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Spring bean 生命周期

blacklad大约 13 分钟SpringSpring

Spring bean 生命周期

1 bean 加载流程

在 Spring 中，当调用 BeanFactory 的 getBean 方法时，就会触发 bean 的加载流程，getBean 内部会调用 doGetBean 方法。

// org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean

/**
 * Return an instance, which may be shared or independent, of the specified bean.
 * @param name the name of the bean to retrieve
 * @param requiredType the required type of the bean to retrieve
 * @param args arguments to use when creating a bean instance using explicit arguments
 * (only applied when creating a new instance as opposed to retrieving an existing one)
 * @param typeCheckOnly whether the instance is obtained for a type check,
 * not for actual use
 * @return an instance of the bean
 * @throws BeansException if the bean could not be created
*/
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
    String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
    throws BeansException {

    // 对beanName做处理
    String beanName = transformedBeanName(name);
    Object bean;

    // Eagerly check singleton cache for manually registered singletons.
    // 尝试在缓存中获取单例bean
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        if (logger.isTraceEnabled()) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                             "' that is not fully initialized yet - a consequence of a circular reference");
            }
            else {
                logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
            }
        }
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else {
        // Fail if we're already creating this bean instance:
        // We're assumably within a circular reference.
        // 如果是多例模式，且已经在创建中，说明发生了循环依赖，则抛出异常
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }

        // Check if bean definition exists in this factory.
        // 递归使用parent BeanFactory加载bean
        BeanFactory parentBeanFactory = getParentBeanFactory();
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            // Not found -> check parent.
            String nameToLookup = originalBeanName(name);
            if (parentBeanFactory instanceof AbstractBeanFactory) {
                return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                    nameToLookup, requiredType, args, typeCheckOnly);
            }
            else if (args != null) {
                // Delegation to parent with explicit args.
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            }
            else if (requiredType != null) {
                // No args -> delegate to standard getBean method.
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
            else {
                return (T) parentBeanFactory.getBean(nameToLookup);
            }
        }

        // 标记bean正在创建中
        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);
        }

        try {
            // 获取bean的定义beanDefinition
            RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);

            // Guarantee initialization of beans that the current bean depends on.
            // 递归加载定义的显示依赖，如果有显示的循环依赖，抛出异常
            String[] dependsOn = mbd.getDependsOn();
            if (dependsOn != null) {
                for (String dep : dependsOn) {
                    if (isDependent(beanName, dep)) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                                        "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                    }
                    registerDependentBean(dep, beanName);
                    try {
                        getBean(dep);
                    }
                    catch (NoSuchBeanDefinitionException ex) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                                        "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                    }
                }
            }

            // Create bean instance.
            if (mbd.isSingleton()) {
                // 单例加载bean实例
                sharedInstance = getSingleton(beanName, () -> {
                    try {
                        return createBean(beanName, mbd, args);
                    }
                    catch (BeansException ex) {
                        // Explicitly remove instance from singleton cache: It might have been put there
                        // eagerly by the creation process, to allow for circular reference resolution.
                        // Also remove any beans that received a temporary reference to the bean.
                        destroySingleton(beanName);
                        throw ex;
                    }
                });
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }

            else if (mbd.isPrototype()) {
                // 多例 创建一个新的实例
                // It's a prototype -> create a new instance.
                Object prototypeInstance = null;
                try {
                    beforePrototypeCreation(beanName);
                    prototypeInstance = createBean(beanName, mbd, args);
                }
                finally {
                    afterPrototypeCreation(beanName);
                }
                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
            }

            else {
                // 其他类型scope
                String scopeName = mbd.getScope();
                if (!StringUtils.hasLength(scopeName)) {
                    throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
                }
                Scope scope = this.scopes.get(scopeName);
                if (scope == null) {
                    throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
                }
                try {
                    Object scopedInstance = scope.get(beanName, () -> {
                        beforePrototypeCreation(beanName);
                        try {
                            return createBean(beanName, mbd, args);
                        }
                        finally {
                            afterPrototypeCreation(beanName);
                        }
                    });
                    bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                }
                catch (IllegalStateException ex) {
                    throw new BeanCreationException(beanName,
                                                    "Scope '" + scopeName + "' is not active for the current thread; consider " +
                                                    "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
                                                    ex);
                }
            }
        }
        catch (BeansException ex) {
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
    }

