24--Spring解決bean之間的迴圈依賴

通過前幾節的分析，已經成功將bean例項化，但是大家一定要將bean的例項化和完成bean的建立區分開，bean的例項化僅僅是獲得了bean的例項，該bean仍在繼續建立之中，之後在該bean例項的基礎之上，還要做很多額外的操作，例如bean的屬性填充、處理器的應用、bean的迴圈依賴解決等，今天我們就來分析下Spring是如何解決bean之間的迴圈依賴。

當ClassA引用ClassB，ClassB又引用ClassA，那麼兩個類之間就會形成一個閉環，導致迴圈依賴的出現。大家只需記住一點，Spring只能解決單例模式下的Setter迴圈依賴。

1.測試用例

• 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;
    }
}

<!--迴圈依賴-->
<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>

• 結果

========測試方法開始=======

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

========測試方法結束=======

當scope="singleton"時結果是正常的，Spring為我們解決了bean之間的迴圈依賴，再將scope改為prototype，執行測試用例（摘取部分異常資訊）：

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

從異常資訊中可以看到Is there an unresolvable circular reference?,有迴圈依賴異常，這也證明了Spring是不能解決prototype作用域的bean之間的迴圈依賴的。

下面我們從原始碼角度去分析，Spring是如何解決bean之間的迴圈依賴問題的。

引

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

    // Instantiate the bean.
    // ① 例項化bean
    BeanWrapper instanceWrapper = null;
    // 注意factoryBeanInstanceCache是ConcurrentMap,remove方法會返回刪除的鍵值(如果不存在返回null)
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    // 如果factoryBeanInstanceCache沒有快取對應的BeanWrapper,則重新建立bean例項
    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 允許MergedBeanDefinitionPostProcessor後處理器修改已合併的bean定義。
    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 提前快取ObjectFactory以解決bean之間的迴圈依賴
    // mbd.isSingleton()->是否單例,Spring只解決單例bean的迴圈依賴問題
    // allowCircularReferences->是否允許迴圈依賴
    // isSingletonCurrentlyInCreation->該bean是否建立中
    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例項 這裡大家要與第①步區分開,到這裡bean已經完成了例項化,但是還沒有完成初始化的操作,例如bean的屬性填充
    Object exposedObject = bean;
    try {
        // 填充bean屬性
        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 迴圈依賴檢查
    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 根據bean的作用域註冊bean
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }
    // ⑦ 返回bean例項
    return exposedObject;
}

通過第一步已經獲得了bean的例項（第二步留在以後再講解），直接看第三步：提前快取ObjectFactory以解決bean之間的迴圈依賴。

1.提前曝光物件

這裡涉及到一個非常重要的介面ObjectFactory，該介面是一個函式式介面且只有一個方法：T getObject() throws BeansException;，該方法用於返回一個bean的例項，此時的bean已經完成初始化，但是尚未完成建立。

如果當前的bean滿足條件，則將當前正在建立的bean和其ObjectFactory物件提前曝光，加入到正在建立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.迴圈依賴的解決

在完成bean的例項建立之後，還要填充bean的屬性，針對ClassA，其屬性是ClassB，如果要填充ClassA的屬性則勢必先要例項化ClassB，那麼這裡又涉及到一個概念，RuntimeBeanReference-->執行時引用。

開啟BeanDefinitionValueResolver類的resolveValueIfNecessary方法。摘取程式碼片段（該方法會在以後全部分析）

• 判斷RuntimeBeanReference屬性

// ① RuntimeBeanReference->執行時引用
//   例如BeanA依賴BeanB,那麼在配置檔案中有通過配置ref標籤進行引用的,在解析BeanDefinition的時候,是不會直接例項化BeanB的,那麼這個引用就是RuntimeBeanReference
if (value instanceof RuntimeBeanReference) {
    RuntimeBeanReference ref = (RuntimeBeanReference) value;
    return resolveReference(argName, ref);
}

• 解析RuntimeBeanReference（執行時引用）

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、判斷引用bean是否屬於父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、從當前beanFactory獲取引用beanName例項
        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);
    }
}

該過程很簡單，首先解析refBeanName，然後通過getBean方法獲取其例項，此時當前建立的bean是ClassA，引用bean是ClassB。

獲取到ClassB例項之後，又要填充ClassB的屬性，此時又會出現對RuntimeBeanReference的解析，即ClassA，再去獲取ClassA的例項，此時的ClassA的例項已經被提前曝光，會從快取中獲取ClassA的例項。

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    // 1、從快取中獲取bean
    Object singletonObject = this.singletonObjects.get(beanName);
    // 2、未能獲取到bean,但是允許對當前建立的單例的早期引用(解決迴圈引用)
    // isSingletonCurrentlyInCreation-->判斷指定的單例bean是否當前正在建立(Spring只解決單例bean的迴圈依賴問題)
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        synchronized (this.singletonObjects) {
            // 從earlySingletonObjects獲取提前曝光的bean
            singletonObject = this.earlySingletonObjects.get(beanName);
            // 未能獲取到提前曝光的bean且當前的bean允許被建立早期依賴
            if (singletonObject == null && allowEarlyReference) {
                // 從快取中獲取BeanFactory
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    // 通過getObject()方法獲取提前曝光的bean
                    singletonObject = singletonFactory.getObject();
                    // 將獲取到的singletonObject快取至earlySingletonObjects
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    // 從singletonFactories移除bean
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return singletonObject;
}

從singletonObjects中無法獲取到bean的例項，因為此時bean尚未完成全部建立，但是由於我們提前曝光了ObjectFactory，所以通過singletonObject = singletonFactory.getObject();是可以獲取到bean的例項的。這樣就解決了Spring的迴圈依賴問題。

3.總結

• Spring只能解決Setter方法注入的單例bean之間的迴圈依賴
• ClassA依賴ClassB，ClassB又依賴ClassA，形成依賴閉環。Spring在獲取ClassA的例項時，不等ClassA完成建立就將其曝光加入正在建立的bean快取中。在解析ClassA的屬性時，又發現依賴於ClassB，再次去獲取ClassB，當解析ClassB的屬性時，又發現需要ClassA的屬性，但此時的ClassA已經被提前曝光加入了正在建立的bean的快取中，則無需建立新的的ClassA的例項，直接從快取中獲取即可。從而解決迴圈依賴問題。