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>