    // Check if required type matches the type of the actual bean instance.
    // 检查类型
    if (requiredType != null && !requiredType.isInstance(bean)) {
        try {
            T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
            if (convertedBean == null) {
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
            return convertedBean;
        }
        catch (TypeMismatchException ex) {
            if (logger.isTraceEnabled()) {
                logger.trace("Failed to convert bean '" + name + "' to required type '" +
                             ClassUtils.getQualifiedName(requiredType) + "'", ex);
            }
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
    }
    return (T) bean;
}

1.1 单例

定义了单例对象的初始化框架，包括多个缓存、创建中列表等临时状态的维护。

// org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton
/**
 * Return the (raw) singleton object registered under the given name,
 * creating and registering a new one if none registered yet.
 * @param beanName the name of the bean
 * @param singletonFactory the ObjectFactory to lazily create the singleton
 * with, if necessary
 * @return the registered singleton object
*/
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "Bean name must not be null");
    synchronized (this.singletonObjects) {
        // 先尝试在缓存中获取bean
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            if (this.singletonsCurrentlyInDestruction) {
                throw new BeanCreationNotAllowedException(beanName,
                                                          "Singleton bean creation not allowed while singletons of this factory are in destruction " +
                                                          "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
            }
            // 将beanName放入singletonsCurrentlyInCreation set中
            beforeSingletonCreation(beanName);
            boolean newSingleton = false;
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet<>();
            }
            try {
                // 通过传入的的方法获取对象
                singletonObject = singletonFactory.getObject();
                newSingleton = true;
            }
            catch (IllegalStateException ex) {
                // Has the singleton object implicitly appeared in the meantime ->
                // if yes, proceed with it since the exception indicates that state.
                singletonObject = this.singletonObjects.get(beanName);
                if (singletonObject == null) {
                    throw ex;
                }
            }
            catch (BeanCreationException ex) {
                if (recordSuppressedExceptions) {
                    for (Exception suppressedException : this.suppressedExceptions) {
                        ex.addRelatedCause(suppressedException);
                    }
                }
                throw ex;
            }
            finally {
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = null;
                }
                afterSingletonCreation(beanName);
            }
            if (newSingleton) {
                // 如果是新创建的对象，将对象放入缓存中
                addSingleton(beanName, singletonObject);
            }
        }
        return singletonObject;
    }
}

子类 AbstractAutowireCapableBeanFactory 实现了 singletonFactory.getObject() 中的 createBean 方法。

// org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean
//---------------------------------------------------------------------
// Implementation of relevant AbstractBeanFactory template methods
//---------------------------------------------------------------------

/**
 * Central method of this class: creates a bean instance,
 * populates the bean instance, applies post-processors, etc.
 * @see #doCreateBean
*/
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
    throws BeanCreationException {

    if (logger.isTraceEnabled()) {
        logger.trace("Creating instance of bean '" + beanName + "'");
    }
    RootBeanDefinition mbdToUse = mbd;

    // Make sure bean class is actually resolved at this point, and
    // clone the bean definition in case of a dynamically resolved Class
    // which cannot be stored in the shared merged bean definition.
    // 获取bean的class对象
    Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }

    // Prepare method overrides.
    // 处理覆盖方法
    try {
        mbdToUse.prepareMethodOverrides();
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                                               beanName, "Validation of method overrides failed", ex);
    }

    try {
        // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
        // InstantiationAwareBeanPostProcessor提前对bean进行aop生成代理对象
        Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
        if (bean != null) {
            return bean;
        }
    }
    catch (Throwable ex) {
        throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
                                        "BeanPostProcessor before instantiation of bean failed", ex);
    }

    try {
        // 创建bean
        Object beanInstance = doCreateBean(beanName, mbdToUse, args);
        if (logger.isTraceEnabled()) {
            logger.trace("Finished creating instance of bean '" + beanName + "'");
        }
        return beanInstance;
    }
    catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
        // A previously detected exception with proper bean creation context already,
        // or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
        throw ex;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
            mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
    }
}

doCreateBean定义了创建 bean 的主要流程，包括创建 bean 实例、属性填充和初始化。

// org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean
/**
 * Actually create the specified bean. Pre-creation processing has already happened
 * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
 * <p>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
*/
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
    throws BeanCreationException {

