循环依赖其实就是循环引用，也就是两个或则两个以上的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，由于对象之间的依赖关系造成了死循环。
Spring中循环依赖场景有：
- 构造器的循环依赖
- field属性的循环依赖
Spring怎么解决循环依赖
Spring循环依赖主要基于Java引用传递，当获取到对象时，对象的field或者属性可以延后设置
创建Bean
/**
* 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
* 实际创建指定的bean。 预创建处理已经发生
* 在这一点上，例如 检查{@code postProcessBeforeInstantiation}回调。
* <p>区分默认的bean实例化，使用
* 工厂方法，并自动装配构造函数。
* @param beanName bean的名称
* @param mbd该bean的合并bean定义
* @param args用于构造函数或工厂方法调用的显式参数
* @返回bean的新实例
* @throws BeanCreationException如果无法创建bean
* @请参阅#instantiateBean
* @see #instantiateUsingFactoryMethod
* @请参阅#autowireConstructor
*/
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
//从缓存中移除bean，因为要创建新的bean了
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//根据指定bean的使用策略创建新的实例，如：工厂方法，构造函数自动注入，简单初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
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.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
//是否需要提早曝光：单例&允许循环依赖&当前bean正在创建中，检测循环依赖
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");
}
//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，其他bean引用此bean时直接从ObjectFactory获取bean
//getEarlyBeanReference对bean再一次依赖引用，主要应用SmartInstantiationAware BeanPost Processor其中我们熟悉的AOP就是在这里将advance动态织入bean中的，如果没有就直接返回bean，不做任何处理
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
//对bean进行填充,将各个属性值注入，其中，可能存在依赖于其他bean的属性，则会递归给初始依赖bean
populateBean(beanName, mbd, instanceWrapper);
//调用初始化方法，如init-method
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);
//earlySingletonReference只有在检测到有依赖循环的情况下才会不为空
if (earlySingletonReference != null) {
//如果exposedObject没有在初始化方法中被改变，也就是没有被增强
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) {
//检测依赖
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
//因为bean创建后其所依赖的bean一定是已经创建的，actualDependentBeans不为空则表示当前bean创建后其依赖的bean却没有创建完，也就是存在循环依赖
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.
try {
//根据scopse注册bean
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}以上是Spring创建bean的方法，代码很多但是核心思路就是实例化，填充属性，初始化，而循环依赖主要发生在实例化和填充属性这两部分
//根据指定bean的使用策略创建新的实例，如：工厂方法，构造函数自动注入，简单初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
//对bean进行填充,将各个属性值注入，其中，可能存在依赖于其他bean的属性，则会递归给初始依赖bean
populateBean(beanName, mbd, instanceWrapper);
//调用初始化方法，如init-method
exposedObject = initializeBean(beanName, exposedObject, mbd);Spring为了解决循环依赖，设置了三级缓存
一级缓存singletonObjects
用于保存BeanName和创建bean实例之间的关系,beanName -> bean instance
private final Map<String, Object> singletonObjects = new ConcurrentHashMap(256);
二级缓存earlySingletonObjects
保存提前曝光的单例bean对象
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
三级缓存singletonFactories
保存beanName和创建bean实例之间的关系,与singletonObjects不同的地方在于，当一个单例bean被放到这里面后，bean在创建过程中，可以通过getBean方法获取到,目的是用来检测循环引用
private final Map<String, Object> singletonFactories = new HashMap(16);
在创建bean的时候，首先从缓存中获取单例的bean，这个缓存就是singletonObjects，如果获取不到且bean正在创建中，就再从earlySingletonObjects中获取，如果还是获取不到且允许从singletonFactories中通过getObject拿到对象，就从singletonFactories中获取，如果获取到了就存入earlySingletonObjects并从singletonFactories中移除。
getSingleton
//用于保存BeanName和创建bean实例之间的关系,beanName -> bean instance
private final Map<String, Object> singletonObjects = new ConcurrentHashMap(256);
//保存提前曝光的单例bean对象
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
//保存beanName和创建bean实例之间的关系,与singletonObjects不同的地方在于，当一个单例bean被放到这里面后，bean在创建过程中，可以通过getBean方法获取到,目的是用来检测循环引用
private final Map<String, Object> singletonFactories = new HashMap(16);
//保存当前所有已注册的bean
private final Set<String> registeredSingletons = new LinkedHashSet(256);
/**
* 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
* 返回以给定名称注册的（原始）单例对象。
* <p>检查已实例化的单例，并允许早期
* 引用当前创建的单例（解析循环引用）。
* @param beanName bean的名称
* @param allowEarlyReference 是否允许从singletonFactories中通过getObject拿到对象
* @返回注册的单例对象；如果找不到，则返回{@code null}
*/
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//检查缓存中是否存在实例
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
//如果为空,且正在创建中,锁定全局变量并进行处理
synchronized (this.singletonObjects) {
//如果bean正在加载则不处理
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
//当某些方法需要提前初始化的时候会调用addSingletonFactory方法将对于ObjectFactory初始化策略存储在singletonFactories
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//调用预设的getObject方法
singletonObject = singletonFactory.getObject();
//记录在缓存中earlySingletonObjects和singletonFactories互斥
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}addSingletonFactory
/**
* 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) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}现在有A的field或者setter依赖B的实例对象，同时B的field或者setter依赖了A的实例，A首先开始创建，并将自己暴露到 singletonFactories 中，开始填充属性，此时发现自己依赖B的属性，尝试去get(B)，发现B还没有被创建，所以开始创建B，在进行属性填充时初始化A，就从singletonObjects中获取到了初始化但没有任何属性的A，B拿到A后完成了初始化阶段，将自己放到singletonObjects中,此时返回A，A拿到B的对象继续完成初始化，完成后将自己放到singletonObjects中，由A与B中所表示的A的属性地址是一样的，所以A的属性填充完后，B也获取了A的属性，这样就解决了循环的问题。