HttpSession解决表单的重复提交

1). 重复提交的情况: 

①. 在表单提交到一个 Servlet, 而 Servlet 又通过请求转发的方式响应一个 JSP(HTML) 页面,
此时地址栏还保留着 Serlvet 的那个路径, 在响应页面点击 "刷新"
②. 在响应页面没有到达时重复点击 "提交按钮".
③. 点击 "返回", 再点击 "提交"

2). 不是重复提交的情况:

点击 "返回", "刷新" 原表单页面, 再 "提交"。

3). 如何避免表单的重复提交:

在表单中做一个标记, 提交到 Servlet 时, 检查标记是否存在且是否和预定义的标记一致, 若一致, 则受理请求,
并销毁标记, 若不一致或没有标记, 则直接响应提示信息: "重复提交"

①.调用 RequestDispatcher.forward() 方法，浏览器所保留的URL 是先前的表单提交的 URL，此时点击”刷新”, 浏览器将再次提交用户先前输入的数据，引起重复提交;如果采用 HttpServletResponse.sendRedirct() 方法将客户端重定向到成功页面，将不会出现重复提交问题

②.利用Session对表单进行token标识

  1. 在原表单页面, 生成一个随机值 token
  2. 在原表单页面, 把 token 值放入 session 属性中
  3. 在原表单页面, 把 token 值放入到 隐藏域 中.
  4.  在目标的 Servlet 中: 获取 session 和 隐藏域 中的 token 值
  5.  比较两个值是否一致: 若一致, 受理请求, 且把 session 域中的 token 属性清除
  6. 若不一致, 则直接响应提示页面: "重复提交"

表单页面

<%@ page import="java.util.Date" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%
    //随机生成一个标识
    String tokenValue = new Date().getTime()+"";
    session.setAttribute("token",tokenValue);
%>
<form action="<%=request.getContextPath()%>/tokenServlet" method="post">
    <input  type="hidden"  name="tokenValue" value=<%=tokenValue%> />
    Username:<input  type="text" name="username"/>
    <input type="submit" value="登录"/>
</form>
</body>
</html>

TokenServlet.java

package yang.mybatis.servlet.token;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;

/**
 * Created by yangshijing on 2017/11/22 0022.
 */
public class TokenServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //模仿后台延时
        try {
            Thread.sleep(300);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        HttpSession session = request.getSession();
        //从Session域中获得标识
        String token = (String)session.getAttribute("token");
        //从表单隐藏域中获取token value
        String tokenValue = request.getParameter("tokenValue");
        //如果标识不为空且隐藏域中值和Session域中的值一致，则处理该表单请求
        if(token != null && tokenValue.equals(token)){
            session.removeAttribute("token");
        }else{
            response.sendRedirect(request.getContextPath()+"/jsp/token/token.jsp");
            return;
        }
        request.getRequestDispatcher("/jsp/token/success.jsp").forward(request,response);
    }
}

 TokenProcessor.java

用于管理表单标识号的工具类，它主要用于产生、比较和清除存储在当前用户Session中的表单标识号。为了保证表单标识号的唯一性，每次将当前SessionID和系统时间的组合值按MD5算法计算的结果作为表单标识号，并且将TokenProcessor类设计为单例类

package yang.mybatis.servlet.token;


import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
/**
 * Created by yangshijing on 2017/11/23 0023.
 */
public class TokenProcessor {
    //表单隐藏域的name属性值
    private static final String TOKEN_KEY = "TOKEN_KEY";
    //Session域中的属性值
    private static final String TRANSACTION_TOKEN_KEY = "TRANSACTION_TOKEN_KEY";

    private static TokenProcessor instance = new TokenProcessor();

    private long previous;

    protected TokenProcessor() {
        super();
    }
    //单例模式
    public static TokenProcessor getInstance() {
        return instance;
    }
    //判断表单提交请求是否重复
    public synchronized boolean isTokenValid(HttpServletRequest request) {
        HttpSession session = request.getSession(false);