    // Instantiate the bean.
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        // 创建bean实例
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    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 {
                // MergedBeanDefinitionPostProcessor可以对bean进行处理
                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.
    // 判断是否能提前暴露对象，  单例、允许循环引用、当前正在创建中
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                                      isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        if (logger.isTraceEnabled()) {
            logger.trace("Eagerly caching bean '" + beanName +
                         "' to allow for resolving potential circular references");
        }
        // 如果需要提前暴露，将创建EarlyBeanReference的工厂方法方式三级缓存中
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }

    // Initialize the bean instance.
    // 初始化bean
    Object exposedObject = bean;
    try {
        // 填充bean的属性，包括依赖注入
        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);
        }
    }

    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 " +
                                                               "'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
                }
            }
        }
    }

    // Register bean as disposable.
    try {
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(
            mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

1.1.1 创建 bean 实例

// org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBeanInstance
/**
 * Create a new instance for the specified bean, using an appropriate instantiation strategy:
 * factory method, constructor autowiring, or simple instantiation.
 * @param beanName the name of the bean
 * @param mbd the bean definition for the bean
 * @param args explicit arguments to use for constructor or factory method invocation
 * @return a BeanWrapper for the new instance
 * @see #obtainFromSupplier
 * @see #instantiateUsingFactoryMethod
 * @see #autowireConstructor
 * @see #instantiateBean
*/
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
    // Make sure bean class is actually resolved at this point.
    Class<?> beanClass = resolveBeanClass(mbd, beanName);

    // 检查权限
    if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                        "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
    }

    // 自定义实现创建逻辑
    Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
    if (instanceSupplier != null) {
        return obtainFromSupplier(instanceSupplier, beanName);
    }

    // 通过factory method 初始化
    if (mbd.getFactoryMethodName() != null) {
        return instantiateUsingFactoryMethod(beanName, mbd, args);
    }

    // Shortcut when re-creating the same bean...
    boolean resolved = false;
    boolean autowireNecessary = false;
    if (args == null) {
        synchronized (mbd.constructorArgumentLock) {
            if (mbd.resolvedConstructorOrFactoryMethod != null) {
                resolved = true;
                autowireNecessary = mbd.constructorArgumentsResolved;
            }
        }
    }
    if (resolved) {
        if (autowireNecessary) {
            return autowireConstructor(beanName, mbd, null, null);
        }
        else {
            return instantiateBean(beanName, mbd);
        }
    }

    // Candidate constructors for autowiring?
    // 构造函数注入
    Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
        mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
        return autowireConstructor(beanName, mbd, ctors, args);
    }

    // Preferred constructors for default construction?
    ctors = mbd.getPreferredConstructors();
    if (ctors != null) {
        return autowireConstructor(beanName, mbd, ctors, null);
    }

    // No special handling: simply use no-arg constructor.
    // 无参构造函数创建对象
    return instantiateBean(beanName, mbd);
}

1.1.2 依赖注入

对于 @Autowired 注解的属性，主要是通过 InstantiationAwareBeanPostProcessor 的子类 AutowiredAnnotationBeanPostProcessor中的postProcessProperties 方法对属性进行注入。

