spring-getBean方法流程
流程图
（1）调用AbstractBeanFactory的doGetBean方法
（2）转化beanName，如果包含&，删除之
（3）根据beanName从缓存中获取Bean
（4）Bean 是否 不为null并且args为null
（5）调用getObjectForBeanInstance方法
（6） 判断BeanInstance是否为FactoryBean类型并且不以name为&开头
（7）判断BeanInstance不是FactoryBean类型的或者是FactoryBean类型的并且以&开头
（8）从FactoryBean生成的实例缓存中获取obean
（9）判断bean是否为null
（10）根据FactoryBean的getObject方法返回Bean
（11）检查BeanFactory是否包含此Bean定义,如果不包含，继续自己的父BeanFactory查找
（12）获取bean的依赖项
（13）调用getObjectForBeanInstance方法
（14）抛出异常
（15）返回Bean
代码注释
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;
// 从缓存中获取BeanInstance，此BeanInstance在我看来，这才是真正创建Bean实例的工厂，根据不同的情况这个Bean实例做处理，或直接返回，如果是FactoryBean并且并且那么没有以&开头则调用其方法getObject（）返回实例
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 + "'");
}
}
//真正的bean 返回处理
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// 假设循环依赖的时候，如果我们已经开始在当前线程中创建此bean实例，但是还没有创建完成，则失败；例如此时是A依赖于B，但是B不依赖于A，B也没有任何属性依赖于A，则不存在循环依赖，那么无论B初始化，未初始化都不会有以下情况。但是如果B依赖于A，A在获取依赖的Bean是激活创建B的方法，那么B创建过程中就会出现以下情况。就会出现循环依赖错误。如果A，B 是单例的并且A的构造函数不包含B，B的构造函数不包含A，spring还是可以通过提前暴露实例地址处理这种依赖，但是其它情况spring也无能为力了。循环依赖也是一个大问题。。。
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 判断工厂中是否含有此Bean的定义
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// 如果没有，查询父工厂
String nameToLookup = originalBeanName(name);
if (args != null) {
// 执行带有args参数的getbean方法
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
//如果没有参数，执行标准的getbean方法
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
//如果不仅仅是做类型检查，那么需要标记此Bean正在创建之中
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 获取依赖的Bean
String[] dependsOn = mbd.getDependsOn();
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);
}
}
// 这里终于开始创建Bean实例了，如果是单例的，那么会创建一个单例的匿名工厂，如果是原型模式的，则不需要创建单例的工厂的，其他的如request、session作用域的，则根据自身的需要
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 {
//把当前的beanName加入正在创建beanName的集合中，会在第一步判断
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
//把当前的beanName从正在创建beanName的集合中移除
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, 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;
}
}
// 检查类型，如果不满足可以进行类型转换，如果转换器进行不了这两种类型的转换，则抛出异常
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;
}