package com.tjsoft.system.login;


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 javax.servlet.http.HttpSession;

public class ValidateServlet extends HttpServlet {

    /**
     * Constructor of the object.
     */
    public ValidateServlet() {
        super();
    }

    /**
     * Destruction of the servlet. <br>
     */
    public void destroy() {
        super.destroy(); // Just puts "destroy" string in log
        // Put your code here
    }

    /**
     * The doGet method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to get.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        BufferedImage bi=new BufferedImage(75,25,BufferedImage.TYPE_INT_RGB);
        Graphics g=bi.getGraphics();        
        g.fillRect(3, 3, 70, 20);
        String line="";
        Random rnd=new Random();
        for(int i=0;i<5;i++){
            line+=rnd.nextInt(10);
        }
        HttpSession session=request.getSession();
        session.setAttribute("line", line);
        g.setFont(new Font("黑体",Font.BOLD,20));
        int x1 = 0;
        int x2 = 0;
        int y1 = 0;
        int y2 = 0;
        g.setColor(Color.gray);
        for(int i = 0; i < 150; i ++){
            x1 = rnd.nextInt(60)+3;
            x2 = rnd.nextInt(70)+3;
            y1 = rnd.nextInt(10)+3;
            y2 = rnd.nextInt(20)+3;
            
            g.drawLine(x1, y1, x2, y2);
        }
        g.setColor(Color.RED);
        g.drawString(line, 3, 20);
        response.setContentType("image/jpeg");//将servlet设置成图片类型
        //禁止缓存
        response.setHeader("pragma", "no-cache");
        response.setHeader("cache-control", "no-cache");
        response.setDateHeader("expires", 0);
        
        OutputStream os = response.getOutputStream();
        //将图片写到客户端
        ImageIO.write(bi, "jpg", os);
        
    }
}

------解决方案--------------------
JScript code

var code; // 在全局 定义验证码
function createCode() {
    code = "";
    var codeLength = 3;// 验证码的长度
    var checkCode = document.getElementById("checkCode");
    var selectChar = new Array(0, 1, 2, 3, 4, 5, 6, 7, 8, 9);
    // , 'A', 'B',
    // 'C', 'D', 'E', 'F', 'G', 'G', 'I', 'J', 'K', 'L', 'M', 'N',
    // 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y',
    // 'Z');//所有候选组成验证码的字符，当然也可以用中文的
    for ( var i = 0; i < codeLength; i++) {
        var charIndex