如果尖锐的批评完全消失，温和的批评将会变得刺耳。

如果温和的批评也不被允许，沉默将被认为居心叵测。

如果沉默也不再允许，赞扬不够卖力将是一种罪行。

如果只允许一种声音存在，那么，唯一存在的那个声音就是谎言。

谢飞机，小记！，总感觉 Spring 也没啥看的，怎么面试官一问就能问出花？

面试官：Spring 的 getBean 中，transformedBeanName 的作用是什么？

谢飞机：不知道呀，看单词意思好像是改变Bean名称。

面试官：那这么说，你的 Bean 如果有 alias 别名，Spring 在获取 Bean 时候要怎么处理？

谢飞机：这！

面试官：那如果用了 depends-on 呢？

谢飞机：啊， 我没用过 depends-on 我不知道！

面试官：那你调试代码时候，看见过BeanName前面有 & 的情况吗，为啥会出现？

谢飞机：我不配知道！再见！

对于刚接触看 Spring 源码的伙伴来说，可能很疑惑于怎么就获取一个 Bean 就这么多流程呢？

所以，它为了适应各类的需求，变得越来越复杂了。而这部分知识的深入学习绝对不只是为了应付八股文，更多的是考虑到在日常的 Spring 使用中遇到复杂问题时有没有一个大致知晓的流程，可以快速定位问题，以及此类需求的技术实现方案是否能在以后的应用开发中起到一定的指导作用，因为它是一种设计方案的具体实现。

小傅哥，getBean 核心流程图

接下来，我们就依次的把关于获取 Bean 实例的重点代码列举出来做分析，读者伙伴也可以结合流程图一起看，这样会更方便理解。

@Testpublic void test_getBean() { BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config.xml"); UserDao userDao = beanFactory.getBean("userDao", UserDao.class); logger("获取 Bean：{}", userDao);}在日常应用到 Spring 的开发中基本都是基于注解，几乎不会自己去使用 beanFactory.getBean 的方式去获取一个 Bean 实例。所以在你学习的时候如果找不到查看 getBean 源码的入口，也不方便调试熟悉源码时，可以写这样一个单元测试类，点入到 getBean 就可以阅读源码了。3. getBean 源码全局预览

源码位置：AbstractBeanFactory -> getBean() -> doGetBean()

@Overridepublic <T> T getBean(String name, Class<T> requiredType) throws BeansException { // getBean 就像你的领导其实没做啥，都在 doGetBean 里 return doGetBean(name, requiredType, null, false);}

protected <T> T doGetBean(final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException { // 处理别名BeanName、处理带&符的工厂BeanName final String beanName = transformedBeanName(name); Object bean; // 先尝试从缓存中获取Bean实例，这个位置就是三级缓存解决循环依赖的方法 Object sharedInstance = getSingleton(beanName); if (sharedInstance != null && args == null) { if (logger.isDebugEnabled()) { if (isSingletonCurrentlyInCreation(beanName)) { logger.debug("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference"); } else { logger.debug("Returning cached instance of singleton bean '" + beanName + "'"); } } // 1. 如果 sharedInstance 是普通的 Bean 实例，则下面的方直接返回 // 2. 如果 sharedInstance 是工厂Bean类型，则需要获取 getObject 方法，可以参考关于 FactoryBean 的实现类 bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); } else { // 循环依赖有三种，setter注入、多实例和构造函数，Spring 只能解决 setter 注入，所以这里是 Prototype 则会抛出异常 if (isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } // 1. 父 bean 工厂存在 // 2. 当前 bean 不存在于当前bean工厂，则到父工厂查找 bean 实例 BeanFactory parentBeanFactory = getParentBeanFactory(); if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { // 获取 name 对应的 beanName，如果 name 是以 & 开头，则返回 & + beanName String nameToLookup = originalBeanName(name); // 根据 args 参数是否为空，调用不同的父容器方法获取 bean 实例 if (args != null) { return (T) parentBeanFactory.getBean(nameToLookup, args); } else { return parentBeanFactory.getBean(nameToLookup, requiredType); } } // 1. typeCheckOnly，用于判断调用 getBean 方法时，是否仅是做类型检查 // 2. 如果不是只做类型检查，就会调用 markBeanAsCreated 进行记录 if (!typeCheckOnly) { markBeanAsCreated(beanName); } try { // 从容器 getMergedLocalBeanDefinition 获取 beanName 对应的 GenericBeanDefinition，转换为 RootBeanDefinition final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); // 检查当前创建的 bean 定义是否为抽象 bean 定义 checkMergedBeanDefinition(mbd, beanName, args); // 处理使用了 depends-on 注解的依赖创建 bean 实例 String[] dependsOn = mbd.getDependsOn(); if (dependsOn != null) { for (String dep : dependsOn) { // 监测是否存在 depends-on 循环依赖，若存在则会抛出异常 if (isDependent(beanName, dep)) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'"); } // 注册依赖记录 registerDependentBean(dep, beanName); try { // 加载 depends-on 依赖（dep 是 depends-on 缩写） getBean(dep); } catch (NoSuchBeanDefinitionException ex) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "'" + beanName + "' depends on missing bean '" + dep + "'", ex); } } } // 创建单例 bean 实例 if (mbd.isSingleton()) { // 把 beanName 和 new ObjectFactory 匿名内部类传入回调 sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() { @Override public Object getObject() throws BeansException { try { // 创建 bean return createBean(beanName, mbd, args); } catch (BeansException ex) { // 创建失败则销毁 destroySingleton(beanName); throw ex; } } }); bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); } // 创建其他类型的 bean 实例 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 = thisopes.get(scopeName); if (scope == null) { throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'"); } try { Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() { @Override public Object getObject() throws BeansException { 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; } } // 如果需要类型转换，这里会进行操作 if (requiredType != null && bean != null && !requiredType.isInstance(bean)) { try { return getTypeConverter()nvertIfNecessary(bean, requiredType); } catch (TypeMismatchException ex) { if (logger.isDebugEnabled()) { logger.debug("Failed to convert bean '" + name + "' to required type '" + ClassUtils.getQualifiedName(requiredType) + "'", ex); } throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } } // 返回 Bean return (T) bean;}

