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本篇文章学习下spring循环依赖的源码与思想。
建议参照本文，跟踪源码进行学习。
1、什么是循环依赖
通过以下一个例子，我们来看下什么是循环依赖：
有两个类A和B，其代码分别如下所示：
/**
* 循环依赖A
* @date: 2020/12/24
* @author weirx
* @version 3.0
*/
public class A {
public B getB() {
return b;
}
public void setB(B b) {
this.b = b;
}
private B b;
}
复制代码/**
* 循环依赖B
* @date: 2020/12/24
* @author weirx
* @version 3.0
*/
public class B {
public A getA() {
return a;
}
public void setA(A a) {
this.a = a;
}
private A a;
}
复制代码A中有属性b，B中有属性a，相互依赖如下图所示：
循环依赖导致了什么问题？
在学习bean的生命周期时，bean在实例化后要经历初始化的过程，初始化需要对bean的属性进行赋值，而A的实例进行初始化的时候，B还没有没实例化，导致A的属性赋值失败。
2、spring解决方案
了解过相关内容的同学，一定知道spring解决循环依赖的方案是采用三级缓存，sping源码中有个重要的类DefaultSingletonBeanRegistry，在这里面定义了三级缓存，如下所示：
//一级缓存
/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
//三级缓存
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
//二级缓存
/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
复制代码其本质在于bean实例化和初始化分开处理，借助缓存实现了半成品（未经过初始化）的提前暴露。
这里面需要有一个点需要注意下，spring常用的注入方式有set方式和构造器方式，那么这两种方式能否解决循环依赖的问题呢？
main方法：
import org.springframework.context.support.ClassPathXmlApplicationContext;
/**
* main
* @date: 2020/12/23
* @author weirx
* @version 3.0
*/
public class TestSpring {
public static void main(String[] args) {
ClassPathXmlApplicationContext applicationContext
= new ClassPathXmlApplicationContext("spring-beans.xml");
}
}
复制代码set方式：
/**
* 循环依赖A
* @date: 2020/12/24
* @author weirx
* @version 3.0
*/
public class A {
public B getB() {
return b;
}
public void setB(B b) {
this.b = b;
}
private B b;
}
复制代码/**
* 循环依赖B
* @date: 2020/12/24
* @author weirx
* @version 3.0
*/
public class B {
public A getA() {
return a;
}
public void setA(A a) {
this.a = a;
}
private A a;
}
复制代码xml文件：
<bean id="a" class="com.demo.A">
<property name="b" ref="b"></property>
</bean>
<bean id="b" class="com.demo.B">
<property name="a" ref="a"></property>
</bean>
复制代码执行结果，经测试可以解决，得到的对象是循环依赖的:
构造器方式：
/**
* 循环依赖A
* @date: 2020/12/24
* @author weirx
* @version 3.0
*/
public class A {
private B b;
public A(B b) {
this.b = b;
}
}
复制代码/**
* 循环依赖B
* @date: 2020/12/24
* @author weirx
* @version 3.0
*/
public class B {
private A a;
public B(A a) {
this.a = a;
}
}
复制代码xml配置：
<bean id="a" class="com.demo.A">
<constructor-arg ref="b"></constructor-arg>
</bean>
<bean id="b" class="com.demo.B">
<constructor-arg ref="a"></constructor-arg>
</bean>
复制代码结果是不能解决，会得到以下报错信息：
十二月 25, 2020 9:38:00 上午 org.springframework.context.support.AbstractApplicationContext refresh
警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'a' defined in class path resource [spring-beans.xml]: Cannot resolve reference to bean 'b' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'b' defined in class path resource [spring-beans.xml]: Cannot resolve reference to bean 'a' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'a' defined in class path resource [spring-beans.xml]: Cannot resolve reference to bean 'b' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'b' defined in class path resource [spring-beans.xml]: Cannot resolve reference to bean 'a' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:342)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:113)
at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:704)
at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:196)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1356)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1203)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:556)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516)
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:897)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:879)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:551)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:144)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:85)
at com.demo.TestSpring.main(TestSpring.java:14)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'b' defined in class path resource [spring-beans.xml]: Cannot resolve reference to bean 'a' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:342)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:113)
at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:704)
at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:196)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1356)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1203)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:556)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516)
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:330)
... 17 more
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:355)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:227)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:330)
... 29 more
Disconnected from the target VM, address: '127.0.0.1:60287', transport: 'socket'
Process finished with exit code 1
复制代码3、源码跟踪
先扔一张我跟踪源码过程的流程图：
