总体流程

protected 
    
      T doGetBean(		final String name, final Class
     
       requiredType, final Object[] args, boolean typeCheckOnly)		throws BeansException {	// 1、转换beanName	final String beanName = transformedBeanName(name);	Object bean;  	// 2、获取缓存中的singleton类型的实例，也有可能是提早暴露的没有完全初始化的实例	// Eagerly check singleton cache for manually registered singletons.	Object sharedInstance = getSingleton(beanName);	if (sharedInstance != null && args == null) {// singleton类型的bean，args必须是null，否则报错        2.1	}	else {		2.2	}	// Check if required type matches the type of the actual bean instance.	if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {		3	}	return (T) bean;}
     
    

1、处理beanName

2、获取缓存中的singleton类型的实例，这里有两种情况：

• 已经初始化完成的bean实例，从singletonObjects中获取

  bean name --> bean instanceMap
        
          singletonObjects
        

• 提早暴露的正在创建中的bean，没有完全初始化的bean（这个是用来解决循环依赖）

2.1、getSingleton(beanName) 不为空

PS：getBean()的参数arg在singleton类型的bean时必须为null，否则报错。

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 + "'");	}}// 处理FactoryBean类型bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);

2.2、getSingleton(beanName)为空

1、检查prototype类型的bean，如果是循环依赖就报错

if (isPrototypeCurrentlyInCreation(beanName)) {	throw new BeanCurrentlyInCreationException(beanName);}

2、如果当前找不到当前bean的BeanDefinition，就从父类BeanFactory中获取

// Check if bean definition exists in this factory.BeanFactory parentBeanFactory = getParentBeanFactory();if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {	// Not found -> check parent.	String nameToLookup = originalBeanName(name);	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);	}}

3、获取BeanDefinition

final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);// 如果是singleton类型的bean，args不为null就报错checkMergedBeanDefinition(mbd, beanName, args);

4、处理singleton类型的创建

// Create bean instance.if (mbd.isSingleton()) {	sharedInstance = getSingleton(beanName, new ObjectFactory

通过getSingleton()方法，传入ObjectFactory的匿名实现。

4.1、getSingleton()

Assert.notNull(beanName, "'beanName' must not be null");synchronized (this.singletonObjects) {	Object singletonObject = this.singletonObjects.get(beanName);	if (singletonObject == null) {		...		beforeSingletonCreation(beanName);		boolean newSingleton = false;		boolean recordSuppressedExceptions = (this.suppressedExceptions == null);		if (recordSuppressedExceptions) {			this.suppressedExceptions = new LinkedHashSet
    
     ();		}		try {			singletonObject = singletonFactory.getObject();			newSingleton = true;		}		...		finally {			if (recordSuppressedExceptions) {				this.suppressedExceptions = null;			}			afterSingletonCreation(beanName);		}		if (newSingleton) {			addSingleton(beanName, singletonObject);		}	}	return (singletonObject != NULL_OBJECT ? singletonObject : null);}
    

总体分为这几步：

1. beforeSingletonCreation：将beanName加入到正在创建的集合中
2. singletonFactory.getObject()：调用之前传入的ObjectFactory实现，进入创建bean的流程n
3. afterSingletonCreation：将beanName从正在创建的集合中移除
4. addSingleton：加入到缓存中

5、其他类型的不看了

3、对bean类型转换

// Check if required type matches the type of the actual bean instance.if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {	try {		return getTypeConverter().convertIfNecessary(bean, requiredType);	}	catch (TypeMismatchException ex) {		if (logger.isDebugEnabled()) {			logger.debug("Failed to convert bean '" + name + "' to required type [" +					ClassUtils.getQualifiedName(requiredType) + "]", ex);		}		throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());	}}return (T) bean;