我在 JavaFX 异常方面遇到一些问题。我的项目在我的 Eclipse 中运行，但现在我的 friend 也尝试访问该项目。我们已共享并直接保存到保管箱文件夹中。但他根本无法让它发挥作用。他在控制台中收到此错误。真的很感谢一些解释这意味着什么..:)

jun 12, 2014 9:19:37 EM application.Main gotoLogin
SEVERE: null
javafx.fxml.LoadException: 
/C:/Users/Administrator/Dropbox/Project%20BlueRift/bin/application/LoginGUI.fxml

 at javafx.fxml.FXMLLoader.constructLoadException(Unknown Source)
 at javafx.fxml.FXMLLoader.loadImpl(Unknown Source)
 at javafx.fxml.FXMLLoader.load(Unknown Source)
 at application.Main.replaceSceneContent(Main.java:53)
 at application.Main.gotoLogin(Main.java:68)
 at application.Main.start(Main.java:39)
 at com.sun.javafx.application.LauncherImpl$8.run(Unknown Source)
 at com.sun.javafx.application.PlatformImpl$7.run(Unknown Source)
 at com.sun.javafx.application.PlatformImpl$6$1.run(Unknown Source)
 at com.sun.javafx.application.PlatformImpl$6$1.run(Unknown Source)
 at java.security.AccessController.doPrivileged(Native Method)
 at com.sun.javafx.application.PlatformImpl$6.run(Unknown Source)
 at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
 at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
 at com.sun.glass.ui.win.WinApplication.access$300(Unknown Source)
 at com.sun.glass.ui.win.WinApplication$4$1.run(Unknown Source)
 at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
 at application.DBManager.connect(DBManager.java:54)
 at controllers.LoginController.initialize(LoginController.java:48)
 ... 16 more

数据库连接:

package application;

import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import com.mysql.jdbc.Connection;
import com.mysql.jdbc.DatabaseMetaData;
import com.mysql.jdbc.PreparedStatement;
import com.mysql.jdbc.Statement;

import controllers.IReportStatus;

public class DBManager {

    private static DBManager INSTANCE;

    private final String MYSQL_URL = "jdbc:mysql://213.65.171.115:3306";
    private final String MYSQL_DATABASE = "/bluerift";
    private final String MYSQL_USER = "test";
    private final String MYSQL_PW = "test";
    private String CURRENT_USER = null;
    private String CURRENT_TIMELINE = null;

    private Connection conn = null;
    private Statement st = null;
    private ResultSet rs = null;
    private PreparedStatement preparedStatement = null;;
    private DatabaseMetaData dbm = null;

    private IReportStatus reportStatus;

    private DBManager() {
    }

    public boolean connect() {
        thử {
            if (conn != null) {
                trả về giá trị đúng;
            }

            Class.forName("com.mysql.jdbc.Driver");

            conn = (Connection) DriverManager.getConnection(MYSQL_URL
                    + MYSQL_DATABASE, MYSQL_USER, MYSQL_PW);
            st = (Statement) conn.createStatement();
            dbm = (DatabaseMetaData) conn.getMetaData();
            System.out.println("DevCon: Connected");
            trả về giá trị đúng;
        }

        catch (Exception ex) {
            reportStatus.setMsg("Could not reach database.");
            conn = null;
            st = null;
            rs = null;
            trả về false;
        }
    }

    public boolean userLogin(String aUserName, String aPassword) {
        aUserName = aUserName.trim();

        if (aUserName.isEmpty()) {
            reportStatus.setMsg("Empty Username");
            trả về false;
        }

        if (aUserName.contains(" ")) {
            reportStatus.setMsg("Username can not contain whitespaces");
            trả về false;
        }

        if (aPassword.isEmpty()) {
            reportStatus.setMsg("Empty Password");
            trả về false;
        }

        thử {
            String query = "SELECT userName,userPassword FROM users";
            rs = st.executeQuery(query);

            while (rs.next()) {
                if (rs.getString("userName").equals(aUserName)) {

                    if (rs.getString("userPassword").equals(aPassword)) {
                        System.out.println("DevCon: User "
                                + MyUtil.quote(aUserName) + " logged in");
                        CURRENT_USER = aUserName;

                        trả về giá trị đúng;
                    }

                    else {
                        reportStatus.setMsg("Wrong Password");
                        trả về false;
                    }
                }
            }

            reportStatus.setMsg("Username not found in database");
            trả về false;
        }

        catch (Exception ex) {
            reportStatus.setMsg("Could not reach database.");
            conn = null;
            st = null;
            rs = null;
            trả về false;
        }
    }

    public boolean userRegister(String aName, String aPassword,
            String confirmPassword, String aEmail) {

        aEmail = aEmail.trim();
        aName = aName.trim();

        if (aName.length() < 5 || aName.length() > 25) {
            reportStatus
            .setMsg("Username has to be between 5 and 25 characters long");
            trả về false;
        }

