AJAX请求的responseXML始终返回null

Question

我正在使用servlet和ajax来传递数据，但是，servlet返回的xml文件总是等于＆＃39; Null＆＃39;而且我无法弄明白为什么！

这是我处理数据的Servlet函数：

      public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {

          // Set response content type
            response.setContentType("text/xml");
            PrintWriter out = response.getWriter();
            String action = request.getParameter("action");
            String searchQuery = request.getParameter("q");


            //String searchQuery = request.getParameter("query");
            StringBuffer sb = new StringBuffer();

            boolean change = false;

            if(searchQuery != null){
                //remove spaces and convert to lower case
                searchQuery = searchQuery.trim().toLowerCase();
            }

            if(action.equals("complete")){
                // check if user sent empty string
              if (!searchQuery.equals("")) {
                  for (String key : comp.keySet()) { 
                     if (comp.get(key).startsWith(searchQuery)){
                          sb.append("<result>");
                          sb.append("<id>" + comp.get(key) + "</id>");
                          sb.append("</result>");
                          change = true;
                          }
                      }
                  }     
              }
            if (change) {
                  response.setHeader("Cache-Control", "no-cache");
                  out.println("<?xml version=\"1.0\" encoding=\"ISO-8859-1\"?>");
                  out.write("<results>" + sb.toString() + "</results>");
              } else {
                  //indicating that the request succeeded but that there was no new information to return.
                  response.setStatus(HttpServletResponse.SC_NO_CONTENT);
              }




}

这是我收到请求的地方：

  xhttp.onreadystatechange = function() {

    if (xhttp.readyState == 4 && xhttp.status == 200) {

             parseMessages(xhttp.responseXML);
         }

}

parseMessages中的代码抛出错误，因为我尝试调用null参数。 任何帮助将不胜感激！