開發過程中，經常存在類似於A->B，B->A的情況；詳見spring源碼 DefaultSingletonBeanRegistry的
getSingleton(String beanName, boolean allowEarlyReference)方法：
/**
* spring循環引用的處理思路： 允許提前執行引用當前創建的單例對象，
* 此時提前引用的單例對象的屬性注入還未完成，因此可以解決循環引用的問題
* 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).
* 檢查已經實例化的單例對象，並允許提前執行引用當前創建的單例(解析循環引用)。
* @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) {
// 先從單例緩存中找，沒有找到會先判斷是否是正在創建的bean
// isSingletonCurrentlyInCreation 判斷對應的單例對象是否在創建中
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單例，嘗試從earlySingletonObjects中找
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
// 如果允許早期依賴，可以嘗試從singletonFactories中找到對應的單例工廠
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//創建bean，並緩存提前曝光的bean，就是還未進行屬性注入的bean，用於解決循環依賴
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
說明：
/** Cache of singleton objects: bean name to bean instance. */
/** 緩存單例bean */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
/** 緩存單例工廠 */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name to bean instance. */
/** 緩存提前曝光的單例bean 即還未完成屬性注入的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工廠管理的bean的實例
* 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、首先創建A的實例a，然后對a做屬性填充B
2、此時發現需要創建B的實例b，創建實例b，
3、對b做屬性填充，發現需要A的實例；此時A的實例對象a正在創建中，此時B的對象b會設置屬性A a=null;以用來完成B的實例化成功
4、B實例化對象b完成；設置A實例化對象a的b屬性；完成a的實例化；
循環引用的解決方案：
1 使用@Autowired 注解，由spring決定對象屬性的注入時機，先暴露對象A的引用，在需要的時候在注入對象B；
2 基於setter方法注入屬性B