input check復選框選擇后修改標簽超鏈接href

1. 給復選框添加onclick事件 獲取標簽id

    <tbody>
                        <c:forEach var="file" items="${files}">
                        <tr class="er" onMouseMove="changeTrColor(this)">
                            <td class="xuanze"><input type="checkbox" name="r" value="${file.id}" onclick="aa()"/></td>
                            <td class="name">${file.filename}</td>
                            <td class="daxiao">15k</td>
                            <td class="time"><a href="#">2015-01-01</a> <select><option>刪除</option><option>移動到</option><option>復制到</option><option>下載</option></select></td>
                        </tr>
                      </c:forEach>
                    </tbody>

2。給a標簽添加id

  <div class="btn2">
            <a href="#" id="xiazai">下載</a>
            <a href="#">刪除</a>
            <a href="#">打印</a>
        </div>

3.獲取復選框id修改href屬性

function aa(){
    alert("aa");
    var r=document.getElementsByName("r"); 
    for(var i=0;i<r.length;i++){
         if(r[i].checked){
         alert(r[i].value);
         var num = r[i].value;
         document.getElementById("xiazai").href="file/download?id="+num;
       }
    }     
}