注：本文分析的内容，针对的是Spring 2.5.6的版本
ApplicationContext是spring IoC容器的顶级接口，其类结构图如下：

    从上面的类图中可以看出， ApplicationContext继承了ResourceLoader接口，便于获取外部资源；也间接继承了 BeanFactory接口，这样可以在Spring容器中创建Bean对象；同时也继承了ApplicationEventPublisher接口，用于发送一些事件消息。
    通常我们使用这样的一行代码来创建并启动Spring容器：

ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");

 虽然是简单的一行代码，但背后却做了很多事情，先看一下ClassPathXmlApplicationContext的构造方法：

	public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
		this(new String[] {configLocation}, true, null);
	}
	public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
			throws BeansException {

		super(parent);
		setConfigLocations(configLocations);
		if (refresh) {
			refresh();
		}
	}

在该构造方法中，最后调用了AbstractApplicationContext.refresh()方法，该方法的实现代码如下：

public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			//1、 Prepare this context for refreshing.
			prepareRefresh();

			//2、 Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			//3、 Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				//4、 Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				//5、 Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				//6、 Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				//7、 Initialize message source for this context.
				initMessageSource();

				//8、 Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				//9、 Initialize other special beans in specific context subclasses.
				onRefresh();

				//10、 Check for listener beans and register them.
				registerListeners();

				//11、 Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				//12、 Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				// Destroy already created singletons to avoid dangling resources.
				beanFactory.destroySingletons();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}
		}
	}

这个方法非常关键，通过调用12个其他的方法，完成了整个IoC容器的构建。详细了解这个方法里面的每一行代码，基本上就掌握了Spring大部分的原理、功能和扩展点。下面分析每一个方法的所做的事情。

1、prepareRefresh()
该方法所做的事情相对比较简单：记录容器启动的时间，并设置容器处于活跃状态。

2、obtainFreshBeanFactory()
该方法的作用：创建BeanFactory实例，并解析Spring的xml配置文件。beanFactory的实现类是：ConfigurableListableBeanFactory。方法的实现如下：

	protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
		refreshBeanFactory();
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();

		if (logger.isInfoEnabled()) {
			logger.info("Bean factory for application context [" &#