servlet中获得表单值为空

ajax代码：
$(":button").click(function(){
$.post("../servlet/itemAddAltSelServlet?type=alter",
$("#alterItem form").serialize(), function(data){
alert("xx");
})
})

表单代码（部分）：

 <div id="alterItem">
                    <form method="post" action="/Exchange/servlet/itemAddAltSelServlet?type=alter">
                        <div id="ID">
                            I&nbspD:&nbsp<input type="text" name="ID">
                        </div>
                        <div id="tie2">
                            标题:&nbsp<input type="text" name="tie">
                        </div>
                        <div id="num2">
                            数量:&nbsp<input type="text" name="num">
                        </div>

servlet代码：if(type.equals("alter")){
String ID=request.getParameter("ID");
//int ID=Integer.parseInt(request.getParameter("ID"));
System.out.println(ID);//输出的值为空
}

http://bbs.csdn.net/topics/390470394

http://bbs.csdn.net/topics/390470394