综上基本就是 getBean 过程涉及到的核心处理方法，基本包括；

处理 & 符：transformedBeanName() -> BeanFactoryUtils.transformedBeanName(name)

public static String transformedBeanName(String name) { Assert.notNull(name, "'name' must not be null"); String beanName = name; while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) { beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length()); } return beanName;}使用 FactoryBean 创建出的对象，会在 DefaultListableBeanFactory 初始化的时候，使用 getBean(FACTORY_BEAN_PREFIX + beanName) 给 beanName 加上 & (String FACTORY_BEAN_PREFIX = "&")这里是使用 while 循环逐步的把 & 去掉，只要截取首个字符是 & 符，就继续循环截取。&&&userService -> &&userService -> &userService -> userService

别名转换：transformedBeanName() -> canonicalName

public String canonicalName(String name) { String canonicalName = name; // Handle aliasing... String resolvedName; do { resolvedName = this.aliasMap.get(canonicalName); if (resolvedName != null) { canonicalName = resolvedName; } } while (resolvedName != null); return canonicalName;}

<bean id="userService" class="org.itstackterview.UserService"/><alias name="userService" alias="userService-alias01"/><alias name="userService-alias01" alias="userService-alias02"/>首先 Spring 对 Bean 的存放并不会使用别名作为Map中的key，所以遇到所有别名获取 Bean 都需要查到对应原来名字，才可以。如果你知道这个事，是不遇到此类问题时，就知道从哪下手查了do...while 循环会依次像链条一样不断的寻找别名对应的名称，直到当前这个名称没有别名了，就返回对应 BeanName5. depends-on 依赖 Bean

AbstractBeanFactory -> isDependent(beanName, dep) -> DefaultSingletonBeanRegistry

protected boolean isDependent(String beanName, String dependentBeanName) { synchronized (this.dependentBeanMap) { return isDependent(beanName, dependentBeanName, null); }

<bean id="userService" class="org.itstackterview.UserService" depends-on="userDao"/><bean id="userDao" class="org.itstackterview.UserDao"/>isDependent 处理的是使用了 depends-on 配置的 Bean 定义。

private boolean isDependent(String beanName, String dependentBeanName, Set<String> alread if (alreadySeen != null && alreadySeenntains(beanName)) { return false; } String canonicalName = canonicalName(beanName); Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName); if (dependentBeans == null) { return false; } if (dependentBeansntains(dependentBeanName)) { return true; } for (String transitiveDependency : dependentBeans) { if (alreadySeen == null) { alreadySeen = new HashSet<String>(); } alreadySeen.add(beanName); if (isDependent(transitiveDependency, dependentBeanName, alreadySeen)) { return true; } } return false;}alreadySeen != null，监测已经依赖的 BeancanonicalName，处理别名配置，找到最原来是的 BeanNameSet<String> dependentBeans，获取依赖的 Bean 集合for 循环递归检测依赖的 Bean，并添加到 alreadySeen 中

AbstractBeanFactory -> registerDependentBean(dep, beanName) -> DefaultSingletonBeanRegistry

public void registerDependentBean(String beanName, String dependentBeanName) { String canonicalName = canonicalName(beanName); synchronized (this.dependentBeanMap) { Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName); if (dependentBeans == null) { dependentBeans = new LinkedHashSet<String>(8); this.dependentBeanMap.put(canonicalName, dependentBeans); } dependentBeans.add(dependentBeanName); } synchronized (this.dependenciesForBeanMap) { Set<String> dependenciesForBean = this.dependenciesForBeanMap.get(dependentBeanName if (dependenciesForBean == null) { dependenciesForBean = new LinkedHashSet<String>(8); this.dependenciesForBeanMap.put(dependentBeanName, dependenciesForBean); } dependenciesForBean.add(canonicalName); }}canonicalName(beanName)，获取原始的 beanNamesynchronized (this.dependentBeanMap)，添加 <canonicalName, dependentBeanName> 到 dependentBeanMap 中synchronized (this.dependenciesForBeanMap)，添加 <dependentBeanName, canonicalName> 到 dependenciesForBeanMap 中

