如何更改spring源码_最强源码分析，带你一步步弄清楚Spring如何解决循环依赖
写在前面
憋了好几天了，在经历了感冒的悲伤之后，终于大脑变得清晰起来。在《聊聊Spring的IOC以及JVM的类加载》中，之前留下了一个坑，不知道大家还有没有印象。那就是一个考查源码时必然会问到的一个面试题 - -Spring循环依赖是如何解决的。今天，我们就来好好分析下这个话题，我会很细致的讲解。预计，本文会很长很长，希望大家有耐心，去读完，相信你读完之后，对于Spring的IOC部分将有更深的源码理解。采用我的逻辑去把这个问题去解读明白。大致分为以下几部分：
何为循环依赖？
Spring管理bean对象
Spring如何解决循环依赖
何为循环依赖
想要理解这个问题，那么首先呢，需要有基础的知识储备。那就是Spring的IOC。IOC，是控制反转，后来出现更容易的理解 DI，依赖注入。大致上就是，一个A对象内有一个B对象属性，无需A对象显式创建B对象，可以通过Spring容器进行注入B对象到A对象中。这便是依赖注入的含义。
循环依赖的出现，是因为A对象中依赖B对象作为其中属性，B对象中依赖A对象作为其中属性。如下图所示：
举个例子描述：
小明喜欢小红，小红喜欢小黑，小黑喜欢小丽，小丽喜欢小明，如果中间所有人都不放弃喜欢的人，那么每个人都将陷入爱情循环中，无法自拔。
类似于这种依赖关系，在自然界中屡见不鲜，因此，映射到我们java的对象世界中，就必然会存在。
那么，如何处理循环依赖，保证合理的程序运行，是我们需要思考的问题。Spring作为一个技术框架，必然考虑到了循环依赖的问题。
Spring管理Bean对象
理解Spring作为容器管理bean对象之前，我们可以尝试思考下，我们自主，如何实现对于依赖对象的注入
场景描述
现有一个场景，蜜蜂采蜜。蜜蜂是一个对象，蜜蜂采蜜，需要作用于花，因此依赖花的存在。
蜜蜂对象，代码如下：
public class Bee {
/** * 性别 */ private String sex; /** * 年龄 */ private Integer age; @Autowired private Flower flower; /** * 采蜜 */ public void pickingHoney() {
// 花产蜜 flower.product(); } public String getSex() {
return sex; } public void setSex(String sex) {
this.sex = sex; } public Integer getAge() {
return age; } public void setAge(Integer age) {
this.age = age; }}上述代码，是Spring采用注解注入的方式实现。
那么，试想，如果不使用Spring，我们如何实现呢？
一 直接简单粗暴地创建对象(可以实现，但是明显不符合自然界对象定义，因为蜜蜂不能创造出花这个对象)
public class Bee {
/** * 性别 */ private String sex; /** * 年龄 */ private Integer age; /** * 采蜜 */ public void pickingHoney() {
// 花产蜜 Flower flower = new Flower(); flower.product(); } public String getSex() {
return sex; } public void setSex(String sex) {
this.sex = sex; } public Integer getAge() {
return age; } public void setAge(Integer age) {
this.age = age; }}二 反射获取对象
/** * 采蜜 */ public void pickingHoney() {
Class clazz = Class.forName("com.example.demo.test.Flower"); Flower flower = (Flower)clazz.getDeclaredConstructor().newInstance(); flower.product(); }可见，相比第一种来说，第二种反射获取对象，很优雅的实现了对象的处理。
注：反射作为Java的一个典型技术，非常重要
上述示例代码，通过反射实现，获取类对象，然后创建类实例。且上述采用的类名形式，那么我们就可以通过配置文件读取、自定义注解等多种方式，来实现反射的实现。
有了上述的技术积累，我们可以很开心的完成第一步，反射获取对象，实现对象的注入。
再思考，在每个类对象中，处理反射逻辑，会造成代码的冗余，且会造成，对象的创建频繁，没法保证单一等问题。那么，通过学习设计模式中的单例模式、工厂模式，我们可以发现，如果应用上述设计模式，或许更优雅。就这样，我们慢慢贴近了Spring的实现。
Spring框架就是应用了各种设计模式，同时采用反射技术，来实现对象的创建、初始化等操作的。
Spring管理bean对象
Bean对象定义方式
Bean对象的定义，正如我们掌握的，在Spring中，可以通过几种方式完成：
XML文件定义
property> bean> property> bean>beans> 如代码展示，可以直接通过xml文件，管理bean对象，实现bean对象的定义，以及属性的注入
注解形式可以通过各类注解，比如@Component @Controller @Service 等等
Bean对象读取
在Spring管理下，对于Bean对象的读取，会形成一个抽象层，BeanDefinitionReader，然后形成BeanDefinition，完成对于Bean对象描述的定义
BeanFactory
Bean对象的创建，由创建工程BeanFactory完成。BeanFactory会完成BeanDefinition的转换，然后构建bean对象，完成bean对象的实例化、初始化。
工厂，通过反射技术，根据BeanDefinition创建Bean对象。
FactoryBean
FactoryBean是特殊的BeanFactory
BeanFactoryPostPocessor
BeanFactory的后置处理器，完成BeanFactory创建后的一系列补充
BeanPostPocessor
Bean对象初始化前后的处理
流程描述：
Spring读取XML文件等形式对于Bean对象的定义，构建BeanDefinition
Spring的BeanFactory，读取BeanDefinition，完成Bean对象的读取，通过反射技术进行实例化
BeanFactoryPostPocessor作为BeanFactory的后置处理器，完成补充处理
BeanPostPocessor - before 完成对于bean对象初始化前的对象处理
BeanPostPocessor - after完成对于bean对象初始化后的对象处理
context.getBean 进行bean对象的使用
通过以上的分析，我们掌握了，Spring对于bean对象的读取、创建实例化、初始化的过程。了解了这些，我们才能去看循环依赖的事情。如若不然，个人觉得理解循环依赖问题，就比较困难。
Spring如何解决循环依赖
当我们能够明确了上述学习之后，我想，我们可以进入到源码的世界中去分析了。
查看解决循环依赖，也就是了解Spring是如何对Bean对象进行实例化、初始化的。
知识点：
对象的创建分为两部分，一部分为实例化，此时，在堆内存中分配了内存空间，但是未对其属性进行赋值，另一部分为初始化，完成对于实例化后对象的属性的赋值操作。因此类对象在内存空间中，两种状态，实例化状态(实例化完成、未初始化)，完全状态(实例化完成、初始化完成)
一切的开始
以XML文件为例，代码如下，进入debug模式，走起，跟上队伍！
public class SpringTestApplication {
public static void main(String[] args) {
ApplicationContext ac = new ClassPathXmlApplicationContext("bean.xml"); }}第一站
