JSP 驗證碼大全：數字驗證碼實現

驗證碼廣泛用於註冊、登入、發布內容等場景，防止自動化程式濫用。原理：隨機字符繪製成圖片，存入 Session，提交時比對。

Num.jsp 完整代碼

<%@ page contentType="image/jpeg" import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*" %>
<%!
  Color getRandColor(int fc, int bc) {
    Random random = new Random();
    if (fc > 255) fc = 255;
    if (bc > 255) bc = 255;
    int r = fc + random.nextInt(bc - fc);
    int g = fc + random.nextInt(bc - fc);
    int b = fc + random.nextInt(bc - fc);
    return new Color(r, g, b);
  }
%>
<%
  response.setHeader("Pragma", "No-cache");
  response.setHeader("Cache-Control", "no-cache");
  response.setDateHeader("Expires", 0);
  int width = 80, height = 30;
  BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
  Graphics g = image.getGraphics();
  Random random = new Random();
  g.setColor(getRandColor(200, 250));
  g.fillRect(0, 0, width, height);
  g.setFont(new Font("Times New Roman", Font.PLAIN, 18));
  g.setColor(getRandColor(160, 200));
  for (int i = 0; i < 155; i++) {
    int x = random.nextInt(width);
    int y = random.nextInt(height);
    int xl = random.nextInt(12);
    int yl = random.nextInt(12);
    g.drawLine(x, y, x + xl, y + yl);
  }
  String sRand = "";
  for (int i = 0; i < 4; i++) {
    String rand = String.valueOf(random.nextInt(10));
    sRand += rand;
    g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
    g.drawString(rand, 13 * i + 6, 16);
  }
  session.setAttribute("rand", sRand);
  g.dispose();
  ImageIO.write(image, "JPEG", response.getOutputStream());
  out.clear();
  out = pageContext.pushBody();
%>

下篇將介紹中文驗證碼實現。