前言
在 finishBeanFactoryInitialization 中介绍了创立 Bean 的流程大略流程，这里进入单例 Bean 的创立过程。
这里次要分为三个局部创立单例 Bean
- getSingleton
- createBean
- getObjectForBeanInstance
上面进入源码：
getSingleton
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) { Assert.notNull(beanName, "Bean name must not be null"); // 加锁 synchronized (this.singletonObjects) { // 查看 singletonObjects 缓存中是否有 Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null) { // 查看是否在执行销毁 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 Bean 增加到 singletonsCurrentlyInCreation 汇合中, 示意正在创立 beforeSingletonCreation(beanName); boolean newSingleton = false; boolean recordSuppressedExceptions = (this.suppressedExceptions == null); if (recordSuppressedExceptions) { this.suppressedExceptions = new LinkedHashSet<>(); } try { // 调用工厂办法 // 也就是调用 createBean(beanName, mbd, args) 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; } // 创立胜利, 从 singletonsCurrentlyInCreation 移除 afterSingletonCreation(beanName); } if (newSingleton) { // 将给定的单例对象增加到该工厂的单例缓存中 // this.singletonObjects.put(beanName, singletonObject); // this.singletonFactories.remove(beanName); // this.earlySingletonObjects.remove(beanName); // this.registeredSingletons.add(beanName); addSingleton(beanName, singletonObject); } } return singletonObject; }}返回以给定名称注册的（原始）单例对象，如果尚未注册，则创立并注册一个新对象。
这一块一共能够拆成三局部来了解：
1. 从缓存中获取 singletonObjects
singletonObject 是什么？
/** Cache of singleton objects: bean name to bean instance. */private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);singletonObjects 是一个 ConcurrentHashMap， 用来缓存单例对象的实例。
2. 创立 singletonObject
在从缓存中没有获取到 singletonObject ，创立新的对象
singletonObject = singletonFactory.getObject();
这一步其实就是调用外边的 createBean(beanName, mbd, args) 办法，这是一个工厂办法。 通过 createBean 办法，会创立一个新的 singletonObject。
3. 将创立的 singletonObject 增加到缓存中
protected void addSingleton(String beanName, Object singletonObject) { synchronized (this.singletonObjects) { this.singletonObjects.put(beanName, singletonObject); this.singletonFactories.remove(beanName); this.earlySingletonObjects.remove(beanName); // 曾经胜利创立的单例 this.registeredSingletons.add(beanName); }}这一步波及到三个缓存，以及一个胜利创立的单例列表。
/** Cache of singleton objects: bean name to bean instance. *//** 缓存单例对象， K-V -> BeanName - Bean 实例 */private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);/** Cache of singleton factories: bean name to ObjectFactory. *//** 缓存 Bean 工厂 */private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);/** Cache of early singleton objects: bean name to bean instance. *//** 缓存晚期单例对象 */private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);/** Set of registered singletons, containing the bean names in registration order. *//** 已注册的单例列表，按注册程序保留 BeanName。 */private final Set<String> registeredSingletons = new LinkedHashSet<>(256);将创立的单例对象，增加到单例缓存中，同时将工厂缓存以及晚期单例对象缓存中的对应对象删除。
createBean
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. // 获取实在的类型 Class<?> resolvedClass = resolveBeanClass(mbd, beanName); if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) { // 创立新的 mbd 避免 其余线程批改 mbdToUse = new RootBeanDefinition(mbd); mbdToUse.setBeanClass(resolvedClass); } // Prepare method overrides. try { // 验证并筹备为此bean定义的办法代替。 查看是否存在具备指定名称的办法。 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. // 利用实例化之前的后处理器，以解决指定的bean是否存在实例化快捷方式。 // InstantiationAwareBeanPostProcessor 后置处理器 // postProcessBeforeInstantiation 办法可能会曾经实例化 Bean 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 { // 实例化 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); }}这个办法中波及到：创立 Bean 实例 , 填充 Bean , 利用 PostProcessor。
