在Spring中如何实现加载Bean的

TestBean bean = factory.getBean("testBean");

@Override
public Object getBean(String name) throws BeansException {
  return doGetBean(name, null, null, false);
}

protected  T doGetBean(
    final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
    throws BeansException {
  // 提取beanName
  final String beanName = transformedBeanName(name);
  Object bean;

  // 试着从缓存或实例工厂中获取
  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 + "'");
      }
    }
    // 返回实例，有时存在如BeanFactory这样情况时并不是直接返回实例本身而是返回指定方法返回的实例
    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);
    }

    // Check if bean definition exists in this factory.
    BeanFactory parentBeanFactory = getParentBeanFactory();
    // 如果在所有已经加载的类中没有beanName则会尝试从parentBeanFactory中检测
    if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
      // Not found -> check parent.
      String nameToLookup = originalBeanName(name);
      // 到BeanFactory查找
      if (args != null) {
        // Delegation to parent with explicit args.
        return (T) parentBeanFactory.getBean(nameToLookup, args);
      }
      else {
        // No args -> delegate to standard getBean method.
        return parentBeanFactory.getBean(nameToLookup, requiredType);
      }
    }
    // 如果不是只做类型检查则是创建bean
    if (!typeCheckOnly) {
      markBeanAsCreated(beanName);
    }

    try {
      // 将存储XML配置文件的GernericBeanDefinition转换成RootBeanDefinition，如果BeanName是子Bean的话会合并父类的相关属性
      final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
      checkMergedBeanDefinition(mbd, beanName, args);

      // Guarantee initialization of beans that the current bean depends on.
      String[] dependsOn = mbd.getDependsOn();
      // 如果存在依赖的话要递归实例化依赖的bean
      if (dependsOn != null) {
        for (String dependsOnBean : dependsOn) {
          if (isDependent(beanName, dependsOnBean)) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                "Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
          }
          registerDependentBean(dependsOnBean, beanName);
          getBean(dependsOnBean);
        }
      }

      // Create bean instance.
      // 依赖的bean实例化完后就可以实例化mbd了
      if (mbd.isSingleton()) {
        sharedInstance = getSingleton(beanName, new ObjectFactory