java - 即使连接成功后，org.apache.derby.client.net.NetConnection40 也无法转换为 com.mysql.jdbc.Connection 错误？

Question

我正在 Netbeans 中制作一个简单的 JavaSE 登录 jFrame。我创建了一个内部 sql 数据库，但问题是启动应用程序时它具有以下控制台输出:

run:
CONNECTION DONE!
Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: org.apache.derby.client.net.NetConnection40 cannot be cast to com.mysql.jdbc.Connection
    at DBConnection.connect(DBConnection.java:31)
    at mainFrame.<init>(mainFrame.java:29)
    at mainFrame$7.run(mainFrame.java:360)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
BUILD SUCCESSFUL (total time: 1 second)

它说连接已完成，但随后出现错误。我还在库中导入了 derbyclient.jar 文件。我在网上搜索了解决方案并尝试替换:

jdbc:derby://localhost:1527/JavaDB;

这样:

jdbc:derby://localhost:1527/JavaDB;create=true

代码:主框架

import com.mysql.jdbc.Connection;
import com.mysql.jdbc.PreparedStatement;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Toolkit;
import java.sql.ResultSet;
import javax.swing.ImageIcon;
import javax.swing.JOptionPane;
import javax.swing.SwingConstants;

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 *
 * @author sniper
 */
public class mainFrame extends javax.swing.JFrame {

    Dimension dim = Toolkit.getDefaultToolkit().getScreenSize(); // Getting dimentions for screen size
    int xMouse; // for movement x axis
    int yMouse; // for movement y axis

    // FOR SQL CONNECTION
    Connection conn = new DBConnection().connect();

    /**
     * Creates new form mainFrame
     */
    public mainFrame() {
        initComponents();
        this.setLocation(dim.width/2-this.getSize().width/2, dim.height/2-this.getSize().height/2); // setting screen in center
        txtUsername.setHorizontalAlignment(SwingConstants.CENTER); // center align txtUsername text
        txtPassword.setHorizontalAlignment(SwingConstants.CENTER); // center align txtPassword text
    }

    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        jPanel1 = new javax.swing.JPanel();
        jPanel2 = new javax.swing.JPanel();
        lblHeaderText = new javax.swing.JLabel();
        txtUsername = new javax.swing.JTextField();
        txtPassword = new javax.swing.JPasswordField();
        lblLoginTypeText = new javax.swing.JLabel();
        cbType = new javax.swing.JComboBox<>();
        btnEnter = new javax.swing.JLabel();
        lblTopText = new javax.swing.JLabel();
        lblIcon = new javax.swing.JLabel();
        lblExit = new javax.swing.JLabel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("SGMS");
        setUndecorated(true);
        setResizable(false);

        jPanel1.setBackground(new java.awt.Color(0, 0, 0));
        jPanel1.setForeground(new java.awt.Color(255, 255, 255));
        jPanel1.addMouseMotionListener(new java.awt.event.MouseMotionAdapter() {
            public void mouseDragged(java.awt.event.MouseEvent evt) {
                jPanel1MouseDragged(evt);
            }
        });
        jPanel1.addMouseListener(new java.awt.event.MouseAdapter() {
            public void mousePressed(java.awt.event.MouseEvent evt) {
                jPanel1MousePressed(evt);
            }
        });

        jPanel2.setBackground(new java.awt.Color(0, 51, 51));
        jPanel2.setForeground(new java.awt.Color(153, 204, 255));

        lblHeaderText.setBackground(new java.awt.Color(0, 51, 51));
        lblHeaderText.setFont(new java.awt.Font("Arial Black", 1, 36)); // NOI18N
        lblHeaderText.setForeground(new java.awt.Color(102, 102, 255));
        lblHeaderText.setText("SGMS - LOGIN");

        txtUsername.setBackground(new java.awt.Color(0, 0, 0));
        txtUsername.setFont(new java.awt.Font("Tahoma", 1, 18)); // NOI18N
        txtUsername.setForeground(new java.awt.Color(255, 255, 255));
        txtUsername.setText("Username");
        txtUsername.setToolTipText("");
        txtUsername.setBorder(null);
        txtUsername.addMouseListener(new java.awt.event.MouseAdapter() {
            public void mouseEntered(java.awt.event.MouseEvent evt) {
                txtUsernameMouseEntered(evt);
            }
            public void mouseExited(java.awt.event.MouseEvent evt) {
                txtUsernameMouseExited(evt);
            }
        });

        txtPassword.setBackground(new java.awt.Color(0, 0, 0));
        txtPassword.setFont(new java.awt.Font("Tahoma", 1, 18)); // NOI18N
        txtPassword.setForeground(new java.awt.Color(255, 255, 255));
        txtPassword.setText("Password");
        txtPassword.setBorder(null);
        txtPassword.addMouseListener(new java.awt.event.MouseAdapter() {
            public void mouseEntered(java.awt.event.MouseEvent evt) {
                txtPasswordMouseEntered(evt);
            }
            public void mouseExited(java.awt.event.MouseEvent evt) {
                txtPasswordMouseExited(evt);
            }
        });