        if (!MyUtil.isAlphaNumericUs(aName)) {
            reportStatus
            .setMsg("Username can only contain a-Z, A-Z, 0-9 and _");
            trả về false;
        }

        if (aEmail.isEmpty()) {
            reportStatus.setMsg("Email can not be empty");
            trả về false;
        }

        if (!MyUtil.isVaildEmail(aEmail)) {
            reportStatus.setMsg("Email incorrect");
            trả về false;
        }

        if (aPassword.length() < 5 || aPassword.length() > 30
                || confirmPassword.length() < 5
                || confirmPassword.length() > 30) {
            reportStatus
            .setMsg("Password must be between 5 and 30 characters long");
            trả về false;
        }

        if (!aPassword.equals(confirmPassword)) {
            reportStatus.setMsg("Password does not match");
            trả về false;
        }

        thử {
            String query = "SELECT userName, userEmail FROM users";
            rs = st.executeQuery(query);

            while (rs.next()) {
                if (aName.equals(rs.getString("userName"))) {
                    reportStatus.setMsg("Username already registred");
                    trả về false;
                }

                if (aEmail.equals(rs.getString("userEmail"))) {
                    reportStatus.setMsg("Email already registred");
                    trả về false;
                }
            }

            query = "INSERT INTO users (userName, userPassword, userEmail)"
                    + " VALUES (?, ?, ?)";

            preparedStatement = (PreparedStatement) conn
                    .prepareStatement(query);
            preparedStatement.setString(1, aName);
            preparedStatement.setString(2, aPassword);
            preparedStatement.setString(3, aEmail);
            preparedStatement.execute();
        }

        catch (Exception ex) {
            System.out.println("Error: " + ex);
            trả về false;
        }

        trả về giá trị đúng;
    }

    public boolean createTimeline(String timelineName) {
        timelineName = timelineName.trim();

        if (timelineName.length() < 3 || timelineName.length() > 15) {
            reportStatus
            .setMsg("Name must be between 3 and 15 characters long");
            trả về false;
        }

        System.out.println("timeline name in: " + timelineName);
        if (!MyUtil.isAlphaNumeric(timelineName) || timelineName.isEmpty()) {
            reportStatus
            .setMsg("Invaild timeline name. Allowed symbols are a-z, A-Z and 0-9");
            trả về false;
        }

        if (timelineExists(timelineName)) {

            reportStatus.setMsg("Timeline called " + MyUtil.quote(timelineName)
                    + " already exists");
            trả về false;
        }

        else {

            String query = "CREATE TABLE `bluerift`.`"
                    + getCurrentUser()
                    + "-"
                    + timelineName
                    + "` (`id` INT NOT NULL AUTO_INCREMENT,`eventName` VARCHAR(45) NULL,`eventYear` INT NULL,`eventDesc` LONGBLOB NULL,PRIMARY KEY (`id`));";

            thử {
                st.executeUpdate(query);
            } catch (SQLException e) {

                e.printStackTrace();
            }
        }
        trả về giá trị đúng;
    }

    public ObservableList receiveTimelines() {
        ObservableList temp = FXCollections.observableArrayList();

        thử {
            rs = dbm.getTables(null, null, "%", null);
            while (rs.next()) {

                if (rs.getString(3).contains(getCurrentUser() + "-")) {
                    temp.add(rs.getString(3).substring(
                            getCurrentUser().length() + 1));
                }
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }

        return temp;

    }

    private boolean timelineExists(String t) {

        thử {
            rs = dbm.getTables(null, null, CURRENT_USER + "-" + t, null);

            if (rs.next()) {
                trả về giá trị đúng;
            }
        }

        catch (SQLException e) {
            e.printStackTrace();
        }

        trả về false;
    }

    public void deleteTimeline(String timelineName) {
        thử {
            String query = "DROP TABLE `bluerift`.`" + getCurrentUser() + "-"
                    + timelineName + "`;";
            st.executeUpdate(query);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public String getCurrentUser() {
        if (CURRENT_USER == null) {
            return "DevCon: No user online";
        }

        return CURRENT_USER;
    }

    public void userLogout() {
        if (CURRENT_USER != null) {
            System.out.println("DevCon: User " + CURRENT_USER + " logged out");
            CURRENT_USER = null;
        }

        else {
            System.out.println("DevCon: No user online");
        }
    }

    public void setReportTo(IReportStatus reportStatus) {
        System.out.println("new ReportStatus is: " + reportStatus);
        this.reportStatus = reportStatus;
    }

    public String getCurrentTimeline() {
        return CURRENT_TIMELINE;
    }

    public static DBManager getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new DBManager();
        }
        return INSTANCE;
    }

}

我真的不知道是什么导致了空指针以及为什么

1 Câu trả lời

异常输出准确地指出了行号:

Caused by: java.lang.NullPointerException
 at application.DBManager.connect(DBManager.java:54)

假设问题中的代码与您的生产代码相同，第 54 行包含:

reportStatus.setMsg("Could not reach database.");

reportStatus 可能未初始化，从而导致 NullPointerException.

关于JavaFX 异常，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24206480/