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>
<body>
<form action="/form/servlet/DoFormServlet" method="post">
<input type="hidden" name="token" value="${token }">
用户名：<input type="text" name="username"><br />
<input type="submit" value="提交">
</form>
</body>
</html>