Spring中的ApplicationContext是父子层次结构的，在存在多个上下文时，会有一个根上下文，作为其他上下文的“父亲”。：
<Listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
如上，如果在web.xml文件中使用Listener监听器来加载Spring的配置，Spring会创建一个全局的WebApplicationContext上下文，此上下文被称为根上下文，保存在ServletContext中。
public interface WebApplicationContext extends ApplicationContext {
/**
* Context attribute to bind root WebApplicationContext to on successful startup.
* <p>Note: If the startup of the root context fails, this attribute can contain
* an exception or error as value. Use WebApplicationContextUtils for convenient
* lookup of the root WebApplicationContext.
* @see org.springframework.web.context.support.WebApplicationContextUtils#getWebApplicationContext
* @see org.springframework.web.context.support.WebApplicationContextUtils#getRequiredWebApplicationContext
*/
String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
/**
* Scope identifier for request scope: "request".
* Supported in addition to the standard scopes "singleton" and "prototype".
*/
String SCOPE_REQUEST = "request";
/**
* Scope identifier for session scope: "session".
* Supported in addition to the standard scopes "singleton" and "prototype".
*/
String SCOPE_SESSION = "session";
/**
* Scope identifier for global session scope: "globalSession".
* Supported in addition to the standard scopes "singleton" and "prototype".
*/
String SCOPE_GLOBAL_SESSION = "globalSession";
/**
* Scope identifier for the global web application scope: "application".
* Supported in addition to the standard scopes "singleton" and "prototype".
*/
String SCOPE_APPLICATION = "application";
/**
* Name of the ServletContext environment bean in the factory.
* @see javax.servlet.ServletContext
*/
String SERVLET_CONTEXT_BEAN_NAME = "servletContext";
/**
* Name of the ServletContext/PortletContext init-params environment bean in the factory.
* <p>Note: Possibly merged with ServletConfig/PortletConfig parameters.
* ServletConfig parameters override ServletContext parameters of the same name.
* @see javax.servlet.ServletContext#getInitParameterNames()
* @see javax.servlet.ServletContext#getInitParameter(String)
* @see javax.servlet.ServletConfig#getInitParameterNames()
* @see javax.servlet.ServletConfig#getInitParameter(String)
*/
String CONTEXT_PARAMETERS_BEAN_NAME = "contextParameters";
/**
* Name of the ServletContext/PortletContext attributes environment bean in the factory.
* @see javax.servlet.ServletContext#getAttributeNames()
* @see javax.servlet.ServletContext#getAttribute(String)
*/
String CONTEXT_ATTRIBUTES_BEAN_NAME = "contextAttributes";
/**
* Return the standard Servlet API ServletContext for this application.
* <p>Also available for a Portlet application, in addition to the PortletContext.
*/
ServletContext getServletContext();
}
其中，key为org.springframework.web.context.WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUT属性的值。
在很多情况下，我们还会配置一个或者多个DispatcherServlet，每个DispatcherServlet都有一个自己的WebApplicationContext上下文。这个上下文是私有的，继承了根上下文的所有内容，该上下文保存在ServletContext中，key是org.springframework.web.servlet.FrameworkServlet#SERVLET_CONTEXT_PREFIX的值+servlet名称。当一个Request对象产生时，会把这个WebApplicationContext上下文保存在Request对象中。
DispatcherServlet加载Srping配置。