进入到，上下文的处理构造器中，看代码，找重点。
public ClassPathXmlApplicationContext( String[] configLocations, boolean refresh, @Nullable ApplicationContext parent) throws BeansException {
super(parent); setConfigLocations(configLocations); if (refresh) {
// 此处是我们的第一站 refresh(); }}refresh方法，在AbstractApplicationContext内。代码内部，做了点什么事情呢，可以详细的看看代码注释。
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 完成准备，设置时间啊等等，我们无需太关心 prepareRefresh(); // 无需多关心 ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); // bean factory 创建前的准备阶段，对应我们上述图的流程 prepareBeanFactory(beanFactory); try {
//BeanFactory的处理过程，跟我们的对象创建关系不大 开始 // Allows post-processing of the bean factory in context subclasses. postProcessBeanFactory(beanFactory); // Invoke factory processors registered as beans in the context. invokeBeanFactoryPostProcessors(beanFactory); // Register bean processors that intercept bean creation. registerBeanPostProcessors(beanFactory); // Initialize message source for this context. initMessageSource(); // Initialize event multicaster for this context. initApplicationEventMulticaster(); // Initialize other special beans in specific context subclasses. onRefresh(); // Check for listener beans and register them. registerListeners(); //BeanFactory的处理过程，跟我们的对象创建关系不大 完成 // Instantiate all remaining (non-lazy-init) singletons. //实例化，所有的非懒加载的单例bean对象，可见第二站入口来了，开启 finishBeanFactoryInitialization(beanFactory); // Last step: publish corresponding event. finishRefresh(); } catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " + "cancelling refresh attempt: " + ex); } // Destroy already created singletons to avoid dangling resources. destroyBeans(); // Reset 'active' flag. cancelRefresh(ex); // Propagate exception to caller. throw ex; } finally {
// Reset common introspection caches in Spring's core, since we // might not ever need metadata for singleton beans anymore... resetCommonCaches(); } }}第二站，真正的入口
看这个前，我们有必要了解一个类对象，其中提供了三个缓存对象
DefaultSingletonBeanRegistry
/** Cache of singleton objects: bean name to bean instance. */ // 一级缓存 private final Map singletonObjects = new ConcurrentHashMap<>(256); /** Cache of singleton factories: bean name to ObjectFactory. */ // 三级缓存 private final Map> singletonFactories = new HashMap<>(16); /** Cache of early singleton objects: bean name to bean instance. */ // 二级缓存 private final Map earlySingletonObjects = new HashMap<>(16); 记住，对应的对象名，后续在源码中会依次出现。
回到第二站，查看相关源码，直接查看注释中中文标记点，继续追踪
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context. if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) && beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService( beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)); } // Register a default embedded value resolver if no bean post-processor // (such as a PropertyPlaceholderConfigurer bean) registered any before: // at this point, primarily for resolution in annotation attribute values. if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal)); } // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early. String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false); for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName); } // Stop using the temporary ClassLoader for type matching. beanFactory.setTempClassLoader(null); // Allow for caching all bean definition metadata, not expecting further changes. beanFactory.freezeConfiguration(); // Instantiate all remaining (non-lazy-init) singletons. // 实例化，所有的非懒加载的单例bean对象 beanFactory.preInstantiateSingletons();}DefaultListableBeanFactory
