency_cycle\hans\ControllerA.java
package com.hans.bean_dependency_cycle.hans;import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;@RestController
public class ControllerA {@Autowiredprivate ControllerB cb;public ControllerB getCb() {return cb;}@RequestMapping("/controllerA")public String index() {return "hello controllerA!";}
}// src\main\java\com\hans\bean_dependency_cycle\hans\ControllerB.java
package com.hans.bean_dependency_cycle.hans;import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;@RestController
public class ControllerB {@Autowiredprivate ControllerA ca;public ControllerA getCa() {return ca;}@RequestMapping("/controllerB")public String index() {return "hello controllerB!";}
}

期望输出：

com.hans.bean_dependency_cycle.hans.ControllerA@39f82681
com.hans.bean_dependency_cycle.hans.ControllerB@4bd9e7fd
com.hans.bean_dependency_cycle.hans.ControllerB@4bd9e7fd
com.hans.bean_dependency_cycle.hans.ControllerA@39f82681

脉络

根据参考链接2，有一个经验：“do”开头的方法名是真正含有大量逻辑的方法。参考链接2~~Java之父精选的~~脉络函数如下：

getBean
doGetBean
createBean
doCreateBean
createBeanInstance
populateBean

场景1到递归点为止的调用链：preInstantiateSingletons > getBean > doGetBean > getSingleton函数有多个，其中调用了beforeSingletonCreation的函数调用singletonFactory.getObject()时才真正调用了createBean > createBean > doCreateBean > createBeanInstance, populateBean 都在 doCreateBean 的实现里 > populateBean调用了applyPropertyValues > resolveValueIfNecessary > resolveReference > getBean。

场景2的调用链到populateBean开始和场景1的调用链岔开，两个场景的差异放在后文《Controller和普通Bean解决循环依赖过程的相同点与不同点》分析。

场景1递归到A -> B -> A时的调用链：getBean > doGetBean > 未调用beforeSingletonCreation的getSingleton，操作第2级缓存后离开。

操作三级缓存的函数都位于https://github.com/spring-projects/spring-framework/blob/502997d8e986dcfde1f49b2b2f443a32b5488b13/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java：

放入第3级缓存的函数：doCreateBean调用的addSingletonFactory。
放入第2级缓存的函数：首先要知道getSingleton在DefaultSingletonBeanRegistry.java里本质上的实现有两个，一个public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory)调用了beforeSingletonCreation并间接调用了createBean；另一个protected Object getSingleton(String beanName, boolean allowEarlyReference)则是操作了第2级缓存。
放入第1级缓存的函数：addSingleton。在间接调用了createBean函数的getSingleton处调用。

场景1的执行过程

真正意义上的入口：spring-context/src/main/java/org/springframework/context/support/AbstractApplicationContext.java的beanFactory.preInstantiateSingletons();。这里的beanFactory就有singletonObjects那3级缓存的对象。于是跳到https://github.com/spring-projects/spring-framework/blob/bbde68c49e66c3c531920cb80a55742262507be7/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultListableBeanFactory.java：

	@Overridepublic void preInstantiateSingletons() throws BeansException {if (logger.isTraceEnabled()) {logger.trace("Pre-instantiating singletons in " + this);}// Iterate over a copy to allow for init methods which in turn register new bean definitions.// While this may not be part of the regular factory bootstrap, it does otherwise work fine.List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);// 触发所有非延迟加载的（non-lazy）单例 bean 的初始化// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) {// 假如先遍历 A 再遍历 B 那么遍历到 B 的时候，因为循环引用解决的关系，B 已经放到了第1级缓存，所以 doGetBean 的 getSingleton 可以直接从第1级缓存取到值，不用再走一遍 createBean 方法// 合并父类 BeanDefinitionRootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);// 非抽象、是单例、非懒加载if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {// 如果实现了 FactoryBean 接口则是 FactoryBeanif (isFactoryBean(beanName)) {Object bean = getBean(FACTORY_BEAN_PREFIX + beanName); // 比如：FACTORY_BEAN_PREFIX + beanName = "&A"if (bean instanceof SmartFactoryBean<?> smartFactoryBean && smartFactoryBean.isEagerInit()) {getBean(beanName);}}else {// 不是 FactoryBean，只是普通 Bean，则走这个分支getBean(beanName);}}}// 触发所有 SmartInitializingSingleton 的后初始化回调// Trigger post-initialization callback for all applicable beans...for (String beanName : beanNames) {Object singletonInstance = getSingleton(beanName);if (singletonInstance instanceof SmartInitializingSingleton smartSingleton) {StartupStep smartInitialize = getApplicationStartup().start("spring.beans.smart-initialize").tag("beanName", beanName);smartSingleton.afterSingletonsInstantiated();smartInitialize.end();}}}

顺便看下isFactoryBean的实现https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractBeanFactory.java：

	@Overridepublic boolean isFactoryBean(String name) throws NoSuchBeanDefinitionException {String beanName = transformedBeanName(name);Object beanInstance = getSingleton(beanName, false);if (beanInstance != null) {return (beanInstance instanceof FactoryBean);}// No singleton instance found -> check bean definition.if (!containsBeanDefinition(beanName) && getParentBeanFactory() instanceof ConfigurableBeanFactory cbf) {// No bean definition found in this factory -> delegate to parent.return cbf.isFactoryBean(name);}return isFactoryBean(beanName, getMergedLocalBeanDefinition(beanName));}

getBean和isFactoryBean都位于https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractBeanFactory.java，getBean只有1行

