Spring bean的循环依赖以及解决方式

什么是循环依赖

循环依赖其实就是循环引用，也就是两个或则两个以上的bean互相持有对方，最终形成闭环。比如A依赖于B，B依赖于C，C又依赖于A，由于对象之间的依赖关系造成了死循环。

Spring中循环依赖场景有：

• 构造器的循环依赖
• field属性的循环依赖

Spring怎么解决循环依赖

Spring循环依赖主要基于Java引用传递，当获取到对象时，对象的field或者属性可以延后设置

创建Bean

    /**
     * Actually create the specified bean. Pre-creation processing has already happened
     * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
     * 
Differentiates between default bean instantiation, use of a
     * factory method, and autowiring a constructor.
     * @param beanName the name of the bean
     * @param mbd the merged bean definition for the bean
     * @param args explicit arguments to use for constructor or factory method invocation
     * @return a new instance of the bean
     * @throws BeanCreationException if the bean could not be created
     * @see #instantiateBean
     * @see #instantiateUsingFactoryMethod
     * @see #autowireConstructor
     * 实际创建指定的bean。 预创建处理已经发生
     * 在这一点上，例如 检查{@code postProcessBeforeInstantiation}回调。
     * 
区分默认的bean实例化，使用
     * 工厂方法，并自动装配构造函数。
     * @param beanName bean的名称
     * @param mbd该bean的合并bean定义
     * @param args用于构造函数或工厂方法调用的显式参数
     * @返回bean的新实例
     * @throws BeanCreationException如果无法创建bean
     * @请参阅#instantiateBean
     * @see #instantiateUsingFactoryMethod
     * @请参阅#autowireConstructor
     */
    protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
            throws BeanCreationException {

        // Instantiate the bean.
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
             //从缓存中移除bean，因为要创建新的bean了
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
              //根据指定bean的使用策略创建新的实例，如：工厂方法，构造函数自动注入，简单初始化
            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.
        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.
         //是否需要提早曝光：单例&允许循环依赖&当前bean正在创建中，检测循环依赖
        boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                isSingletonCurrentlyInCreation(beanName));
        if (earlySingletonExposure) {
            if (logger.isDebugEnabled()) {
                logger.debug("Eagerly caching bean '" + beanName +
                        "' to allow for resolving potential circular references");
            }
             //为避免后期循环依赖，可以在bean初始化完成前将创建实例的ObjectFactory加入工厂，其他bean引用此bean时直接从ObjectFactory获取bean
             //getEarlyBeanReference对bean再一次依赖引用，主要应用SmartInstantiationAware BeanPost Processor其中我们熟悉的AOP就是在这里将advance动态织入bean中的，如果没有就直接返回bean，不做任何处理
            addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
        }

        // Initialize the bean instance.
        Object exposedObject = bean;
        try {
             //对bean进行填充,将各个属性值注入，其中，可能存在依赖于其他bean的属性，则会递归给初始依赖bean
            populateBean(beanName, mbd, instanceWrapper);
             //调用初始化方法，如init-method
            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);
            }
        }

        if (earlySingletonExposure) {
            Object earlySingletonReference = getSingleton(beanName, false);
             //earlySingletonReference只有在检测到有依赖循环的情况下才会不为空
            if (earlySingletonReference != null) {
                  //如果exposedObject没有在初始化方法中被改变，也就是没有被增强
                if (exposedObject == bean) {
                    exposedObject = earlySingletonReference;
                }
                else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                    String[] dependentBeans = getDependentBeans(beanName);
                    Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
                    for (String dependentBean : dependentBeans) {
                          //检测依赖
                        if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                            actualDependentBeans.add(dependentBean);
                        }
                    }
                      //因为bean创建后其所依赖的bean一定是已经创建的，actualDependentBeans不为空则表示当前bean创建后其依赖的bean却没有创建完，也就是存在循环依赖
                    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 {
              //根据scopse注册bean
            registerDisposableBeanIfNecessary(beanName, bean, mbd);
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
        }

        return exposedObject;
    }

以上是Spring创建bean的方法，代码很多但是核心思路就是实例化，填充属性，初始化，而循环依赖主要发生在实例化和填充属性这两部分

        //根据指定bean的使用策略创建新的实例，如：工厂方法，构造函数自动注入，简单初始化
        instanceWrapper = createBeanInstance(beanName, mbd, args);
        //对bean进行填充,将各个属性值注入，其中，可能存在依赖于其他bean的属性，则会递归给初始依赖bean
        populateBean(beanName, mbd, instanceWrapper);
        //调用初始化方法，如init-method
        exposedObject = initializeBean(beanName, exposedObject, mbd);

