spring学习（五）ClassPathXmlApplicationContext的getBean获取实例bean

上一篇简单说明了一下spring读取xml文件加载bean的流程，读取xml文件之后，我们一般会通过ApplicationContext.getBean(String beanName)方法获取指定的名称的bean类，这里简单介绍一个这个方法的源码。

1、getBean方法是统一调用的AbstractApplicationContext.getBean方法，这个方法的实现就是调用BeanFactory.getBean方法，之前讲过ApplicationContext里的beanFactory是实例化的DefaultListableBeanFactory。通过DefaultListableBeanFactory可以找到对应的getBean方法的实现是在AbstractBeanFactory.getBean(String beanName)

	public Object getBean(String name) throws BeansException {
		return getBeanFactory().getBean(name);
	}

2、AbstractBeanFactory.getBean调用了自己类中的doGetBean方法。

public Object getBean(String name) throws BeansException {
		return doGetBean(name, null, null, false);
}

因为doGetBean的方法的代码很多，这里主要说明一下bean的scope是single的创建方法，完整代码如下

@SuppressWarnings("unchecked")
	protected  T doGetBean(
			final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
			throws BeansException {

		final String beanName = transformedBeanName(name);
		Object bean;

		// Eagerly check singleton cache for manually registered singletons.
		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.
			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);
				}
			}

			if (!typeCheckOnly) {
				markBeanAsCreated(beanName);
			}

			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) {
					getBean(dependsOnBean);
					registerDependentBean(dependsOnBean, beanName);
				}
			}

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