        if (session == null) {
            return false;
        }
        //从Session域中获取标识值
        String saved = (String) session.getAttribute(TRANSACTION_TOKEN_KEY);

        if (saved == null) {
            return false;
        }

        String token = request.getParameter(TOKEN_KEY);

        if (token == null) {
            return false;
        }

        return saved.equals(token);
    }
    //移除Session域中的token
    public synchronized void resetToken(HttpServletRequest request) {
        HttpSession session = request.getSession(false);

        if (session == null) {
            return;
        }

        session.removeAttribute(TRANSACTION_TOKEN_KEY);
    }
    //将token保存到Session域中并返回token
    public synchronized String saveToken(HttpServletRequest request) {
        HttpSession session = request.getSession();
        String token = generateToken(request);

        if (token != null) {
            session.setAttribute(TRANSACTION_TOKEN_KEY, token);
        }

        return token;
    }

    public synchronized String generateToken(HttpServletRequest request) {
        HttpSession session = request.getSession();

        return generateToken(session.getId());
    }

    public synchronized String generateToken(String id) {
        try {
            long current = System.currentTimeMillis();

            if (current == previous) {
                current++;
            }

            previous = current;

            byte[] now = new Long(current).toString().getBytes();
            MessageDigest md = MessageDigest.getInstance("MD5");

            md.update(id.getBytes());
            md.update(now);

            return toHex(md.digest());
        } catch (NoSuchAlgorithmException e) {
            return null;
        }
    }

    private String toHex(byte[] buffer) {
        StringBuffer sb = new StringBuffer(buffer.length * 2);

        for (int i = 0; i < buffer.length; i++) {
            sb.append(Character.forDigit((buffer[i] & 0xf0) >> 4, 16));
            sb.append(Character.forDigit(buffer[i] & 0x0f, 16));
        }

        return sb.toString();
    }
}

重构的index.jsp

<%@ page import="java.util.Date" %>
<%@ page import="yang.mybatis.servlet.token.TokenProcessor" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%--<%
    String tokenValue = new Date().getTime()+"";
    session.setAttribute("token",tokenValue);
%>--%>
<form action="<%=request.getContextPath()%>/tokenServlet" method="post">
    <input  type="hidden"  name="TOKEN_KEY" value=<%=TokenProcessor.getInstance().saveToken(request)%> />
    Username:<input  type="text" name="username"/>
    <input type="submit" value="登录"/>
</form>
</body>
</html>

重构的TokenServlet

package yang.mybatis.servlet.token;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;

/**
 * Created by yangshijing on 2017/11/22 0022.
 */
public class TokenServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //模仿后台延时
        try {
            Thread.sleep(300);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
     /*   HttpSession session = request.getSession();
        //从Session域中获得标识
        String token = (String)session.getAttribute("token");
        //从表单隐藏域中获取token value
        String tokenValue = request.getParameter("tokenValue");
        //如果标识不为空且隐藏域中值和Session域中的值一致，则处理该表单请求
        if(token != null && tokenValue.equals(token)){
            session.removeAttribute("token");
        }else{
            response.sendRedirect(request.getContextPath()+"/jsp/token/token.jsp");
            return;
        }
        request.getRequestDispatcher("/jsp/token/success.jsp").forward(request,response);*/
        boolean tokenValid = TokenProcessor.getInstance().isTokenValid(request);
        //如果不是重复的表单请求，移除token并处理请求
        if(tokenValid){
            TokenProcessor.getInstance().resetToken(request);
        }else {
            //重复的表单请求，结束方法，并重定向到提示页面
            response.sendRedirect(request.getContextPath()+"/jsp/token/token.jsp");
            return;
        }
        response.sendRedirect(request.getContextPath()+"/jsp/token/success.jsp");
    }
}
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原文地址：https://www.cnblogs.com/realshijing/p/7882026.html