@Service
public class AServiceImpl implements AService {
    @Autowired
    private BService bService;
    ...
}
@Service
public class BServiceImpl implements BService {
    @Autowired
    private AService aService;
    ...
}

@Service
public class A {
    public A(B b) {
    }
}
@Service
public class B {
    public B(A a) {
    }
}

Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:339)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:215)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)

@Service
public class A {
    @Autowired
    private B b;
}
@Service
public class B {
    @Autowired
    private A a;
}

@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
@Service
public class A {
    @Autowired
    private B b;
}
@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
@Service
public class B {
    @Autowired
    private A a;
}

// 在單例Bean內注入
    @Autowired
    private A a;

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'mytest.TestSpringBean': Unsatisfied dependency expressed through field 'a'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'a': Unsatisfied dependency expressed through field 'b'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'b': Unsatisfied dependency expressed through field 'a'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:596)
	at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:90)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:374)

    @Lazy
    @Autowired
    private A a;

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
	...
	// 從上至下 分表代表這“三級緩存”
	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256); //一級緩存
	private final Map<String, Object> earlySingletonObjects = new HashMap<>(16); // 二級緩存
	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); // 三級緩存
	...
	/** Names of beans that are currently in creation. */
	// 這個緩存也十分重要：它表示bean創(chuàng)建過程中都會在里面呆著~
	// 它在Bean開始創(chuàng)建時放值，創(chuàng)建完成時會將其移出~
	private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16));
	/** Names of beans that have already been created at least once. */
	// 當這個Bean被創(chuàng)建完成后，會標記為這個 注意：這里是set集合 不會重復
	// 至少被創(chuàng)建了一次的  都會放進這里~~~~
	private final Set<String> alreadyCreated = Collections.newSetFromMap(new ConcurrentHashMap<>(256));
}

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
	...
	@Override
	@Nullable
	public Object getSingleton(String beanName) {
		return getSingleton(beanName, true);
	}
	@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			synchronized (this.singletonObjects) {
				singletonObject = this.earlySingletonObjects.get(beanName);
				if (singletonObject == null && allowEarlyReference) {
					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
					if (singletonFactory != null) {
						singletonObject = singletonFactory.getObject();
						this.earlySingletonObjects.put(beanName, singletonObject);
						this.singletonFactories.remove(beanName);
					}
				}
			}
		}
		return singletonObject;
	}
	...
	public boolean isSingletonCurrentlyInCreation(String beanName) {
		return this.singletonsCurrentlyInCreation.contains(beanName);
	}
	protected boolean isActuallyInCreation(String beanName) {
		return isSingletonCurrentlyInCreation(beanName);
	}
	...
}

// 它可以將創(chuàng)建對象的步驟封裝到ObjectFactory中 交給自定義的Scope來選擇是否需要創(chuàng)建對象來靈活的實現(xiàn)scope。  具體參見Scope接口
@FunctionalInterface
public interface ObjectFactory<T> {
	T getObject() throws BeansException;
}

