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

<%!
    Color getRandColor(int fc, int bc) {//給定范圍獲得隨機顏色
        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);
    }
%>
<%
    //設置頁面不緩存
    response.setHeader("Pragma", "No-cache");
    response.setHeader("Cache-Control", "no-cache");
    response.setDateHeader("Expires", 0);

    // 在內存中創建圖象
    int width = 60, height = 20;
    BufferedImage image = new BufferedImage(width, height,
            BufferedImage.TYPE_INT_RGB);

    // 獲取圖形上下文
    Graphics g = image.getGraphics();

    //生成隨機類
    Random random = new Random();

    // 設定背景色
    g.setColor(getRandColor(200, 250));
    g.fillRect(0, 0, width, height);

    //設定字體
    g.setFont(new Font("Times New Roman", Font.PLAIN, 18));

   

    // 隨機產生155條干擾線，使圖象中的認證碼不易被其它程序探測到
    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);
    }

    // 取隨機產生的認證碼(4位數字)
 
    String sRand = "";
    for (int i = 0; i < 4; i++) {
        String rand = String.valueOf(random.nextInt(10));
        sRand += rand;
        // 將認證碼顯示到圖象中
        //調用函數出來的顏色相同，可能是因為種子太接近，所以只能直接生成
        g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
        g.drawString(rand, 13 * i + 6, 16);
    }

    // 將認證碼存入SESSION
    //關鍵點，可以與其認證，來驗證是否正確
    session.setAttribute("rand", sRand);

    // 圖象生效
    g.dispose();

    // 輸出圖象到頁面
    out.clear();
    out = pageContext.pushBody();
    ImageIO.write(image, "JPEG", response.getOutputStream());
%>