contextLoader的源码
 public WebApplicationContext initWebApplicationContext(ServletContext servletContext)
   throws BeansException {

  long startTime = System.currentTimeMillis();
  if (logger.isInfoEnabled()) {
   logger.info("Root WebApplicationContext: initialization started");
  }
  servletContext.log("Loading Spring root WebApplicationContext");

  try {
   // Determine parent for root web application context, if any.
   ApplicationContext parent = loadParentContext(servletContext);

   WebApplicationContext wac = createWebApplicationContext(servletContext, parent);
   servletContext.setAttribute(
     WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, wac);

   if (logger.isInfoEnabled()) {
    logger.info("Using context class [" + wac.getClass().getName() +
      "] for root WebApplicationContext");
   }
   if (logger.isDebugEnabled()) {
    logger.debug("Published root WebApplicationContext [" + wac +
      "] as ServletContext attribute with name [" +
      WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
   }

   if (logger.isInfoEnabled()) {
    long elapsedTime = System.currentTimeMillis() - startTime;
    logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
   }

   return wac;
  }
  catch (RuntimeException ex) {
   logger.error("Context initialization failed", ex);
   servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
   throw ex;
  }
  catch (Error err) {
   logger.error("Context initialization failed", err);
   servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
   throw err;
  }
 }
 注意WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE,这里面放了WebApplicationContext,需要使用时从ServletContext取出
 可以使用WebApplicationContextUtils得到WebApplicationContext
 public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
  Object attr = sc.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
  if (attr == null) {
   return null;
  }
  if (attr instanceof RuntimeException) {
   throw (RuntimeException) attr;
  }
  if (attr instanceof Error) {
   throw (Error) attr;
  }
  if (!(attr instanceof WebApplicationContext)) {
   throw new IllegalStateException("Root context attribute is not of type WebApplicationContext: " + attr);
  }
  return (WebApplicationContext) attr;
 }
 关键的问题在于struts如何启动的spring的,ContextLoaderPlugIn的源码
 
 // Publish the context as a servlet context attribute.
  String attrName = getServletContextAttributeName();
  getServletContext().setAttribute(attrName, wac);
 
 public String getServletContextAttributeName() {
  return SERVLET_CONTEXT_PREFIX + getModulePrefix();
 }
 不同加载的Key竟然不同,原因就是WebApplicationContext放在那里的问题,可spring调用的时候会根据WebApplicationContext里面定义的那个名字去找的,问题出在这里

 在struts-config.xml中配置
    <plug-in className="org.springframework.web.struts.ContextLoaderPlugIn">
      <set-property property="contextConfigLocation" value="/WEB-INF/applicationContext.xml" />
    </plug-in>

    <controller>
        <set-property property="processorClass" value="org.springframework.web.struts.DelegatingRequestProcessor" />
    </controller>

 原理是这样的,Struts虽然只能有一个ActionServlet实例,但是对于不同的子应用分别能有自己的RequestProcessor实例每个RequestProcessor实例分别对应不同的struts配置文件。
   子应用的ProcessorClass类必须重写一般就是继承RequestProcessor类，然后再其配置文件的controller元素中的<processorClass>属性中作出修改。那么当
  getRequestProcessor(getModuleConfig(request)).process(request,response);就能根据request选择相应的moduleconfig,再根据其<processorClass>属性选择相应的RequestProcessor子类来处理相应的请求了。