通过前几节的分析，已经成功将bean实例化，但是大家一定要将bean的实例化和完成bean的创建区分开，bean的实例化仅仅是获得了bean的实例，该bean仍在继续创建之中，之后在该bean实例的基础之上，还要做很多额外的操作，例如bean的属性填充、处理器的应用、bean的循环依赖解决等，今天我们就来分析下Spring是如何解决bean之间的循环依赖。

当ClassA引用ClassB，ClassB又引用ClassA，那么两个类之间就会形成一个闭环，导致循环依赖的出现。大家只需记住一点，Spring只能解决单例模式下的Setter循环依赖。

1.测试用例

• bean和xml

package com.lyc.cn.v2.day01.cycle;

/**
 * @author: LiYanChao
 * @create: 2018-10-16 23:59
 */
public class ClassA {
    private ClassB classB;

    public ClassB getClassB() {
        return classB;
    }

    public void setClassB(ClassB classB) {
        this.classB = classB;
    }
}

package com.lyc.cn.v2.day01.cycle;

/**
 * @author: LiYanChao
 * @create: 2018-10-16 23:59
 */
public class ClassB {
    private ClassA classA;

    public ClassA getClassA() {
        return classA;
    }

    public void setClassA(ClassA classA) {
        this.classA = classA;
    }
}

<!--循环依赖-->
<bean id="classA" class="com.lyc.cn.v2.day01.cycle.ClassA" scope="singleton">
    <property name="classB" ref="classB"></property>
</bean>
<bean id="classB" class="com.lyc.cn.v2.day01.cycle.ClassB" scope="singleton">
    <property name="classA" ref="classA"></property>
</bean>

• 结果

========测试方法开始=======

com.lyc.cn.v2.day01.cycle.ClassB@2d6a9952
com.lyc.cn.v2.day01.cycle.ClassA@22a71081

========测试方法结束=======

当scope="singleton"时结果是正常的，Spring为我们解决了bean之间的循环依赖，再将scope改为prototype，运行测试用例（摘取部分异常信息）：

Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'classA': Requested bean is currently in creation: Is there an unresolvable circular reference?
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:255)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:198)
    at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:372)
    ... 40 more

从异常信息中可以看到Is there an unresolvable circular reference?,有循环依赖异常，这也证明了Spring是不能解决prototype作用域的bean之间的循环依赖的。

下面我们从源码角度去分析，Spring是如何解决bean之间的循环依赖问题的。

引

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException {

    // Instantiate the bean.
    // ① 实例化bean
    BeanWrapper instanceWrapper = null;
    // 注意factoryBeanInstanceCache是ConcurrentMap,remove方法会返回删除的键值(如果不存在返回null)
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    // 如果factoryBeanInstanceCache没有缓存对应的BeanWrapper,则重新创建bean实例
    if (instanceWrapper == null) {
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    final Object bean = instanceWrapper.getWrappedInstance();
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }

    // Allow post-processors to modify the merged bean definition.
    // ② 允许MergedBeanDefinitionPostProcessor后处理器修改已合并的bean定义。
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            }
            catch (Throwable ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Post-processing of merged bean definition failed", ex);
            }
            mbd.postProcessed = true;
        }
    }

    // Eagerly cache singletons to be able to resolve circular references
    // even when triggered by lifecycle interfaces like BeanFactoryAware.
    // ③ 提前缓存ObjectFactory以解决bean之间的循环依赖
    // mbd.isSingleton()->是否单例,Spring只解决单例bean的循环依赖问题
    // allowCircularReferences->是否允许循环依赖
    // isSingletonCurrentlyInCreation->该bean是否创建中
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences && isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }

    // Initialize the bean instance.
    // ④ 初始化bean实例 这里大家要与第①步区分开,到这里bean已经完成了实例化,但是还没有完成初始化的操作,例如bean的属性填充
    Object exposedObject = bean;
    try {
        // 填充bean属性
        populateBean(beanName, mbd, instanceWrapper);
        // 初始化bean
        exposedObject = initializeBean(beanName, exposedObject, mbd);
    }
    catch (Throwable ex) {
        if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
            throw (BeanCreationException) ex;
        }
        else {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
        }
    }

    // ⑤ 循环依赖检查
    if (earlySingletonExposure) {
        Object earlySingletonReference = getSingleton(beanName, false);
        if (earlySingletonReference != null) {
            if (exposedObject == bean) {
                exposedObject = earlySingletonReference;
            }
            else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
                for (String dependentBean : dependentBeans) {
                    if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                        actualDependentBeans.add(dependentBean);
                    }
                }
                if (!actualDependentBeans.isEmpty()) {
                    throw new BeanCurrentlyInCreationException(beanName,
                            "Bean with name '" + beanName + "' has been injected into other beans [" +
                            StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                            "] in its raw version as part of a circular reference, but has eventually been " +
                            "wrapped. This means that said other beans do not use the final version of the " +
                            "bean. This is often the result of over-eager type matching - consider using " +
                            "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
                }
            }
        }
    }

    // Register bean as disposable.
    try {
        // ⑥ 根据bean的作用域注册bean
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }
    // ⑦ 返回bean实例
    return exposedObject;
}

