生成校验码

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<%@ page contentType="image/jpeg"
         import="javax.imageio.ImageIO,java.awt.*,java.awt.image.BufferedImage,java.util.Random" %>

<%!
    Color getRandColor(int fc, int bc) {//给定范围获得随机颜色
        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);
    }
%>

<%
    //设置页面不缓存
    response.setHeader("Pragma", "No-cache";
    response.setHeader("Cache-Control", "no-cache";
    response.setDateHeader("Expires", 0);

    //在内存中创建图象
    int width = 60, height = 20;
    BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
    //获取图形上下文
    Graphics g = image.getGraphics();
    //生成随机类
    Random random = new Random();
    //设定背景色
    g.setColor(getRandColor(200, 250));
    g.fillRect(0, 0, width, height);
    //设定字体
    g.setFont(new Font("Times New Roman", Font.PLAIN, 18));

    //画边框
    //g.setColor(new Color());
    //g.drawRect(0,0,width-1,height-1);
    // 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到
    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);
    }

    // 取随机产生的认证码(4位数字)
    String sRand = "";
    for (int i = 0; i < 4; i++) {
        String rand = String.valueOf(random.nextInt(10));
        sRand += rand;
        // 将认证码显示到图象中
        g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
        //调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成
        g.drawString(rand, 13 * i + 6, 16);
    }

    // 将认证码存入SESSION
    session.setAttribute("rand", sRand);

    // 图象生效
    g.dispose();

    // 输出图象到页面
    ImageIO.write(image, "JPEG", response.getOutputStream());
%>

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需要校验码的页面上如此使用：