public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
	...
	protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType, @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
		...
		// Eagerly check singleton cache for manually registered singletons.
		// 先去獲取一次，如果不為null，此處就會走緩存了~~
		Object sharedInstance = getSingleton(beanName);
		...
		// 如果不是只檢查類型，那就標記這個Bean被創(chuàng)建了~~添加到緩存里 也就是所謂的  當前創(chuàng)建Bean池
		if (!typeCheckOnly) {
			markBeanAsCreated(beanName);
		}
		...
		// Create bean instance.
		if (mbd.isSingleton()) {
			// 這個getSingleton方法不是SingletonBeanRegistry的接口方法  屬于實現(xiàn)類DefaultSingletonBeanRegistry的一個public重載方法~~~
			// 它的特點是在執(zhí)行singletonFactory.getObject();前后會執(zhí)行beforeSingletonCreation(beanName);和afterSingletonCreation(beanName);  
			// 也就是保證這個Bean在創(chuàng)建過程中，放入正在創(chuàng)建的緩存池里  可以看到它實際創(chuàng)建bean調用的是我們的createBean方法~~~~
			sharedInstance = getSingleton(beanName, () -> {
				try {
					return createBean(beanName, mbd, args);
				} catch (BeansException ex) {
					destroySingleton(beanName);
					throw ex;
				}
			});
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
		}
	}
	...
}
// 抽象方法createBean所在地  這個接口方法是屬于抽象父類AbstractBeanFactory的   實現(xiàn)在這個抽象類里
public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory implements AutowireCapableBeanFactory {
	...
	protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException {
		...
		// 創(chuàng)建Bean對象，并且將對象包裹在BeanWrapper 中
		instanceWrapper = createBeanInstance(beanName, mbd, args);
		// 再從Wrapper中把Bean原始對象（非代理~~~）  這個時候這個Bean就有地址值了，就能被引用了~~~
		// 注意：此處是原始對象，這點非常的重要
		final Object bean = instanceWrapper.getWrappedInstance();
		...
		// earlySingletonExposure 用于表示是否”提前暴露“原始對象的引用，用于解決循環(huán)依賴。
		// 對于單例Bean，該變量一般為 true   但你也可以通過屬性allowCircularReferences = false來關閉循環(huán)引用
		// isSingletonCurrentlyInCreation(beanName) 表示當前bean必須在創(chuàng)建中才行
		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");
			}
			// 上面講過調用此方法放進一個ObjectFactory，二級緩存會對應刪除的
			// getEarlyBeanReference的作用：調用SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference()這個方法  否則啥都不做
			// 也就是給調用者個機會，自己去實現(xiàn)暴露這個bean的應用的邏輯~~~
			// 比如在getEarlyBeanReference()里可以實現(xiàn)AOP的邏輯~~~  參考自動代理創(chuàng)建器AbstractAutoProxyCreator  實現(xiàn)了這個方法來創(chuàng)建代理對象
			// 若不需要執(zhí)行AOP的邏輯，直接返回Bean
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}
		Object exposedObject = bean; //exposedObject 是最終返回的對象
		...
		// 填充屬于，解決@Autowired依賴~
		populateBean(beanName, mbd, instanceWrapper);
		// 執(zhí)行初始化回調方法們~~~
		exposedObject = initializeBean(beanName, exposedObject, mbd);
		// earlySingletonExposure：如果你的bean允許被早期暴露出去 也就是說可以被循環(huán)引用  那這里就會進行檢查
		// 此段代碼非常重要~~~~~但大多數(shù)人都忽略了它
		if (earlySingletonExposure) {
			// 此時一級緩存肯定還沒數(shù)據(jù)，但是呢此時候二級緩存earlySingletonObjects也沒數(shù)據(jù)
			//注意，注意：第二參數(shù)為false  表示不會再去三級緩存里查了~~~
			// 此處非常巧妙的一點：：：因為上面各式各樣的實例化、初始化的后置處理器都執(zhí)行了，如果你在上面執(zhí)行了這一句
			//  ((ConfigurableListableBeanFactory)this.beanFactory).registerSingleton(beanName, bean);
			// 那么此處得到的earlySingletonReference 的引用最終會是你手動放進去的Bean最終返回，完美的實現(xiàn)了"偷天換日" 特別適合中間件的設計
			// 我們知道，執(zhí)行完此doCreateBean后執(zhí)行addSingleton()  其實就是把自己再添加一次  **再一次強調，完美實現(xiàn)偷天換日**
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
				// 這個意思是如果經(jīng)過了initializeBean()后，exposedObject還是木有變，那就可以大膽放心的返回了
				// initializeBean會調用后置處理器，這個時候可以生成一個代理對象，那這個時候它哥倆就不會相等了 走else去判斷吧
				if (exposedObject == bean) {
					exposedObject = earlySingletonReference;
				} 
				// allowRawInjectionDespiteWrapping這個值默認是false
				// hasDependentBean：若它有依賴的bean 那就需要繼續(xù)校驗了~~~(若沒有依賴的 就放過它~)
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
					// 拿到它所依賴的Bean們~~~~ 下面會遍歷一個一個的去看~~
					String[] dependentBeans = getDependentBeans(beanName);
					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					// 一個個檢查它所以Bean
					// removeSingletonIfCreatedForTypeCheckOnly這個放見下面  在AbstractBeanFactory里面
					// 簡單的說，它如果判斷到該dependentBean并沒有在創(chuàng)建中的了的情況下,那就把它從所有緩存中移除~~~  并且返回true
					// 否則（比如確實在創(chuàng)建中） 那就返回false 進入我們的if里面~  表示所謂的真正依賴
					//（解釋：就是真的需要依賴它先實例化，才能實例化自己的依賴）
					for (String dependentBean : dependentBeans) {
						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
							actualDependentBeans.add(dependentBean);
						}
					}
					// 若存在真正依賴，那就報錯（不要等到內存移除你才報錯，那是非常不友好的） 
					// 這個異常是BeanCurrentlyInCreationException，報錯日志也稍微留意一下，方便定位錯誤~~~~
					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.");
					}
				}
			}
		}
		return exposedObject;
	}
	// 雖然是remove方法 但是它的返回值也非常重要
	// 該方法唯一調用的地方就是循環(huán)依賴的最后檢查處~~~~~
	protected boolean removeSingletonIfCreatedForTypeCheckOnly(String beanName) {
		// 如果這個bean不在創(chuàng)建中  比如是ForTypeCheckOnly的  那就移除掉
		if (!this.alreadyCreated.contains(beanName)) {
			removeSingleton(beanName);
			return true;
		}
		else {
			return false;
		}
	}
}

