当有两个类A、B相互依赖时便会形成循环依赖，多个类之间相互依赖也会存在循环依赖。
@Service
public class ServiceA {
@Autowired
private ServiceB serviceB;
public String say(){
return "hello";
}
}
@Service
public class ServiceB {
@Autowired
private ServiceA serviceA;
}当构造方法注入依赖时循环无解，会有如下提示
The dependencies of some of the beans in the application context form a cycle:
cycleController (field com.paw.cycle.dependency.service.ServiceA com.paw.cycle.dependency.controller.CycleController.serviceA)
┌─────┐
| serviceA defined in file [/Users/.../target/classes/com/paw/cycle/dependency/service/ServiceA.class]
↑ ↓
| serviceB defined in file [/Users/.../target/classes/com/paw/cycle/dependency/service/ServiceB.class]
└─────┘看下spring是如何解决循环依赖的
开启日志
logging:
level:
org.springframework: trace运行查看日志
main] o.s.b.f.s.DefaultListableBeanFactory : Finished creating instance of bean 'cycleDependencyApplication'
main] o.s.b.f.s.DefaultListableBeanFactory : Returning cached instance of singleton bean 'org.springframework.boot.autoconfigure.internalCachingMetadataReaderFactory'
main] o.s.b.f.s.DefaultListableBeanFactory : Creating shared instance of singleton bean 'cycleController'
main] o.s.b.f.s.DefaultListableBeanFactory : Creating instance of bean 'cycleController'
main] o.s.b.f.s.DefaultListableBeanFactory : Eagerly caching bean 'cycleController' to allow for resolving potential circular references
main] o.s.b.f.s.DefaultListableBeanFactory : Finished creating instance of bean 'cycleController'
main] o.s.b.f.s.DefaultListableBeanFactory : Creating shared instance of singleton bean 'serviceA'
main] o.s.b.f.s.DefaultListableBeanFactory : Creating instance of bean 'serviceA'
main] o.s.b.f.annotation.InjectionMetadata : Registered injected element on class [com.paw.cycle.dependency.service.ServiceA]: AutowiredFieldElement for private com.paw.cycle.dependency.service.ServiceB com.paw.cycle.dependency.service.ServiceA.serviceB
main] o.s.b.f.s.DefaultListableBeanFactory : Eagerly caching bean 'serviceA' to allow for resolving potential circular references
main] o.s.b.f.annotation.InjectionMetadata : Processing injected element of bean 'serviceA': AutowiredFieldElement for private com.paw.cycle.dependency.service.ServiceB com.paw.cycle.dependency.service.ServiceA.serviceB
main] o.s.b.f.s.DefaultListableBeanFactory : Creating shared instance of singleton bean 'serviceB'
main] o.s.b.f.s.DefaultListableBeanFactory : Creating instance of bean 'serviceB'
main] o.s.b.f.annotation.InjectionMetadata : Registered injected element on class [com.paw.cycle.dependency.service.ServiceB]: AutowiredFieldElement for private com.paw.cycle.dependency.service.ServiceA com.paw.cycle.dependency.service.ServiceB.serviceA
main] o.s.b.f.s.DefaultListableBeanFactory : Eagerly caching bean 'serviceB' to allow for resolving potential circular references
main] o.s.b.f.annotation.InjectionMetadata : Processing injected element of bean 'serviceB': AutowiredFieldElement for private com.paw.cycle.dependency.service.ServiceA com.paw.cycle.dependency.service.ServiceB.serviceA
main] o.s.b.f.s.DefaultListableBeanFactory : Returning eagerly cached instance of singleton bean 'serviceA' that is not fully initialized yet - a consequence of a circular reference
main] f.a.AutowiredAnnotationBeanPostProcessor : Autowiring by type from bean name 'serviceB' to bean named 'serviceA'
main] o.s.b.f.s.DefaultListableBeanFactory : Finished creating instance of bean 'serviceB'
main] f.a.AutowiredAnnotationBeanPostProcessor : Autowiring by type from bean name 'serviceA' to bean named 'serviceB'
