开发过程中，经常存在类似于A->B，B->A的情况；详见spring源码 DefaultSingletonBeanRegistry的
getSingleton(String beanName, boolean allowEarlyReference)方法：
/**
* spring循环引用的处理思路： 允许提前执行引用当前创建的单例对象，
* 此时提前引用的单例对象的属性注入还未完成，因此可以解决循环引用的问题
* Return the (raw) singleton object registered under the given name.
* <p>Checks already instantiated singletons and also allows for an early
* reference to a currently created singleton (resolving a circular reference).
* 检查已经实例化的单例对象，并允许提前执行引用当前创建的单例(解析循环引用)。
* @param beanName the name of the bean to look for
* @param allowEarlyReference whether early references should be created or not
* @return the registered singleton object, or {@code null} if none found
*/
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 先从单例缓存中找，没有找到会先判断是否是正在创建的bean
// isSingletonCurrentlyInCreation 判断对应的单例对象是否在创建中
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
// earlySingletonObjects中保存所有提前曝光的单例，尝试从earlySingletonObjects中找
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
// 如果允许早期依赖，可以尝试从singletonFactories中找到对应的单例工厂
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//创建bean，并缓存提前曝光的bean，就是还未进行属性注入的bean，用于解决循环依赖
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
说明：
/** Cache of singleton objects: bean name to bean instance. */
/** 缓存单例bean */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
/** 缓存单例工厂 */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name to bean instance. */
/** 缓存提前曝光的单例bean 即还未完成属性注入的bean */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
/** Set of registered singletons, containing the bean names in registration order. */
private final Set<String> registeredSingletons = new LinkedHashSet<>(256);
ObjectFactory 接口的getObject()方法：
public interface ObjectFactory<T> {
/**
* 此方法返回被bean工厂管理的bean的实例
* Return an instance (possibly shared or independent)
* of the object managed by this factory.
* @return the resulting instance
* @throws BeansException in case of creation errors
*/
T getObject() throws BeansException;
}
@Component
public class A {
@Autowired
private B b;
}
@Component
public class B {
@Autowired
private A a;
}
1、首先创建A的实例a，然后对a做属性填充B
2、此时发现需要创建B的实例b，创建实例b，
3、对b做属性填充，发现需要A的实例；此时A的实例对象a正在创建中，此时B的对象b会设置属性A a=null;以用来完成B的实例化成功
4、B实例化对象b完成；设置A实例化对象a的b属性；完成a的实例化；
循环引用的解决方案：
1 使用@Autowired 注解，由spring决定对象属性的注入时机，先暴露对象A的引用，在需要的时候在注入对象B；
2 基于setter方法注入属性B