关于前台获取不到后台传来值的问题

我要做的事点击按钮，不刷新页面，将页面上姓名卡号插入值显示
现在问题是后台过来的值前台获取不到，打印check值时一大段的404代码
js代码
function readCard(){//按钮点击方法

/* document.getElementById("name").value="人人";
document.getElementById("idcode").value="3222222222222"; */

/* ajax(); */

var url = "validate.do?id="+1;
if(window.XMLHttpRequest){
req = new XMLHttpRequest();
}else if(window.ActiveXObject){
req = new ActiveXObject("Microsort.XMLHTTP");
}
req.open("GET",url,true);
req.onreadystatechange = callback;
req.send(null);
};

function callback(){
if(req.readyState == 4 && req.status ==200){
var check = req.responseText;
/* document.getElementById("name").value=${registration.id}; */
document.getElementById("idcode").value=check;
}
}
java代码
public class ReturnName extends HttpServlet{


public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
int a =1111;
resp.setContentType("text/html;charset=UTF-8");
PrintWriter out = resp.getWriter();
//req.setAttribute("namecode", "好人");
out.write(a);
}

public void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
doGet(req, resp);
}
}

xml配置
<servlet>
<servlet-name>DoAjaxServlet</servlet-name>
<servlet-class>com.zyiot.gdbms.modules.bms.web.ReturnName</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>DoAjaxServlet</servlet-name>
<url-pattern>/validate.do</url-pattern>
</servlet-mapping>