request.getParameter（“nickname”）为

Question

我是网络开发的初学者。我想将数据发送到servlet，但是我得到了一个N​​ullPointerException。

例如，nickname = admin的值，我可以看到它。当我按下按钮发布到servlet时，我无法传递数据，所有数据都为空。我不明白为什么。我已将另一个（jsp + servlet）的数据加载到我发布的数据中。

JSP

         <form name="admin_view_user" action="${pageContext.request.contextPath}/admin/users/user/activate" method="POST">
        <div id="leftcolumn">
        <div>
            <img src="${pageContext.request.contextPath}/images/imageholder_detailspage.jpg" alt="" width="220" height="220" class="previewimg" />
        </div>

        <c:choose>
            <c:when test="${active =='1'}">
                <h2>Ok</h2>
                <br />
            </c:when>    
            <c:otherwise>
                <h2>Post</h2>

                <div>
                    <input class="commitbutton" type="submit" alt="buttonActivete" name="buttonActivete" id="buttonActivete" value="Activet" />
                </div>

                <br />
            </c:otherwise>
        </c:choose>
    </div>

    <div id="rightcolumn">
        <table class="table">
            <tr>

                <td width="10%" class="td_attributes">Nickname</td>
                <td name="nickname" id="nickname" class="text longfield" value="${nickname}">
                    <c:out value="${nickname}"/>
                </td>

            </tr>

        </table>
    </div>
    <div class="clear">&nbsp;</div>
</form>
</div>

的Servlet

@WebServlet(name = "AdminActivate", urlPatterns = {"/admin/users/user/activate"})
 public class AdminActivate extends HttpServlet {

/**
 * Processes requests for both HTTP <code>GET</code> and <code>POST</code>
 * methods.
 *
 * @param request servlet request
 * @param response servlet response
 * @throws ServletException if a servlet-specific error occurs
 * @throws IOException if an I/O error occurs
 */
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    ServletContext servletContext = getServletContext();
    try (PrintWriter out = response.getWriter()) {
        ViewUserAdminActivet controller = new ViewUserAdminActivet();
        Map<String, String[]> parameterMap = request.getParameterMap();
        String nickname = parameterMap.get("nickname")[0];
        System.out.println(nickname + "________________________________");
       // controller.execute(servletContext, parameterMap, request, response);
    }
    }

在这一行中，我得到了异常Map<String, String[]> parameterMap = request.getParameterMap();