	@Overridepublic Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);}

doGetBean和getBean, isFactoryBean都在AbstractBeanFactory.java：

	/*** Return an instance, which may be shared or independent, of the specified bean.* @param name the name of the bean to retrieve* @param requiredType the required type of the bean to retrieve* @param args arguments to use when creating a bean instance using explicit arguments* (only applied when creating a new instance as opposed to retrieving an existing one)* @param typeCheckOnly whether the instance is obtained for a type check,* not for actual use* @return an instance of the bean* @throws BeansException if the bean could not be created*/@SuppressWarnings("unchecked")protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)throws BeansException {String beanName = transformedBeanName(name);Object beanInstance;// 提前检查单例缓存中是否有手动注册的单例对象，跟循环依赖有关。如果是初次进这里，比如 controllerA 初次进这个方法，肯定是拿不到值的，就会是 null// 对于最简单的循环依赖，controllerA -> controllerB -> controllerA 之后，需要进入 getSingleton 了，这里的逻辑就是要从第3级缓存里拿到工厂函数，调用后得到 controllerA 半成品，从而可以直接 return// Eagerly check singleton cache for manually registered singletons.Object sharedInstance = getSingleton(beanName);if (sharedInstance != null && args == null) {if (logger.isTraceEnabled()) {if (isSingletonCurrentlyInCreation(beanName)) {logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +"' that is not fully initialized yet - a consequence of a circular reference");}else {logger.trace("Returning cached instance of singleton bean '" + beanName + "'");}}beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);}else {// Fail if we're already creating this bean instance:// We're assumably within a circular reference.if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// Check if bean definition exists in this factory.BeanFactory parentBeanFactory = getParentBeanFactory();if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// Not found -> check parent.String nameToLookup = originalBeanName(name);if (parentBeanFactory instanceof AbstractBeanFactory abf) {return abf.doGetBean(nameToLookup, requiredType, args, typeCheckOnly);}else if (args != null) {// Delegation to parent with explicit args.return (T) parentBeanFactory.getBean(nameToLookup, args);}else if (requiredType != null) {// No args -> delegate to standard getBean method.return parentBeanFactory.getBean(nameToLookup, requiredType);}else {return (T) parentBeanFactory.getBean(nameToLookup);}}if (!typeCheckOnly) {markBeanAsCreated(beanName);}StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate").tag("beanName", name);try {if (requiredType != null) {beanCreation.tag("beanType", requiredType::toString);}RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);// Guarantee initialization of beans that the current bean depends on.String[] dependsOn = mbd.getDependsOn();if (dependsOn != null) {for (String dep : dependsOn) {if (isDependent(beanName, dep)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");}registerDependentBean(dep, beanName);try {getBean(dep);}catch (NoSuchBeanDefinitionException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"'" + beanName + "' depends on missing bean '" + dep + "'", ex);}}}// Create bean instance.if (mbd.isSingleton()) {// 返回 beanName 的原始单例对象。如果尚未注册，则使用 singletonFactory 创建并注册一个对象sharedInstance = getSingleton(beanName, () -> {try {// 为给定的合并后的 BeanDefinition 和参数创建一个 bean 实例// createBean 真实执行时机是调用了 beforeSingletonCreation 方法的 getSingleton 方法执行 singletonObject = singletonFactory.getObject() 时// 首次运行到 beanName = "controllerA" 时 args = nullreturn createBean(beanName, mbd, args);}catch (BeansException ex) {// Explicitly remove instance from singleton cache: It might have been put there// eagerly by the creation process, to allow for circular reference resolution.// Also remove any beans that received a temporary reference to the bean.// 显式地从单例缓存中删除bean实例：因为这个实例可能是由创建过程急切地放在那里的，放在那里的目的是允许循环引用解析。// 还要删除所有被这个bean临时引用的所有bean。如果找到相应的一次性bean实例，则委托给 destroyBeandestroySingleton(beanName);throw ex;}});beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}else if (mbd.isPrototype()) {// It's a prototype -> create a new instance.Object prototypeInstance = null;try {beforePrototypeCreation(beanName);prototypeInstance = createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}else {String scopeName = mbd.getScope();if (!StringUtils.hasLength(scopeName)) {throw new IllegalStateException("No scope name defined for bean '" + beanName + "'");}Scope scope = this.scopes.get(scopeName);if (scope == null) {throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");}try {Object scopedInstance = scope.get(beanName, () -> {beforePrototypeCreation(beanName);try {return createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}});beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);}catch (IllegalStateException ex) {throw new ScopeNotActiveException(beanName, scopeName, ex);}}}catch (BeansException ex) {beanCreation.tag("exception", ex.getClass().toString());beanCreation.tag("message", String.valueOf(ex.getMessage()));cleanupAfterBeanCreationFailure(beanName);throw ex;}finally {beanCreation.end();}}return adaptBeanInstance(name, beanInstance, requiredType);}

我们先只关注createBean，所以需要关注getSingleton。注意：

getSingleton的函数在DefaultSingletonBeanRegistry.java里本质上的实现有两个，一个调用了beforeSingletonCreation并间接调用了createBean；另一个则是操作了第2级缓存。