        lblLoginTypeText.setFont(new java.awt.Font("Arial Black", 0, 18)); // NOI18N
        lblLoginTypeText.setForeground(new java.awt.Color(102, 204, 255));
        lblLoginTypeText.setText("LOGIN TYPE");

        cbType.setBackground(new java.awt.Color(0, 51, 51));
        cbType.setModel(new javax.swing.DefaultComboBoxModel<>(new String[] { "CHOOSE LOGIN TYPE", "Administrator", "Academic Staff", "Administrative Staff", "Student" }));

        btnEnter.setIcon(new javax.swing.ImageIcon("C:\\Users\\sniper\\Documents\\NetBeansProjects\\SDMS\\enter.png")); // NOI18N
        btnEnter.setCursor(new java.awt.Cursor(java.awt.Cursor.HAND_CURSOR));
        btnEnter.addMouseListener(new java.awt.event.MouseAdapter() {
            public void mouseClicked(java.awt.event.MouseEvent evt) {
                btnEnterMouseClicked(evt);
            }
            public void mouseEntered(java.awt.event.MouseEvent evt) {
                btnEnterMouseEntered(evt);
            }
            public void mouseExited(java.awt.event.MouseEvent evt) {
                btnEnterMouseExited(evt);
            }
        });

        javax.swing.GroupLayout jPanel2Layout = new javax.swing.GroupLayout(jPanel2);
        jPanel2.setLayout(jPanel2Layout);
        jPanel2Layout.setHorizontalGroup(
            jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel2Layout.createSequentialGroup()
                .addGap(66, 66, 66)
                .addComponent(lblHeaderText)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
            .addGroup(jPanel2Layout.createSequentialGroup()
                .addGap(25, 25, 25)
                .addGroup(jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addComponent(txtUsername, javax.swing.GroupLayout.DEFAULT_SIZE, 389, Short.MAX_VALUE)
                    .addComponent(txtPassword))
                .addGroup(jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(jPanel2Layout.createSequentialGroup()
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(lblLoginTypeText)
                        .addGap(86, 86, 86))
                    .addGroup(jPanel2Layout.createSequentialGroup()
                        .addGap(104, 104, 104)
                        .addComponent(cbType, javax.swing.GroupLayout.PREFERRED_SIZE, 241, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addContainerGap(26, Short.MAX_VALUE))))
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel2Layout.createSequentialGroup()
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .addComponent(btnEnter)
                .addGap(256, 256, 256))
        );
        jPanel2Layout.setVerticalGroup(
            jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel2Layout.createSequentialGroup()
                .addGap(34, 34, 34)
                .addComponent(lblHeaderText)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addGroup(jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(txtUsername, javax.swing.GroupLayout.PREFERRED_SIZE, 37, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(lblLoginTypeText))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addGroup(jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(txtPassword, javax.swing.GroupLayout.PREFERRED_SIZE, 37, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(cbType, javax.swing.GroupLayout.PREFERRED_SIZE, 34, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 87, Short.MAX_VALUE)
                .addComponent(btnEnter, javax.swing.GroupLayout.PREFERRED_SIZE, 92, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(19, 19, 19))
        );

        lblTopText.setBackground(new java.awt.Color(0, 0, 0));
        lblTopText.setFont(new java.awt.Font("Tahoma", 0, 18)); // NOI18N
        lblTopText.setForeground(new java.awt.Color(255, 255, 255));
        lblTopText.setText("SGMS");

        lblIcon.setBackground(new java.awt.Color(51, 51, 255));
        lblIcon.setFont(new java.awt.Font("Tahoma", 0, 18)); // NOI18N
        lblIcon.setForeground(new java.awt.Color(255, 255, 51));
        lblIcon.setIcon(new javax.swing.ImageIcon("C:\\Users\\sniper\\Documents\\NetBeansProjects\\SDMS\\logo.png")); // NOI18N

        lblExit.setIcon(new javax.swing.ImageIcon("C:\\Users\\sniper\\Documents\\NetBeansProjects\\SDMS\\exit.png")); // NOI18N
        lblExit.setCursor(new java.awt.Cursor(java.awt.Cursor.HAND_CURSOR));
        lblExit.addMouseListener(new java.awt.event.MouseAdapter() {
            public void mouseClicked(java.awt.event.MouseEvent evt) {
                lblExitMouseClicked(evt);
            }
            public void mouseEntered(java.awt.event.MouseEvent evt) {
                lblExitMouseEntered(evt);
            }
            public void mouseExited(java.awt.event.MouseEvent evt) {
                lblExitMouseExited(evt);
            }
        });

