1、根据beanName可以从Spring的三级缓存中获取到bean对象

  1. protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
  2.                           @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
  4.     // 如果这个 name 是 FactoryBean 的beanName (&+beanName),就删除& , 返回beanName ,传入的name也可以是别名,也需要做转换
  5.     // 注意 beanName 和 name 变量的区别,beanName是经过处理的,经过处理的beanName就直接对应singletonObjects中的key
  6.     final String beanName = transformedBeanName(name);
  7.     Object bean;
  9.     // Eagerly check singleton cache for manually registered singletons.
  10.     // 根据beanName尝试从singletonObjects获取Bean
  11.     // 获取不到则再尝试从earlySingletonObjects,singletonFactories 从获取Bean
  12.     // 这段代码和解决循环依赖有关
  13.     Object sharedInstance = getSingleton(beanName);
  14.     // 第一次进入sharedInstance肯定为null
  15.     if (sharedInstance != null && args == null) {
  16.         // 如果sharedInstance不为null,也就是非第一次进入
  17.         // 为什么要调用 getObjectForBeanInstance 方法,判断当前Bean是不是FactoryBean,如果是,那么要不要调用getObject方法
  18.         // 因为传入的name变量如果是(&+beanName),那么beanName变量就是(beanName),也就是说,程序在这里要返回FactoryBean
  19.         // 如果传入的name变量(beanName),那么beanName变量也是(beanName),但是,之前获取的sharedInstance可能是FactoryBean,需要通过sharedInstance来获取对应的Bean
  20.         // 如果传入的name变量(beanName),那么beanName变量也是(beanName),获取的sharedInstance就是对应的Bean的话,就直接返回Bean
  21.         bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);

2、如果是首次进入sharedInstance==null

  1.         // 判断是否循环依赖
  2.         if (isPrototypeCurrentlyInCreation(beanName)) {
  3.             throw new BeanCurrentlyInCreationException(beanName);
  4.         }
  6.         // Check if bean definition exists in this factory.
  7.         // 获取父BeanFactory,一般情况下,父BeanFactory为null,如果存在父BeanFactory,就先去父级容器去查找
  8.         BeanFactory parentBeanFactory = getParentBeanFactory();
  9.         if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
  10.             // Not found -> check parent.
  11.             String nameToLookup = originalBeanName(name);
  12.             if (parentBeanFactory instanceof AbstractBeanFactory) {
  13.                 return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
  14.                         nameToLookup, requiredType, args, typeCheckOnly);
  15.             } else if (args != null) {
  16.                 // Delegation to parent with explicit args.
  17.                 return (T) parentBeanFactory.getBean(nameToLookup, args);
  18.             } else if (requiredType != null) {
  19.                 // No args -> delegate to standard getBean method.
  20.                 return parentBeanFactory.getBean(nameToLookup, requiredType);
  21.             } else {
  22.                 return (T) parentBeanFactory.getBean(nameToLookup);
  23.             }
  24.         }

关于判断循环依赖，可以参考