@Override public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this); } // Iterate over a copy to allow for init methods which in turn register new bean definitions. // While this may not be part of the regular factory bootstrap, it does otherwise work fine. List beanNames = new ArrayList<>(this.beanDefinitionNames); // Trigger initialization of all non-lazy singleton beans... // 根据BeanDefinition 获取bean定义信息 for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName); // 首先 A对象 此处非抽象类 单例 非懒加载才能进入，A对象符合条件，进入 if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName); if (bean instanceof FactoryBean) {
final FactoryBean> factory = (FactoryBean>) bean; boolean isEagerInit; if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction) ((SmartFactoryBean>) factory)::isEagerInit, getAccessControlContext()); } else {
isEagerInit = (factory instanceof SmartFactoryBean && ((SmartFactoryBean>) factory).isEagerInit()); } if (isEagerInit) {
getBean(beanName); } } } else {
//A对象 进入 getBean(beanName); } } } // 省略} AbstractBeanFactory
@Override public Object getBean(String name) throws BeansException {
// Spring源码内 do开头，都是真正做事情的方法 return doGetBean(name, null, null, false);}第三站 创建bean
AbstractBeanFactory
查看中文代码注释部分，快速定位。按照代码执行序号，查看逻辑。
protected T doGetBean(final String name, @Nullable final Class requiredType, @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
final String beanName = transformedBeanName(name); Object bean; // Eagerly check singleton cache for manually registered singletons. // 1. 获取单例bean对象 第四站入口 Object sharedInstance = getSingleton(beanName); // 根据第四站代码分析，此时A对象为空，返回空，将继续执行 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. if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName); } // Check if bean definition exists in this factory. 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); } } if (!typeCheckOnly) {
markBeanAsCreated(beanName); } try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); checkMergedBeanDefinition(mbd, beanName, args); // Guarantee initialization of beans that the current bean depends on. 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. // 3. 创建对象 if (mbd.isSingleton()) {
// 进入方法 sharedInstance = getSingleton(beanName, () -> {
try {
// 5. 创建对象 实现A 对象实例 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()) {
// 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 {
String scopeName = mbd.getScope(); final 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. 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()); } } // 返回 return (T) bean; }第四站
DefaultSingletonBeanRegistry
@Nullable protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 2. // 一级缓存中查找，是否存在bean A对象，此时为空 Object singletonObject = this.singletonObjects.get(beanName); // 判断一级缓存为空并且是否在创建中，我们在知识点中提及，类对象两种状态，实例化、完全状态，此时均未发生，因此不会进入判断 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); } } } } // 直接返回 空对象 return singletonObject;} public Object getSingleton(String beanName, ObjectFactory> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null"); // 4. 执行对象处理 synchronized (this.singletonObjects) {
// 一级缓存中获取A 对象，无 Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null) {
//进入判断 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 + "'"); } beforeSingletonCreation(beanName); boolean newSingleton = false; boolean recordSuppressedExceptions = (this.suppressedExceptions == null); if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>(); } try {
// 获取对象 ObjectFactory> 通过匿名内部类执行，匿名内部类 ，获取到A实例化对象，此时A 为实例化状态，创建中 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) {
