---ValidationCodeServlet.java---
package servlet;

import java.awt.*;
import java.awt.image.BufferedImage;
import *;
import java.util.Random;
import javax.imageio.ImageIO;
import javax.servlet.*;
import javax.servlet.http.*;

public class ValidationCodeServlet extends HttpServlet
{
    private static String codeChars = null;

    private static Color getRandomColor(int minColor, int maxColor)
    {
        Random random = new Random();
        int red = minColor + random.nextInt(maxColor - minColor);
        int green = minColor + random.nextInt(maxColor - minColor);
        int blue = minColor + random.nextInt(maxColor - minColor);
        return new Color(red, green, blue);
    }

    @Override
    protected void service(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException
    {
        // 如果未加载验证码字符串，从code.txt文件中读取验证码字符串
        if(codeChars == null)
        {
            FileInputStream fis = new FileInputStream(this.getServletContext()
                    .getRealPath("/WEB-INF/code.txt"));
            InputStreamReader isr = new InputStreamReader(fis);
            BufferedReader br = new BufferedReader(isr);
            String s = "";
            while ((s = br.readLine()) != null)
            {
                codeChars = s;
            }
        }
        // 获得验证码集合的长度
        int charsLength = codeChars.length();

        // 禁止客户端缓存网页
        response.setHeader("ragma", "No-cache");
        response.setHeader("Cache-Control", "no-cache");
        response.setDateHeader("Expires", 0);

        // 设置图形验证码的长和宽（验证码图像的大小）
        int width = 150, height = 30;
        BufferedImage image = new BufferedImage(width, height,
                BufferedImage.TYPE_INT_RGB);
        Graphics g = image.getGraphics();
        Random random = new Random();
        // 随机设置要填充的颜色
        g.setColor(getRandomColor(190, 250));
        // 填充图形背景
        g.fillRect(0, 0, width, height);
        // 随机设置字体颜色
        g.setColor(getRandomColor(80, 160));
        // 用于保存最后随机生成的验证码
        StringBuilder validationCode = new StringBuilder();
        String[] fontNames = new String[]{"宋体", "楷体", "隶书"};
        // 随机生成3个到5个验证码
        for (int i = 0; i < 3 + random.nextInt(3); i++)
        {
            // 随机设置当前验证码的字符的字体
            g.setFont(new Font(fontNames[random.nextInt(3)], Font.ITALIC, height));
            // 随机获得当前验证码的字符
            
            char codeChar = codeChars.charAt(random.nextInt(charsLength));
            validationCode.append(codeChar);
            // 随机设置当前验证码字符的颜色
            g.setColor(getRandomColor(10, 100));
            // 在图形上输出验证码字符
            g.drawString(String.valueOf(codeChar), 25 * i + 6,
                    height - random.nextInt(5));

        }

        HttpSession session = request.getSession();

        // 设置session对象30分钟失效
        session.setMaxInactiveInterval(30 * 60);
        // 将验证码保存在session对象中，key为vcode
        session.setAttribute("vCode", validationCode.toString());

        g.dispose();
        OutputStream os = response.getOutputStream();
        ImageIO.write(image, "JPEG", os);
        
    }
}
--
调用，错误如图。
<img hint="a" id="img_validation_code"    src=" validationCode"  />，用 <% out.println(session.getAttribute("vCode")); %>，显示为null。

试了下，是可以的，应该是你没有放置code.txt文件，这个文件是你算法中生成随机码的输入条件。
你的图形上的数字和那个要session中的串值不同，你可以看看算法。

code.txt是放了的。我测试过，在另一个纯servlet的程序中，能显示。就是用ssh的不行。很奇怪。