从ClassPathXmlApplicationContext的继承关系可以看出，getBean（）方法调用的是AbstractApplicationContext中的。
public Object getBean(String name) throws BeansException {
//确定bean容器是否active
assertBeanFactoryActive();
//从bean工厂中取出bean
return getBeanFactory().getBean(name);
}
完成的功能：
1.确定bean容器是否active
2.从bean工厂中取出bean
1.1 getBeanFactory().getBean(name)中的getBean方法有很多的实现：
那么现在调用是哪个呢？我们先看一下getBeanFactory()方法：
public final ConfigurableListableBeanFactory getBeanFactory() {
synchronized (this.beanFactoryMonitor) {
if (this.beanFactory == null) {
throw new IllegalStateException("BeanFactory not initialized or already closed - " +
"call 'refresh' before accessing beans via the ApplicationContext");
}
return this.beanFactory;
}
}
返回的是ConfigurableListableBeanFactory对象。但通过之前我们对配置文件读取的分析知道实际的对象是DefaultListableBeanFactory。我们再来看一下DefaultListableBeanFactory的继承关系：
可知实际上调用的是AbstractBeanFactory中的getBean方法：
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
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.
//从单例缓存中取值
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 + "'");
}
}
//获取一个普通的bean或FactoryBean
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
//判断是否正在创建beanName的实例，是则报错
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
//检查BeanFactory中是否已经有了bean，但此时的parentBeanFactory 应该是null
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);
}
}
//判断是否只是类型检查，如果不是则把beanName放入标识为已创建bean的set集合中
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
//获取bean定义
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
//检查bean定义是否是抽象类，是则报错
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//取出bean的依赖，如@autowired修饰的成员变量
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dependsOnBean : dependsOn) {
//判断是否多次依赖
if (isDependent(beanName, dependsOnBean)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
}
//把依赖bean放入dependentBeanMap，dependenciesForBeanMap中
registerDependentBean(dependsOnBean, beanName);
//获取依赖的bean实例
getBean(dependsOnBean);
}
}
// Create bean instance.
//创建bean实例
//bean是单例时，创建danlibean
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);
}
//是prototype时，重新创建一个实例
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);
}
//其他scope时，使用相应的规则进行创建实例
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.
//检查bean实例是否是requiredType 类型，是则转成requiredType 类型，不是则报错
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;
}
此方法完成的功能：
1.从单例缓存中取值
2.获取一个普通的bean或FactoryBean
3.判断是否正在创建beanName的实例，是则报错
4.检查BeanFactory中是否已经有了bean，但此时的parentBeanFactory 应该是null
5.判断是否只是类型检查，如果不是则把beanName放入标识为已创建bean的set集合中
6.获取bean定义
7.检查bean定义是否是抽象类，是则报错
8.取出bean的依赖，如@autowired修饰的成员变量
9.判断是否多次依赖
10.把依赖bean放入dependentBeanMap，dependenciesForBeanMap中
11.获取依赖的bean实例
12.创建bean实例，bean是单例时，创建danlibean
13.是prototype时，重新创建一个实例
14.其他scope时，使用相应的规则进行创建实例