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jPanel2, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addComponent(lblIcon)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(lblTopText)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(lblExit)))
                .addContainerGap())
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addGap(6, 6, 6)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(lblTopText)
                    .addComponent(lblIcon)
                    .addComponent(lblExit))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addComponent(jPanel2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        );

        pack();
    }// </editor-fold>                        

    private void lblExitMouseClicked(java.awt.event.MouseEvent evt) {                                     
        // EXIT BUTTON
        System.exit(0);
    }                                    

    private void lblExitMouseEntered(java.awt.event.MouseEvent evt) {                                     
        // MOUSEOVER ICON CHANGE
        lblExit.setIcon(new ImageIcon("exit_hover.png"));
    }                                    

    private void lblExitMouseExited(java.awt.event.MouseEvent evt) {                                    
        // MOUSE EXIT ICON CHANGE
        lblExit.setIcon(new ImageIcon("exit.png"));
    }                                   

    private void jPanel1MouseDragged(java.awt.event.MouseEvent evt) {                                     
        // FOR MOVEMENT OF JFRAME
        int x  = evt.getXOnScreen();
        int y = evt.getYOnScreen();

        this.setLocation(x - xMouse,y - yMouse);      
    }                                    

    private void jPanel1MousePressed(java.awt.event.MouseEvent evt) {                                     
        // Saving mouse to start from frames 0,0 position
        xMouse = evt.getX();
        yMouse = evt.getY();
    }                                    

    private void txtUsernameMouseEntered(java.awt.event.MouseEvent evt) {                                         
        txtUsername.setBackground(Color.GRAY);
    }                                        

    private void txtUsernameMouseExited(java.awt.event.MouseEvent evt) {                                        
        txtUsername.setBackground(Color.BLACK);
    }                                       

    private void txtPasswordMouseEntered(java.awt.event.MouseEvent evt) {                                         
        txtPassword.setBackground(Color.GRAY);
    }                                        

    private void txtPasswordMouseExited(java.awt.event.MouseEvent evt) {                                        
        txtPassword.setBackground(Color.BLACK);
    }                                       

    private void btnEnterMouseEntered(java.awt.event.MouseEvent evt) {                                      
        // Setting Icon on hover
        btnEnter.setIcon(new ImageIcon("enter_hover.png"));
    }                                     

    private void btnEnterMouseExited(java.awt.event.MouseEvent evt) {                                     
        // Setting icon on hover off
        btnEnter.setIcon(new ImageIcon("enter.png"));
    }                                    

    private void btnEnterMouseClicked(java.awt.event.MouseEvent evt) {                                      
        // LOGIN CODE HERE
        String sql = "select * from ROOT.ADMINLOGIN where username=? and password=?";
        try
        {
            PreparedStatement ps = (PreparedStatement) conn.prepareStatement(sql);
            ps.setString(1, txtUsername.getText());
            ps.setString(2, txtPassword.getText());
            ResultSet rs = ps.executeQuery();

            if(rs.next())
            {
                JOptionPane.showMessageDialog(this,"SUCCESSFULL");
            }
            else
            {
                JOptionPane.showMessageDialog(this,"WRONG");
            }
        }
        catch (Exception e)
        {

        }
    }                                     

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(mainFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(mainFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(mainFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(mainFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new mainFrame().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JLabel btnEnter;
    private javax.swing.JComboBox<String> cbType;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JPanel jPanel2;
    private javax.swing.JLabel lblExit;
    private javax.swing.JLabel lblHeaderText;
    private javax.swing.JLabel lblIcon;
    private javax.swing.JLabel lblLoginTypeText;
    private javax.swing.JLabel lblTopText;
    private javax.swing.JPasswordField txtPassword;
    private javax.swing.JTextField txtUsername;
    // End of variables declaration                   
}

代码:DBConnection.java

import com.mysql.jdbc.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 *
 * @author sniper
 */
public class DBConnection {
    private Connection DBConnection;
    public Connection connect()
    {
            try
            {
                Class.forName("com.mysql.jdbc.Driver");
                System.out.println("CONNECTION DONE!");
            }
            catch(ClassNotFoundException cnfe)
            {
                System.out.println("FAILED CONNECTION >> " + cnfe);
            }
            String url = "jdbc:derby://localhost:1527/JavaDB;create=true";
            try{
                DBConnection = (Connection) DriverManager.getConnection(url, "root", "toor");
                System.out.println("Database Connected");
            }
            catch(SQLException se)
            {
                System.out.println("No Database >> " + se);
            }
            return DBConnection;
    }
}

关于如何使其再次运行有什么建议吗？包括图像。

Image 1 Image 2

Answer 1

您正在连接到 Derby，并尝试将其转换为 com.mysql.jdbc.Connection (这是 MySQL 驱动程序的一部分，而不是 Derby)，这当然不会工作。将import com.mysql.jdbc.Connection替换为import java.sql.Connection，import com.mysql.jdbc.PreparedStatement也是如此code>，将其替换为 import java.sql.PreparedStatement。

您几乎不必强制转换为特定于驱动程序的类，而应始终尝试使用 JDBC API 接口(interface)。