inserito i controlli lato server,ora forse i problemi sono solo html,infatti non mi reindirizza alla pagina index.html e l'alert: riempi tutti i campi compare solo se non inserisco il primo campo.posto cmq la pagina modificata:
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<%@ page language="java" import="java.util.*" import="java.sql.*"%>
<html>  <head><title>Registrazione.jsp</title>
	  <script type="text/javascript">
		function Controlla() {
			var controllo = new RegExp ("^[0-9]+$");
			for (var i=0; i < 4; i++) {
				if (document.getElementById("s"+i).value == "") {
					alert("Riempi tutti i campi");
					return false;					                          }
				else {
					location.href = ("index.html");
					return;
				}	
			}	
		}
	</script>
  </head>
  <body>
<form method="POST" onsubmit="Controlla()">
	<table  width="85%" id="table1">
		<tr>
			<td width="275">
			<p align="center">Nome</td>
                       		<td>
			

<input type="text" size="60" id="s0" name=nome></p>
			</td>
		</tr>
		<tr>
			<td width="275">
			<p align="center">Cognome</td>
			<td>
			

<input type="text" size="60" id="s1" name=cogn></p>
			</td>
		</tr>
		<tr>
			<td width="275">
			<p align="center">Username</td>
			<td>
			

<input type="text" size="60" id="s2" name=user></p>
			</td>
		</tr>
		<tr>
			<td width="275">
			<p align="center">Password</td>
			<td>
			

<input type="text" size="60" id="s3" name=pwd></p>
			</td>
		</tr>
		<tr>
			<td width="275">
			<p align="center"></p>
			<p align="center">
			<input type=Submit value=registrami>
			</p>
			</td>
  		</tr>
	</table>
</form>
<%  
if ((request.getParameter("nome") != "")&&(request.getParameter("cogn")!= "")&&(request.getParameter("user")!= "")&&(request.getParameter("pwd")!= "")){  
     	String strDriver="sun.jdbc.odbc.JdbcOdbcDriver";
     	String strNomeConnessione="jdbc:odbc:database";
 	Class.forName(strDriver);
 	Connection conn=null;
 	conn=DriverManager.getConnection(strNomeConnessione);
 	Statement stmDb=conn.createStatement();
  	 	
 	Enumeration parametri = request.getParameterNames();
 	if(parametri.hasMoreElements()){
 		String strNome = request.getParameter("nome");
 		String strCognome = request.getParameter("cogn");
 		String strUser = request.getParameter("user");
 		String strPwd = request.getParameter("pwd");
 		stmDb.executeUpdate(
		"INSERT INTO UTENTI (\"nome\", \"cognome\", \"user\", \"password\") VALUES ('"+strNome+"','"+strCognome+"','"+strUser+"','"+strPwd+"')");
 	}  	 
 }	
%>
  </body></html>