session 防止表单重复提交

防止表单重复提交应该现在前台做一遍，再在后台做一遍。这样双重安全而且减轻服务器负担。

代码：

package flying.form;

import java.io.IOException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Random;

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

import sun.misc.BASE64Encoder;
//防止表单重复提交
public class FormServlet extends HttpServlet {

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

        //产生随机数（表单号）
        TokenProcessor tp = TokenProcessor.getInstance();
        String token = tp.generateToken();

        request.getSession().setAttribute("token",token);

        request.getRequestDispatcher("/WEB-INF/form.jsp").forward(request,response);

    }

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

}

//产生随机数，相当于表单的令牌
class TokenProcessor{ //令牌

    private TokenProcessor() {}
    private static final TokenProcessor instance = new TokenProcessor();

    public static TokenProcessor getInstance() {
        return instance;

    }

    public String generateToken() {
        String token = System.currentTimeMillis()+new Random().nextInt()+"";

        //随机数长短不一，获取数据摘要,数据摘要始终只有128位
        try {
            MessageDigest md = MessageDigest.getInstance("md5");
            byte[] md5 = md.digest(token.getBytes());

            //base64编码,因为MD5返回乱码，所以用base64编码。base64把数据三个字节变成四个字节：六位一组截取数据的，前面补俩零，这样三个字节24位就会变成32位。base64中的数据从0-63共64种情况。
            BASE64Encoder encoder = new BASE64Encoder();
            return encoder.encode(md5);

        } catch (NoSuchAlgorithmException e) {
            // TODO Auto-generated catch block
            throw new RuntimeException(e);
        }

}
}

package flying.form;

import java.io.IOException;
import java.io.PrintWriter;

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

public class DoFormServlet extends HttpServlet {

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

}

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        boolean b = isTokenValid(request);
        if(!b) {
            response.getWriter().write("<script type='text/javascript'>alert('Form repeat!');</script>");
            return;
        }

        request.getSession().removeAttribute("token");

        //doSomething
        response.getWriter().write("Success");
    }

    //判断表单号是否有效
    private boolean isTokenValid(HttpServletRequest request) {
        String client_token = request.getParameter("token");
        if(client_token==null) {
            return false;
        }
        String server_token = (String)request.getSession().getAttribute("token");
        if(server_token==null) {//已经提交过一次，被服务器删掉了session
            return false;
        }

        return client_token.equals(server_token);

    }

}

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>

<title>My JSP 'form.jsp' starting page</title>

</head>