进 入 正 题 ， 在 Spring 创 建 Bean 的 核 心 代 码
doGetBean 中，在实例化 bean 之前，会先尝试从三级
缓存获取 bean，这也是 Spring 解决循环依赖的开始
(一) 缓存中获取 bean
//AbstractBeanFactory.java
protected<T>TdoGetBean(nalStringname,@Nullable
nalClass<T>requiredType,
@NullablenalObject[]args,booleantypeCheckOnly)thro
wsBeansException{
nalStringbeanName=transformedBeanName(name);
Objectbean;
//2.尝试从缓存中获取 bean
ObjectsharedInstance=getSingleton(beanName);
...
}
protectedObjectgetSingleton(StringbeanName,booleana
llowEarlyReference){
//从一级缓存获取，key=beanNamevalue=bean
ObjectsingletonObject=this.singletonObjects.get(beanName
);
if(singletonObject==null&&isSingletonCurrentlyInCreation
(beanName)){
synchronized(this.singletonObjects){
//从二级缓存获取，key=beanNamevalue=bean
singletonObject=this.earlySingletonObjects.get(beanName
);
//是否允许循环引用
if(singletonObject==null&&allowEarlyReference){
/**
