循环依赖
循环依赖其实就是循环引⽤，也就是两个或者两个以上的 Bean 互相持有对⽅，最终形成闭环。⽐如A
依赖于B，B依赖于C，C⼜依赖于A。
这⾥不是函数的循环调⽤，是对象的相互依赖关系。循环调⽤其实就是⼀个死循环，除⾮有终结条件
Spring中循环依赖场景有：
- 构造器的循环依赖（构造器注⼊）
- Field 属性的循环依赖（set注⼊）
其中，构造器的循环依赖问题⽆法解决，只能拋出 BeanCurrentlyInCreationException 异常，在解决
属性循环依赖时，spring采⽤的是提前暴露对象的⽅法。
循环依赖处理机制
- 单例 bean 构造器参数循环依赖（⽆法解决）
- prototype 原型 bean循环依赖（⽆法解决）
对于原型bean的初始化过程中不论是通过构造器参数循环依赖还是通过setXxx⽅法产⽣循环依
赖，Spring都 会直接报错处理。
AbstractBeanFactory：
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
//根据指定的名称获取被管理Bean的名称，剥离指定名称中对容器的相关依赖
//如果指定的是别名，将别名转换为规范的Bean名称
//（对FactoryBean处理，FactoryBean前面需要加一个&，这里需要处理掉这个符号）
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
//先从缓存中取是否已经有被创建过的单态类型的Bean
//对于单例模式的Bean整个IOC容器中只创建一次，不需要重复创建
Object sharedInstance = getSingleton(beanName);
//IOC容器创建单例模式Bean实例对象
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
//如果指定名称的Bean在容器中已有单例模式的Bean被创建
//直接返回已经创建的Bean
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
//获取给定Bean的实例对象，主要是完成FactoryBean的相关处理
//注意：BeanFactory是管理容器中Bean的工厂，而FactoryBean是
//创建创建对象的工厂Bean，两者之间有区别
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
//缓存没有正在创建的单例模式Bean
//缓存中已经有已经创建的原型模式Bean
//但是由于循环引用的问题导致实例化对象失败
//@@@判断这个类是否在创建过程中prototypeCurrentlyInCreation需要联系getSingleton方法
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
//对IOC容器中是否存在指定名称的BeanDefinition进行检查，首先检查是否
//能在当前的BeanFactory中获取的所需要的Bean，如果不能则委托当前容器
//的父级容器去查找，如果还是找不到则沿着容器的继承体系向父级容器查找
BeanFactory parentBeanFactory = getParentBeanFactory();
//当前容器的父级容器存在，且当前容器中不存在指定名称的Bean
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
//解析指定Bean名称的原始名称
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.
//委派父级容器根据指定名称和显式的参数查找
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
//委派父级容器根据指定名称和类型查找
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
//创建的Bean是否需要进行类型验证，一般不需要
if (!typeCheckOnly) {
//向容器标记指定的Bean已经被创建
markBeanAsCreated(beanName);
}
try {
//根据指定Bean名称获取其父级的Bean定义
//主要解决Bean继承时子类合并父类公共属性问题
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//获取当前Bean所有依赖Bean的名称
String[] dependsOn = mbd.getDependsOn();
//如果当前Bean有依赖Bean
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 + "'");
}
//递归调用getBean方法，获取当前Bean的依赖Bean
registerDependentBean(dep, beanName);
//把被依赖Bean注册给当前依赖的Bean
getBean(dep);
}
}
// Create bean instance.
//创建单例模式Bean的实例对象
if (mbd.isSingleton()) {
//这里使用了一个匿名内部类，创建Bean实例对象，并且注册给所依赖的对象
sharedInstance = getSingleton(beanName, () -> {
try {
//创建一个指定Bean实例对象，如果有父级继承，则合并子类和父类的定义
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.
//显式地从容器单例模式Bean缓存中清除实例对象
destroySingleton(beanName);
throw ex;
}
});
//获取给定Bean的实例对象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
//IOC容器创建原型模式Bean实例对象
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
//原型模式(Prototype)是每次都会创建一个新的对象
Object prototypeInstance = null;
try {
//回调beforePrototypeCreation方法，默认的功能是注册当前创建的原型对象
beforePrototypeCreation(beanName);
//创建指定Bean对象实例
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
//回调afterPrototypeCreation方法，默认的功能告诉IOC容器指定Bean的原型对象不再创建
afterPrototypeCreation(beanName);
}
//获取给定Bean的实例对象
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
//要创建的Bean既不是单例模式，也不是原型模式，则根据Bean定义资源中
//配置的生命周期范围，选择实例化Bean的合适方法，这种在Web应用程序中
//比较常用，如：request、session、application等生命周期
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
//Bean定义资源中没有配置生命周期范围，则Bean定义不合法
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
//这里又使用了一个匿名内部类，获取一个指定生命周期范围的实例
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
//获取给定Bean的实例对象
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
//对创建的Bean实例对象进行类型检查
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.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
AbstractAutowireCapableBeanFactory：
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
//封装被创建的Bean对象
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//实例化bean对象 相当类似于new一个bean 而不是初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
//获取实例化对象的类型
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
//调用PostProcessor后置处理器
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//@@@第三次后置处理器（MergedBeanDefinitionPostProcessor）对注解信息进行缓余
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.
