循環依賴其實就是循環引用，很多地方都說需要兩個或則兩個以上的bean互相持有對方最終形成閉環才是循環依賴，比如A依賴于B，B依賴于C，C又依賴于A。其實一個bean持有自己類型的屬性也會產生循環依賴。

setter singleton循環依賴

使用

SingleSetterBeanA依賴SingleSetterBeanB，SingleSetterBeanB依賴SingleSetterBeanA。

1. @Data
2. public class SingleSetterBeanA {
3. @Autowired
4. private SingleSetterBeanB singleSetterBeanB;
5. }

1. @Data
2. public class SingleSetterBeanB {
3. @Autowired
4. private SingleSetterBeanA singleSetterBeanA;
5. }

源碼分析

spring是通過三級緩存來解決循環依賴的，那么三級緩存是怎么工作的呢？

三級緩存對應org.springframework.beans.factory.support.DefaultSingletonBeanRegistry類的三個屬性：

1. /** Cache of singleton objects: bean name to bean instance. */
2. private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256); // 一級緩存
3.  
4. /** Cache of singleton factories: bean name to ObjectFactory. */
5. private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); // 二級緩存
6.  
7. /** Cache of early singleton objects: bean name to bean instance. */
8. private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16); // 三級緩存

對于setter注入造成的依賴是通過Spring容器提前暴露剛完成實例化但未完成初始化的bean來完成的，而且只能解決單例作用域的bean循環依賴。通過提前暴露一個單例工廠方法，從而使其他bean能引用到該bean，關鍵源碼如下所示：

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean

1. // 處理循環依賴，實例化后放入三級緩存
2. boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
3. isSingletonCurrentlyInCreation(beanName));
4. if (earlySingletonExposure) {
5. if (logger.isTraceEnabled()) {
6. logger.trace("Eagerly caching bean '" + beanName +
7. "' to allow for resolving potential circular references");
8. }
9. addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
10. }

bean實例化后放入三級緩存中：

org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#addSingletonFactory

1. protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
2. Assert.notNull(singletonFactory, "Singleton factory must not be null");
3. synchronized (this.singletonObjects) {
4. if (!this.singletonObjects.containsKey(beanName)) {
5. this.singletonFactories.put(beanName, singletonFactory); // 三級緩存
6. this.earlySingletonObjects.remove(beanName);
7. this.registeredSingletons.add(beanName);
8. }
9. }
10. }

放入三級緩存中的是ObjectFactory類型的lambda表達式：

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#getEarlyBeanReference

1. protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
2. Object exposedObject = bean;
3. if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
4. for (BeanPostProcessor bp : getBeanPostProcessors()) {
5. if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
6. SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
7. /**
8. * @see org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#getEarlyBeanReference(java.lang.Object, java.lang.String)
9. */
10. // 使用AbstractAutoProxyCreator#getEarlyBeanReference創建代理對象
11. exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
12. }
13. }
14. }
15. return exposedObject;
16. }

構造器參數循環依賴

通過構造器注入構成的循環依賴，此依賴是無法解決的，只能拋出BeanCurrentlyInCreationException異常表示循環依賴。

使用

1. @Data
2. public class SingleConstrutorBeanA {
3. public SingleConstrutorBeanA(SingleConstrutorBeanB singleConstrutorBeanB) {
4. }
5. }

1. @Data
2. public class SingleConstrutorBeanB {
3. public SingleConstrutorBeanB(SingleConstrutorBeanA singleConstrutorBeanA) {
4. }
5. }

上面的代碼運行時會拋出如下異常：

... ...
Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'singleConstrutorBeanB': Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'singleConstrutorBeanA': Requested bean is currently in creation: Is there an unresolvable circular reference?
 at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:805)
 at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:228)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1403)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1245)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:579)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:538)
 at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:329)
 at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
 at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:323)
 at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
 at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1321)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1240)
 at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:892)
 at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:796)
 ... 76 more
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'singleConstrutorBeanA': 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:323)
 at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
 at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1321)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1240)
 at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:892)
 at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:796)
 ... 90 more

源碼分析

Spring容器會將每一個正在創建的Bean標識符放在一個“當前創建Bean池”中，Bean標識符在創建過程中將一直保持在這個池中，因此如果在創建Bean過程中發現自己已經在“當前創建Bean池”里時將拋出BeanCurrentlyInCreationException異常表示循環依賴；而對于創建完畢的Bean將從“當前創建Bean池”中清除掉。

1. protected void beforeSingletonCreation(String beanName) {
2. if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
3. throw new BeanCurrentlyInCreationException(beanName);
4. }
5. }