Spring为了解决循环依赖，设置了三级缓存

一级缓存singletonObjects

用于保存BeanName和创建bean实例之间的关系,beanName -> bean instance

private final Map singletonObjects = new ConcurrentHashMap(256);

二级缓存earlySingletonObjects

保存提前曝光的单例bean对象

private final Map earlySingletonObjects = new HashMap<>(16);

三级缓存singletonFactories

保存beanName和创建bean实例之间的关系,与singletonObjects不同的地方在于，当一个单例bean被放到这里面后，bean在创建过程中，可以通过getBean方法获取到,目的是用来检测循环引用

private final Map singletonFactories = new HashMap(16);

在创建bean的时候，首先从缓存中获取单例的bean，这个缓存就是singletonObjects，如果获取不到且bean正在创建中，就再从earlySingletonObjects中获取，如果还是获取不到且允许从singletonFactories中通过getObject拿到对象，就从singletonFactories中获取，如果获取到了就存入earlySingletonObjects并从singletonFactories中移除。

getSingleton

    //用于保存BeanName和创建bean实例之间的关系,beanName -> bean instance
    private final Map singletonObjects = new ConcurrentHashMap(256);
    //保存提前曝光的单例bean对象
    private final Map earlySingletonObjects = new HashMap<>(16);
    //保存beanName和创建bean实例之间的关系,与singletonObjects不同的地方在于，当一个单例bean被放到这里面后，bean在创建过程中，可以通过getBean方法获取到,目的是用来检测循环引用
    private final Map singletonFactories = new HashMap(16);
    //保存当前所有已注册的bean
    private final Set registeredSingletons = new LinkedHashSet(256);

    /**
     * Return the (raw) singleton object registered under the given name.
     * 
Checks already instantiated singletons and also allows for an early
     * reference to a currently created singleton (resolving a circular reference).
     * @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
     * 返回以给定名称注册的（原始）单例对象。
     * 
检查已实例化的单例，并允许早期
     * 引用当前创建的单例（解析循环引用）。
     * @param beanName bean的名称
     * @param allowEarlyReference 是否允许从singletonFactories中通过getObject拿到对象
     * @返回注册的单例对象；如果找不到，则返回{@code null}
     */
    @Nullable
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
         //检查缓存中是否存在实例
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
             //如果为空,且正在创建中,锁定全局变量并进行处理
            synchronized (this.singletonObjects) {
                  //如果bean正在加载则不处理
                singletonObject = this.earlySingletonObjects.get(beanName);
                if (singletonObject == null && allowEarlyReference) {
                      //当某些方法需要提前初始化的时候会调用addSingletonFactory方法将对于ObjectFactory初始化策略存储在singletonFactories
                    ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                          //调用预设的getObject方法
                        singletonObject = singletonFactory.getObject();
                          //记录在缓存中earlySingletonObjects和singletonFactories互斥
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return singletonObject;
    }

addSingletonFactory

    /**
     * Add the given singleton factory for building the specified singleton
     * if necessary.
     * 
To be called for eager registration of singletons, e.g. to be able to
     * resolve circular references.
     * @param beanName the name of the bean
     * @param singletonFactory the factory for the singleton object
     */
    protected void addSingletonFactory(String beanName, ObjectFactory singletonFactory) {
        Assert.notNull(singletonFactory, "Singleton factory must not be null");
        synchronized (this.singletonObjects) {
            if (!this.singletonObjects.containsKey(beanName)) {
                this.singletonFactories.put(beanName, singletonFactory);
                this.earlySingletonObjects.remove(beanName);
                this.registeredSingletons.add(beanName);
            }
        }
    }

现在有A的field或者setter依赖B的实例对象，同时B的field或者setter依赖了A的实例，A首先开始创建，并将自己暴露到 singletonFactories 中，开始填充属性，此时发现自己依赖B的属性，尝试去get(B)，发现B还没有被创建，所以开始创建B，在进行属性填充时初始化A，就从singletonObjects中获取到了初始化但没有任何属性的A，B拿到A后完成了初始化阶段，将自己放到singletonObjects中,此时返回A，A拿到B的对象继续完成初始化，完成后将自己放到singletonObjects中，由A与B中所表示的A的属性地址是一样的，所以A的属性填充完后，B也获取了A的属性，这样就解决了循环的问题。