我之前一直以为ajax就是局部刷新，现在才略微明白他是前端与后端异步交互的一种方式，其最大的优势在于不需要整个页面向后端发送请求，而可以提供我们需要与数据库比对的特定值给Servlet，
让Servlet一层一层往后调用方法获取数据库中的值进行比对，然后返回JSON给前端，前端利用后端传来的JSON进行判断并展示特定内容(JS实现)

JSON大概的样子--->{"":"","":""},类型于Map的形式，我们可以通过阿里巴巴的fastJSON依赖里的JSONArray.toJSONString()方法将Map转为JSON

下面代码以修改密码为例，利用ajax向Sevlet传递了旧密码的值，Servlet通过Session获取用户原密码，两者进行比较，将对应的情况值存入map，然后转为JSON传回前端，前端通过.键名获取键值，进行判断，最后展示

代码部分仅供参考
页面展示

 <form action="userUpdate" method="get" id="userForm">
        <input type="hidden" name="method" value="savepwd">
        <label for="oldPassword">旧密码：</label><input type="password" name="oldPassword" id="oldPassword"><span style="color: red"></span><br/>
        <label for="newPassword">新密码：</label><input type="password" name="newPassword" id="newPassword"><span style="color: red"></span><br/>
        <label for="conNewPassword">确认新密码：</label><input type="password" name="conNewPassword" id="conNewPassword"><span style="color: red"></span><br/>
        <input type="button" name="save" id="save" value="确认修改">
    </form>

核心代码1，Jquery

$(function () {
            var oldPassword = $("#oldPassword");
            var newPassword = $("#newPassword");
            var conNewPassword = $("#conNewPassword");
            var saveBtn = $("#save");
            oldPassword.on("blur",function () {
                $.ajax({
                    type:"GET",
                    url:"userUpdate",
                    //传参数给后台，参数名：参数值
                    data:{method:"pwdmodify",oldPassword:oldPassword.val()},
                    dataType:"json",
                    success:function (data) {
                        if(data.result == "true"){
                            oldPassword.next().css({"color":"green"});
                            oldPassword.next().html("正确");
                            oldPassword.VALIDATE_STATUS = true;
                        }else if(data.result == "false"){
                            oldPassword.next().css({"color":"red"});
                            oldPassword.next().html("原密码输入不正确");
                            oldPassword.VALIDATE_STATUS = false;
                        }else if(data.result == "sessionerror"){
                            oldPassword.next().css({"color":"red"});
                            oldPassword.next().html("当前用户session过期，请重新登录");
                            oldPassword.VALIDATE_STATUS = false;
                        }else if(data.result == "error"){
                            oldPassword.next().css({"color":"red"});
                            oldPassword.next().html("旧密码不能为空");
                            oldPassword.VALIDATE_STATUS = false;
                        }
                    },
                    error:function (data) {
                        oldPassword.next().css({"color":"red"});
                        oldPassword.next().html("请求错误");
                        oldPassword.VALIDATE_STATUS = false;
                    }
                });
            }).on("focus",function () {
                oldPassword.next().css({"color":"#666666"});
                oldPassword.next().html("请输入旧密码");
                oldPassword.VALIDATE_STATUS = false;
            })
            saveBtn.on("click",function () {
                oldPassword.blur();
                newPassword.blur();
                conNewPassword.blur();
                if(oldPassword.VALIDATE_STATUS==true){
                    if(confirm("确认要修改密码？")){
                        $("#userForm").submit();
                    }
                }
           })
    });

核心代码2，Servlet部分

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getParameter("method");
        if(method.equals("pwdmodify")){
            this.pwdModify(req,resp);
        }else if(method.equals("savepwd")){
            this.savePwd(req,resp);
        }
    }
private void pwdModify(HttpServletRequest req, HttpServletResponse resp) {
        Object o = req.getSession().getAttribute(Constant.USER_SESSION);
        String oldPassword = req.getParameter("oldPassword");
        HashMap<String,String> resultMap = new HashMap<String,String>();
        if(o==null){
            resultMap.put("result","error");
        }else if(StringUtils.isNullOrEmpty(oldPassword)){
            resultMap.put("result","error");
        }else {
            String userPassword = ((User) o).getPassword();
            if (oldPassword.equals(userPassword)) {
                resultMap.put("result", "true");
            }else{
                resultMap.put("result","false");
            }
        }
        try {
            resp.setContentType("application/json");
            PrintWriter writer = resp.getWriter();
            writer.write(JSONArray.toJSONString(resultMap));
            writer.flush();
            writer.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    private void savePwd(HttpServletRequest req, HttpServletResponse resp) {
        UserService userService = new UserServiceImpl();
        String newPassword = req.getParameter("newPassword");
        int id = ((User)req.getSession().getAttribute(Constant.USER_SESSION)).getId();
        int result = userService.savePwd(newPassword,id);
        if(result>0){
            req.getSession().removeAttribute(Constant.USER_SESSION);
            try {
                resp.sendRedirect(req.getContextPath()+"/Login.jsp");
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

实体类

public class User {
    private int id;
    private String username;
    private String password;
    private String remark;
    private int role_id;

    public User() {
    }

    public User(int id, String username, String password, String remark, int role_id) {
        this.id = id;
        this.username = username;
        this.password = password;
        this.remark = remark;
        this.role_id = role_id;
    }

    public User(String username, String password, String remark, int role_id) {
        this.username = username;
        this.password = password;
        this.remark = remark;
        this.role_id = role_id;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getRemark() {
        return remark;
    }

    public void setRemark(String remark) {
        this.remark = remark;
    }

    public int getRole_id() {
        return role_id;
    }

    public void setRole_id(int role_id) {
        this.role_id = role_id;
    }

    @Override
    public String toString() {
        return id+"--"+username+"--"+password+"--"+remark+"--"+role_id;
    }
}

大致效果

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