关于spring循环依赖问题及解决方案

if (isPrototypeCurrentlyInCreation(beanName)) 
{  throw new BeanCurrentlyInCreationException(beanName);}

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
    BeanWrapper instanceWrapper = null;
	
    if (instanceWrapper == null) {
        // ① 实例化对象
        instanceWrapper = this.createBeanInstance(beanName, mbd, args);
    }
    final Object bean = instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null;
    Class<?> beanType = instanceWrapper != null ? instanceWrapper.getWrappedClass() : null;
   
    // ② 判断是否允许提前暴露对象，如果允许，则直接添加一个 ObjectFactory 到三级缓存
	boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        // 添加三级缓存的方法详情在下方
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }
    // ③ 填充属性
    this.populateBean(beanName, mbd, instanceWrapper);
    // ④ 执行初始化方法，并创建代理
    exposedObject = initializeBean(beanName, exposedObject, mbd);
   
    return exposedObject;
}

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(singletonFactory, "Singleton factory must not be null");
    synchronized (this.singletonObjects) {
        if (!this.singletonObjects.containsKey(beanName)) { // 判断一级缓存中不存在此对象
            this.singletonFactories.put(beanName, singletonFactory); // 添加至三级缓存
            this.earlySingletonObjects.remove(beanName); // 确保二级缓存没有此对象
            this.registeredSingletons.add(beanName);
        }
    }
}
@FunctionalInterface
public interface ObjectFactory<T> {
	T getObject() throws BeansException;
}

protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
    Object exposedObject = bean;
    if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
                SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
                // 如果需要代理，这里会返回代理对象；否则返回原始对象
                exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
            }
        }
    }
    return exposedObject;
}

public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
		implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {
    @Override
    public Object getEarlyBeanReference(Object bean, String beanName) {
        Object cacheKey = getCacheKey(bean.getClass(), beanName);
        // 记录已被代理的对象
        this.earlyProxyReferences.put(cacheKey, bean);
        return wrapifNecessary(bean, beanName, cacheKey);
    }
}

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) {
                // 三级缓存
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    // Bean 工厂中获取 Bean
                    singletonObject = singletonFactory.getObject();
                    // 放入到二级缓存中
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return singletonObject;
}

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(singletonFactory, "Singleton factory must not be null");
    synchronized (this.singletonObjects) {
        if (!this.singletonObjects.containsKey(beanName)) { // 判断一级缓存中不存在此对象
            object o = singletonFactory.getObject(); // 直接从工厂中获取 Bean
            this.earlySingletonObjects.put(beanName, o); // 添加至二级缓存中
            this.registeredSingletons.add(beanName);
        }
    }
}

@RestController
public class AccountController {
    private static final Logger LOG = LoggerFactory.getLogger(AccountController.class);
    private AccountService accountService;
    // 构造函数依赖注入
    // 不管是否设置为required为true，都会出现循环依赖问题
    @Autowire
    // @Autowired(required = false)
    public AccountController(AccountService accountService) {
        this.accountService = accountService;
    }
    
}
@Service
public class AccountService {
    private static final Logger LOG = LoggerFactory.getLogger(AccountService.class);
    
    // 属性值依赖注入
    @Autowired
    private AccountController accountController;
   } 

@RestController
public class AccountController {
    private static final Logger LOG = LoggerFactory.getLogger(AccountController.class);
    // 属性值注入
    @Autowired
    private AccountService accountService;
    
}
@Service
public class AccountService {
    private AccountController accountController;
    // 构造函数注入
    @Autowired
    public AccountService(AccountController accountController) {
        this.accountController = accountController;
    }
    
}

    // bean对象实例创建的核心实现方法
    protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
    		throws BeanCreationException {
    		// 省略其他代码
    		// 1. 调用构造函数创建该bean对象，若不存在构造函数注入，顺利通过
    		instanceWrapper = createBeanInstance(beanName, mbd, args);
    		// 2. 在singletonFactories缓存中，放入该bean对象，以便解决循环依赖问题
    		addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    		// 3. populateBean方法：bean对象的属性赋值
    		populateBean(beanName, mbd, instanceWrapper); 		
    		// 省略其他代码
    	return exposedObject;
    }