其中实例化 Bean 是在 doCreateBean 中。当初重点看一下 doCreateBean 办法。
doCreateBean
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException { // Instantiate the bean. // Bean 的 对象包装 BeanWrapper instanceWrapper = null; if (mbd.isSingleton()) { // 从缓存中获取 instanceWrapper = this.factoryBeanInstanceCache.remove(beanName); } if (instanceWrapper == null) { // 缓存中获取不到则间接创立, 这里创立的 BeanInstance !!! instanceWrapper = createBeanInstance(beanName, mbd, args); } // 获取 Bean 实例以及类型 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 { // 如果容许批改 mbd // 调用 MergedBeanDefinitionPostProcessor 后置处理器的 // postProcessMergedBeanDefinition(mbd, beanType, beanName); 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. // mbd 是单例 且 容许循环援用, (默认 true) 且在创立 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"); } // 先获取 之前的 Bean 的援用, 从 beanPostProcessorCache 中 获取 SmartInstantiationAwareBeanPostProcessor // 而后从 SmartInstantiationAwareBeanPostProcessor#getEarlyBeanReference 获取之前的援用 addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean)); } // Initialize the bean instance. Object exposedObject = bean; try { // 属性赋值 populateBean(beanName, mbd, instanceWrapper); // 执行 init 办法 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); } } // 这里容许循环依赖 if (earlySingletonExposure) { // 获取晚期的 Bean, 如果没有循环依赖 则获取不到 Object earlySingletonReference = getSingleton(beanName, false); // 有循环依赖 if (earlySingletonReference != null) { // 创立的是不是同一个，可能会有代理对象 if (exposedObject == bean) { exposedObject = earlySingletonReference; } else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) { // 获取依赖的 Bean 并 循环放入到 actualDependentBeans 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 { // 注册销毁办法 registerDisposableBeanIfNecessary(beanName, bean, mbd); } catch (BeanDefinitionValidationException ex) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex); } return exposedObject;}同样是代码很长很长！
分步骤浏览：
如果这个 Bean 是单例 Bean 且容许循环援用且在创立中，则阐明在有循环援用。则调用：
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
这一行代码波及到两个办法，别离是 getEarlyBeanReference 和 addSingletonFactory
- getEarlyBeanReference
- addSingletonFactory
这一块能够看到将创立的一个单例对象的 singletonFactory 增加到了 singletonFactories 缓存中。
同时将 earlySingletonObjects 缓存中的单例对象移除。
那什么时候增加到 earlySingletonObjects 缓存中的呢？
这块能够参考 Spring 源码学习 15：finishBeanFactoryInitialization 在 getSingleton 办法中 put 进去的。
为了不便，我把这一小块代码也贴出来：
在这里将缓存从 singletonFactories 移到了 earlySingletonObjects。
Spring 的 Bean 实例化的时候用到的三级缓存其实是：
singletonObjects： 一级缓存，存储单例对象，Bean 曾经实例化，初始化实现。
earlySingletonObjects： 二级缓存，存储 singletonObject，这个 Bean 实例化了，还没有初始化。
singletonFactories： 三级缓存，存储 singletonFactory
上面会初始化 Bean
这里关注重点关注上面一部分：
- populateBean
对 Bean 的属性进行赋值。
这块须要留神的是，在对属性进行赋值时，发现依赖了其余 Bean，就会去先创立其余 Bean。
我这边应用的注解 @Autowired 就会执行上面一部分：
在这里解析属性的时候，就会去创立外部依赖的 Bean。
- initializeBean
getObjectForBeanInstance
获取给定bean实例的对象，如果是FactoryBean，则为bean实例自身或其创立的对象。
这一块逻辑绝对比较简单，就是依据后面你创立的 beanInstance ， 判断其类型，从而创立 Bean 实例。
总结
本文次要介绍了一个 单例 Bean 的创立，当然都是大块大块的源码，须要急躁的啃。
浏览完源码，基本上对循环依赖能有个具体的理解，晓得 Spring 在初始化 Bean 的时候是应用三级缓存来解决循环依赖的额，而前面则会独自筹备一篇文章对循环依赖做介绍。
相干举荐
- Spring 源码学习 15：finishBeanFactoryInitialization（重点）
- Spring 源码学习 14：initApplicationEventMulticaster
- Spring 源码学习 13：initMessageSource