上图中有个关键点，在spring的源码中，通常会看到getBean，doGetBean等方法，在解决循环依赖过程中，重要的流程如下图所示：
代码跟踪开始：
类A和B采用set方式
执行main方法：
/**
* main
* @date: 2020/12/23
* @author weirx
* @version 3.0
*/
public class TestSpring {
public static void main(String[] args) {
ClassPathXmlApplicationContext applicationContext
= new ClassPathXmlApplicationContext("spring-beans.xml");
}
}
复制代码跟踪refresh方法的finishBeanFactoryInitialization方法，直接跟踪器内部的最后一行代码beanFactory.preInstantiateSingletons()：
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
// 获取bean名称
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// 遍历实例化所有非延时bean
for (String beanName : beanNames) {
//获取bean的定义
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
//bean是否不是抽象&是否单例&是否不是懒加载
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//是否是工厂bean
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(
(PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
//获取bean
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
复制代码上述代码忽略不重要的流程，断点直接走到了getBean方法：
public Object getBean(String name) throws BeansException {
//真正实例化bean的方法
return doGetBean(name, null, null, false);
}
复制代码跟踪进入doGetBean，两个关键点，检查已经注册的缓存（getSingleton(beanName)），创建bean实例，在下面的代码中都有注释，需要重点关注:
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
String beanName = transformedBeanName(name);
Object bean;
// 检查已经注册的缓存
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 + "'");
}
}
bean = 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) {
return ((AbstractBeanFactory) parentBeanFactory).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);
}
try {
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);
}
}
}
// 创建bean实例
if (mbd.isSingleton()) {
//从缓存获取bean实例，此处使用函数式接口(ObjectFactory是函数式接口，当调用其默认方法，会执行函数内的createBean)
sharedInstance = getSingleton(beanName, () -> {
try {
return 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.
destroySingleton(beanName);
throw ex;
}
});
bean = 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);
}
bean = 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);
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new ScopeNotActiveException(beanName, scopeName, ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isTraceEnabled()) {
logger.trace("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
复制代码我们分别去看下这两个关键点做了什么？首先是到缓存获取bean实例getSingleton，此时一定是null，因为还没有进行实例化：
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//检查一级缓存是否存在当前bean的实例
Object singletonObject = this.singletonObjects.get(beanName);
//判断当前对象在缓存是否为null，且是否是当前正在创建的bean
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
//从二级缓存获取bean实例
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
//从三级缓存获取bean实例
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//执行函数式接口内的方法
singletonObject = singletonFactory.getObject();
//添加bean到二级缓存
this.earlySingletonObjects.put(beanName, singletonObject);
//移除三级缓存
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
复制代码查询缓存不存在该实例，则需要创建实例，创建实例的部分代码块如下，这里重点关注下getSingletion方法，此处是一个lambda表达式:
// 创建bean实例
if (mbd.isSingleton()) {
//从缓存获取bean实例，此处使用函数式接口(ObjectFactory是函数式接口，当调用其默认方法，会执行函数内的createBean)
sharedInstance = getSingleton(beanName, () -> {
try {
return 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.
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
复制代码进入getSingleton方法看看，第二个参数ObjectFactory<?>，这个对象是一个函数式接口，当执行器内部的默认方法getObject，则会调用之前getSingleton的lambda表达式的内容createBean；下面的方法经过获取一级缓存，发现未取到bean实例，后面经过一些判断执行了singletonFactory.getObject()，进入createBean阶段：
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
//从一级缓存获取bean
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 + "'");
}
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
//调用ObjectFactory的默认方法，执行createBean
singletonObject = singletonFactory.getObject();
newSingleton = 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) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
复制代码进入createBean方法，这里的主要方法就是doCreateBean：
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
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 {
//让BeanPostProcessors有机会返回一个代理而不是目标bean实例
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 {
//真正创建bean的方法
Object 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);
}
}
复制代码进入doCreateBean，这里面有很多关键步骤： 1)通过反射方式创建bean实例 2)设置三级缓存，删除二级缓存，key是beanName,value是当前bean的一个引用（getEarlyBeanReference方法），这个引用方法需要重点记下，是aop的关键。 3)bean的初始化：填充属性populateBean和初始化initializeBean
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) {
//通过反射方式创建bean实例
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//获取bean实例对象
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
//允许BeanDefinitionPostProcessors修改合并后的bean定义
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.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
//是否单例bean、允许循环引用、当前创建是单例
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");
}
//设置三级缓存，此处也是使用函数式接口ObjectFactory<?>
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// 初始化bean实例.
