关于Filter的疑问,该如何解决

package com.chk.cms.checkcode;import java.awt.Color;import java.awt.Font;import java.awt.Graphics2D;import java.awt.image.BufferedImage;import java.io.IOException;import java.io.OutputStream;import java.io.PrintWriter;import java.util.Random;import javax.servlet.ServletConfig;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import com.sun.image.codec.jpeg.JPEGCodec;import com.sun.image.codec.jpeg.JPEGImageEncoder;import com.sun.xml.internal.ws.api.ha.StickyFeature;public class CheckCodeServlet extends HttpServlet {    // 定义验证码 图片的宽度    private int width;    // 定义验证码图片的高度。    private int height;    // 显示多少位字符    private int number;    // 有哪些字符备选    private String codes;    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        response.setContentType("image/jepg");        // 创建一张图片        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);        Graphics2D g = image.createGraphics();        // 设置背景色        g.setColor(Color.WHITE);        g.fillRect(0, 0, width, height);        // 设置黑色边框        g.setColor(Color.BLACK);        g.drawRect(0, 0, width - 1, height - 1);        // 每个字符占据的宽度和高度        int x = (width - 1) / number;        int y = height - 4;        // 定义一个StringBuffer,用于存储code字符串，将来放置到session中        StringBuffer sb = new StringBuffer();        // 随机生成字符        Random random = new Random();        for (int i = 0; i < number; i++) {            String code = String.valueOf(codes.charAt(random.nextInt(codes.length())));            // 随机生成颜色            int red = random.nextInt(255);            int green = random.nextInt(255);            int blue = random.nextInt(255);            g.setColor(new Color(red, green, blue));            // 设置字体            g.setFont(new Font("宋体", Font.BOLD, 14));            g.drawString(code, i * x + 2, y);            sb.append(code);        }        // 将字符串放置到Http Session中        request.getSession().setAttribute("codes", sb.toString());        // 随机生成一些干扰点        for (int i = 0; i < 50; i++) {            int red = random.nextInt(255);            int green = random.nextInt(255);            int blue = random.nextInt(255);            g.setColor(new Color(red, green, blue));            g.drawOval(random.nextInt(width), random.nextInt(height), 1, 1);        }        OutputStream out = response.getOutputStream();        JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);        encoder.encode(image);        out.flush();        out.close();    }    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        doGet(request, response);    }    public void init() throws ServletException {        width = Integer.parseInt(this.getInitParameter("width"));        height = Integer.parseInt(this.getInitParameter("height"));        number = Integer.parseInt(this.getInitParameter("number"));        codes = this.getInitParameter("codes");    }}

package com.chk.cms.checkcode;import java.io.IOException;import java.sql.Connection;import java.sql.DriverManager;import java.sql.PreparedStatement;import java.sql.ResultSet;import java.sql.SQLException;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class LoginServlet extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        doPost(request, response);    }    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        String username = request.getParameter("username");        String password = request.getParameter("password");        String checkcode = request.getParameter("checkcode");// 这个是用户输入的验证码        // 判断验证码是否正确        // 系统生成的验证码        String sessioncodes = (String) request.getSession().getAttribute("codes");        if (!sessioncodes.equalsIgnoreCase(checkcode)) {            // 重定向到登录页面            // response.sendRedirect("index.jsp");            request.setAttribute("error", "验证码不正确");            request.getRequestDispatcher("/index.jsp").forward(request, response);            return;        }        // 判断用户名是否存在，密码是否正确        Connection conn = null;        PreparedStatement ps = null;        ResultSet rs = null;        try {            Class.forName("com.mysql.jdbc.Driver");            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/cms", "root", "chengke168");            String sql = "select * from users where username = ?";            ps = conn.prepareStatement(sql);            ps.setString(1, username);            rs = ps.executeQuery();            if (rs.next()) {                // 进入这一步，说明用户名已经存在                // 判断用户密码是否正确，如果不正确就给出提示“用户密码不正确”                // 返回登陆页面                String pass = rs.getString("password");// 这里的password指的是数据库表中的字段名                if (!password.equals(pass)) {                    request.setAttribute("error", "用户 " + username + " 密码错误！");                    request.getRequestDispatcher("/index.jsp").forward(request, response);                    return;                }            } else {                // 说明用户名不存在，提示用户检查用户名，给出提示“用户名不存在”                // 返回登陆页面                request.setAttribute("error", "用户 " + username + " 不存在！");                request.getRequestDispatcher("/index.jsp").forward(request, response);                return;            }        } catch (ClassNotFoundException e) {            // TODO Auto-generated catch block            e.printStackTrace();        } catch (SQLException e) {            // TODO Auto-generated catch block            e.printStackTrace();        } finally {            try {                if (rs != null)                    rs.close();                if (ps != null)                    ps.close();                if (conn != null)                    conn.close();            } catch (SQLException e) {                e.printStackTrace();            }        }        // 将登陆用户的名称保存在Session中，以便将来检查用户是否是正常登陆的用户        request.getSession().setAttribute("login_username", username);        // 如果都通过了，就转向到主页面        request.getRequestDispatcher("/main.jsp").forward(request, response);    }}