SpringBoot IoC（5）循環依賴與實例化總結

所謂的循環依賴，是指在實例化A類時，注入B類，實例化B類時，注入A類。

依賴注入分爲構造器注入和字段注入，下面依次對這兩種狀況分析。

構造器注入時

@Configuration
@RequiredArgsConstructor
public class Adam {
    private final Eve eve;
}
複製代碼

@RequiredArgsConstructor
@Configuration
public class Eve {
    private final Adam adam;
}
複製代碼

AbstractAutowireCapableBeanFactory->createBeanInstance():
    Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
            mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
        return autowireConstructor(beanName, mbd, ctors, args);
    }
複製代碼

在選擇構造器注入的時候，會遍歷字段，根據beanClass反推beanName，而後實例化。 一路debug，最終找到了拋出異常的調用棧：

DefaultSingletonBeanRegistry->getSingleton():
    beforeSingletonCreation(beanName);
DefaultSingletonBeanRegistry->beforeSingletonCreation():
    if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
        throw new BeanCurrentlyInCreationException(beanName);
    }
複製代碼

與之對應的是afterSingletonCreation方法

if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
        throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
    }
複製代碼

結論

singletonsCurrentlyInCreation是一個Set集合，表示當前正在建立的bean名稱，建立完成後，會移除。
原來，當同一個bean未完成實例化，又再一次實例化時，singletonsCurrentlyInCreation.add(beanName)返回false，進而拋出了異常，致使程序報錯

字段注入時

@Component
public class Adam {
    @Autowired
    private eve;
}
複製代碼

@Component
public class Eve {
    @Autowired
    private Adam adam;
}
複製代碼

這樣寫發現並無報錯，一路debug，發現是這段代碼起的做用：

AbstractBeanFactory->doGetBean():
    Object sharedInstance = getSingleton(beanName);
AbstractBeanFactory->getSingleton():
    return getSingleton(beanName, true);
AbstractBeanFactory->getSingleton(): 
    // 此時singletonObjects未完成，因此確定是null
    Object singletonObject = this.singletonObjects.get(beanName);
    // 根據前面的分析，singletonsCurrentlyInCreation確定包含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);
                }
            }
        }
    }
複製代碼

此時發現，earlySingletonObjects.get(beanName)爲空，singletonFactories.get(beanName)獲取到了值，繼而獲取到了singletonObject（這裏看出earlySingletonObjects建立，對應的singletonFactory會移除）。從新debug，發現是這裏給singletonFactories賦了值：

AbstractAutowireCapableBeanFactory->doCreateBean():
    if (earlySingletonExposure) {
        if (logger.isTraceEnabled()) {
            logger.trace("Eagerly caching bean '" + beanName +
                "' to allow for resolving potential circular references");
        }
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }
DefaultSingletonBeanRegistry->addSingletonFactory():
    synchronized (this.singletonObjects) {
        if (!this.singletonObjects.containsKey(beanName)) {
            this.singletonFactories.put(beanName, singletonFactory);
            this.earlySingletonObjects.remove(beanName);
            this.registeredSingletons.add(beanName);
        }
    }
複製代碼

這段代碼是在實例化完成以後進行的，也就是說，當構造器注入的時候，並無添加到singletonFactories，因此getSingleton(beanName)返回空，進行了第二次的實例化，從而致使報錯（這裏看出singletonFactories建立，對應的earlySingletonObjects會移除）。繼續debug，最終singletonFactories和earlySingletonObjects都被清除了：

DefaultSingletonBeanRegistry->getSingleton():
    addSingleton(beanName, singletonObject);
DefaultSingletonBeanRegistry->addSingleton():
    synchronized (this.singletonObjects) {
        this.singletonObjects.put(beanName, singletonObject);
        this.singletonFactories.remove(beanName);
        this.earlySingletonObjects.remove(beanName);
        this.registeredSingletons.add(beanName);
    }
複製代碼

當singletonObjects建立的時候，earlySingletonObjects和singletonFactories的使命就完成了~

結論

對於字段注入，IoC容器使用了singletonFactories和earlySingletonObjects來預防循環依賴。

附錄：正常狀況下一個bean的實例化過程

此處漏掉了AoP的邏輯，後續補充