当网络有延迟时，用户提交的表单等数据还没有完成此次提交，但用户又多次点击提交，造成用户数据在数据库或存储中被提交多次。
利用线程延迟，简单模拟重复提交。
表单页面为form.html
[html] view plain copy
form.html
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="this is my page">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<!--<link rel="stylesheet" type="text/css" href="./styles.css">-->
用户名 ：
处理提交请求的servlet为DoFormServlet.java
[java] view plain copy
package SessionDemo;
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
{
String username = request.getParameter("username");
//利用线程休眠，模拟网络延迟
try
{
Thread.sleep(1000*3);
} catch (InterruptedException e)
{
e.printStackTrace();
}
System.out.println("向数据库中注册数据。");
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
doGet(request, response);
}
}
在浏览器中加载form.html
当用户反复点击“提交”时，就造成了重复提交。
二、解决方法一：利用javascript阻止
其他不变，将form.html修改为
[html] view plain copy
form.html
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="this is my page">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<!--<link rel="stylesheet" type="text/css" href="./styles.css">-->
var isCommitted = false;
function dosubmit()
{
if(!isCommitted)
{
isCommitted=true;
return true;
}
else
{
return false;
}
}
用户名 ：
这样在浏览器中加载form.html后，点击“提交”，终端中只会输出一次”向数据库中注册数据。”，表明成功阻止表单反复提交。
以上form.html可以进一步优化，当用户点击“提交”后，“提交”按钮应该变为灰色不可用。
[html] view plain copy
form.html
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="this is my page">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<!--<link rel="stylesheet" type="text/css" href="./styles.css">-->
function dosubmit()
{
var input = document.getElementById("submit");
input.disabled = 'disabled';
return true;
}
用户名 ：
利用javascript的方法不能完全防止用户恶意重复提交，例如：用户可以将form.html保存后修改，还可以在点击“提交”后重复刷新页面，从而实现反复提交。
三、解决方法二：利用服务器端Session防止表单重复提交。
其中FormServlet.java为
[java] view plain copy
package SessionDemo;
import java.io.IOException;
import java.io.PrintWriter;
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;
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("/form.jsp").forward(request, response);
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
doGet(request, response);
}
}
//设计为单例模式
class TokenProcessor
{
private TokenProcessor(){};
private static final TokenProcessor instance = new TokenProcessor();
public static TokenProcessor getInstance()
{
return instance;
}
public String generateToken()
{
//获得随机数字符串
String token = System.currentTimeMillis() + new Random().nextInt() + "";
//获得数据摘要
try
{
MessageDigest md = MessageDigest.getInstance("md5");
byte[] md5 = md.digest(token.getBytes());
//利用base64编码防止乱码。
BASE64Encoder encoder = new BASE64Encoder();
return encoder.encode(md5);
} catch (NoSuchAlgorithmException e)
{
throw new RuntimeException(e);
}
}
}
添加form.jsp
[plain] view plain copy
<%@ page language=”java” import=”java.util.*” pageEncoding=”UTF-8”%>
用户名
将DoFormServlet.java修改为
[java] view plain copy
package SessionDemo;
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
{
String username = request.getParameter("username");
boolean b = isTokenValue(request);
if (!b)
{
System.out.println("请不要重复提交。");
return;
}
request.getSession().removeAttribute("token");
System.out.println("向数据库中注册数据。");
}
//判断表单号是否有效
private boolean isTokenValue(HttpServletRequest request)
{
String clientToken = request.getParameter("token");
if(clientToken==null)
{
return false;
}
String serverToken = (String) request.getSession().getAttribute("token");
if(serverToken==null)
{
return false;
}
if (!clientToken.equals(serverToken))
{
return false;
}
return true;
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
doGet(request, response);
}
}
再浏览器中加载FormServlet
点击“提交”后跳转
终端显示用户提交
这时即使用户点击“刷新”，也不能实现重复提交。