1、js的编写
???????????????????????????var xmlHttpRequest = null; ???????????????????????????//1、创建XMLHttpRequest对象 ???????????????????????????if (window.XMLHttpRequest) {//新版本返回为TRUE ???????????????????????????????xmlHttpRequest = new XMLHttpRequest(); ???????????????????????????} else { ???????????????????????????????xmlHttpRequest = new ActiveXObject( ???????????????????????????????????????"Microsoft.XMLHTTF"); ???????????????????????????} ???????????????????????????//2、设置回调函数 ???????????????????????????xmlHttpRequest.onreadystatechange = callBack; ???????????????????????????var username = $("#username").val(); ???????????????????????????//3、初始化XMLHttpRequest组件 ???????????????????????????var url = "UserServlet?username=" + username; ???????????????????????????xmlHttpRequest.open("GET", url, true); ???????????????????????????//4、发送请求 ???????????????????????????xmlHttpRequest.send(null); ???????????????????????????//回调函数callBack的编写 ???????????????????????????function callBack() { ???????????????????????????????if (xmlHttpRequest.readyState == 4 ???????????????????????????????????????&& xmlHttpRequest.status == 200) { ???????????????????????????????????var data = xmlHttpRequest.responseText; ???????????????????????????????????if (data == "true") { ???????????????????????????????????????$("#errMsg").html("用户名已被注册"); ???????????????????????????????????} else { ???????????????????????????????????????$("#errMsg").html("用户可以注册"); ???????????????????????????????????} ???????????????????????????????} ???????????????????????????} ???????????????????????});2、servlet的编写
package com.niu.servlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class UserServlet extends HttpServlet { ???@Override ???protected void doGet(HttpServletRequest req, HttpServletResponse resp) ???????????throws ServletException, IOException { ???????doPost(req, resp); ???} ???@Override ???protected void doPost(HttpServletRequest req, HttpServletResponse resp) ???????????throws ServletException, IOException { ???????req.setCharacterEncoding("utf-8"); ???????String name = req.getParameter("username"); ???????boolean used = false; ???????if ("ajax".equals(name)) { ???????????used = true; ???????} ???????resp.setContentType("text/html;charset=utf-8"); ???????PrintWriter out = resp.getWriter(); ???????out.print(used); ???????out.flush(); ???????out.close(); ???}}ajax ?异步处理 Get请求
原文地址：http://www.cnblogs.com/binglong180/p/7967083.html