1.1.3 初始化实例

主要包括以下步骤

  1. 对实现了 Aware 接口的 bean 进行织入， BeanNameAware、BeanFactoryAware
  2. 依次调用 beanPostProcessor 的 postProcessBeforeInitialization 方法
    1. CommonAnnotationBeanPostProcessor 会对 @PostConstruct、@PreDestroy 注解进行解析，postProcessBeforeInitialization 中会调用被 PostConstruct 注解的函数。
  3. 如果 bean 实现了 InitializingBean 接口，调用 afterPropertiesSet 方法
  4. 调用 beanDefinition 中定义的 init-method 方法
  5. 依次调用 beanPostProcessor 的 postProcessAfterInitialization 方法
// org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBean
/**
 * Initialize the given bean instance, applying factory callbacks
 * as well as init methods and bean post processors.
 * <p>Called from {@link #createBean} for traditionally defined beans,
 * and from {@link #initializeBean} for existing bean instances.
 * @param beanName the bean name in the factory (for debugging purposes)
 * @param bean the new bean instance we may need to initialize
 * @param mbd the bean definition that the bean was created with
 * (can also be {@code null}, if given an existing bean instance)
 * @return the initialized bean instance (potentially wrapped)
 * @see BeanNameAware
 * @see BeanClassLoaderAware
 * @see BeanFactoryAware
 * @see #applyBeanPostProcessorsBeforeInitialization
 * @see #invokeInitMethods
 * @see #applyBeanPostProcessorsAfterInitialization
*/
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
    if (System.getSecurityManager() != null) {
        AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
            invokeAwareMethods(beanName, bean);
            return null;
        }, getAccessControlContext());
    }
    else {
        // 织入
        invokeAwareMethods(beanName, bean);
    }

    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        // 调用postProcessBeforeInitialization
        wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    }

    try {
        // 调用init方法
        invokeInitMethods(beanName, wrappedBean, mbd);
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
            (mbd != null ? mbd.getResourceDescription() : null),
            beanName, "Invocation of init method failed", ex);
    }
    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }

    return wrappedBean;
}


/**
 * Give a bean a chance to react now all its properties are set,
 * and a chance to know about its owning bean factory (this object).
 * This means checking whether the bean implements InitializingBean or defines
 * a custom init method, and invoking the necessary callback(s) if it does.
 * @param beanName the bean name in the factory (for debugging purposes)
 * @param bean the new bean instance we may need to initialize
 * @param mbd the merged bean definition that the bean was created with
 * (can also be {@code null}, if given an existing bean instance)
 * @throws Throwable if thrown by init methods or by the invocation process
 * @see #invokeCustomInitMethod
*/
protected void invokeInitMethods(String beanName, Object bean, @Nullable RootBeanDefinition mbd)
    throws Throwable {

    // 调用InitializingBean接口的afterPropertiesSet方法
    boolean isInitializingBean = (bean instanceof InitializingBean);
    if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
        if (logger.isTraceEnabled()) {
            logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
        }
        if (System.getSecurityManager() != null) {
            try {
                AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
                    ((InitializingBean) bean).afterPropertiesSet();
                    return null;
                }, getAccessControlContext());
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            ((InitializingBean) bean).afterPropertiesSet();
        }
    }

    // 自定义的init方法
    if (mbd != null && bean.getClass() != NullBean.class) {
        String initMethodName = mbd.getInitMethodName();
        if (StringUtils.hasLength(initMethodName) &&
            !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
            !mbd.isExternallyManagedInitMethod(initMethodName)) {
            invokeCustomInitMethod(beanName, bean, mbd);
        }
    }
}

2 循环依赖

2.1 循环依赖场景

两个 bean 之间发生的循环依赖包含多种情况。

  1. 属性依赖。
  2. 构造函数参数依赖。
  3. 显示 depend 依赖。

循环依赖中，对于多例的属性依赖、构造函数参数依赖以及显示依赖，Spring 无法判断处理，会直接抛出异常。只有单例模式之间的循环依赖，Spring通过三级缓存保证了两个bean的正常初始化和注入。

2.2 Spring 解决方案

Spring 定义了三级缓存用来存放不同阶段的 bean 实例。

  1. singletonObjects：用来存放已经被实例化且初始化完成的 bean。
  2. earlySingletonObjects：提前暴露实例化的对象，但并未初始化完成。
  3. singletonFactories：存放创建对象的工厂方法。
// org.springframework.beans.factory.support.DefaultSingletonBeanRegistry

