在服务端实现验证码代码

package com.nlte;

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

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

/**
 * Created by hp on 2016/10/26.
 */
public class IdentityServlet extends HttpServlet{
    //显示的字符
    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','@', '#', '%', '&'};
    public static Random random = new Random();//随机数

    /**
     * 获取随机字符串
     * @return
     */
    public static String getRandomString(){
        StringBuffer buffer = new StringBuffer();

        //显示6个字符
        for (int i = 0; i < 6; i++) {
            buffer.append(CHARS[random.nextInt(CHARS.length)]);
        }
        return buffer.toString();
    }

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

    /**
     * 随机颜色取反
     * @param c
     * @return
     */
    public static Color getReverseColor(Color c){
        return new Color(255-c.getRed(), 255-c.getGreen(), 255-c.getBlue());
    }

    @Override
    protected void doGet(HttpServletRequest req,
                         HttpServletResponse resp)
            throws ServletException, IOException {

        resp.setContentType("image/jpeg");//设置输出类型，必须的

        String randomString = getRandomString();
        req.getSession(true).setAttribute("randomString", randomString);//放到 session 中

        int width = 100;//图片宽度
        int height = 30;//图片高度

        Color color = getRandomColor();//随机颜色，用于背景颜色
        Color reverse = getReverseColor(color);//反色，用于前景色

        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(Font.SANS_SERIF, Font.BOLD, 16));//设置字体
        g.fillRect(0, 0, width, height);//绘制背景
        g.setColor(color);//设置颜色
        g.drawString(randomString, 18, 20);//绘制随机字符串
        for (int i = 0, n= random.nextInt(100); i<n; i++) {
            g.drawRect(random.nextInt(width), random.nextInt(height), 1, 1);//绘制随即噪音点
        }

        ServletOutputStream out = resp.getOutputStream();//转换成JPEG格式
        JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);//编码器
        encoder.encode(bi);//对图片进行编码
        out.flush();//输出到客户端
    }

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

结果