session绑定的验证码返回的是null

else if(action.equals("/login")){

			//number1:用户提交的验证码

			String number1=request.getParameter("number");

			//number2:session对象上绑定的验证码

			HttpSession session=request.getSession();

			String number2=(String)session.getAttribute("number");

			if(!number1.equalsIgnoreCase(number2)){

				request.setAttribute("number_error", "验证码错误");

				request.getRequestDispatcher("login.jsp")

				.forward(request,response);

				return;

			}

			//只有在验证码正确的情况下，才会比较用户名和密码是否正确

			String username = 

		  		request.getParameter("username");

		  	String pwd = 

		  		request.getParameter("pwd");

		  	UserDAO dao = new UserDAO();

		  	try {

	        User user = dao.findByUserName(username);

//	        System.out.println(user.getPwd()+",");

	        try {

				if(user != null && user.getPwd().equals(MD5Util.encrypt(pwd.toUpperCase()))){

					//绑订数据到session对象上，用于验证

					session.setAttribute("user", user);

					//登录成功,跳转到主功能页面

					response.sendRedirect("list.do");

				}else{

					//登录失败,提示用户

					request.setAttribute("login_error", 

							"用户名或密码错误");

					request.getRequestDispatcher("login.jsp")

					.forward(request, response);

				}

			} catch (Exception e) {

				// TODO Auto-generated catch block

				e.printStackTrace();

			}

        } catch (SQLException e) {

	        e.printStackTrace();

	        //将系统异常扔给容器来处理

	        throw new ServletException(e);

        }		

		}

package web;



import java.awt.Color;

import java.awt.Font;

import java.awt.Graphics;

import java.awt.image.BufferedImage;

import java.io.IOException;

import java.io.OutputStream;

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;

import javax.servlet.http.HttpSession;



public class CheckcodeServlet extends HttpServlet {

		private int WIDTH= 80;

		private int HEIGHT= 30;

		public void service(HttpServletRequest request, HttpServletResponse response)

	    throws ServletException, IOException {

			//一、绘图

			//step1,创建一个内存映像对象(画布)

			BufferedImage image = 

				new BufferedImage(WIDTH,HEIGHT,

						BufferedImage.TYPE_INT_RGB);

			//step2,获得画笔

			Graphics g = image.getGraphics();

			//step3,

			Random r = new Random();

			//step3,给画笔上一个随机的颜色

			g.setColor(new Color(r.nextInt(255),

					r.nextInt(255),r.nextInt(255)));

			//step4,填充画布

			g.fillRect(0, 0, WIDTH, HEIGHT);

			//step5,在画布上绘图

//			String number = r.nextInt(99999) + "";

			String number ="";

//			g.setColor(new Color(0,0,0));

			for (int i = 0; i < 4; i++) {

				g.setColor(new Color(r.nextInt(255),

				r.nextInt(255),r.nextInt(255)));

				int h=(int)(HEIGHT*0.3+(HEIGHT*0.7)*r.nextDouble());

				g.setFont(new Font(null,Font.ITALIC|Font.BOLD,h));

				String str=getNumber(1);

				number+=str;

				g.drawString(str, i*WIDTH/5, h);

			}

			//将number 绑定到session上

			HttpSession session =request.getSession();

			session.setAttribute("number", number);

			

			//step6,设置字体的大小、风格

//			g.setFont(new Font(null,

//					Font.ITALIC|Font.BOLD,20));

//			g.drawString(number, 5, 22);

			//加干扰线

			for (int i = 0; i < 3; i++) {

				g.setColor(new Color(r.nextInt(255),r.nextInt(255),r.nextInt(255),r.nextInt(255)));

			    g.drawLine(r.nextInt(WIDTH), r.nextInt(HEIGHT),r.nextInt(WIDTH), r.nextInt(HEIGHT));

			}

			//二、压缩图片，输出

			response.setContentType("image/jpeg");

			OutputStream ops = 

				response.getOutputStream();

			//write方法:将原始图片(image)使用

			//指定的压缩算法(jpeg)进行压缩，然后

			//使用指定的输出流(ops)输出。

			ImageIO.write(image, "jpeg",	ops);

			ops.close();

	}/*返回一个指定长度的字符串

	  要求从A~Z，0~9中随机选取字符

	 */

	   

		private String getNumber(int size) {

			Random r=new Random();

			String strs="ABCDEFGHIJKLMNOPQRSTUVWXYZ"

				+"0123456789";

			String number="";

			for (int i = 0; i < size; i++) {

				number+=strs.charAt(r.nextInt(strs.length()));

			}	

			

			return number;		

		}



}