/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

2.2.1 循环依赖处理流程

先假设有两个单例类 Cat 和 Dog，类 Cat 中包含 Dog 类型的属性，Dog 中包含 Cat 类型的属性，两个类之间发生了循环依赖。

1. 创建 bean cat（未完成）

首先会创建 cat，在 bean 加载过程中的对象实例化后，填充属性前，如果当前 bean 支持循环引用，通过addSingletonFactory将getEarlyBeanReference方法作为工厂方法，放入三级缓存earlySingletonObjects中。

addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));

然后对 cat 进行属性的注入，此对象依赖 dog，则会对 dog 进行 bean 的创建。

2. 创建 bean dog（未完成）

在对 dog 的创建过程中，填充属性的时候，又发现依赖了 cat ，又通过 getBean 获取 cat 对象。在 doGetBean 的 getSingleton 中，会尝试从三个缓存中获取对象cat，而

3. 通过三级缓存获取了 cat 实例
  1. 尝试从一级缓存singletonObjects中获取对象
  2. 尝试从二级缓存earlySingletonObjects中获取对象
  3. 如果一二级缓存中都没有，会通过三级缓存singletonFactories中获取创建 cat 的工厂方法，通过工厂方法创建对象的实例，由于此时 cat 类并未做初始化，所以将生成的实例放在二级缓存中，并移出三级缓存。
org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton

/**
 * Return the (raw) singleton object registered under the given name.
 * <p>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
 */
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    // Quick check for existing instance without full singleton lock
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        singletonObject = this.earlySingletonObjects.get(beanName);
        if (singletonObject == null && allowEarlyReference) {
            synchronized (this.singletonObjects) {
                // Consistent creation of early reference within full singleton lock
                singletonObject = this.singletonObjects.get(beanName);
                if (singletonObject == null) {
                    singletonObject = this.earlySingletonObjects.get(beanName);
                    if (singletonObject == null) {
                        ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                        if (singletonFactory != null) {
                            singletonObject = singletonFactory.getObject();
                            this.earlySingletonObjects.put(beanName, singletonObject);
                            this.singletonFactories.remove(beanName);
                        }
                    }
                }
            }
        }
    }
    return singletonObject;
}
4. 完成 dog 的初始化

dog 拿到了 cat 的实例，完成了属性的注入，最终将生成的对象放入了一级缓存中。

5. 完成cat的初始化

cat 也拿到了 dog 的实例，完成了属性注入，以及后续的初始化，最终将二级缓存中的 bean 移出，放入了以及缓存中。

3. 问题

可以看到 Spring 额外用到了两个缓存解决了循环依赖，但看代码过程中就会有个疑问，正常情况下，额外使用一个缓存存放为做属性注入的实例就可以了，为什么这里要用两个缓存呢?

在三级缓存的工厂方法内调用了 getEarlyBeanReference 方法，会通过 SmartInstantiationAwareBeanPostProcessor 的 getEarlyBeanReference 方法。

/**
 * Obtain a reference for early access to the specified bean,
 * typically for the purpose of resolving a circular reference.
 * @param beanName the name of the bean (for error handling purposes)
 * @param mbd the merged bean definition for the bean
 * @param bean the raw bean instance
 * @return the object to expose as bean reference
*/
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
    Object exposedObject = bean;
    if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
                SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
                exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
            }
        }
    }
    return exposedObject;
}

其中 AbstractAutoProxyCreator 对需要动态代理的对象进行封装代理并返回。也就是 Spring 为了保证最终被注入的对象，不能是原始对象（如果该对象需要动态代理）。

//  org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator
public Object getEarlyBeanReference(Object bean, String beanName) {
    Object cacheKey = this.getCacheKey(bean.getClass(), beanName);
    this.earlyProxyReferences.put(cacheKey, bean);
    return this.wrapIfNecessary(bean, beanName, cacheKey);
}

当然这里也还有个疑问是为什么不提前判断是否需要动态代理直接放入二级缓存中也可解决问题，这个问题结合网上的搜索以及自己的理解。

  1. Spring 对没有循环依赖的对象不想提前创建代理对象，影响改变之前的创建代理时机，尽量扩展而不是修改。
  2. 也有可能是 Spring 为了满足一些设计原则，同时也为循环依赖的创建提供了一个钩子，以便更好的扩展。
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贡献者: blacklad