Object exposedObject = bean;
try {
//填充属性
populateBean(beanName, mbd, instanceWrapper);
//初始化bean
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) {
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;
}
复制代码添加三级缓存的方法：
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
//设置三级缓存，key是beanName,value是当前bean的一个引用（getEarlyBeanReference方法）
this.singletonFactories.put(beanName, singletonFactory);
//删除二级缓存
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
复制代码进入populateBean，这个类主要用作给bean的属性赋值，其中主要是获取属性值和其设置bean属性值的方法applyPropertyValues：
@SuppressWarnings("deprecation") // for postProcessPropertyValues
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;
}
}
// 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;
}
}
}
//获取当前bean的属性
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;
}
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
PropertyDescriptor[] filteredPds = null;
if (hasInstAwareBpps) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
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;
}
}
if (needsDepCheck) {
if (filteredPds == null) {
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
}
checkDependencies(beanName, mbd, filteredPds, pvs);
}
if (pvs != null) {
//设置属性值
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
复制代码进入applyPropertyValues方法，这里面主要是对bean进行赋值的，我们重点关注resolveValueIfNecessary(pv, originalValue)方法，这是获取属性值得关键方法：
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
if (pvs.isEmpty()) {
return;
}
if (System.getSecurityManager() != null && bw instanceof BeanWrapperImpl) {
((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
}
MutablePropertyValues mpvs = null;
List<PropertyValue> original;
if (pvs instanceof MutablePropertyValues) {
mpvs = (MutablePropertyValues) pvs;
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);
}
}
original = mpvs.getPropertyValueList();
}
else {
original = Arrays.asList(pvs.getPropertyValues());
}
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
//创建一个深拷贝,解析任何值的引用
List<PropertyValue> deepCopy = new ArrayList<>(original.size());
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
if (pv.isConverted()) {
deepCopy.add(pv);
}
else {
//获取属性名
String propertyName = pv.getName();
//获取属性值，此处是引用
Object originalValue = pv.getValue();
if (originalValue == AutowiredPropertyMarker.INSTANCE) {
Method writeMethod = bw.getPropertyDescriptor(propertyName).getWriteMethod();
if (writeMethod == null) {
throw new IllegalArgumentException("Autowire marker for property without write method: " + pv);
}
originalValue = new DependencyDescriptor(new MethodParameter(writeMethod, 0), true);
}
//解析值里面的引用
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
Object convertedValue = resolvedValue;
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
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) originalValue).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 {
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
复制代码进入resolveValueIfNecessary(pv, originalValue)方法，因为我们是bean里面引用bean，所以bean属性的值是RunTimeBeanPreference，即运行时bean引用，我们只需要关注这个方法的前几行代码：
@Nullable
public 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.
//我们必须检查每个值，看它是否需要对另一个bean的运行时引用来解析
if (value instanceof RuntimeBeanReference) {
RuntimeBeanReference ref = (RuntimeBeanReference) value;
//解析引用
return resolveReference(argName, ref);
}
···
}
复制代码进入方法resolveReference解析引用内部，这里面我们只关注一行重要的代码： bean = this.beanFactory.getBean(resolvedName) 又去获取bean了，只不过这回获取的是属性中的B的实例。
@Nullable
private Object resolveReference(Object argName, RuntimeBeanReference ref) {
try {
Object bean;
Class<?> beanType = ref.getBeanType();
if (ref.isToParent()) {
BeanFactory parent = this.beanFactory.getParentBeanFactory();
if (parent == null) {
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) {
NamedBeanHolder<?> namedBean = this.beanFactory.resolveNamedBean(beanType);
bean = namedBean.getBeanInstance();
resolvedName = namedBean.getBeanName();
}
else {
resolvedName = String.valueOf(doEvaluate(ref.getBeanName()));
//根据引用中的beanName获取bean
bean = this.beanFactory.getBean(resolvedName);
}
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);
}
}
复制代码之后的步骤是相同的，去doGetBean获取bean，发现没有，开始创建B的实例，并将B的实例放入第三级缓存，然后对B的实例进行初始化，这里面可以关注下代码，初始化B时对B的属性A进行赋值，需要去获取A的实例，A的实例在第一次就创建了并且放在了第三级缓存中，去缓存获取A的代码如下：
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//检查一级缓存是否存在当前bean的实例
Object singletonObject = this.singletonObjects.get(beanName);
//判断当前对象在缓存是否为null，且是否是当前正在创建的bean
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
//从二级缓存获取bean实例
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
//从三级缓存获取bean实例
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//执行函数式接口内的方法
singletonObject = singletonFactory.getObject();
//添加bean到二级缓存
this.earlySingletonObjects.put(beanName, singletonObject);
//移除三级缓存
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
复制代码在上面取A实例的过程中，将第三级缓存中的a取出来放到了二级缓存中，并删除了三级缓存中的a。二级缓存的key是beanName,value是半成品。此时的b已经成功的拿到了a的半成品。则b的初始化已经完成了并且放入了一级缓存中：
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
复制代码b初始化完成后，下一步就是继续去初始化a了。重复代码不在粘贴了，最终也会将a放入一级缓存中，清除二级缓存中的a。
因为实例化的时候时候a、b进行遍历的，当a初始化完成后，就开始了b的实例化，此时a和b都已经在一级缓存中完成实例化和初始化了，所以无论是b的实例化，还是b初始化属性a时，都从一级缓存直接获取就可以了，以上就是spring解决循环依赖的整个过程。
流程很长，建议配合源码读取，不然很枯燥且难以读下去。
总结思考
分析下三级缓存的变化
根据上面的源码过程，整个三级缓存的变化过程如下所示：
根据上图，在a和b的实例化过程中，按照以上步骤，缓存一步步的变化，当A或A的缓存由下一级移动至高一级，则清除下一级额缓存，始终保持实例对象只在其中一层缓存实现。
三级缓存存在的意义是什么?
