spring中获取bean的常见的方式，一般从BeanFactory中或者ApplicationContext中获取。因为ApplicationContext接口继承了BeanFactory接口，所以直接讲BeanFactory.getBean(String beanName)
AbstractBeanFactory的getBean调用的是该类的doGetBean方法

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {final String beanName = transformedBeanName(name);Object bean;// 从spring的三级缓存中获取bean---解决循环依赖的关键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 + "'");}}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);}//忽略部分代码.....try {final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);//忽略部分代码,getDependsOn依赖bean先加载.....// Create bean instance.if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, () -> {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {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;}}//忽略部分代码--类型的校验.....}return (T) bean;}
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上述的代码中大致的流程如下：

1. 从Spring维护的三级缓存中获取一个singleton bean，获取成功直接返回
2. 判断该bean是否处于创建中，如果是说明出现了循环依赖，抛异常
3. 根据bean的作用域，分别用不同的方式获取bean，如果获取不到最终调用的都是同一个createBean方式
4. getObjectForBeanInstance 如果上述方法获取到的是一个FactoryBean的情况下，需要从里面获取对应的我们需要的Bean

下面分析下singleton bean的获取

// Create bean instance.
if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, () -> {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {destroySingleton(beanName);throw ex;}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
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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);//定义异常...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) {//封装异常...}finally {...afterSingletonCreation(beanName);}if (newSingleton) {addSingleton(beanName, singletonObject);}}return singletonObject;}}
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1. 因为singleton的bean只能存在一个，所以对singletonObjects这个缓存做加锁获取
2. 做循环依赖判断
3. beforeSingletonCreation(beanName);//也就是把这个beanName标记成正在创建中
4. 通过singletonFactory的方法获取bean --> 也就是调用createBean方法
5. afterSingletonCreation(beanName);//去除该beanName正在创建中的标记
6. 如果是新创建的singleton的bean的话，把他加入singletonObjects缓存中
7. 这个singletonObjects缓存也就是上面所说的三级缓存的中的第一级（具体后续再说）

不管是singleton还是Prototype还是其他的scope，在从作用域中获取不到bean的情况下，都是调用的createBean

• 知识点梳理：

1. FactoryBean：www.cnblogs.com/davidwang45…
2. bean的作用域：
   1. singleton:整个容器中保留一份
   2. prototype:每次使用的时候，创建新的
   3. 其他scope
      1. SimpleThreadScope
      2. RequestScope
      3. SessionScope