Detailed explanation of the code example of java graphic verification code generation tool web page verification code

import javax.imageio.ImageIO;  
import java.awt.*;  
import java.awt.image.BufferedImage;  
import java.io.FileOutputStream;  
import java.io.IOException;  
import java.io.OutputStream;  
import java.util.Date;  
import java.util.Random;  
  
/**  
 * 验证码生成器  
 *  
 * @author  
 */  
public class ValidateCode {  
  // 图片的宽度。  
  private int width = 160;  
  // 图片的高度。  
  private int height = 40;  
  // 验证码字符个数  
  private int codeCount = 5;  
  // 验证码干扰线数  
  private int lineCount = 150;  
  // 验证码  
  private String code = null;  
  // 验证码图片Buffer  
  private BufferedImage buffImg = null;  
  
  // 验证码范围,去掉0(数字)和O(拼音)容易混淆的(小写的1和L也可以去掉,大写不用了)  
  private char[] codeSequence = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',  
      'K', 'L', 'M', 'N', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W',  
      'X', 'Y', 'Z', '1', '2', '3', '4', '5', '6', '7', '8', '9'};  
  
  /**  
   * 默认构造函数,设置默认参数  
   */  
  public ValidateCode() {  
    this.createCode();  
  }  
  
  /**  
   * @param width 图片宽  
   * @param height 图片高  
   */  
  public ValidateCode(int width, int height) {  
    this.width = width;  
    this.height = height;  
    this.createCode();  
  }  
  
  /**  
   * @param width   图片宽  
   * @param height  图片高  
   * @param codeCount 字符个数  
   * @param lineCount 干扰线条数  
   */  
  public ValidateCode(int width, int height, int codeCount, int lineCount) {  
    this.width = width;  
    this.height = height;  
    this.codeCount = codeCount;  
    this.lineCount = lineCount;  
    this.createCode();  
  }  
  
  public void createCode() {  
    int x = 0, fontHeight = 0, codeY = 0;  
    int red = 0, green = 0, blue = 0;  
  
    x = width / (codeCount + 2);//每个字符的宽度(左右各空出一个字符)  
    fontHeight = height - 2;//字体的高度  
    codeY = height - 4;  
  
    // 图像buffer  
    buffImg = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);  
    Graphics2D g = buffImg.createGraphics();  
    // 生成随机数  
    Random random = new Random();  
    // 将图像填充为白色  
    g.setColor(Color.WHITE);  
    g.fillRect(0, 0, width, height);  
    // 创建字体,可以修改为其它的  
    Font font = new Font("Fixedsys", Font.PLAIN, fontHeight);  
//    Font font = new Font("Times New Roman", Font.ROMAN_BASELINE, fontHeight);  
    g.setFont(font);  
  
    for (int i = 0; i < lineCount; i++) {  
      // 设置随机开始和结束坐标  
      int xs = random.nextInt(width);//x坐标开始  
      int ys = random.nextInt(height);//y坐标开始  
      int xe = xs + random.nextInt(width / 8);//x坐标结束  
      int ye = ys + random.nextInt(height / 8);//y坐标结束  
  
      // 产生随机的颜色值，让输出的每个干扰线的颜色值都将不同。  
      red = random.nextInt(255);  
      green = random.nextInt(255);  
      blue = random.nextInt(255);  
      g.setColor(new Color(red, green, blue));  
      g.drawLine(xs, ys, xe, ye);  
    }  
  
    // randomCode记录随机产生的验证码  
    StringBuffer randomCode = new StringBuffer();  
    // 随机产生codeCount个字符的验证码。  
    for (int i = 0; i < codeCount; i++) {  
      String strRand = String.valueOf(codeSequence[random.nextInt(codeSequence.length)]);  
      // 产生随机的颜色值，让输出的每个字符的颜色值都将不同。  
      red = random.nextInt(255);  
      green = random.nextInt(255);  
      blue = random.nextInt(255);  
      g.setColor(new Color(red, green, blue));  
      g.drawString(strRand, (i + 1) * x, codeY);  
      // 将产生的四个随机数组合在一起。  
      randomCode.append(strRand);  
    }  
    // 将四位数字的验证码保存到Session中。  
    code = randomCode.toString();  
  }  
  
  public void write(String path) throws IOException {  
    OutputStream sos = new FileOutputStream(path);  
    this.write(sos);  
  }  
  
  public void write(OutputStream sos) throws IOException {  
    ImageIO.write(buffImg, "png", sos);  
    sos.close();  
  }  
  
  public BufferedImage getBuffImg() {  
    return buffImg;  
  }  
  
  public String getCode() {  
    return code;  
  }  
  
  /**  
   * 测试函数,默认生成到d盘  
   * @param args  
   */  
  public static void main(String[] args) {  
    ValidateCode vCode = new ValidateCode(160,40,5,150);  
    try {  
      String path="D:/"+new Date().getTime()+".png";  
      System.out.println(vCode.getCode()+" >"+path);  
      vCode.write(path);  
    } catch (IOException e) {  
      e.printStackTrace();  
    }  
  }  
}

// 刷新图片  
    function changeImg() {  
      var imgSrc = $("#imgObj");  
      var url = imgSrc.attr("src");  
      imgSrc.attr("src", changeUrl(url));  
    }  
    //为了使每次生成图片不一致，即不让浏览器读缓存，所以需要加上时间戳  
    function changeUrl(url) {  
      var timestamp = (new Date()).valueOf();  
      var index = url.indexOf("?"); 
      console.log(index);  
      if (index > 0) {  
        url = url.substring(0, url.indexOf("?"));  
      } 
      console.log(url); 
      if ((url.indexOf("&") > 0)) {  
        url = url + "×tamp=" + timestamp;  
        console.log(url); 
      } else {  
        url = url + "?timestamp=" + timestamp; 
        console.log(url); 
        }  
      return url;  
    }

/** 
 * 响应验证码页面 
 * @return 
 */  
@RequestMapping(value="/validateCode")  
public String validateCode(HttpServletRequest request,HttpServletResponse response) throws Exception{  
  // 设置响应的类型格式为图片格式  
  response.setContentType("image/jpeg");  
  //禁止图像缓存。  
  response.setHeader("Pragma", "no-cache");  
  response.setHeader("Cache-Control", "no-cache");  
  response.setDateHeader("Expires", 0);  
  
  HttpSession session = request.getSession();  
  
  ValidateCode vCode = new ValidateCode(120,40,5,100);  
  session.setAttribute("code", vCode.getCode());  
  vCode.write(response.getOutputStream());  
  return null;  
}

String code = request.getParameter("code");  
HttpSession session = request.getSession();  
String sessionCode = (String) session.getAttribute("code");  
if (!StringUtils.equalsIgnoreCase(code, sessionCode)) { //忽略验证码大小写  
  throw new RuntimeException("验证码对应不上code=" + code + " sessionCode=" + sessionCode);  
}