所以目前我们需要关注的是调用了beforeSingletonCreation的getSingleton方法。路径：https://github.com/spring-projects/spring-framework/blob/502997d8e986dcfde1f49b2b2f443a32b5488b13/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java

	/*** Return the (raw) singleton object registered under the given name,* creating and registering a new one if none registered yet.* @param beanName the name of the bean* @param singletonFactory the ObjectFactory to lazily create the singleton* with, if necessary* @return the registered singleton object*/public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(beanName, "Bean name must not be null");synchronized (this.singletonObjects) {Object singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {if (this.singletonsCurrentlyInDestruction) {throw new BeanCreationNotAllowedException(beanName,"Singleton bean creation not allowed while singletons of this factory are in destruction " +"(Do not request a bean from a BeanFactory in a destroy method implementation!)");}if (logger.isDebugEnabled()) {logger.debug("Creating shared instance of singleton bean '" + beanName + "'");}// 创建单例前的回调，默认实现为：将单例注册为当前正在创建中，实现只有3行，可以看下。beforeSingletonCreation(beanName);// flag 表示是否生成了新的单例对象boolean newSingleton = false;// flag 表示是否有抑制异常的记录，true表示没有boolean recordSuppressedExceptions = (this.suppressedExceptions == null);if (recordSuppressedExceptions) {// 若没有抑制异常记录，则对抑制异常列表进行初始化this.suppressedExceptions = new LinkedHashSet<>();}try {// 从单例工厂获取对象。注意 singletonFactory 是本方法第二个参数，之前也介绍了// ObjectFactory 对象通过调 getObject 来正式执行函数，所以 createBean 在此时才真正执行singletonObject = singletonFactory.getObject();// 获取后，就已经生成了新的单例对象，标记为 truenewSingleton = true;}catch (IllegalStateException ex) {// Has the singleton object implicitly appeared in the meantime ->// if yes, proceed with it since the exception indicates that state.singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {throw ex;}}catch (BeanCreationException ex) {if (recordSuppressedExceptions) {for (Exception suppressedException : this.suppressedExceptions) {ex.addRelatedCause(suppressedException);}}throw ex;}finally {if (recordSuppressedExceptions) {this.suppressedExceptions = null;}afterSingletonCreation(beanName);}if (newSingleton) {// 操作第1级缓存addSingleton(beanName, singletonObject);}}return singletonObject;}}

在调用singletonObject = singletonFactory.getObject();以间接调用createBean后，会调用addSingleton，将bean加入第1级缓存，这标志着bean变为成品。接下来我们看下addSingleton的代码，它和getSingleton定义于同一个文件。

	/*** Add the given singleton object to the singleton cache of this factory.* <p>To be called for eager registration of singletons.* @param beanName the name of the bean* @param singletonObject the singleton object*/protected void addSingleton(String beanName, Object singletonObject) {// addSingleton 在调用了 createBean 的 getSingleton 方法中被调用，标志着 bean 变为成品对象synchronized (this.singletonObjects) {// 第1级缓存添加，第2、3级缓存移除this.singletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);this.earlySingletonObjects.remove(beanName);// 添加到已注册的单例集合。 registeredSingletons 为 Set<String>this.registeredSingletons.add(beanName);}}

接下来看createBean。https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java

	/*** Central method of this class: creates a bean instance,* populates the bean instance, applies post-processors, etc.* @see #doCreateBean*/@Overrideprotected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {// 首次看源码，直接看 doCreateBean 调用处if (logger.isTraceEnabled()) {logger.trace("Creating instance of bean '" + beanName + "'");}RootBeanDefinition mbdToUse = mbd;// Make sure bean class is actually resolved at this point, and// clone the bean definition in case of a dynamically resolved Class// which cannot be stored in the shared merged bean definition.Class<?> resolvedClass = resolveBeanClass(mbd, beanName);if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {mbdToUse = new RootBeanDefinition(mbd);mbdToUse.setBeanClass(resolvedClass);}// Prepare method overrides.try {mbdToUse.prepareMethodOverrides();}catch (BeanDefinitionValidationException ex) {throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),beanName, "Validation of method overrides failed", ex);}try {// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.Object bean = resolveBeforeInstantiation(beanName, mbdToUse);if (bean != null) {return bean;}}catch (Throwable ex) {throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,"BeanPostProcessor before instantiation of bean failed", ex);}try {// 实际创建 beanObject beanInstance = doCreateBean(beanName, mbdToUse, args);if (logger.isTraceEnabled()) {logger.trace("Finished creating instance of bean '" + beanName + "'");}return beanInstance;}catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {// A previously detected exception with proper bean creation context already,// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.throw ex;}catch (Throwable ex) {throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);}}

createBean目前唯一值得关注的点就是调用了doCreateBean。doCreateBean做了几件值得本文关注的事：

调用了createBeanInstance完成bean的实例化。
调用了addSingletonFactory，即加入了第3级缓存。
调用了populateBean完成bean的属性赋值操作。

doCreateBean和createBean都定义在https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java。

