服务器ActionServlet代码如下：
public void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("utf-8");
response.setContentType("text/html;charset=utf-8");
PrintWriter out = response.getWriter();
out.println("successful");
这里的successful会被ajax对象的responseText接收，为什么不会被responseXML接收??什么时候才用
responseXML??服务器返回的信息什么时候才会被ajax对象的responseXML接收??
------解决思路----------------------
response.setContentType("text/xml");