if (exposedObject == bean) {
	exposedObject = earlySingletonReference;
}

protected <T> T doGetBean(...){
	... 
	// 標記beanName a是已經(jīng)創(chuàng)建過至少一次的~~~ 它會一直存留在緩存里不會被移除（除非拋出了異常）
	// 參見緩存Set<String> alreadyCreated = Collections.newSetFromMap(new ConcurrentHashMap<>(256))
	if (!typeCheckOnly) {
		markBeanAsCreated(beanName);
	}
	// 此時a不存在任何一級緩存中，且不是在創(chuàng)建中  所以此處返回null
	// 此處若不為null，然后從緩存里拿就可以了(主要處理FactoryBean和BeanFactory情況吧)
	Object beanInstance = getSingleton(beanName, false);
	...
	// 這個getSingleton方法非常關鍵。
	//1、標注a正在創(chuàng)建中~
	//2、調用singletonObject = singletonFactory.getObject();（實際上調用的是createBean()方法）  因此這一步最為關鍵
	//3、此時實例已經(jīng)創(chuàng)建完成  會把a移除整整創(chuàng)建的緩存中
	//4、執(zhí)行addSingleton()添加進去。（備注：注冊bean的接口方法為registerSingleton，它依賴于addSingleton方法）
	sharedInstance = getSingleton(beanName, () -> { ... return createBean(beanName, mbd, args); });
}protected <T> T doGetBean(...){
	... 
	// 標記beanName a是已經(jīng)創(chuàng)建過至少一次的~~~ 它會一直存留在緩存里不會被移除（除非拋出了異常）
	// 參見緩存Set<String> alreadyCreated = Collections.newSetFromMap(new ConcurrentHashMap<>(256))
	if (!typeCheckOnly) {
		markBeanAsCreated(beanName);
	}
	// 此時a不存在任何一級緩存中，且不是在創(chuàng)建中  所以此處返回null
	// 此處若不為null，然后從緩存里拿就可以了(主要處理FactoryBean和BeanFactory情況吧)
	Object beanInstance = getSingleton(beanName, false);
	...
	// 這個getSingleton方法非常關鍵。
	//1、標注a正在創(chuàng)建中~
	//2、調用singletonObject = singletonFactory.getObject();（實際上調用的是createBean()方法）  因此這一步最為關鍵
	//3、此時實例已經(jīng)創(chuàng)建完成  會把a移除整整創(chuàng)建的緩存中
	//4、執(zhí)行addSingleton()添加進去。（備注：注冊bean的接口方法為registerSingleton，它依賴于addSingleton方法）
	sharedInstance = getSingleton(beanName, () -> { ... return createBean(beanName, mbd, args); });
}