//向容器中缓存单例模式的Bean对象，以防循环引用
//@@@判断是否允许循环依赖 (allowCircularReferences是用来判断是否允许开启循环依赖的属性 默认为true)
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
//这里是一个匿名内部类，为了防止循环引用，尽早持有对象的引用
//@@@getEarlyBeanReference中包含第四次后置处理器（SmartInstantiationAwareBeanPostProcessor）（判断是否需要aop！）（为了处理循环依赖时会把bean：是对象还不是完整的bean 放到临时缓存当中）
/**
* 如果使用aop的话 会有一个类AbstractAutoProxyCreator继承SmartInstantiationAwareBeanPostProcessor后置处理器，其中getEarlyBeanReference()方法会 为了解决单例bean之间的循环依赖问题，提前将aop代理对象暴露出去。
* （initializeBean中再执行aop代理时，会直接从这里生产的缓存中获取，不需要再生成代理类了）
*/
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
//Bean对象的初始化，依赖注入在此触发
//这个exposedObject在初始化完成之后返回作为依赖注入完成后的Bean
Object exposedObject = bean;
try {
//将Bean实例对象封装，并且Bean定义中配置的属性值赋值给实例对象
//@@@populateBean中包含第五次后置处理器（InstantiationAwareBeanPostProcessor：postProcessAfterInstantiation方法：是否自动装配属性 返回值是boolean）
populateBean(beanName, mbd, instanceWrapper);
//初始化Bean对象 和一些后置处理器的执行 （包含aop代理）
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) {
//获取指定名称的已注册的单例模式Bean对象
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
//根据名称获取的已注册的Bean和正在实例化的Bean是同一个
if (exposedObject == bean) {
//当前实例化的Bean初始化完成
exposedObject = earlySingletonReference;
}
//当前Bean依赖其他Bean，并且当发生循环引用时不允许新创建实例对象
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
//获取当前Bean所依赖的其他Bean
for (String dependentBean : dependentBeans) {
//对依赖Bean进行类型检查
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
//注册完成依赖注入的Bean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
DefaultSingletonBeanRegistry：
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
//@@@isSingletonCurrentlyInCreation判断bean是否在创建当中
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
//@@@singletonFactories这个缓存的作用同时为了解决在循环依赖时候的aop，当循环依赖时 要保证能实现aop需要用工厂来生产对象，而不能直接new出来，所以这里的二级缓存singletonFactories map是工厂的原因
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
//放到三级缓存 earlySingletonObjects作用：多个类不止2个类循环依赖时候不需要通过singletonFactories再产生一次了，解决性能问题 每次都从singletonFactories获取还需要产生效率慢
this.earlySingletonObjects.put(beanName, singletonObject);
//从二级缓存中清除？为什么需要清除：为了GC 效率啊！！
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
/**
* 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 + "'");
}
//把beanName添加到集合中，表示bean正在创建当中
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
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;
}
}
Spring 的循环依赖的理论依据基于 Java 的引⽤传递，当获得对象的引⽤时，对象的属性是可以延
后设置的，但是构造器必须是在获取引⽤之前
Spring通过setXxx或者@Autowired⽅法解决循环依赖其实是通过提前暴露⼀个ObjectFactory对
象来完成的，简单来说ClassA在调⽤构造器完成对象初始化之后，在调⽤ClassA的setClassB⽅法
之前就把ClassA实例化的对象通过ObjectFactory提前暴露到Spring容器中。
- Spring容器初始化ClassA通过构造器初始化对象后提前暴露到Spring容器。
- ClassA调⽤setClassB⽅法，Spring⾸先尝试从容器中获取ClassB，此时ClassB不存在Spring
容器中。 - Spring容器初始化ClassB，同时也会将ClassB提前暴露到Spring容器中
- ClassB调⽤setClassA⽅法，Spring从容器中获取ClassA ，因为第⼀步中已经提前暴露了
ClassA，因此可以获取到ClassA实例 - ClassA通过spring容器获取到ClassB，完成了对象初始化操作。
这样ClassA和ClassB都完成了对象初始化操作，解决了循环依赖问题。