jsp中的javascript的$(document).ready( function() { $("#loginForm").validate(）

转自：https://bbs.csdn.net/topics/392459787?list=71147533

下面是jsp页面中的JavaScript代码

$(document).ready(
  function()
     {
$("#loginForm").validate(
                                  {
             rules: {
          validateCode: {remote: "${pageContext.request.contextPath}/servlet/validateCodeServlet"}
                },
         messages: {
                  username:       {required: "请填写用户名."},
                                                   password:       {required: "请填写密码."},
                   validateCode:   {remote: "验证码不正确.", required: "请填写验证码."}
             },
         errorLabelContainer: "#messageBox",
         errorPlacement:   function(error, element)
                                                                              {
                                    error.appendTo($("#loginError").parent());
                                            }
           }
                               ); // validate
   } // function
                  );// ready

请问这个$("#loginForm").validate 的工作原理和过程是什么，它参数中的rules messages 等参数有啥用 ？非常感谢大家。
以下是validateCodeServlet.java的代码

/**
 * Copyright &copy; 2015-2020 <a href=" ">JeePlus</a> All rights reserved.
 */
package com.jeeplus.core.servlet;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.commons.lang3.math.NumberUtils;
import org.apache.commons.lang3.StringUtils;

/**
 * 生成随机验证码
 * @author jeeplus
 * @version 2017-7-27
 */
@SuppressWarnings("serial")
public class ValidateCodeServlet extends HttpServlet {

    public static final String VALIDATE_CODE = "validateCode";

    private int w = ;
    private int h = ;

    public ValidateCodeServlet() {
        super();
    }

    public void destroy() {
        super.destroy();
    }

    public static boolean validate(HttpServletRequest request, String validateCode){
        String code = (String)request.getSession().getAttribute(VALIDATE_CODE);
        return validateCode.toUpperCase().equals(code);
    }

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String validateCode = request.getParameter(VALIDATE_CODE); // AJAX验证，成功返回true
        if (StringUtils.isNotBlank(validateCode)){
            response.getOutputStream().print(validate(request, validateCode)?"true":"false");
        }else{
            this.doPost(request, response);
        }
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        createImage(request,response);
    }

    private void createImage(HttpServletRequest request,
            HttpServletResponse response) throws IOException {

        response.setHeader("Pragma", "no-cache");
        response.setHeader("Cache-Control", "no-cache");
        response.setDateHeader("Expires", );
        response.setContentType("image/jpeg");

        /*
         * 得到参数高，宽，都为数字时，则使用设置高宽，否则使用默认值
         */
        String width = request.getParameter("width");
        String height = request.getParameter("height");
        if (StringUtils.isNumeric(width) && StringUtils.isNumeric(height)) {
            w = NumberUtils.toInt(width);
            h = NumberUtils.toInt(height);
        }

        BufferedImage image = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
        Graphics g = image.getGraphics();

        /*
         * 生成背景
         */
        createBackground(g);

        /*
         * 生成字符
         */
        String s = createCharacter(g);
        request.getSession().setAttribute(VALIDATE_CODE, s);

        g.dispose();
        OutputStream out = response.getOutputStream();
        ImageIO.write(image, "JPEG", out);
        out.close();

    }

    private Color getRandColor(int fc,int bc) {
        int f = fc;
        int b = bc;
        Random random=new Random();
        if(f>) {
            f=;
        }
        if(b>) {
            b=;
        }
        return new Color(f+random.nextInt(b-f),f+random.nextInt(b-f),f+random.nextInt(b-f));
    }

    private void createBackground(Graphics g) {
        // 填充背景
        g.setColor(getRandColor(,));
        g.fillRect(, , w, h);
        // 加入干扰线条
        for (int i = ; i < ; i++) {
            g.setColor(getRandColor(,));
            Random random = new Random();
            int x = random.nextInt(w);
            int y = random.nextInt(h);
            int x1 = random.nextInt(w);
            int y1 = random.nextInt(h);
            g.drawLine(x, y, x1, y1);
        }
    }

    private String createCharacter(Graphics g) {
        char[] codeSeq = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'J',
                'K', 'M', 'N', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W',
                'X', 'Y', 'Z', '', '', '', '', '', '', '', '' };
        String[] fontTypes = {"Arial","Arial Black","AvantGarde Bk BT","Calibri"};
        Random random = new Random();
        StringBuilder s = new StringBuilder();
        for (int i = ; i < ; i++) {
            String r = String.valueOf(codeSeq[random.nextInt(codeSeq.length)]);//random.nextInt(10));
            g.setColor(new Color( + random.nextInt(),  + random.nextInt(),  + random.nextInt()));
            g.setFont(new Font(fontTypes[random.nextInt(fontTypes.length)],Font.BOLD,));
            g.drawString(r,  * i + ,  + random.nextInt());
//            g.drawString(r, i*w/4, h-5);
            s.append(r);
        }
        return s.toString();
    }

}

答案：

$("#loginForm").validate是validate插件提供的api,是前端做表单验证的；rules是每一项的验证规则； messages 则是验证不通过时的提示信息；remote是指远程验证，插件都有规定的，如果是远程验证，api接口必须返回规定格式的数据，一般是返回boolean值;
其实就是一个表单拦截功能，在表单数据提交之前先根据提供的规则进行校验，不通过就进行错误提示，通过后才允许提交；

追答：

非常感谢，还要请教一下：
messages: {
                  username:       {required: "请填写用户名."},
                                                   password:       {required: "请填写密码."},
                   validateCode:   {remote: "验证码不正确.", required: "请填写验证码."}
             },
这个错误提示，如果只是没填写密码，这3个提示都显示吗，如果都显示，那肯定不对呀。

追答rules里面的key对应messages里面的key,同时对应form表单里面name为key的选项或者输入框；有些验证插件的机制是遇到一个不通过就会停下，并提示当前的错误信息；有些插件的机制是全部验证完，把所有不通过的错误信息都提示出来；