对下拉框设置只读

function setReadOnly(id){
$("#"+id).wrap("<span id='"+id+"wrap'></span>");
var obj = document.getElementById(id+"wrap");
obj.onmouseover = function(){
obj.setCapture();
}
obj.onmouseout = function(){
obj.releaseCapture();
}
obj.onfocus = function(){
obj.blur();
}
obj.onbeforeactivate = function(){
return false;
}
}

id为元素的id