JSON在jsp页面中返回空值或未定义

Question

我正在尝试使用数据库中的数据绘制散点图。我正在从我的 servlet 中检索数据，当我试图打印出来时，它给了我一个 JSON 对象。但是，在我的 jsp 页面中，它给了我一个未定义的值或 null。有人可以帮我解决这个问题吗？谢谢。

Servlet

try {
         // Register JDBC driver
         Class.forName("com.mysql.jdbc.Driver");
         // Open a connection
         Connection conn = (Connection) DriverManager.getConnection(DB_URL, USER, PASS);
         // Execute SQL query
         Statement stmt = (Statement) conn.createStatement();
         String sql;

         sql = "SELECT userName as userName, fpogx, fpogy\r\n"+
                 "FROM affecttutor.gazedata WHERE fpogv = 1;";

        /*sql = "select exercise_working_on as exeWork, sum(no_of_errors) as countErr, idtopic\r\n" + 
                "from affecttutor.fexdb left join affecttutor.exercises on affecttutor.fexdb.exercise_working_on=affecttutor.exercises.idexercises\r\n" + 
                "where user_id LIKE '%student08%' && no_of_errors!=0 && exercise_working_on!=0  group by exeWork;";*/
         JSONArray list = new JSONArray();
         ResultSet rs = stmt.executeQuery(sql);

         // Extract data from result set
         while(rs.next()) {      
                    //Retrieve by column name
                        JSONObject object=new JSONObject(); 

                        float fpogx = rs.getFloat("fpogx");
                        float fpogy = rs.getFloat("fpogy");
                        String userName = rs.getString("userName");

                     object.put("fpogx", fpogx);
                     object.put("fpogy", fpogy);      
                     object.put("userName", userName);

                    list.put(object);

         }
         out.println(list);
         System.out.println(list);
         // Clean-up environment
        // rs.close();
         //stmt.close();
         //conn.close();
      } catch(SQLException se) {
         //Handle errors for JDBC
         se.printStackTrace();
      } catch(Exception e) {
         //Handle errors for Class.forName
         e.printStackTrace();
      }

JSP

d3.json("analysisDataServlet", function(error, data) {
        console.log(data);
      if (error) throw error;

JSON

[{"fpogy":0.0348300002515316,"fpogx":-2.7372400760650635,"userName":"student01"},{"fpogy":0.03328999876976013,"fpogx":-2.73757004737854,"userName":"student01"},{"fpogy":0.03189000114798546,"fpogx":-2.73786997795105,"userName":"student01"},{"fpogy":0.5096700191497803,"fpogx":0.4750800132751465,"userName":"student01"},{"fpogy":0.5096700191497803,"fpogx":0.4750800132751465,"userName":"student01"},{"fpogy":0.5096700191497803,"fpogx":0.4750800132751465,"userName":"student01"},{"fpogy":0.5096700191497803,"fpogx":0.4750800132751465,"userName":"student01"},{"fpogy":0.5096700191497803,"fpogx":0.4750800132751465,"userName":"student01"},{"fpogy":0.5012699961662292,"fpogx":0.4617300033569336,"userName":"student01"},{"fpogy":0.5012699961662292,"fpogx":0.4617300033569336,"userName":"student01"},{"fpogy":0.5012699961662292,"fpogx":0.4617300033569336,"userName":"student01"},{"fpogy":0.5012699961662292,"fpogx":0.4617300033569336,"userName":"student01"},{"fpogy":0.5012699961662292,"fpogx":0.4617300033569336,"userName":"student01"},{"fpogy":0.4968700110912323,"fpogx":0.4556199908256531,"userName":"student01"},{"fpogy":0.4968700110912323,"fpogx":0.4556199908256531,"userName":"student01"}]

Answer 1

<%@page import="java.io.PrintWriter"%>
<%@page contentType="text/html; charset=UTF-8"%>
<%@page import="newpackage.ChartDataController,java.util.*"%>

<%
String json=ChartDataController.getChartData();  

response.setContentType("application/json");
response.getWriter().write(json);
  
%>