servlet 产生随机验证码

servlet实现代码：

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.sun.image.codec.jpeg.*;

public class PrintServlet extends javax.servlet.http.HttpServlet{

    //设置要随机生成的数字
    public static final char[] CHARS = {'1', '2', '3', '4', '5', '6', '7', '8', '9',
                                        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'L', 'M',
                                        'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'};

    //随机器
    public static Random random = new Random();     
    
    //随即获取4位数
    public static String getRandomString() {
        StringBuffer sb = new StringBuffer();
        for(int i=0; i<4; i++) {
            sb.append(CHARS[random.nextInt(CHARS.length)]);
        }
        return sb.toString();
    }
    
    //获取随机产生的颜色
    public static Color getRandomColor() {
        return new Color(random.nextInt(255), random.nextInt(255), random.nextInt(255));
    }
    
    //返回颜色的反色
    public static Color getReverseColor(Color c) {
        return new Color(255-c.getRed(), 255-c.getGreen(), 255-c.getBlue());
    }    

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        this.doGet(req, resp);
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        resp.setContentType("image/jpeg");        //设置输出的类型
        
        String randomString = getRandomString();
        req.getSession(true).setAttribute("randomString", randomString);    //将获得的随机字符放回session
        
        int width = 120;
        int height = 30;
        
        Color color = getRandomColor();
        Color reverse = getReverseColor(color);
        
        BufferedImage bi = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);    //创建一个彩色图片
        Graphics2D g = bi.createGraphics();                                //获取绘图对象
        g.setFont(new Font("宋体", Font.ITALIC, 16));                        //设置随机字符的字体
        Color c = g.getColor();
        g.setColor(color);                                            //设置绘制随机字符背景的颜色
        g.fillRect(0, 0, width, height);                            //绘制随机字符的背景
        g.setColor(reverse);
        g.drawString(randomString, 18, 20);                            //绘制字符
        g.setColor(c);
        
        //设置随机字符中的干扰点
        for(int i=0, n=random.nextInt(80); i<n; i++) {
            g.drawRect(random.nextInt(width), random.nextInt(height), 1, 1);         //随机干扰点
        }
        
        ServletOutputStream out = resp.getOutputStream();            
        JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);        //设置转码器
        encoder.encode(bi);                                //对图片进行编码
        out.flush();
    }
}

然后web.xml配置好就行.