jsp：
<script type="text/javascript">
 function check(){
  var userName = document.getElementById("name").value;
  var pass1 = document.getElementById("pass1").value;
  var pass2 = document.getElementById("pass2").value;
  var email = document.getElementById("email").value;  
  if(userName == ""|| pass1 == ""|| pass2 == ""|| email == ""){
   alert("请将登录信息填写完整");
   return false;
  }
 }
</script>
 
 
 
 <form action="#" method="post" onsubmit="return check();" />
 <input type="text" name="user.name" id="pass1" />
 <input type="password" name="user.pass" id="pass2" />
 <input type="password" id="name">
 <input type="text" name="user.email" id="email" />
 <input type="reset" value="重置" />
 <input type="submit" value="注册" />
 </form>
 
   if(userName.length<15||userName.length>3){  
    pName.innerHTML = "";
    return false;
   }

 

看demo：