代码如下：

package cn.response;

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

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@SuppressWarnings("serial")
public class validatePicture extends HttpServlet {

 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  try {
   validateimage(request, response);
  } catch (Exception e) {
   e.printStackTrace();
  }
 }

 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  doGet(request, response);
 }

 public static final int WIDTH = 120;
 public static final int HEIGHT = 25;

 public void validateimage(HttpServletRequest request,
   HttpServletResponse response) throws Exception {

  BufferedImage bi = new BufferedImage(WIDTH, HEIGHT,
    BufferedImage.TYPE_INT_RGB);// 在内存中构建一幅图象
  Graphics g = bi.getGraphics();

  setBackGround(g);// 设置背景色
  setBorder(g);// 设置边框
  drawRandomLine(g);// 画干扰线
  String random = drawRandomNum((Graphics2D) g);// 写随机数

  request.getSession().setAttribute("random", random);
  //request.getSession(false);
  response.setContentType("image/jpeg");
  // 设置不要缓存
  response.setDateHeader("expries", -1);
  response.setHeader("Cache-Control", "no-cache");

  ImageIO.write(bi, "jpg", response.getOutputStream());
 }

 private String drawRandomNum(Graphics2D g) {
  g.setColor(Color.RED);
  g.setFont(new Font("宋体", Font.BOLD, 20));
  // [/u4e00-/u9fa5]中文数字区间
  String base = "/u4e00/u4f00/u5e00/u4e50/u4e89/u4f10/u4e09";

  StringBuffer sb = new StringBuffer();

  int x = 5;
  for (int i = 0; i < 4; i++) {
   int degree = new Random().nextInt() % 30;

   String ch = base.charAt(new Random().nextInt(base.length())) + "";
   sb.append(ch);
   g.rotate(degree * Math.PI / 180, x, 20);
   g.drawString(ch, x, 20);
   g.rotate(-degree * Math.PI / 180, x, 20);

   x = x + 30;
  }
  System.out.println(sb.toString());
  return sb.toString();
 }

 private void drawRandomLine(Graphics g) {
  g.setColor(Color.GREEN);
  for (int i = 0; i < 3; i++) {
   int x1 = new Random().nextInt(WIDTH);
   int y1 = new Random().nextInt(HEIGHT);

   int x2 = new Random().nextInt(WIDTH);
   int y2 = new Random().nextInt(HEIGHT);

   g.drawLine(x1, y1, x2, y2);
  }
 }

 private void setBorder(Graphics g) {
  g.setColor(Color.BLUE);
  g.drawRect(1, 1, WIDTH - 2, HEIGHT - 2);
 }

 private void setBackGround(Graphics g) {
  g.setColor(Color.WHITE);
  g.fillRect(0, 0, WIDTH, HEIGHT);
 }
}