About the solution to java graphic verification code

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);
}
}