通过第一步已经获得了bean的实例（第二步留在以后再讲解），直接看第三步：提前缓存ObjectFactory以解决bean之间的循环依赖。

1.提前曝光对象

这里涉及到一个非常重要的接口ObjectFactory，该接口是一个函数式接口且只有一个方法：T getObject() throws BeansException;，该方法用于返回一个bean的实例，此时的bean已经完成初始化，但是尚未完成创建。

如果当前的bean满足条件，则将当前正在创建的bean和其ObjectFactory对象提前曝光，加入到正在创建bean池中。

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
    synchronized (this.singletonObjects) {
        if (!this.singletonObjects.containsKey(beanName)) {
            this.singletonFactories.put(beanName, singletonFactory);
            this.earlySingletonObjects.remove(beanName);
            this.registeredSingletons.add(beanName);
        }
    }
}

2.循环依赖的解决

在完成bean的实例创建之后，还要填充bean的属性，针对ClassA，其属性是ClassB，如果要填充ClassA的属性则势必先要实例化ClassB，那么这里又涉及到一个概念，RuntimeBeanReference-->运行时引用。

打开BeanDefinitionValueResolver类的resolveValueIfNecessary方法。摘取代码片段（该方法会在以后全部分析）

• 判断RuntimeBeanReference属性

// ① RuntimeBeanReference->运行时引用
//   例如BeanA依赖BeanB,那么在配置文件中有通过配置ref标签进行引用的,在解析BeanDefinition的时候,是不会直接实例化BeanB的,那么这个引用就是RuntimeBeanReference
if (value instanceof RuntimeBeanReference) {
    RuntimeBeanReference ref = (RuntimeBeanReference) value;
    return resolveReference(argName, ref);
}

• 解析RuntimeBeanReference（运行时引用）

private Object resolveReference(Object argName, RuntimeBeanReference ref) {
    try {
        // 1、解析引用beanName
        Object bean;
        String refName = ref.getBeanName();
        refName = String.valueOf(doEvaluate(refName));
        // 2、判断引用bean是否属于父BeanFactory
        if (ref.isToParent()) {
            if (this.beanFactory.getParentBeanFactory() == null) {
                throw new BeanCreationException(
                        this.beanDefinition.getResourceDescription(), this.beanName,
                        "Can't resolve reference to bean '" + refName +
                        "' in parent factory: no parent factory available");
            }
            bean = this.beanFactory.getParentBeanFactory().getBean(refName);
        }
        // 3、从当前beanFactory获取引用beanName实例
        else {
            bean = this.beanFactory.getBean(refName);
            this.beanFactory.registerDependentBean(refName, this.beanName);
        }
        if (bean instanceof NullBean) {
            bean = null;
        }
        return bean;
    }
    catch (BeansException ex) {
        throw new BeanCreationException(
                this.beanDefinition.getResourceDescription(), this.beanName,
                "Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
    }
}

该过程很简单，首先解析refBeanName，然后通过getBean方法获取其实例，此时当前创建的bean是ClassA，引用bean是ClassB。

获取到ClassB实例之后，又要填充ClassB的属性，此时又会出现对RuntimeBeanReference的解析，即ClassA，再去获取ClassA的实例，此时的ClassA的实例已经被提前曝光，会从缓存中获取ClassA的实例。

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    // 1、从缓存中获取bean
    Object singletonObject = this.singletonObjects.get(beanName);
    // 2、未能获取到bean,但是允许对当前创建的单例的早期引用(解决循环引用)
    // isSingletonCurrentlyInCreation-->判断指定的单例bean是否当前正在创建(Spring只解决单例bean的循环依赖问题)
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        synchronized (this.singletonObjects) {
            // 从earlySingletonObjects获取提前曝光的bean
            singletonObject = this.earlySingletonObjects.get(beanName);
            // 未能获取到提前曝光的bean且当前的bean允许被创建早期依赖
            if (singletonObject == null && allowEarlyReference) {
                // 从缓存中获取BeanFactory
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    // 通过getObject()方法获取提前曝光的bean
                    singletonObject = singletonFactory.getObject();
                    // 将获取到的singletonObject缓存至earlySingletonObjects
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    // 从singletonFactories移除bean
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return singletonObject;
}

从singletonObjects中无法获取到bean的实例，因为此时bean尚未完成全部创建，但是由于我们提前曝光了ObjectFactory，所以通过singletonObject = singletonFactory.getObject();是可以获取到bean的实例的。这样就解决了Spring的循环依赖问题。

3.总结

• Spring只能解决Setter方法注入的单例bean之间的循环依赖
• ClassA依赖ClassB，ClassB又依赖ClassA，形成依赖闭环。Spring在获取ClassA的实例时，不等ClassA完成创建就将其曝光加入正在创建的bean缓存中。在解析ClassA的属性时，又发现依赖于ClassB，再次去获取ClassB，当解析ClassB的属性时，又发现需要ClassA的属性，但此时的ClassA已经被提前曝光加入了正在创建的bean的缓存中，则无需创建新的的ClassA的实例，直接从缓存中获取即可。从而解决循环依赖问题。