Question

我希望在提交表单后再由于某种原因再次转发到同一页面时，下拉列表中的所选选项应保留我第一次输入的选定值。

<form action="CitySelection" method="POST">

     <select name="cityname" id="myselect" onchange="this.form.submit()">
         <option value="england">england</option>
         <option value="france">france</option>
         <option value="spain">spain</option>
     </select>

</form>

如何通过此功能增强我的上述表单代码，任何建议？

Answer 1

为什么要在选择菜单中放置硬编码城市列表？要做到这一点，您的jsp页面应该使用c taglib：

<%@ taglib prefix="c" uri="java.sun.com/jsp/jstl/core" %>

<form action="<SUBMITTING_URL>" method="post">
      <select name="cityname" id="myselect" onchange="this.form.submit()">
         <c:foreach var="cityname" items="${cityList}">
            <c:choose>
               <c:when test="${not empty selectedCity && selectedCity eq cityname}">
                   <option value="${cityname}" selected = "true">${cityname}</option>
               </c:when>
               <c:otherwise>
                   <option value="${cityname}">${cityname}</option>
               </c:otherwise>
            </c:choose>
         </c:foreach>
      </select>
</form>

在与<SUBMITTING_URL>映射的servlet中应该有一个doPost方法，如下所示：

 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

   String cityName = request.getParameter("cityname"); //retrieving value from post request.
   //do your necessary coding.
   if(validation error or as your wish){
      List<String> cityList = new ArrayList<String>();
      cityList.add("england");
      cityList.add("france");
      cityList.add("spain");
      request.setAttribute("cityList",cityList);
      request.setAttribute("selectedCity",cityName);

      RequestDispatcher dispatcher = request.getRequestDispatcher("cityform.jsp");  
      dispatcher.forward(request, response);
   } else {
      //do other things
   }
}

你的doGet方法看起来应该如下：

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

      List<String> cityList = new ArrayList<String>();
      cityList.add("england");
      cityList.add("france");
      cityList.add("spain");
      request.setAttribute("cityList",cityList);
      request.setAttribute("selectedCity",null);

      RequestDispatcher dispatcher = request.getRequestDispatcher("cityform.jsp");  
      dispatcher.forward(request, response);
}

现在，当您想第二次访问该页面时，您将看到所选的值。

如何在同一个jsp页面的下拉列表中保留所选值？

1 个答案: