java - Servlet Jsp 和空指针

Question

大家好，我有一个网站，讲师为实验室分配导师，目前我有一个导师列表，我可以在其中单击分配，然后将用户带到另一个页面，其中有一个下拉框，他们可以选择他们希望导师教授哪个实验室。它应该将实验室的 id 和用户的 id 提交到数据库。

目前，当用户获取导师列表时，他们会单击一个名为“分配”的链接，该链接会将导师 ID 放在 url 中，然后他们就会有一个可以选择的导师列表，一旦我选择了导师并单击提交我收到以下错误。

描述服务器遇到内部错误()，导致其无法完成此请求。

exception

java.lang.NumberFormatException: null
    java.lang.Integer.parseInt(Unknown Source)
    java.lang.Integer.parseInt(Unknown Source)
    TutorAssign.doPost(TutorAssign.java:97)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

sendBack 方法正在运行，但 doPost 没有运行，它指向我获取请求参数为 null 的行 - int user_id = Integer.parseInt(request.getParameter("id"));

我该如何解决这个问题？为了更好地说明，我将包含整个 servlet

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Servlet implementation class TutorAssign
 */
@WebServlet("/TutorAssign")
public class TutorAssign extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public TutorAssign() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        sendBack(request, response);
    }

    private void sendBack(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         HttpSession session = request.getSession(true);
            //Set data you want to send back to the request (will be forwarded to the page)
            //Can set string, int, list, array etc.
         int user_id = Integer.parseInt((String)request.getParameter("id"));
         String sql = "SELECT l.id,s.name,l.day,l.time,l.room" +
                  " FROM subject s, lab l " +
                    " WHERE s.user_id="+(Integer)session.getAttribute("id");

          try{
            Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/wae","root","");
            System.out.println("got a hj");
            System.out.println(session.getAttribute("id"));

              Statement stmt = con.createStatement();
              ResultSet res = stmt.executeQuery(sql);
              System.out.println(res);
              ArrayList<String> list1 = new ArrayList<String>();
              ArrayList<String> list2 = new ArrayList<String>();
              if (res.next()){
                  do{
                       list1.add(res.getString(1));
                       list2.add(res.getString(2)+" "+res.getString(3)+" "+res.getString(4)+" "+res.getString(5));


                  }while(res.next());
              System.out.println("Outside");
              String[] arr1 = list1.toArray(new String[list1.size()]);
              String[] arr2 = list2.toArray(new String[list2.size()]);
              System.out.println(list1);
              request.setAttribute("res1", arr1);
              request.setAttribute("res2", arr2);
              request.setAttribute("user_id", user_id);

              }



          }catch (SQLException e) {
            } 
            catch (Exception e) {
            } 


            //Decides what page to send the request data to
            RequestDispatcher view = request.getRequestDispatcher("TutorAssign.jsp");
            //Forward to the page and pass the request and response information
            view.forward(request, response); 
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {


        int user_id = Integer.parseInt((String)request.getParameter("id"));
          System.out.println(user_id);
          int lab_id = 0;
          System.out.println("I got a blow job");
          String message = null; 
          try {
              Class.forName("com.mysql.jdbc.Driver");
              Connection con = 
                DriverManager.getConnection("jdbc:mysql://localhost:3306/wae","root","");
            System.out.println("got connection"); 
            System.out.println(user_id);
            Statement s = con.createStatement(); 

            String sql = "INSERT INTO user_lab" + 
                      " (user_id, lab_id)" + 
                      " VALUES" + 
                      " ('" +  user_id + "'," + 
                         " '"  +  lab_id + "')"; 

              System.out.println(sql);
              int i = s.executeUpdate(sql); 
              if (i==1) {
                message = "Successfully assigned a tutor."; 
                response.sendRedirect("Lecturer_labs");
              } 

              s.close(); 
              con.close(); 
            } 
            catch (SQLException e) {
              message = "Error." + e.toString(); 
              boolean error = true; 
            } 
            catch (Exception e) {
              message = "Error." + e.toString(); 
              boolean error = true; 
            } 
            if (message!=null) {
              PrintWriter out = response.getWriter(); 
              out.println("<B>" + message + "</B><BR>"); 
              out.println("<HR><BR>"); 
            } 

          } 
// TODO Auto-generated method stub


}

这是我的jsp代码

<form name ="TutorAssign" ACTION="TutorAssign" method="post">
        <input type="hidden" name="user_id" value="user_id"/>
        <select name="lab_id">
        <%
        for(int i=0; i<list1.length;i++)  
        { 
        out.println("<option value="+list1[i]+"> "+list2[i]+" </option>");

        } %>
        </select>
        <input type=SUBMIT value="Submit" name="Submit" />

        </form>

Answer 1

当发生 HTTP POST 时，id 似乎没有被传递到您的 servlet。您可以通过添加合适的保护声明来防止这种情况:

if (request.getParameter("id") == null) {
   // handle non existance of id
}

未传入 id 的原因是 JSP 表单中没有任何输入字段可传入此值。这看起来像:

<input type="hidden" name="id" value="your_id_here"/>