main] o.s.b.f.s.DefaultListableBeanFactory : Finished creating instance of bean 'serviceA'
main] o.s.b.f.s.DefaultListableBeanFactory : Returning cached instance of sinleton bean 'serviceB'
可以梳理到基本流程：
创建ing serviceA==>需要注入serviceB==>缓存半成品serviceA ==>创建ing serviceB==>需要注入serviceA==>缓存半成品serviceB==>注入缓存的半成品 serviceA==>完成创建serviceB==>service 注入serviceB ==>完成serviceA创建。
DefaultSingletonBeanRegistry的三级缓存
// 一级实例化好的bean
/**Cache of singleton objects: bean name to bean instance.*/
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
// 二级半成品 未进行属性注入
/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
// 三级 生成bean的ObjectFactory
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
获取bean实例对象
先从一级取 》再从二级取 》最后从三级取
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 一级缓存
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
// 二级缓存
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
// 三级缓存
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}加入条件断点，开始顺腾摸瓜
beanName.equals("serviceA")
类的继承关系
SimpleAliasRegistry (org.springframework.core)
|-DefaultSingletonBeanRegistry (org.springframework.beans.factory.support)
|-|-FactoryBeanRegistrySupport (org.springframework.beans.factory.support)
|-|-|-AbstractBeanFactory (org.springframework.beans.factory.support)
|-|-|-|-AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
|-|-|-|-|-DefaultListableBeanFactory (org.springframework.beans.factory.support)
|-|-|-|-|-XmlBeanFactory (org.springframework.beans.factory.xml)
三级缓存objectFactory 通过匿名内部类实现 （匿名类 调用了 AbstractAutowireCapableBeanFactory#getEarlyBeanReference方法） getEarlyBeanReference 获取早期的半成品对象。
AbstractAutowireCapableBeanFactory
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
...
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
...
}singletonFactory AbstractAutowireCapableBeanFactory
// 加入三级缓存
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);
}
}
}// 由ObjectFactory创建bean
AbstractBeanFactory#doGetBean
// 创建bean实例
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
...
}
==> DefaultSingletonBeanRegistry#getSingleton
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bean之前的处理
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
// 通过三级缓存的objectFactory获取bean对象实例
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;
}
// 创建完后的处理
afterSingletonCreation(beanName);
}
if (newSingleton) {
// 创建成功添加到一级缓存
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}AbstractAutowireCapableBeanFactory 创建bean
添加到三级缓存==>创建Eagerly caching bean ===>注入bean属性实例化bean
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.
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");
}
// 添加三级缓存 beanFactory
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
// 填充bean属性 即注入@Autowired字段
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;
}属性注入serviceB
填充属性
//AbstractAutowireCapableBeanFactory
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) AbstractAutowireCapableBeanFactory#postProcessProperties
InjectionMetadata类进行属性注入
public 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;
}
存在AOP的情况singletonObject = singletonFactory.getObject(); 多次调用会生成新的代理对象，（不同代理对象代理的还是同一个bean对象serviceA)。spring 将singletonObject 代理对象放入到二级缓存，注入到serviceB中的也是代理对象singletonObject。
如果没有AOP的话确实可以两级缓存就可以解决循环依赖的问题，如果加上AOP，两级缓存是无法解决的，不可能每次执行singleFactory.getObject()方法都给我产生一个新的代理对象，所以还要借助另外一个缓存来保存产生的代理对象。
总结：
spring中三级缓存存放ObjectFactory(匿名类 调用了 AbstractAutowireCapableBeanFactory #getEarlyBeanReference方法),通过getObect获取半成品对象放入二级缓存，完成属性注入及bean初始化后放入一级缓存。对存在AOP代理的情况，三级缓存每次会生成一个新的代理对象，因此将getObject生成的代理对象singletonObject放入到二级缓存，完成属性注入后，将代理对象添加到一级缓存。
spring-boot 2.6.0中对循环依赖默认报错，需手动开启。