protected Object doCreateBean(){
	...
	// 使用構造器/工廠方法   instanceWrapper是一個BeanWrapper
	instanceWrapper = createBeanInstance(beanName, mbd, args);
	// 此處bean為"原始Bean"   也就是這里的A實例對象：A@1234
	final Object bean = instanceWrapper.getWrappedInstance();
	...
	// 是否要提前暴露（允許循環(huán)依賴）  現(xiàn)在此處A是被允許的
	boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences && isSingletonCurrentlyInCreation(beanName));
	// 允許暴露，就把A綁定在ObjectFactory上，注冊到三級緩存`singletonFactories`里面去保存著
	// Tips:這里后置處理器的getEarlyBeanReference方法會被促發(fā)，自動代理創(chuàng)建器在此處創(chuàng)建代理對象（注意執(zhí)行時機 為執(zhí)行三級緩存的時候）
	if (earlySingletonExposure) {
		addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
	}
	...
	// exposedObject 為最終返回的對象，此處為原始對象bean也就是A@1234,下面會有用處
	Object exposedObject = bean; 
	// 給A@1234屬性完成賦值，@Autowired在此處起作用~
	// 因此此處會調用getBean("b")，so 會重復上面步驟創(chuàng)建B類的實例
	// 此處我們假設B已經(jīng)創(chuàng)建好了 為B@5678
	// 需要注意的是在populateBean("b")的時候依賴有beanA，所以此時候調用getBean("a")最終會調用getSingleton("a")，
	//此時候上面說到的getEarlyBeanReference方法就會被執(zhí)行。這也解釋為何我們@Autowired是個代理對象，而不是普通對象的根本原因
	populateBean(beanName, mbd, instanceWrapper);
	// 實例化。這里會執(zhí)行后置處理器BeanPostProcessor的兩個方法
	// 此處注意：postProcessAfterInitialization()是有可能返回一個代理對象的，這樣exposedObject 就不再是原始對象了  特備注意哦~~~
	// 比如處理@Aysnc的AsyncAnnotationBeanPostProcessor它就是在這個時間里生成代理對象的（有坑，請小心使用@Aysnc）
	exposedObject = initializeBean(beanName, exposedObject, mbd);
	... // 至此，相當于A@1234已經(jīng)實例化完成、初始化完成（屬性也全部賦值了~）
	// 這一步我把它理解為校驗：校驗：校驗是否有循環(huán)引用問題~~~~~
	if (earlySingletonExposure) {
		// 注意此處第二個參數(shù)傳的false，表示不去三級緩存里singletonFactories再去調用一次getObject()方法了~~~
		// 上面建講到了由于B在初始化的時候，會觸發(fā)A的ObjectFactory.getObject()  所以a此處已經(jīng)在二級緩存earlySingletonObjects里了
		// 因此此處返回A的實例：A@1234
		Object earlySingletonReference = getSingleton(beanName, false);
		if (earlySingletonReference != null) {
			// 這個等式表示，exposedObject若沒有再被代理過，這里就是相等的
			// 顯然此處我們的a對象的exposedObject它是沒有被代理過的  所以if會進去~
			// 這種情況至此，就全部結束了~~~
			if (exposedObject == bean) {
				exposedObject = earlySingletonReference;
			}
			// 繼續(xù)以A為例，比如方法標注了@Aysnc注解，exposedObject此時候就是一個代理對象，因此就會進到這里來
			//hasDependentBean(beanName)是肯定為true，因為getDependentBeans(beanName)得到的是["b"]這個依賴
			else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
				String[] dependentBeans = getDependentBeans(beanName);
				Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
				// A@1234依賴的是["b"]，所以此處去檢查b
				// 如果最終存在實際依賴的bean：actualDependentBeans不為空 那就拋出異常  證明循環(huán)引用了~
				for (String dependentBean : dependentBeans) {
					// 這個判斷原則是：如果此時候b并還沒有創(chuàng)建好，this.alreadyCreated.contains(beanName)=true表示此bean已經(jīng)被創(chuàng)建過，就返回false
					// 若該bean沒有在alreadyCreated緩存里，就是說沒被創(chuàng)建過(其實只有CreatedForTypeCheckOnly才會是此倉庫)
					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.");
				}
			}
		}
	}
}