Spring源码学习（-）别怕，老外点餐与AbstractBeanFactory.getBean的流程差不多

//AbstractBeanFactory 中getBean方法第源码
@Override
public Object getBean(String name, Object... args) throws BeansException {
   return doGetBean(name, null, args, false);
}

/**

 * 获得一个实例，可以是共享(原型的)，独立（单例的），指定的bean
 * @param name the name of the bean to retrieve
 * @param requiredType the required type of the bean to retrieve
 * @param args arguments to use when creating a bean instance 
 * using explicit arguments
 * (only applied when creating a new instance as opposed to 
 retrieving an existing one)
 *        args      (仅在创建一个新实例时使用，而不是获取一个已经存在的bean)
 * @param typeCheckOnly whether the instance is obtained 
   for a type check,not for actual use
 *          是否仅仅是类型检查
 * @return an instance of the bean
 * @throws BeansException if the bean could not be created
 */
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
final String name, @Nullable final Class<T> requiredType,
      @Nullable final Object[] args, boolean typeCheckOnly) 
      throws BeansException {
   //1.获取对应的beanName ，BeanFactory去掉&，别名找到对应的BeanName
   final String beanName = transformedBeanName(name);
   Object bean;

   // Eagerly check singleton cache for manually registered singletons.
   //2. 尝试从缓存中获取单例 Spring会，在Bean未创建完成的情况下，创建Bean的ObjectFactory对象，提早曝光，以方便解决循环依赖。
   Object sharedInstance = getSingleton(beanName);
   if (sharedInstance != null && args == null) {
      if (logger.isTraceEnabled()) {
         if (isSingletonCurrentlyInCreation(beanName)) {
            logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                  "' that is not fully initialized yet - a consequence of a circular reference");
         }
         else {
            logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
         }
      }
      //根据bean实例获取对象，如果是FactoryBean 获取它创建的对象
      bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
   }
   else {
      // Fail if we're already creating this bean instance:
      // We're assumably within a circular reference.
      //3.原型依赖的检查，如果是原型，假设存在循环引用，抛出异常
      if (isPrototypeCurrentlyInCreation(beanName)) {
         throw new BeanCurrentlyInCreationException(beanName);
      }
      // Check if bean definition exists in this factory.
      // 4.检查bean，是否定义在了BeanFactory中，
      BeanFactory parentBeanFactory = getParentBeanFactory();
      // 如果父BeanFactory不为空&&当前并没有beanDefinitionMap不包含，
      // 委托给父BeanFactory
      if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
         // Not found -> check parent.
         String nameToLookup = originalBeanName(name);
         if (parentBeanFactory instanceof AbstractBeanFactory) {
            return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                  nameToLookup, requiredType, args, typeCheckOnly);
         }
         else if (args != null) {
            // Delegation to parent with explicit args.
            //委托给，需要参数的 getBean方法
            return (T) parentBeanFactory.getBean(nameToLookup, args);
         }
         else if (requiredType != null) {
            // No args -> delegate to standard getBean method.
            // 委托给标准的getBean方法
            return parentBeanFactory.getBean(nameToLookup, requiredType);
         }
         else {
            return (T) parentBeanFactory.getBean(nameToLookup);
         }
      }

      if (!typeCheckOnly) {
         //不仅是，类型检查，标记bean为创建
         // 允许bean可以重新合并
         markBeanAsCreated(beanName);
      }
      try {
         //5.将GernericBeanDefinition转换为RootBeanDefinition，
         //如果Bean有父Bean，时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.
         // 6.保证bean的依赖先初始化了
         String[] dependsOn = mbd.getDependsOn();
         if (dependsOn != null) {
            for (String dep : dependsOn) {
                 // 对应@DependsOn 注解，
                  /**
                   * 检测是否存在 depends-on 循环依赖，若存在则抛异常。
                   * 比如 A 依赖 B，
                   * B 又依赖 A，他们的配置如下：
                  @Bean(name="a")
                 @DependsOn("b")public A a () {
                          return new A();}

                      @Bean(name = "b")
                      @DependsOn("a")
                     public B b () {
                          return new B();
                      }

                   * a 要求 b 在其之前被创建，但 b 又要求 a 先于它
                   * 创建。这个时候形成了循环，对于 depends-on 循环，Spring 会直接
                   * 抛出异常
                   *
                   *@see org.springframework.context.annotation.ClassPathBeanDefinitionScanner#doScan
                   */
                
               if (isDependent(beanName, dep)) {
                  throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
               }
               //注册，Dependent和Bean的关系
               registerDependentBean(dep, beanName);
               try {
                  //先创建Dependent
                  getBean(dep);
               }
               catch (NoSuchBeanDefinitionException ex) {
                  throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
               }
            }
         }

         // Create bean instance.
         //7.创建Bean的实例
         if (mbd.isSingleton()) {
            sharedInstance = getSingleton(beanName, () -> {
               try {
                  return createBean(beanName, mbd, args);
               }
               catch (BeansException ex) {
                  // Explicitly remove instance from singleton cache: It might have been put there
                  // eagerly by the creation process, to allow for circular reference resolution.
                  // Also remove any beans that received a temporary reference to the bean.
                  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;
      }
   }

   // Check if required type matches the type of the actual bean instance.
   // 8.进行类型转换
   //  requiredType.isInstance（bean） 类似 bean instanceof requiredType
   // 为true表示可以直接返回，或强转
   //clazz.isAssignableFrom(obj.getClass()) == clazz.isInstance(obj)  
   //仅当obj not null
   if (requiredType != null && !requiredType.isInstance(bean)) {
      try {
         T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
         if (convertedBean == null) {
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
         }
         return convertedBean;
      }
      catch (TypeMismatchException ex) {
         if (logger.isTraceEnabled()) {
            logger.trace("Failed to convert bean '" + name + "' to required type '" +
                  ClassUtils.getQualifiedName(requiredType) + "'", ex);
         }
         throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
      }
   }
   return (T) bean;
}

AnnotationAttributes dependsOn = attributesFor(metadata, DependsOn.class);
if (dependsOn != null) {
   abd.setDependsOn(dependsOn.getStringArray("value"));
}

  String[] dependsOn = mbd.getDependsOn();
  if (dependsOn != null) {
       .....
       }