1. 看源码(AbstractBeanFactory)

@SuppressWarnings("unchecked")protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args,boolean typeCheckOnly)	throws BeansException {String beanName = transformedBeanName(name);Object beanInstance;// Eagerly check singleton cache for manually registered singletons.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 + "'");}}beanInstance = 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();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.return (T) parentBeanFactory.getBean(nameToLookup, args);}else if (requiredType != null) {// No args -> delegate to standard getBean method.return parentBeanFactory.getBean(nameToLookup, requiredType);}else {return (T) parentBeanFactory.getBean(nameToLookup);}}if (!typeCheckOnly) {markBeanAsCreated(beanName);}StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate").tag("beanName", name);try {if (requiredType != null) {beanCreation.tag("beanType", requiredType::toString);}RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);// Guarantee initialization of beans that the current bean depends on.String[] dependsOn = mbd.getDependsOn();if (dependsOn != null) {for (String dep : dependsOn) {if (isDependent(beanName, dep)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");}registerDependentBean(dep, beanName);try {getBean(dep);}catch (NoSuchBeanDefinitionException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"'" + beanName + "' depends on missing bean '" + dep + "'", ex);}}}// Create bean instance.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;}});beanInstance = 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);}beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}else {String scopeName = mbd.getScope();if (!StringUtils.hasLength(scopeName)) {throw new IllegalStateException("No scope name defined for bean '" + beanName + "'");}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);}});beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);}catch (IllegalStateException ex) {throw new ScopeNotActiveException(beanName, scopeName, ex);}}}catch (BeansException ex) {beanCreation.tag("exception", ex.getClass().toString());beanCreation.tag("message", String.valueOf(ex.getMessage()));cleanupAfterBeanCreationFailure(beanName);throw ex;}finally {beanCreation.end();}}return adaptBeanInstance(name, beanInstance, requiredType);}

2. 解析

// 这个方法比较长，需要耐心看，没耐心就别看了！
/**
* 这个方法是不对外暴露的，只用于内部使用。
* 
*/
String beanName = transformedBeanName(name);
// 获取到这个name的真实beanName
Object beanInstance;
// 最终要返回的对象
Object sharedInstance = getSingleton(beanName);
//1. 如果缓存中有key为beanName的对象直接拿出来
if (sharedInstance != null && args == null) {beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}//  如果不为空，就获取到beanInstance
//  直接返回 adaptBeanInstance(name, beanInstance, requiredType);// 2.如果在本地缓存中没有此对象
// 可能已经在创建了if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);
}// 如果在创建的过程中，那么就会抛出错误
// 如果本地BeanFactory没有bean定义,那就从父容器中找
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {return ((AbstractBeanFactory) parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);
}
// 找到直接返回
// 如果父容器不是AbstractBeanFactory child
else if (args != null) {// Delegation to parent with explicit args.return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {// No args -> delegate to standard getBean method.return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {return (T) parentBeanFactory.getBean(nameToLookup);
}// 如果只是check类型，就不创建；如果不仅仅check类型，标记创建对象
if (!typeCheckOnly) {markBeanAsCreated(beanName);
}
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);// 先创建当前bean所依赖的属性bean
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {for (String dep : dependsOn) {registerDependentBean(dep, beanName);try {getBean(dep);}catch (NoSuchBeanDefinitionException ex) {// nothing ...}// 接下来是根据mbd的scope去创建对象// 最后
调用adaptBeanInstance(name, beanInstance, requiredType)
// 获取类型转换器,将器转换为目标类型，返回。
Object convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return (T) convertedBean;<<<<<<<<<<<<<<<<<<<<<结束>>>>>>>>>>>>>>>>>>>>>>>>>>