@Lazy打破循環依賴

在上面的例子中只需要在SingleConstrutorBeanA或者SingleConstrutorBeanB的構造方法上面加上@Lazy注解，就會發現不會拋出異常了，這又是為什么呢？

下面假設在SingleConstrutorBeanA的構造方法上面加了@Lazy注解，在構造B時，發現參數A時被@Lazy注解修飾時，那么就不會調用getBean來獲取對象，而是創建了一個代理對象，所以不會構成真正的循環依賴，不會拋出BeanCurrentlyInCreationException異常。

1. /**
2. * 處理懶加載對象
3. * 懶加載返回的又是一個代理對象，不會真正的調用getBean，所以如果構造方法依賴中有循環依賴，那么不會報錯
4. * @see org.springframework.context.annotation.ContextAnnotationAutowireCandidateResolver#getLazyResolutionProxyIfNecessary(org.springframework.beans.factory.config.DependencyDescriptor, java.lang.String)
5. */
6. Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
7. descriptor, requestingBeanName);
8. if (result == null) {
9. // 調用beanFactory.getBean(beanName)從容器中獲取依賴對象
10. result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
11. }
12. return result;

setter prototype循環依賴

對于prototype作用域bean，Spring容器無法完成依賴注入，因為Spring容器不進行緩存"prototype"作用域的bean，因此無法提前暴露一個創建中的bean。

使用

1. @Data
2. @Component
3. @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
4. public class PrototypeBeanA {
5. @Autowired
6. private PrototypeBeanB prototypeBeanB;
7. }

1. @Data
2. @Component
3. @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
4. public class PrototypeBeanB {
5. @Autowired
6. private PrototypeBeanA prototypeBeanA;
7. }

1. @Test
2. public void test3() {
3. AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
4. applicationContext.register(PrototypeBeanA.class);
5. applicationContext.register(PrototypeBeanB.class);
6. applicationContext.refresh();
7. applicationContext.getBean(PrototypeBeanA.class); // 此時必須要獲取Spring管理的實例，因為現在scope="prototype" 只有請求獲取的時候才會實例化對象
8. }

運行結果如下：

Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'prototypeBeanA': Requested bean is currently in creation: Is there an unresolvable circular reference?
 at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:269)
 at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
 at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1322)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1240)
 at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:668)
 ... 89 more

源碼分析

org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean

1. ... ...
2. // 判斷是否在當前創建Bean池中
3. if (isPrototypeCurrentlyInCreation(beanName)) {
4. throw new BeanCurrentlyInCreationException(beanName);
5. }
6. ... ...

異常就是在上面的代碼中拋出來的，那么beanName是什么時候添加至當前創建Bean池中的呢？

org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean

1. else if (mbd.isPrototype()) {
2. // It's a prototype -> create a new instance.
3. // prototype類型的bean的實例化
4. Object prototypeInstance = null;
5. try {
6. beforePrototypeCreation(beanName);
7. prototypeInstance = createBean(beanName, mbd, args);
8. }
9. finally {
10. afterPrototypeCreation(beanName);
11. }
12. bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
13. }

org.springframework.beans.factory.support.AbstractBeanFactory#beforePrototypeCreation

1. protected void beforePrototypeCreation(String beanName) {
2. // ThreadLocal
3. Object curVal = this.prototypesCurrentlyInCreation.get();
4. if (curVal == null) {
5. this.prototypesCurrentlyInCreation.set(beanName);
6. }
7. else if (curVal instanceof String) {
8. Set<String> beanNameSet = new HashSet<>(2);
9. beanNameSet.add((String) curVal);
10. beanNameSet.add(beanName);
11. this.prototypesCurrentlyInCreation.set(beanNameSet);
12. }
13. else {
14. Set<String> beanNameSet = (Set<String>) curVal;
15. beanNameSet.add(beanName);
16. }
17. }

其根本原因就是Spring容器不會對prototype類型的bean進行緩存，因此無法提前利用三級緩存暴露一個代理對象。

循環依賴開關

可以通過allowCircularReferences來禁止循環依賴，這樣的話，singleton bean的setter循環依賴也會報錯。

1. applicationContext.setAllowCircularReferences(false);

二級緩存可行？

理論上二級緩存時可行的，只需要將三級緩存中BeanFactory創建的對象提前放入二級緩存中，這樣三級緩存就可以移除了。

那么spring中為什么還要使用三級緩存呢？如果要使用二級緩存解決循環依賴，意味著所有Bean在實例化后就要完成AOP代理，這樣違背了Spring設計的原則，Spring在設計之初就是通過AnnotationAwareAspectJAutoProxyCreator這個后置處理器來在Bean生命周期的最后一步來完成AOP代理，而不是在實例化后就立馬進行AOP代理。

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原文鏈接：https://blog.csdn.net/u022812849/article/details/114264411