通过上面的代码，我们发现： 一级缓存：存放成品对象 二级缓存：存放半成品对象 三级缓存：存放getEarlyBeanReference方法
其实我们可以发现，真正涉及到循环依赖的缓存只有一级和二级缓存，如果我们去修改代码，不使用三级缓存的话，一样可以解决循环依赖的问题。
那么三级缓存到底干了什么呢？下面我们简单的跟进下getEarlyBeanReference方法的代码。
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
//遍历beanPostProcessor
for (SmartInstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().smartInstantiationAware) {
//获取提前暴露的对象
exposedObject = bp.getEarlyBeanReference(exposedObject, beanName);
}
}
return exposedObject;
}
复制代码跟踪getEarlyBeanReference(exposedObject, beanName)方法：
@Override
public Object getEarlyBeanReference(Object bean, String beanName) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
//添加代理缓存
this.earlyProxyReferences.put(cacheKey, bean);
return wrapIfNecessary(bean, beanName, cacheKey);
}
复制代码跟踪wrapIfNecessary(bean, beanName, cacheKey)，发现了createProxy方法，这里豁然开朗，原来是代理：
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
return bean;
}
if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
return bean;
}
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
// 如果有通知的话，创建一个代理
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
if (specificInterceptors != DO_NOT_PROXY) {
this.advisedBeans.put(cacheKey, Boolean.TRUE);
//创建代理
Object proxy = createProxy(
bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
//设置代理类型
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
复制代码进入createProxy，直接关注最后一行：
protected Object createProxy(Class<?> beanClass, @Nullable String beanName,
@Nullable Object[] specificInterceptors, TargetSource targetSource) {
if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
}
ProxyFactory proxyFactory = new ProxyFactory();
proxyFactory.copyFrom(this);
if (!proxyFactory.isProxyTargetClass()) {
if (shouldProxyTargetClass(beanClass, beanName)) {
proxyFactory.setProxyTargetClass(true);
}
else {
evaluateProxyInterfaces(beanClass, proxyFactory);
}
}
Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
proxyFactory.addAdvisors(advisors);
proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory);
proxyFactory.setFrozen(this.freezeProxy);
if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
}
//获取代理
return proxyFactory.getProxy(getProxyClassLoader());
}
复制代码跟踪proxyFactory.getProxy(getProxyClassLoader())：
public Object getProxy(@Nullable ClassLoader classLoader) {
return createAopProxy().getProxy(classLoader);
}
复制代码跟踪getProxy(classLoader)，发现接口有两个实现类，都是实现aop的代理，到此揭开了三级缓存的作用。是实现aop的本质。
debug跟踪下：
xml配置aop
<bean id="a" class="com.demo.A">
<property name="b" ref="b"></property>
</bean>
<bean id="b" class="com.demo.B">
<property name="a" ref="a"></property>
</bean>
<bean id="c" class="com.demo.C"></bean>
<aop:config>
<aop:aspect id="c" ref="c">
<aop:pointcut id="method" expression="execution(* com.demo.*.*(..))" />
<aop:before pointcut-ref="method" method="before" />
<aop:after pointcut-ref="method" method="after" />
</aop:aspect>
</aop:config>
复制代码C类：
/**
* C
* @date: 2020/12/25
* @author weirx
* @version 3.0
*/
public class C {
private A a;
private B b;
public A getA() {
return a;
}
public void setA(A a) {
this.a = a;
}
public B getB() {
return b;
}
public void setB(B b) {
this.b = b;
}
private void before(){
System.out.println("this is before");
}
private void after(){
System.out.println("this is after");
}
}
复制代码运行代码debug到getEarlyBeanReference，如下图所示，当前A对象的是：
当创建代理后，当前暴露的对象编程了CGLIB的代理对象：
到此为止，整个循环依赖的内容到此结束。