关于struts2类型转换的问题

这种方式能修改成功

<td align="right" > 

<a href="javascript:document.getElementById('updateForm').submit();">保存</a> &nbsp;&nbsp;

<a href="javascript:window.history.back();">退回 </a>

</td>

</tr>

</table>

<br/>

 <br>

 <form id="updateForm" method="post" action="<%=request.getContextPath() %>/department/department_update?did=${update_department.did}" >

<%-- <input type="hidden" name="did" value="${update_department.did} "> --%>

<table style="font-size::16px">

<tr>

  <td>部门名称：</td>

  <td><input type="text" name="dname" value="${update_department.dname} "/></td>

</tr>

<tr>

  <td>部门介绍：</td>

  <td></td>

</tr>

<tr>

  <td width="10%"></td>

  <td>

     <textarea cols="50" rows="5" name="ddesc">${update_department.ddesc}</textarea>

  </td>

</tr>

</table>

这种范是修改失败，setId失败（setId不能使用String参数）

<a href="javascript:document.getElementById('updateForm').submit();">保存</a> &nbsp;&nbsp;

<a href="javascript:window.history.back();">退回 </a>

</td>

</tr>

</table>

<br/>

 <br>

 <form id="updateForm" method="post" action="<%=request.getContextPath() %>/department/department_update" >

<input type="hidden" name="did" value="${update_department.did} ">

<table style="font-size::16px">

<tr>

  <td>部门名称：</td>

  <td><input type="text" name="dname" value="${update_department.dname} "/></td>

</tr>

<tr>

  <td>部门介绍：</td>

  <td></td>

</tr>

<tr>

  <td width="10%"></td>

  <td>

     <textarea cols="50" rows="5" name="ddesc">${update_department.ddesc}</textarea>

  </td>

对应action如下

public String update(){

System.out.println(department.getDid());

System.out.println(department.getDname());

System.out.println(department.getDdesc());

//return NONE;

departmentService.update(department);

return "update_success";

}

这是什么原因？？？