如何避免重复提交

这是More Java Pitfalls中文版上的一个例子。
一、何谓重复提交
   设想一个访问量非常大的订票站点，当购票者点击订票的提交按钮后，服务器的处理速度可能会迟缓，购票者迟迟见不到结果，以为自己的购买请求失败，于是一遍又一遍地点击Submit按钮，如果不加处理，这些请求都会被服务器处理，从而导致错误的结果。

二、防止重复提交

先看一下web.xml(整个例子的目录和文件请下载）

<?xml version="1.0"?> 
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/j2ee/dtds/web-app_2_3.dtd"> 

<web-app> 

  <servlet> 
    <servlet-name>ControllerServlet</servlet-name> 
    <display-name>ControllerServlet</display-name> 
    <description>ControllerServlet</description> 
    <servlet-class>org.javapitfalls.item36.ControllerServlet</servlet-class> 
        <init-param> 
                <param-name>id</param-name> 
                <param-value>id</param-value> 
        </init-param> 
  </servlet> 
   
   
  <servlet> 
        <servlet-name>form</servlet-name> 
        <jsp-file>/ticketForm.jsp</jsp-file> 
  </servlet>  

<servlet> 
        <servlet-name>success</servlet-name> 
        <jsp-file>/success.jsp</jsp-file> 
  </servlet>  
     
  <servlet> 
        <servlet-name>resubmit</servlet-name> 
        <jsp-file>/resubmitError.jsp</jsp-file> 
  </servlet>  
   
   
  <servlet-mapping> 
    <servlet-name>ControllerServlet</servlet-name> 
    <url-pattern>/ControllerServlet</url-pattern> 
  </servlet-mapping> 
   
   
</web-app> 

再看表单ticketForm.jsp
<%@ page contentType="text/html; charset=GBK" %>
<TITLE> Resubmit Demonstration </TITLE> 

<jsp:useBean id="resubmit" class="org.javapitfalls.item36.formBean" scope="request">
<jsp:setProperty name="resubmit" property="*"/>
</jsp:useBean>

<% 
//第一次提交时，在session对象中设置一个id属性,提交成功后将删除。以后的提交将检测这个属性值是否为null来判断是重复提交。
if(session.isNew()) { 
       session.setAttribute("id",session.getId()); 
} 
%> 

<center><H1> 奥齐音乐会在线订票 </H1></center> 

<form method="get" action="/resubmit/ControllerServlet">
<center>
<table border="1" width="50%">
<tr>
<td>票的数目:</td>
<td>
<select name="numTickets">
        <option value="Please enter a Ticket #">Please enter a Ticket #</option>
        <option value="1 Ticket" <% if (resubmit.getNumTickets() == "1 Ticket") out.println("SELECTED"); %>>1 Ticket</option>
        <option value="2 Tickets" <% if (resubmit.getNumTickets() == "2 Tickets") out.println("SELECTED"); %>>2 Tickets</option>
        <option value="3 Tickets" <% if (resubmit.getNumTickets() == "3 Tickets") out.println("SELECTED"); %>>3 Tickets</option>
        <option value="4 Tickets" <% if (resubmit.getNumTickets() == "4 Tickets") out.println("SELECTED"); %>>4 Tickets</option>
        <option value="5 Tickets" <% if (resubmit.getNumTickets() == "5 Tickets") out.println("SELECTED"); %>>5 Tickets</option>
        <option value="6 Tickets" <% if (resubmit.getNumTickets() == "6 Tickets") out.println("SELECTED"); %>>6 Tickets</option>
        <option value="7 Tickets" <% if (resubmit.getNumTickets() == "7 Tickets") out.println("SELECTED"); %>>7 Tickets</option>
        <option value="8 Tickets" <% if (resubmit.getNumTickets() == "8 Tickets") out.println("SELECTED"); %>>8 Tickets</option>
        <option value="9 Tickets" <% if (resubmit.getNumTickets() == "9 Tickets") out.println("SELECTED"); %>>9 Tickets</option>
</select>
</td>
</tr>
<tr> 
<td>体育场的等级:</td>
<td>
<select name="stadiumTier">
        <option value="Please enter a Stadium Tier">Please enter a Stadium Tier</option>
        <option value="Tier A" <% if (resubmit.getStadiumTier() == "Tier A") out.println("SELECTED"); %>>Tier A</option>
        <option value="Tier B" <% if (resubmit.getStadiumTier() == "Tier B") out.println("SELECTED"); %>>Tier B</option>
        <option value="Tier C" <% if (resubmit.getStadiumTier() == "Tier C") out.println("SELECTED"); %>>Tier C</option>
        <option value="Tier D" <% if (resubmit.getStadiumTier() == "Tier D") out.println("SELECTED"); %>>Tier D</option>
        <option value="Tier E" <% if (resubmit.getStadiumTier() == "Tier E") out.println("SELECTED"); %>>Tier E</option>
</select>
</td> 
</tr>
<tr>
<td>票的价格:</td>
<td>
<select name="ticketPrice">
        <option value="Please enter a Ticket Price">Please enter a Ticket Price</option>
        <option value="$12" <% if (resubmit.getTicketPrice() == "$12") out.println("SELECTED"); %>>$12</option>
        <option value="$17" <% if (resubmit.getTicketPrice() == "$17") out.println("SELECTED"); %>>$17</option>
        <option value="$25" <% if (resubmit.getTicketPrice() == "$25") out.println("SELECTED"); %>>$25</option>
        <option value="$35" <% if (resubmit.getTicketPrice() == "$35") out.println("SELECTED"); %>>$35</option>
        <option value="$50" <% if (resubmit.getTicketPrice() == "$50") out.println("SELECTED"); %>>$50</option>
</select>
</td>
</tr>
<tr>
<td colspan="2" align="center"><INPUT TYPE="submit" VALUE="Submit"></td>
</tr>
</table>
</center>

</form> 

</BODY> 
</HTML> 

下面是处理表单的ControllerServlet.java

package org.javapitfalls.item36;

import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
import org.javapitfalls.item36.*;

public class ControllerServlet extends HttpServlet {

        private static String SESSION_ID;

        public void destroy() {} 
  
        public void init() throws ServletException {

              SESSION_ID = getInitParameter("id"); 
        }
    
        protected void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { 
        
             process(req, res); 
    
       } 

    protected void process(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { 

       HttpSession session = req.getSession(false); 
       String numTickets = req.getParameter("numTickets");
       String stadiumTier = req.getParameter("stadiumTier");
       String ticketPrice = req.getParameter("ticketPrice");

       if(session == null) { 

              if( (numTickets == null) || (stadiumTier == null) || (ticketPrice == null) ) { 

                   getServletConfig().getServletContext().getNamedDispatcher("form").forward(req, res);
            
              } else { 
            
                       throw new ServletException("[form] Page Not Found"); 
            
              } 

      } else { 

         if ( (!numTickets.equals("Please enter a Ticket #")) && 
           (!stadiumTier.equals("Please enter a Stadium Tier")) && (!ticketPrice.equals("Please enter a Ticket Price")) ) {
 
          String sessionValidatorID = (String)session.getAttribute(SESSION_ID); 
          if(sessionValidatorID != null ) { //第一次提交的处理
                           session.removeAttribute(SESSION_ID); 
                           getServletConfig().getServletContext().getNamedDispatcher("success").forward(req, res);
            
                  } else { //重复提交的处理
                           getServletConfig().getServletContext().getNamedDispatcher("resubmit").forward(req, res);

                  }
         } else {

                  getServletConfig().getServletContext().getNamedDispatcher("form").forward(req, res);

         }
 
     } 
  } 
  
}