Question

我写了一个由数据库填充的下拉列表。这是我在JSP文件中的下拉代码。

 <td>Category:</td>
 <%
  try {
      dbConnect dbConnect = new dbConnect();
      Connection currentCon = dbConnect.Connect();
      System.out.println("Connection sucess");
      String sql = "SELECT * FROM categories";
      PreparedStatement ps = currentCon.prepareStatement(sql);
      ResultSet rs = ps.executeQuery();%>
 <td><select name="category" class="form-control" required>
      <%
        while (rs.next()) {
           String cname = rs.getString("category_name");
           String id = rs.getString("category_id");
           System.out.println(id);
      %>
      <option value="<%= id%>"><%= cname%></option>
      <%
        }
      %>
      </select>
      <%
      } catch (SQLException sqe) {
            out.println(sqe);
      }
 %>
 </td>

相关的servlet代码是；

String id = request.getParameter("category");

在JSP的下拉列表中，它打印类别中的所有ID。但是，当涉及到servlet时，它将返回null。如何解决此问题？

Answer 1

在您的代码中我没有找到任何<form> tag或submit按钮，请尝试以下代码：

    <td>Category:</td>
     <form method="post"  action="yourservleturl">//form tag for submitting data

         <%
          try {
              dbConnect dbConnect = new dbConnect();
              Connection currentCon = dbConnect.Connect();
              System.out.println("Connection sucess");
              String sql = "SELECT * FROM categories";
              PreparedStatement ps = currentCon.prepareStatement(sql);
              ResultSet rs = ps.executeQuery();%>
         <td><select name="category" class="form-control" required>
              <%
                while (rs.next()) {

              %>
              <option value="<%=rs.getString("category_id")%>"><%=rs.getString("category_name")%></option> // printing option values
              <%
                }
              %>
              </select>
              <%
              } catch (SQLException sqe) {
                    out.println(sqe);
              }
         %>
     <input type="submit" value="submit">///submit btn
  </form>
</td>

还可以通过String id = request.getParameter("category");获取下拉值。应该可以使用。

Answer 2

应该在while循环外发出局部变量，因为在while循环后需要该值。

<%
try {
  dbConnect dbConnect = new dbConnect();
  Connection currentCon = dbConnect.Connect();
  System.out.println("Connection sucess");
  String sql = "SELECT * FROM categories";
  PreparedStatement ps = currentCon.prepareStatement(sql);
  ResultSet rs = ps.executeQuery();%>
  String cname = null;
  String id = null;
 <td><select name="category" class="form-control" required>
  <%
    while (rs.next()) {
       cname = rs.getString("category_name");
       id = rs.getString("category_id");
       System.out.println(id);
  %>

此外，我建议您使用表达式语言而不是<%= id%>

更改 <option value="<%= id%>"><%= cname%></option>

<option value="${id}">${name}</option>

别忘了提交按钮。

动态下拉列表从jsp返回null

2 个答案: