SpringBoot IoC（5）循环依赖与实例化总结

所谓的循环依赖，是指在实例化A类时，注入B类，实例化B类时，注入A类。

依赖注入分为构造器注入和字段注入，下面依次对这两种状况分析。

构造器注入时

@Configuration
@RequiredArgsConstructor
public class Adam {
    private final Eve eve;
}
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@RequiredArgsConstructor
@Configuration
public class Eve {
    private final Adam adam;
}
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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);
    }
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在选择构造器注入的时候，会遍历字段，根据beanClass反推beanName，而后实例化。 一路debug，最终找到了抛出异常的调用栈：

DefaultSingletonBeanRegistry->getSingleton():
    beforeSingletonCreation(beanName);
DefaultSingletonBeanRegistry->beforeSingletonCreation():
    if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
        throw new BeanCurrentlyInCreationException(beanName);
    }
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与之对应的是afterSingletonCreation方法

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

singletonsCurrentlyInCreation是一个Set集合，表示当前正在建立的bean名称，建立完成后，会移除。
原来，当同一个bean未完成实例化，又再一次实例化时，singletonsCurrentlyInCreation.add(beanName)返回false，进而抛出了异常，致使程序报错

字段注入时

@Component
public class Adam {
    @Autowired
    private eve;
}
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@Component
public class Eve {
    @Autowired
    private Adam adam;
}
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这样写发现并无报错，一路debug，发现是这段代码起的做用：

AbstractBeanFactory->doGetBean():
    Object sharedInstance = getSingleton(beanName);
AbstractBeanFactory->getSingleton():
    return getSingleton(beanName, true);
AbstractBeanFactory->getSingleton(): 
    // 此时singletonObjects未完成，因此确定是null
    Object singletonObject = this.singletonObjects.get(beanName);
    // 根据前面的分析，singletonsCurrentlyInCreation确定包含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);
                }
            }
        }
    }
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此时发现，earlySingletonObjects.get(beanName)为空，singletonFactories.get(beanName)获取到了值，继而获取到了singletonObject（这里看出earlySingletonObjects建立，对应的singletonFactory会移除）。从新debug，发现是这里给singletonFactories赋了值：

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);
        }
    }
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这段代码是在实例化完成以后进行的，也就是说，当构造器注入的时候，并无添加到singletonFactories，因此getSingleton(beanName)返回空，进行了第二次的实例化，从而致使报错（这里看出singletonFactories建立，对应的earlySingletonObjects会移除）。继续debug，最终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);
    }
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当singletonObjects建立的时候，earlySingletonObjects和singletonFactories的使命就完成了~

结论

对于字段注入，IoC容器使用了singletonFactories和earlySingletonObjects来预防循环依赖。

附录：正常状况下一个bean的实例化过程

此处漏掉了AoP的逻辑，后续补充