	/*** Actually create the specified bean. Pre-creation processing has already happened* at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.* <p>Differentiates between default bean instantiation, use of a* factory method, and autowiring a constructor.* @param beanName the name of the bean* @param mbd the merged bean definition for the bean* @param args explicit arguments to use for constructor or factory method invocation* @return a new instance of the bean* @throws BeanCreationException if the bean could not be created* @see #instantiateBean* @see #instantiateUsingFactoryMethod* @see #autowireConstructor*/protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {// Instantiate the bean.// instanceWrapper 持有创建出的 bean 对象BeanWrapper instanceWrapper = null;// 获取 factoryBean 实例缓存if (mbd.isSingleton()) {// 如果是单例对象，从 factoryBean 实例缓存中移除当前 bean 的信息// 首次 controllerA 进来会进行移除操作，并且会调用 createBeanInstanceinstanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}// 没有就创建实例if (instanceWrapper == null) {// 根据执行 bean 使用对应的策略创建新的工厂实例，如：工厂方法、构造函数主动注入、简单初始化// 第一次读源码时不需要点进去看 createBeanInstance 。下一个主干方法 populateBean 是在本函数下文调用的instanceWrapper = createBeanInstance(beanName, mbd, args);}// 从包装类中获取原始 bean 。首次执行 controllerA 的时候, bean 就是普通的 controllerA 对象 ControllerA@100{cb=null}Object bean = instanceWrapper.getWrappedInstance();// 获取具体 bean 对象的 Class 属性Class<?> beanType = instanceWrapper.getWrappedClass();if (beanType != NullBean.class) {// 不等于 NullBean 类型时就修改目标类型mbd.resolvedTargetType = beanType;}// Allow post-processors to modify the merged bean definition.synchronized (mbd.postProcessingLock) {if (!mbd.postProcessed) {try {applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);}catch (Throwable ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Post-processing of merged bean definition failed", ex);}mbd.markAsPostProcessed();}}// Eagerly cache singletons to be able to resolve circular references// even when triggered by lifecycle interfaces like BeanFactoryAware.// 判断当前 bean 是否需要提前曝光，条件为：是单例 && 允许循环依赖 spring.main.allow-circular-references 配置为 true && 当前 bean 正在创建中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");}// 为避免后期循环依赖，可以在 bean 初始化完成前将创建实例的 ObjectFactory 加入工厂// 注意，这个方法会操作3级缓存的数据结构，尤其是第3级缓存。在 controllerA -> controllerB -> controllerA 的时候未调用 createBean 的 getSingleton 方法会真正调用这个匿名函数，从而调用 getEarlyBeanReference 进而操作第2级缓存addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));}// Initialize the bean instance.// controllerA 首次执行到这里时，exposedObject = ControllerA@100{cb=null}Object exposedObject = bean;try {// populateBean 是主干方法，给刚刚实例化的 bean （半成品）填充属性populateBean(beanName, mbd, instanceWrapper);// controllerA -> controllerB -> controllerA 之后，getSingleton 返回 controllerA 后回到这里， controllerB 的 ca 属性就有值了// 随后递归返回到这句话且调用栈只有 controllerA 的时候，发现两者都有值了exposedObject = initializeBean(beanName, exposedObject, mbd);}catch (Throwable ex) {if (ex instanceof BeanCreationException bce && beanName.equals(bce.getBeanName())) {throw bce;}else {throw new BeanCreationException(mbd.getResourceDescription(), beanName, ex.getMessage(), ex);}}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.");}}}}// Register bean as disposable.try {registerDisposableBeanIfNecessary(beanName, bean, mbd);}catch (BeanDefinitionValidationException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);}return exposedObject;}

createBeanInstance的细节与本文主题无关，不关注。接下来我们看下addSingletonFactory的实现。addSingletonFactory位于https://github.com/spring-projects/spring-framework/blob/502997d8e986dcfde1f49b2b2f443a32b5488b13/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java，主要动作是操作第3级缓存。

	/*** Add the given singleton factory for building the specified singleton* if necessary.* <p>To be called for eager registration of singletons, e.g. to be able to* resolve circular references.* @param beanName the name of the bean* @param singletonFactory the factory for the singleton object*/protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(singletonFactory, "Singleton factory must not be null");synchronized (this.singletonObjects) {// controllerA 首次执行到这里，第一级缓存肯定是查不到的if (!this.singletonObjects.containsKey(beanName)) {// 第3级缓存放入。再回顾一下， singletonFactory存的是 beanName 到一个延迟执行的函数的映射// controllerA 首次执行到这里的时候， singletonFactory = () -> getEarlyBeanReference(beanName, mbd, bean)this.singletonFactories.put(beanName, singletonFactory);// 从早期单例对象的高速缓存（即第2级缓存）移除当前 beanName 对应的缓存对象this.earlySingletonObjects.remove(beanName);// 添加到已注册的单例集合里，和三级缓存无关。值得注意的是， A 首次加入三级缓存时，就是首次加入已注册的单例集合this.registeredSingletons.add(beanName);}}}

接下来关注位于https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java的populateBean。populateBean方法有一个功能是给bean的属性赋值，包含了递归点。在创建Bean的源码中，递归点指的是递归调用getBean方法。