addSingleton(beanName, singletonObject); } } return singletonObject; }}执行匿名内部类 创建A 对象
AbstractAutowireCapableBeanFactory
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException {
// Instantiate the bean. BeanWrapper instanceWrapper = null; if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName); } if (instanceWrapper == null) {
//实例A创建 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. // 此处 A对象已创建，会进入判断 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"); } //三级缓存，放入实例化对象A 此时A 未完全状态 缓存内状态，见代码下图示 addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean)); } // Initialize the bean instance. Object exposedObject = bean; try {
// 6. 填充bean populateBean(beanName, mbd, instanceWrapper); 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 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;} 缓存内部状态
填充bean属性内容
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
// 省略 if (pvs != null) {
//处理属性内容 applyPropertyValues(beanName, mbd, bw, pvs); }}protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
// 省略代码 // Create a deep copy, resolving any references for values. List deepCopy = new ArrayList<>(original.size()); boolean resolveNecessary = false; for (PropertyValue pv : original) {
if (pv.isConverted()) {
deepCopy.add(pv); } else {
// B对象 String propertyName = pv.getName(); // 值为 null Object originalValue = pv.getValue(); if (originalValue == AutowiredPropertyMarker.INSTANCE) {
Method writeMethod = bw.getPropertyDescriptor(propertyName).getWriteMethod(); if (writeMethod == null) {
throw new IllegalArgumentException("Autowire marker for property without write method: " + pv); } originalValue = new DependencyDescriptor(new MethodParameter(writeMethod, 0), true); } // 7. 解决B对象 Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue); Object convertedValue = resolvedValue; //省略代码} BeanDefinitionValueResolver
@Nullable public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {
// We must check each value to see whether it requires a runtime reference // to another bean to be resolved. if (value instanceof RuntimeBeanReference) {
RuntimeBeanReference ref = (RuntimeBeanReference) value; //B对象作为A对象属性值，类型为RuntimeBeanReference 进入判断，执行方法resolveReference return resolveReference(argName, ref); } // 省略代码@Nullable private Object resolveReference(Object argName, RuntimeBeanReference ref) {
try {
Object bean; Class> beanType = ref.getBeanType(); if (ref.isToParent()) {
BeanFactory parent = this.beanFactory.getParentBeanFactory(); if (parent == null) {
throw new BeanCreationException( this.beanDefinition.getResourceDescription(), this.beanName, "Cannot resolve reference to bean " + ref + " in parent factory: no parent factory available"); } if (beanType != null) {
bean = parent.getBean(beanType); } else {
// A对象依赖B对象，获取B对象 bean = parent.getBean(String.valueOf(doEvaluate(ref.getBeanName()))); } } //省略代码开始B对象的获取
第五站 依赖对象 B生成
重新回到
protected T doGetBean(final String name, @Nullable final Class requiredType, @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
final String beanName = transformedBeanName(name); Object bean; // Eagerly check singleton cache for manually registered singletons. //进入获取 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); } //省略代码 类似A对象的处理，此处忽略
最终到了，处理B对象属性A对象，上述描述6 。此时，缓存情况如下：
进一步补充属性，即完成B对象中属性A对象的获取。
重新进入doGetBean ，进入 getSingleton
@Nullable protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 此时一级缓存中获取A对象，同样 A对象为空，此时一级缓存中不存在 Object singletonObject = this.singletonObjects.get(beanName); // A对象状态为 实例化 因此，在创建中，进入判断 if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
