问HTML:JS:引导；验证多选择下拉列表

EN

<form action="#">
<select id="users" name="username[]"  class="chzn-select span2" multiple="multiple"> 
      <option value=""></option>  
     <option value="sdfsd">sdfsdfs</option>
           .... 
</select>
<input type="submit" id="add_btn">
</form>

$('#add_btn').on('click', function(e) {
        if($('#users').val() == null && $('#users_chosen').is(':visible')) {
        alert('You must choose division');
        }

});

$(function(){
    $('form').submit(function(){
         var options = $('#users > option:selected');
         if(options.length == 0){
             alert('no value selected');
             return false;
         }
    });
});

//You can use Js for this
    var ptype=document.getElementById('username').options[document.getElementById('username').selectIndex].value;

if(ptype=""){
document.getElementById('username').style.display='none';
alert('Please select field');
document.getElementById('username').focus();
return false;
}