	/*** Populate the bean instance in the given BeanWrapper with the property values* from the bean definition.* @param beanName the name of the bean* @param mbd the bean definition for the bean* @param bw the BeanWrapper with bean instance*/protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {if (bw == null) {if (mbd.hasPropertyValues()) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");}else {// Skip property population phase for null instance.return;}}if (bw.getWrappedClass().isRecord()) {if (mbd.hasPropertyValues()) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Cannot apply property values to a record");}else {// Skip property population phase for records since they are immutable.return;}}// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the// state of the bean before properties are set. This can be used, for example,// to support styles of field injection.if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {return;}}}PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);int resolvedAutowireMode = mbd.getResolvedAutowireMode();if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {MutablePropertyValues newPvs = new MutablePropertyValues(pvs);// Add property values based on autowire by name if applicable.if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {autowireByName(beanName, mbd, bw, newPvs);}// Add property values based on autowire by type if applicable.if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {autowireByType(beanName, mbd, bw, newPvs);}pvs = newPvs;}if (hasInstantiationAwareBeanPostProcessors()) {if (pvs == null) {pvs = mbd.getPropertyValues();}for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {// 对于 Controller Bean 自动装配属性产生循环依赖的场景，遍历到 AutowiredAnnotationBeanPostProcessor 时，这句话包含递归点PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {return;}pvs = pvsToUse;}}boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);if (needsDepCheck) {PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);checkDependencies(beanName, mbd, filteredPds, pvs);}if (pvs != null) {// 对于普通 Bean 的场景，这句话包含递归点applyPropertyValues(beanName, mbd, bw, pvs);}}

applyPropertyValues也位于https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java，主要功能为完成bean初始化。其中，Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue)包含了递归点，通过动调可以验证bw.setPropertyValues(new MutablePropertyValues(deepCopy))真正完成了属性赋值工作。

	/*** Apply the given property values, resolving any runtime references* to other beans in this bean factory. Must use deep copy, so we* don't permanently modify this property.* @param beanName the bean name passed for better exception information* @param mbd the merged bean definition* @param bw the BeanWrapper wrapping the target object* @param pvs the new property values*/protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {// applyPropertyValues 是真正完成赋值操作的函数// 如果 pvs 没有 PropertyValue，就直接结束if (pvs.isEmpty()) {return;}MutablePropertyValues mpvs = null;List<PropertyValue> original;if (pvs instanceof MutablePropertyValues _mpvs) {mpvs = _mpvs;if (mpvs.isConverted()) {// Shortcut: use the pre-converted values as-is.try {bw.setPropertyValues(mpvs);return;}catch (BeansException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Error setting property values", ex);}}// 获取 mpvs 的 PropertyValue 列表original = mpvs.getPropertyValueList();}else {// 获取 pvs 的 PropertyValue 对象数组并转为列表original = Arrays.asList(pvs.getPropertyValues());}// 用户自定义的类型转换器，默认转换器为 bean 的包装类对象TypeConverter converter = getCustomTypeConverter();if (converter == null) {converter = bw;}BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);// Create a deep copy, resolving any references for values.List<PropertyValue> deepCopy = new ArrayList<>(original.size());// resolveNecessary flag 含义为是否还需要解析boolean resolveNecessary = false;for (PropertyValue pv : original) {// 属性已解析过则加入 deepCopyif (pv.isConverted()) {deepCopy.add(pv);}else {String propertyName = pv.getName();// 获取未经类型转换的值Object originalValue = pv.getValue();if (originalValue == AutowiredPropertyMarker.INSTANCE) {Method writeMethod = bw.getPropertyDescriptor(propertyName).getWriteMethod();// 如果 setter 方法为空if (writeMethod == null) {// 异常：自动装配标记属性没有写方法throw new IllegalArgumentException("Autowire marker for property without write method: " + pv);}originalValue = new DependencyDescriptor(new MethodParameter(writeMethod, 0), true);}// 交由 valueResolver 根据 pv 解析出 originalValue 所封装的对象。注意：这个函数包含递归点Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);Object convertedValue = resolvedValue;// flag 含义为是否可转换： propertyName 是 bw 中的可写属性 && propertyName 不是索引属性或嵌套属性boolean convertible = bw.isWritableProperty(propertyName) &&!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);if (convertible) {// 可转换则将 resolvedValue 转换为指定的目标属性对象convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);}// Possibly store converted value in merged bean definition,// in order to avoid re-conversion for every created bean instance.if (resolvedValue == originalValue) {if (convertible) {pv.setConvertedValue(convertedValue);}deepCopy.add(pv);}else if (convertible && originalValue instanceof TypedStringValue typedStringValue &&!typedStringValue.isDynamic() &&!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {pv.setConvertedValue(convertedValue);deepCopy.add(pv);}else {resolveNecessary = true;deepCopy.add(new PropertyValue(pv, convertedValue));}}}if (mpvs != null && !resolveNecessary) {mpvs.setConverted();}// Set our (possibly massaged) deep copy.try {// 完成属性赋值工作。咱们做个简单的实验，动调执行此句前后各点击调用栈看一次 populateBean 调用处的下一句的 exposedObject 或者 bean 变量，这就证实了 bean 的属性赋值确实是在这句话完成的bw.setPropertyValues(new MutablePropertyValues(deepCopy));}catch (BeansException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, ex.getMessage(), ex);}}

resolveValueIfNecessary位于https://github.com/spring-projects/spring-framework/blob/30d6ec3398ce41add7bc44d360b8fb86ac0264b1/spring-beans/src/main/java/org/springframework/beans/factory/support/BeanDefinitionValueResolver.java。其实现也很长，但目前只需要关注return resolveReference(argName, ref)，因为resolveReference包含递归点。

