serviceA里注入了serviceB，serviceB里又注入了serviceA，这样在实例化serviceA时，在doCreateBean时的populateBean时会先实例化serviceB，然后实例化serviceB，在serviceB的doCreateBean方法的populateBean又会去找servcieA,这样循环依赖就产生了。
解决办法就是在populateBean之前把当前的serviceA提前暴露，然后在实例化B时可以找到这个提前暴露的serviceA。
关键方法是doCreateBean方法中的addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
1. DefaultSingletonBeanRegistry#getSingleton
这里就是对当前bean做了一个标志，在实例化前表明当前bean在创建中，在实例化后删除这个标志
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 + "'");
}
//关键1 标志A这个Bean在创建中
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
//调用createBean方法
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;
}
//关键2 //this.singletonsCurrentlyInCreation.remove(beanName)
//删除掉Bean A创建中的标识
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
2. AbstractAutowireCapableBeanFactory#doCreateBean
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
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.
//这里如果是单例 并且允许循环依赖 并且是正在创建的bean（1中设置）
//能满足这个三个条件，则Spring会认为这个此时A是需要提前暴漏的单例Bean
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");
}
//关键方法
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
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 " +
"'getBeanNamesOfType' 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;
}
3. DefaultSingletonBeanRegistry#addSingletonFactory
实例化A时，doCreateBean方法populateBean方法执行之前，也就是实例化注入的serviceB之前会调用这个方法。
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
//把当前bean 也就是serviceA关联一个singletonFactory（也就是那个lambda表达式）
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
给A关联了一个ObjectFactory，也就是getEarlyBeanReference对应的lambda函数对象。放入了singletonFactories中，当B去查BeanA的时候就可以通过getEarlyBeanReference方法获取exposedObject这个早期bean对象。
4.AbstractAutowireCapableBeanFactory#getEarlyBeanReference
这里基本就是把传入的bean返回。命名为exposedObject 早期暴露对象
调用逻辑就是 serviceB实例化时需要注入serviceA，然后在doGetBean的最开始调用getSingleton方法时会获取到serviceA的ObjectFactory，然后执行singletonFactory.getObject()就会执行这个方法拿到bean。
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
//这个的作用是当前循环依赖的bean恰巧是AOP类型的bean，这里会获取到该bean的代理对象，
//保证最终实例化的bean和依赖引入的bean是相同的，都是代理类型。
if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
}
}
}
//如果是普通的bean就直接返回了。
return exposedObject;
}
查找
比如在实例化serviceA时需要实例化serviceB，实例化serviceB时populateBean方法会再尝试实例化serviceA,doGetBean方法开始会调用getSingleton方法，这个方法里会找到提前暴露的serviceA，保证serviceB可以顺利创建。
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//这里获取不到 因为servcieA的实例化还没完成 也就是servcieA的createBean方法还没执行完，servcieA的getSingleton方法的
//singletonObject = singletonFactory.getObject()方法也就没执行完，下面的addSingleton方法
//就不会执行（在这里执行this.singletonObjects.put(beanName, singletonObject)），
//singletonObjects中就不会有serviceA。
Object singletonObject = this.singletonObjects.get(beanName);
//判断会通过 第二个判断条件是在getSingleton方法的beforeSingletonCreation(beanName)设置的 最开始实例化A时会调用
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
//这里也获取不到，因为在最开始servcieA实例化时的addSingletonFactory方法会执行
//this.earlySingletonObjects.remove(beanName);
singletonObject = this.earlySingletonObjects.get(beanName);
//判断会进入
if (singletonObject == null && allowEarlyReference) {
//这里能获取到，因为在servcieA实例化时的addSingletonFactory方法会执行
//this.singletonFactories.put(beanName, singletonFactory);
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//执行这个方法会执行那个lambda表达式 () -> getEarlyBeanReference(beanName, mbd, bean)
//获取到serviceA
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
//获取到返回 此时serviceB就可以得到serviceA了，然后继续创建serviceB
return singletonObject;
}
总结
实例化过程：
1.finishBeanFactoryInitialization(beanFactory);
2.beanFactory.preInstantiateSingletons();
3.getBean(beanName);
4.doGetBean
这里实例化serviceA时找不到，但是在实例化注入的serviceB时又需要实例化serviceB注入的serviceA，这时就可以获取到
serviceA了。
5.
这里的getSingleton方法在执行时执行到**singletonObject = singletonFactory.getObject()**时就会执行lambda表达式的createBean方法。
依赖关系的处理是在createBean方法的doCreateBean方法的populateBean方法
6.doCreateBean
关键方法
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
这个方法在实例化serviceA，populateBean之前，把serviceA及关联的ObjectFactory（lambda表达式）加到了singletonFactories，这样serviceB实例化时需要实例化servcieA，在doGetBean一开始的getSingleton方法会获取到serviceA，这样就能保证serviceB能创建完成，然后再创建serviceA。文章来源：https://www.toymoban.com/news/detail-474160.html
springboot2.6之后默认禁用循环依赖。
开启的话需要增加配置：
spring.main.allow-circular-references=true文章来源地址https://www.toymoban.com/news/detail-474160.html
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