codice:
<html>
<head>
<title> Avvio sessione </title>
<script language="JavaScript">
	function controlla()
	{
	
		if(document.getElementById("cod").value == "")
		{
			alert("Inserire id");
			return false;
		}
		return true;
	}
</script>		
</head>
	
<body>
<form name="azione" action="controlla.jsp" method="post" onSubmit="return controlla();">
	<table align="right" border="1" width="30%" height="20%">
		<tr><td> <label> Inserisci Id </label>
			<input type="text" name="id" id="cod">
			<label> 
 Inserisci Password </label>
			<input type="text" name="pass" value="">
			<input type="submit" value="Inserisci">
			</td> 
		</tr>
	</table>
</form>
</body>
</html>