	/*** Given a PropertyValue, return a value, resolving any references to other* beans in the factory if necessary. The value could be:* <li>A BeanDefinition, which leads to the creation of a corresponding* new bean instance. Singleton flags and names of such "inner beans"* are always ignored: Inner beans are anonymous prototypes.* <li>A RuntimeBeanReference, which must be resolved.* <li>A ManagedList. This is a special collection that may contain* RuntimeBeanReferences or Collections that will need to be resolved.* <li>A ManagedSet. May also contain RuntimeBeanReferences or* Collections that will need to be resolved.* <li>A ManagedMap. In this case the value may be a RuntimeBeanReference* or Collection that will need to be resolved.* <li>An ordinary object or {@code null}, in which case it's left alone.* @param argName the name of the argument that the value is defined for* @param value the value object to resolve* @return the resolved object*/@Nullablepublic Object resolveValueIfNecessary(Object argName, @Nullable Object value) {// We must check each value to see whether it requires a runtime reference// to another bean to be resolved.// RuntimeBeanReference：当属性值对象是工厂中另一个 bean 的引用时，使用不可变的占位符类，在运行时进行解析if (value instanceof RuntimeBeanReference ref) {// 解析出对应 ref 所封装的 Bean 的元信息的 Bean 对象。Bean 的元信息：Bean 名，Bean类型。注意，这个函数里包含递归点return resolveReference(argName, ref);}else if (value instanceof RuntimeBeanNameReference ref) {String refName = ref.getBeanName();refName = String.valueOf(doEvaluate(refName));if (!this.beanFactory.containsBean(refName)) {throw new BeanDefinitionStoreException("Invalid bean name '" + refName + "' in bean reference for " + argName);}return refName;}else if (value instanceof BeanDefinitionHolder bdHolder) {// Resolve BeanDefinitionHolder: contains BeanDefinition with name and aliases.return resolveInnerBean(bdHolder.getBeanName(), bdHolder.getBeanDefinition(),(name, mbd) -> resolveInnerBeanValue(argName, name, mbd));}else if (value instanceof BeanDefinition bd) {return resolveInnerBean(null, bd,(name, mbd) -> resolveInnerBeanValue(argName, name, mbd));}else if (value instanceof DependencyDescriptor dependencyDescriptor) {Set<String> autowiredBeanNames = new LinkedHashSet<>(2);Object result = this.beanFactory.resolveDependency(dependencyDescriptor, this.beanName, autowiredBeanNames, this.typeConverter);for (String autowiredBeanName : autowiredBeanNames) {if (this.beanFactory.containsBean(autowiredBeanName)) {this.beanFactory.registerDependentBean(autowiredBeanName, this.beanName);}}return result;}else if (value instanceof ManagedArray managedArray) {// May need to resolve contained runtime references.Class<?> elementType = managedArray.resolvedElementType;if (elementType == null) {String elementTypeName = managedArray.getElementTypeName();if (StringUtils.hasText(elementTypeName)) {try {elementType = ClassUtils.forName(elementTypeName, this.beanFactory.getBeanClassLoader());managedArray.resolvedElementType = elementType;}catch (Throwable ex) {// Improve the message by showing the context.throw new BeanCreationException(this.beanDefinition.getResourceDescription(), this.beanName,"Error resolving array type for " + argName, ex);}}else {elementType = Object.class;}}return resolveManagedArray(argName, (List<?>) value, elementType);}else if (value instanceof ManagedList<?> managedList) {// May need to resolve contained runtime references.return resolveManagedList(argName, managedList);}else if (value instanceof ManagedSet<?> managedSet) {// May need to resolve contained runtime references.return resolveManagedSet(argName, managedSet);}else if (value instanceof ManagedMap<?, ?> managedMap) {// May need to resolve contained runtime references.return resolveManagedMap(argName, managedMap);}else if (value instanceof ManagedProperties original) {// Properties original = managedProperties;Properties copy = new Properties();original.forEach((propKey, propValue) -> {if (propKey instanceof TypedStringValue typedStringValue) {propKey = evaluate(typedStringValue);}if (propValue instanceof TypedStringValue typedStringValue) {propValue = evaluate(typedStringValue);}if (propKey == null || propValue == null) {throw new BeanCreationException(this.beanDefinition.getResourceDescription(), this.beanName,"Error converting Properties key/value pair for " + argName + ": resolved to null");}copy.put(propKey, propValue);});return copy;}else if (value instanceof TypedStringValue typedStringValue) {// Convert value to target type here.Object valueObject = evaluate(typedStringValue);try {Class<?> resolvedTargetType = resolveTargetType(typedStringValue);if (resolvedTargetType != null) {return this.typeConverter.convertIfNecessary(valueObject, resolvedTargetType);}else {return valueObject;}}catch (Throwable ex) {// Improve the message by showing the context.throw new BeanCreationException(this.beanDefinition.getResourceDescription(), this.beanName,"Error converting typed String value for " + argName, ex);}}else if (value instanceof NullBean) {return null;}else {return evaluate(value);}}

resolveReference和resolveValueIfNecessary都位于https://github.com/spring-projects/spring-framework/blob/30d6ec3398ce41add7bc44d360b8fb86ac0264b1/spring-beans/src/main/java/org/springframework/beans/factory/support/BeanDefinitionValueResolver.java。其实现也很长，但目前只需要关注bean = this.beanFactory.getBean(resolvedName)，因为这就是递归点。

