protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
/*检查缓存中或者实例工厂中是否有对应的实例
*因为在创建单例bean的时候会存在依赖注入的情况，为了避免循环引用
*Spring创建bean的原则是不等bean创建完成就会将创建bean的ObjectFactory提早曝光
*即将ObjectFactory加入到缓存之中，一旦下一个bean创建的时候需要依赖上一个bean则直接
*使用beanFactory
*/
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 + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 原型类型的bean是不允许循环依赖的，所以Exception
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 (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);
}
}
//如果不仅仅是做类型检查，则需要进行记录
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final 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 dependsOnBean : dependsOn) {
if (isDependent(beanName, dependsOnBean)) {
throw new BeanCreationException("Circular depends-on relationship between '" +
beanName + "' and '" + dependsOnBean + "'");
}
registerDependentBean(dependsOnBean, beanName);
getBean(dependsOnBean);
}
}
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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 '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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.
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;
}
实际的doGetBean过程很复杂，拆成步骤来分析
1.转换对应的beanName
因为这里传入的参数可能是别名，也可能是FactoryBean，所以要进行一系列的解析，解析的内容包括：
去除FactoryBean的修饰符，也就是如果name=”&aa”,那么会首先去除&而使name=”aa”
取置顶alias所表示的最终beanName，例如别名A指向名称为B的bean则返回B，如果A指向B，B又指向C则返回C
2.尝试从缓存中加载bean
默认allowEarlyReference是设置为true的
这个的作用是为了处理singleton bean的依赖注入的情况，在创建依赖的时候为了避免循环依赖，Spring创建bean的原则是不等bean创建完成就会将创建的ObjectFactory提早曝光加入到缓存中，一旦下一个bean创建时需要依赖上一个bean，则直接使用ObjectFactory
注：定义bean的时候是可以指定scope的，默认是单例
先说一下各个缓存的作用：
singletonObjects:
用于保存BeanName和创建bean实例之间的关系，bean name -> bean instance
singletonFactories:
用于保存BeanName和创建bean的工厂之间的关系，bean name -> ObjectFactory
earlySingletonObjects:
也是保存BeanName和创建bean实例之间的关系，与SingletonObjects的不同之处在于，当一个实例bean被放到这里面后，那么当bean还在创建的过程中，就可以通过getBean方法获取到了，其目的是用来检测循环引用
registeredSingletons:
用来保存当前所有已注册的bean
在缓存之中没有拿到bean
3.Bean的实例化，如果从缓存中得到了bean的原始状态，则需要对bean进行实例化。
强调：缓存中记录的只是最原始的bean状态，并不一定是我们最终想要的bean。
切记
在getBean方法中，getObjectForBeanInstance是个高频率使用的方法，无论是从缓存中获得bean还是根据不同的scope策略加载bean。总之，得到bean的实例后要做的第一件事就是调用这个方法来检测一下正确性。
这里为什么会有&作为前缀？通常使用的FactoryBean也是直接用BeanName来获取的。
当使用&作为前缀的时候表示用户想要的是直接获取FactoryBean工厂实例而不是FactoryBean.getObject()对应的实例。
处理FactoryBean
先是从缓存中拿FactoryBean，如果拿不到真正的获取FactoryBean的逻辑委托给getObjectFromFactoryBean
这里还是就判定一下是否单例，是单例需要保证全局唯一，并且使用缓存。实际的创建逻辑在doGetObjectFromFactoryBean之中
终于看到了factory.getObject()
不过在调用factory.getObject()获取bean之后并没有直接返回bean，还有下面一部分：
调用的是AbstractAutowireCapableBeanFactory之中的postProcessObjectFromFactoryBean()
这里是调用了后处理器，这里可以了解Spring获取bean的规则中的一条：
尽可能保证所有bean初始化后都会调用注册的BeanPostProcessor的postProcessAfterInitialization方法进行处理，在实际的开发工程中大可以针对此特性设计自己的业务逻辑。
同时说一下applyBeanPostProcessorsBeforeInitialization
暂时还没有看到调用的时机
到此全部是从缓存中寻找是否有singletonbean，已经从FactoryBean之中获取bean
如果都失败了，那么进入else
只有在单例情况下才会尝试解决循环依赖，如果A依赖B，B依赖A，那么造成循环依赖，引起BeanCurrentlyInCreationException
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