最后：getBean(dep)，就可以获取到 depends-on 依赖的 Bean 了

AbstractBeanFactory -> mbd.isSingleton()

if (mbd.isSingleton()) { sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() { @Override public Object getObject() throws BeansException { try { return createBean(beanName, mbd, args); } catch (BeansException ex) { destroySingleton(beanName); throw ex; } } }); bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}这一部分是使用 beanName 和 singletonFactory 匿名内部类传入等待回调的方式创建单实例 Bean 实例

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) { Assert.notNull(beanName, "'beanName' 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<Exception>(); } try { singletonObject = singletonFactory.getObject(); newSingleton = true; } catch (IllegalStateException ex) { 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 != NULL_OBJECT ? singletonObject : null); }}this.singletonObjects.get(beanName)，先尝试从缓存池中获取对象，没有就继续往下执行beforeSingletonCreation(beanName)，标记当前 bean 被创建，如果有构造函数注入的循环依赖会报错singletonObject = singletonFactory.getObject()，创建 bean 过程就是调用 createBean() 方法afterSingletonCreation(beanName)，最后把标记从集合中移除addSingleton(beanName, singletonObject)，新创建的会加入缓存集合7. 从缓存中获取 bean 实例

doCreateBean -> if (earlySingletonExposure) -> getSingleton(beanName, false)

protected Object getSingleton(String beanName, boolean allowEarlyReference) { // 从 singletonObjects 获取实例，singletonObjects 中缓存的实例都是完全实例化好的 bean，可以直接使用 Object singletonObject = this.singletonObjects.get(beanName); // 如果 singletonObject 为空，则没有创建或创建中 if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { // 加锁 synchronized (this.singletonObjects) { // 单例缓存池中，没有当前beanName singletonObject = this.earlySingletonObjects.get(beanName); if (singletonObject == null && allowEarlyReference) { ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); if (singletonFactory != null) { // 加入到三级缓存，暴漏早期对象用于解决循环依赖 singletonObject = singletonFactory.getObject(); this.earlySingletonObjects.put(beanName, singletonObject); this.singletonFactories.remove(beanName); } } } } return (singletonObject != NULL_OBJECT ? singletonObject : null);}其实这一段代码主要就是使用三级缓存解决set注入循环依赖的，后面会单独列一个章节对循环依赖做相关实验验证singletonObjects，用于存放初始化好的 bean 实例。earlySingletonObjects，用于存放初始化中的 bean，来解决循环依赖。singletonFactories，用于存放 bean 工厂，bean 工厂所生成的 bean 还没有完成初始化 bean。8. FactoryBean 中获取 bean 实例

AbstractBeanFactory -> getObjectForBeanInstance(sharedInstance, name, beanName, null)

protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, RootBeanDefinition mbd) { // 如果 beanName 以 & 开头，但 beanInstance 却不是 FactoryBean，则会抛出异常 if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) { throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass()); } // 这里判断就是这个 bean 是不是 FactoryBean，不是就直接返回了 if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) { return beanInstance; } Object object = null; if (mbd == null) { // 如果 mbd 为空，则从缓存加载 bean（FactoryBean 生成的单例 bean 实例会缓存到 factoryBeanObjectCache 集合中，方便使用） object = getCachedObjectForFactoryBean(beanName); } if (object == null) { // 到这，beanInstance 是 FactoryBean 类型，所以就强转了 FactoryBean<?> factory = (FactoryBean<?>) beanInstance; // mbd 为空且判断 containsBeanDefinition 是否包含 beanName if (mbd == null && containsBeanDefinition(beanName)) { // 合并 BeanDefinition mbd = getMergedLocalBeanDefinition(beanName); } boolean synthetic = (mbd != null && mbd.isSynthetic()); // 调用 getObjectFromFactoryBean 获取实例 object = getObjectFromFactoryBean(factory, beanName, !synthetic); } return object;}(!(beanInstance instanceof FactoryBean)，这里判断就是这个 bean 是不是 FactoryBean，不是就直接返回了如果 mbd 为空，则从缓存加载 bean（FactoryBean 生成的单例 bean 实例会缓存到 factoryBeanObjectCache 集合中，方便使用）调用 getObjectFromFactoryBean 获取实例，这里会包括一部分对单例以及非单例的处理，以及最终返回 factory.getObject(); 对应的 Bean 实例四、测试案例1. 别名

<bean id="userService" class="org.itstackterview.UserService"/><!-- 起个别名 --><alias name="userService" alias="userService-alias01"/><!-- 别名的别名 --><alias name="userService-alias01" alias="userService-alias02"/>

@Testpublic void test_alias() { BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config-alias.xml"); UserService userService = beanFactory.getBean("userService-alias02", UserService.class); logger("获取 Bean 通过别名：{}", userService);}