	/*** Resolve a reference to another bean in the factory.*/@Nullableprivate Object resolveReference(Object argName, RuntimeBeanReference ref) {try {Object bean;Class<?> beanType = ref.getBeanType();// 如果引用来自父工厂if (ref.isToParent()) {// 获取父工厂BeanFactory parent = this.beanFactory.getParentBeanFactory();if (parent == null) {// 没有父工厂则报错：在父工厂中无法解析对 Bean 的引用，因为父工厂就不存在throw new BeanCreationException(this.beanDefinition.getResourceDescription(), this.beanName,"Cannot resolve reference to bean " + ref +" in parent factory: no parent factory available");}if (beanType != null) {bean = parent.getBean(beanType);}else {bean = parent.getBean(String.valueOf(doEvaluate(ref.getBeanName())));}}else {String resolvedName;if (beanType != null) {// 解析与 beanType 唯一匹配的 bean 实例，包括其 bean 名NamedBeanHolder<?> namedBean = this.beanFactory.resolveNamedBean(beanType);// 让 bean 引用 namedBean 所封装的 bean 对象bean = namedBean.getBeanInstance();resolvedName = namedBean.getBeanName();}else {resolvedName = String.valueOf(doEvaluate(ref.getBeanName()));// 获取 ref 所包装的 Bean 名对应的 Bean 对象// 注意，这就是递归点了bean = this.beanFactory.getBean(resolvedName);}// 注册依赖关系到 Bean 工厂this.beanFactory.registerDependentBean(resolvedName, this.beanName);}if (bean instanceof NullBean) {bean = null;}return bean;}catch (BeansException ex) {throw new BeanCreationException(this.beanDefinition.getResourceDescription(), this.beanName,"Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);}}

B创建过程调用栈是完全一样的，接下来我们假设现在走到了A -> B -> A，回到了doGetBean的Object sharedInstance = getSingleton(beanName);处。此时我们需要关注其实现了：

	/*** Return the (raw) singleton object registered under the given name.* <p>Checks already instantiated singletons and also allows for an early* reference to a currently created singleton (resolving a circular reference).* @param beanName the name of the bean to look for* @param allowEarlyReference whether early references should be created or not* @return the registered singleton object, or {@code null} if none found*/@Nullableprotected Object getSingleton(String beanName, boolean allowEarlyReference) {// Quick check for existing instance without full singleton lock// 该函数主要是调用放入第3级缓存的 getEarlyBeanReference 并放入第2级缓存// 首先从第1级缓存获取 beanObject singletonObject = this.singletonObjects.get(beanName);// 第1级缓存没有，并且已标记为创建中if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {// 从第2级缓存获取 bean.因为controllerA -> controllerB -> controllerA 的时候是创建中的 bean,只放到了第3级缓存,所以是查不到的singletonObject = this.earlySingletonObjects.get(beanName);// controllerA -> controllerB -> controllerA 的时候进来, allowEarlyReference 肯定是 trueif (singletonObject == null && allowEarlyReference) {synchronized (this.singletonObjects) {// Consistent creation of early reference within full singleton lock// 这段做二次确认的代码让我联想到线程安全的单例模式的写法// 从第1级缓存取singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {// 从第2级缓存取singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null) {// 从第3级缓存取。如果是 controllerA 初次进来，因为 not in creation 所以不会进这里，就算进了这里，因为第3级缓存取不到所以还是会直接 return。如果是 controllerA -> controllerB -> controllerA 则会操作第2级缓存ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);if (singletonFactory != null) {// 回顾一下，DefaultSingletonBeanRegistry.java 的 addSingletonFactory 函数在操作第3级缓存的时候，放入的匿名函数就是：// singletonFactory = () -> getEarlyBeanReference(beanName, mbd, bean) 所以 getEarlyBeanReference 返回值会在此被放入第2级缓存singletonObject = singletonFactory.getObject();// 为什么是放第2级缓存？因为 getBean 的递归还没返回// 放入第2级缓存后，三级缓存的就可以移除了this.earlySingletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);}}}}}}return singletonObject;}

A -> B -> A的递归返回后，A, B两个单例bean都已经是成品，beanName遍历到B的时候，进入getSingleton就能命中第1级缓存了，不用再走一遍createBean方法。

		for (String beanName : beanNames) {// 假如先遍历 A 再遍历 B 那么遍历到 B 的时候，因为循环引用解决的关系，B 已经放到了第1级缓存，所以 doGetBean 的 getSingleton 可以直接从第1级缓存取到值，不用再走一遍 createBean 方法// 合并父类 BeanDefinitionRootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);// 非抽象、是单例、非懒加载if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {// 如果实现了 FactoryBean 接口则是 FactoryBeanif (isFactoryBean(beanName)) {Object bean = getBean(FACTORY_BEAN_PREFIX + beanName); // 比如：FACTORY_BEAN_PREFIX + beanName = "&A"if (bean instanceof SmartFactoryBean<?> smartFactoryBean && smartFactoryBean.isEagerInit()) {getBean(beanName);}}else {// 不是 FactoryBean，只是普通 Bean，则走这个分支getBean(beanName);}}}

