程式碼直接拷貝就能用

<%@ page contentType="image/jpeg"
	import="java.util.*,java.awt.*,java.io.*,java.awt.image.*,javax.imageio.*"
	pageEncoding="utf-8"%>
<%!Color getRandColor(int fc, int bc) {//給定範圍獲得隨機顏色
		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);
	}%>
<%
	//設定頁面不快取
	response.setHeader("Pragma", "No-cache");
	response.setHeader("Cache-Control", "no-cache");
	response.setDateHeader("Expires", 0);

	// 在記憶體中建立圖象
	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);

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

	//畫邊框
	//g.setColor(new Color());
	//g.drawRect(0,0,width-1,height-1);

	// 隨機產生155條幹擾線，使圖象中的認證碼不易被其它程式探測到
	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
		g.setColor(new Color(20 + random.nextInt(110), 20 + random
				.nextInt(110), 20 + random.nextInt(110)));// 呼叫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.drawString(rand, 13 * i + 6, 16);
	}

	// 將認證碼存入SESSION
	session.setAttribute("rand", sRand);

	// 圖象生效
	g.dispose();
	OutputStream output = response.getOutputStream();

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