// 二级缓存中获取 A对象 ，A对象为空，进入判断 singletonObject = this.earlySingletonObjects.get(beanName); if (singletonObject == null && allowEarlyReference) {
//三级缓存中获取 A对象，可以读取到值 ObjectFactory> singletonFactory = this.singletonFactories.get(beanName); if (singletonFactory != null) {
singletonObject = singletonFactory.getObject(); // 将A 对象放入 二级缓存 this.earlySingletonObjects.put(beanName, singletonObject); // 将A 对象从三级缓存中删除 this.singletonFactories.remove(beanName); } } } } return singletonObject;}此时，缓存情况如下：
处理完B对象的属性A对象后，B对象此时变为初始化完成、实例化完成，成为完全态
public Object getSingleton(String beanName, ObjectFactory> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null"); synchronized (this.singletonObjects) {
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 + "'"); } beforeSingletonCreation(beanName); boolean newSingleton = false; boolean recordSuppressedExceptions = (this.suppressedExceptions == null); if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>(); } try {
// 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) {
// 8. A对象属性对象 B对象完成初始化后执行 addSingleton(beanName, singletonObject); } } return singletonObject; }} protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
// 一级缓存放入 B对象 完全态 this.singletonObjects.put(beanName, singletonObject); // 三级缓存删除 B对象 this.singletonFactories.remove(beanName); // 二级缓存删除 B对象 this.earlySingletonObjects.remove(beanName); this.registeredSingletons.add(beanName); }}此时，缓存情况如下：
此时A对象中属性B对象，完成初始化，A对象会执行以下代码
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
// 一级缓存放入 A对象 完全态 this.singletonObjects.put(beanName, singletonObject); // 三级缓存删除 A对象 this.singletonFactories.remove(beanName); // 二级缓存删除 A对象 this.earlySingletonObjects.remove(beanName); this.registeredSingletons.add(beanName); }}此时，缓存情况如下：
第六站 遍历Bean对象 B
@Override public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this); } // Iterate over a copy to allow for init methods which in turn register new bean definitions. // While this may not be part of the regular factory bootstrap, it does otherwise work fine. List beanNames = new ArrayList<>(this.beanDefinitionNames); // Trigger initialization of all non-lazy singleton beans... // 根据BeanDefinition 获取bean定义信息 for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName); // 首先 A对象 此处非抽象类 单例 非懒加载才能进入，A对象符合条件，进入 if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName); if (bean instanceof FactoryBean) {
final FactoryBean> factory = (FactoryBean>) bean; boolean isEagerInit; if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction) ((SmartFactoryBean>) factory)::isEagerInit, getAccessControlContext()); } else {
isEagerInit = (factory instanceof SmartFactoryBean && ((SmartFactoryBean>) factory).isEagerInit()); } if (isEagerInit) {
getBean(beanName); } } } else {
//B对象 进入 getBean(beanName); } } } // 省略} 故事结尾
最后直接返回，因为完全态在一级缓存中都存在了。
@Nullable protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 从一级缓存中获取B对象，获取得到，直接返回 Object singletonObject = this.singletonObjects.get(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); } } } } return singletonObject;}终于结束了，带着大家走了一遍源码，不知道你能理解多少。
从以上源码，我们可以看到，Spring采用三级缓存，来实现循环依赖的解决。
常见的面试问题
循环依赖必须使用三级缓存么
其实，从我们解读看来，只需要把对象的两个状态处理好，就能处理好循环依赖的问题，因此实际上来说，不必要使用三级缓存对于一般的bean对象来讲。
为何Spring解决三级缓存要使用三级缓存，而不是二级缓存
为何Spring采用三级缓存，其实主要是因为Bean对象不仅仅是基础的一般对象，还会有一些AOP后期增强对象，通过Cglib或者JDK动态代理手段，织入的对象。二级缓存，只能满足一般Bean对象的处理，无法处理更多的对象。
再看三级缓存
/** Cache of singleton objects: bean name to bean instance. */ // 一级缓存 private final Map以上就比较明确，为何使用三级而不是二级了
总结
看源码就是需要细致认真的解读，因此需要耐心。希望读着也能有耐心看下去。如果你可以，我相信，你对于循环依赖，不会有什么难点。