至此，场景1的递归点和三级缓存的操作时机都已经清楚了。

`@Controller`+自动装配属性和普通Bean解决循环依赖过程的相同点与不同点

场景2的Controller使用了@Autowired注解来构造循环依赖。动调可知，这个场景并不是在populateBean的applyPropertyValues(beanName, mbd, bw, pvs);完成修改的，而是在populateBean的：

			for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {return;}pvs = pvsToUse;}

这个循环里完成属性赋值的。动调发现getBeanPostProcessorCache().instantiationAware有一个元素是AutowiredAnnotationBeanPostProcessor@133（所有元素分别是ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor@129, InfrastructureAdvisorAutoProxyCreator@130, PersistenceExceptionTranslationPostProcessor@131, CommonAnnotationBeanPostProcessor@132, AutowiredAnnotationBeanPostProcessor@133），遍历到这个元素时执行的操作完成了自动装配属性的赋值。那我们跟进去看下。

AutowiredAnnotationBeanPostProcessor.postProcessProperties位于https://github.com/spring-projects/spring-framework/blob/6183f0684684912802021556dce916ba26228c26/spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java

	@Overridepublic PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);try {metadata.inject(bean, beanName, pvs);}catch (BeanCreationException ex) {throw ex;}catch (Throwable ex) {throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);}return pvs;}

显然是在metadata.inject处完成自动装配的。InjectionMetadata.inject位于https://github.com/spring-projects/spring-framework/blob/2f33e77ab49f136d83b6ebf5eeb72d200fe23c0b/spring-beans/src/main/java/org/springframework/beans/factory/annotation/InjectionMetadata.java

	public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {Collection<InjectedElement> checkedElements = this.checkedElements;Collection<InjectedElement> elementsToIterate =(checkedElements != null ? checkedElements : this.injectedElements);if (!elementsToIterate.isEmpty()) {for (InjectedElement element : elementsToIterate) {element.inject(target, beanName, pvs);}}}

element.inject最终跳入的是https://github.com/spring-projects/spring-framework/blob/6183f0684684912802021556dce916ba26228c26/spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java的protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable。AutowiredFieldElement, AutowiredMethodElement都有inject方法，显然这个case里我们调用的是AutowiredFieldElement的inject方法。

		@Overrideprotected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {Field field = (Field) this.member;Object value;if (this.cached) {try {value = resolveCachedArgument(beanName, this.cachedFieldValue);}catch (BeansException ex) {// Unexpected target bean mismatch for cached argument -> re-resolvethis.cached = false;logger.debug("Failed to resolve cached argument", ex);value = resolveFieldValue(field, bean, beanName);}}else {value = resolveFieldValue(field, bean, beanName);}if (value != null) {ReflectionUtils.makeAccessible(field);field.set(bean, value);}}

显然field.set(bean, value);最终完成了属性的自动装配。值得注意的是，动调看到resolveFieldValue获取到的cb是已经装配好的，这里一定存在递归调用。我们用一个简单的动态调试技巧来找到递归点：在执行到value = resolveFieldValue(field, bean, beanName)时，给doGetBean函数下一个临时的断点。得到的调用栈如下：

AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement.inject()
AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement.resolveFieldValue()的value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
DefaultListableBeanFactory.java的resolveDependency方法的result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
DefaultListableBeanFactory.java的doResolveDependency方法的instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
https://github.com/spring-projects/spring-framework/blob/f1fe16e3cda66b164f77489f82287116477197bc/spring-beans/src/main/java/org/springframework/beans/factory/config/DependencyDescriptor.java的resolveCandidate方法的return beanFactory.getBean(beanName);，这就是递归点了，beanName = controllerB。

总而言之，Controller的bean自动装配属性的场景和普通的bean的递归点不一样，但对三级缓存的操作逻辑是完全一致的。谜底已揭晓~

一些扩展结论

【1】为什么Spring不能解决构造器的循环依赖？

在doCreateBean调用createBeanInstance时，一二三级缓存都没有Bean的相关信息，在实例化之后才调用addSingletonFactory放入第3级缓存中，因此当getBean的时候缓存不会命中，所以会抛出循环依赖的异常。

【2】为什么多实例Bean不能解决循环依赖？

多实例Bean是每次创建都会调用doGetBean方法，mbd.isSingleton()是false，不走sharedInstance = getSingleton(beanName, () -> {这个分支，所以也不会使用三级缓存，不能解决循环依赖。

总结

根据常识猜测Spring创建Bean过程解决循环依赖的算法也是Map+记忆化搜索。所以我们可以先找到递归点，再去分析调用栈涉及的那些函数，顺便找出其用到的Map数据结构。
在不了解Spring框架的情况下可以用一个动态调试技巧快速找到递归点：在执行到某条顺序靠后的语句时，给顺序靠前的语句下一个断点，若下断成功，则说明找到了递归点。剩下的工作就是关注调用栈涉及的函数。

参考资料

痛快！SpringBoot终于禁掉了循环依赖！https://juejin.cn/post/7096798740593246222
https://www.bilibili.com/video/BV1J14y1A7eE/?p=82 ~~震惊！这位零基础前端同学在学习Java的第7天就可以研究Spring框架源码了！这是怎么做到的呢？噢！原来是站在了Java之父🐎士兵的肩膀上~~！