校验用户名是否存在
1. 服务器响应的数据，在客户端使用时，要想当做json数据格式使用。有两种解决方案：
1. $.get(type):将最后一个参数type指定为"json"
2. 在服务器端设置MIME类型
response.setContentType("application/json;charset=utf-8");
07-regist.html
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>注册页面</title><script src="js/jquery-3.3.1.min.js"></script><script>//在页面加载完成后$(function () {//给username绑定blur事件$("#username").blur(function () {//获取username文本输入框的值var username = $(this).val();//发送ajax请求//期望服务器响应回的数据格式：{"userExsit":true,"msg":"此用户名太受欢迎,请更换一个"}//{"userExsit":false,"msg":"用户名可用"}$.get("findUserServlet",{username:username},function (data) {//判断userExsit键的值是否是true// alert(data);var span = $("#s_username");if(data.userExsit){//用户名存在span.css("color","red");span.html(data.msg);}else{//用户名不存在span.css("color","green");span.html(data.msg);}});}); });</script></head><body><form><input type="text" id="username" name="username" placeholder="请输入用户名"><span id="s_username"></span><br><input type="password" name="password" placeholder="请输入密码"><br><input type="submit" value="注册"><br></form></body></html>
FindUserServlet.java
package cn.itcast.web.servlet;import com.fasterxml.jackson.databind.ObjectMapper;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import java.io.IOException;import java.util.HashMap;import java.util.Map;@WebServlet("/findUserServlet")public class FindUserServlet extends HttpServlet {protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {//1.获取用户名String username = request.getParameter("username");//2.调用service层判断用户名是否存在//期望服务器响应回的数据格式：{"userExsit":true,"msg":"此用户名太受欢迎,请更换一个"}//{"userExsit":false,"msg":"用户名可用"}//设置响应的数据格式为jsonresponse.setContentType("application/json;charset=utf-8");Map<String,Object> map = new HashMap<String,Object>();if("tom".equals(username)){//存在map.put("userExsit",true);map.put("msg","此用户名太受欢迎,请更换一个");}else{//不存在map.put("userExsit",false);map.put("msg","用户名可用");}//将map转为json，并且传递给客户端//将map转为jsonObjectMapper mapper = new ObjectMapper();//并且传递给客户端mapper.writeValue(response.getWriter(),map);}protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {this.doPost(request, response);}}
