servletResponse生成验证码（图片）

package com.cmcc.java;

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.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGImageEncoder;

/**
 * @author create by zhousq
 * @date 2017年9月25日---上午8:39:20
 * @problem
 * @answer
 * @action
 */
public class IdentityServlet extends HttpServlet {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    public static final char[] CHARS = { '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
            'J', 'K', 'L', 'M', 'N', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z' };

    // 随机数
    public static Random random = new Random();

    // 获取6位随机数
    public static String getRandomString() {
        StringBuffer buffer = new StringBuffer(); // 字符串缓存
        for (int i = 0; i < 6; i++) {
            buffer.append(CHARS[random.nextInt(CHARS.length)]);
        }
        return buffer.toString();
    }

    // 获取随机颜色
    public static Color getRandomColor() {
        return new Color(random.nextInt(255), random.nextInt(255), random.nextInt(255));
    }

    public static Color getReverseColor(Color c) {
        return new Color(255 - c.getRed(), 255 - c.getGreen(), 255 - c.getBlue());
    }

    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //设置输出类型
        resp.setContentType("image/jpg");
        //随机字符串
        String randomString = getRandomString();
        req.getSession(true).setAttribute("randomString", randomString);
        int width = 100;//图片宽度
        int height = 30;//图片高度
        Color color = getRandomColor();//随机颜色
        Color reverse = getReverseColor(color);//反色，用于前景色
        //创建一份彩色图片
        BufferedImage bi = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
        Graphics2D g = bi.createGraphics(); //获取绘图对象
        g.setFont(new Font(Font.SANS_SERIF, Font.BOLD, 16)); //设置字体
        g.setColor(color);
        g.fillRect(0, 0, width, height); //绘制背景
        g.setColor(reverse);
        g.drawString(randomString, 18, 20); //绘制随机字符
        for (int i=0,n=random.nextInt(100);i