與MySQL的Java空指針異常

Question

我已經做了一個與MySQL的數據庫連接日食。當執行我的代碼時，我得到空指針異常錯誤。當我點擊添加按鈕在數據庫中添加新記錄時，會發生這種情況。請幫助

import javax.swing.JFrame; 

import java.awt.Color; 
import java.awt.EventQueue; 
import java.awt.Font; 
import java.awt.GridLayout; 

import javax.swing.BorderFactory; 
import javax.swing.ButtonGroup; 
import javax.swing.ImageIcon; 
import javax.swing.JFrame; 
import javax.swing.JOptionPane; 
import javax.swing.JPanel; 
import javax.swing.JLabel; 
import javax.swing.JTabbedPane; 
import javax.swing.border.TitledBorder; 

import org.eclipse.wb.swing.FocusTraversalOnArray; 

import com.toedter.calendar.JDateChooser; 

import java.awt.Component; 
import java.awt.BorderLayout; 
import java.awt.event.ActionEvent; 
import java.awt.event.ActionListener; 
import java.sql.Connection; 
import java.sql.DriverManager; 
import java.sql.PreparedStatement; 
import java.sql.Statement; 

import javax.swing.JCheckBox; 
import javax.swing.JButton; 
import javax.swing.JTable; 
import javax.swing.JTextField; 
import javax.swing.JComboBox; 
import javax.swing.JRadioButton; 
import javax.swing.DefaultComboBoxModel; 

public class PassF extends JFrame{ 

private JTextField fname; 
private JTextField lname; 
private JTextField address; 
private JTextField resId; 
private JComboBox ori, des; 
private JButton btnAdd, btnUpdate, btnDelete, btnClear; 
private JRadioButton premium, economy, business; 
private JDateChooser dt; 



private Connection con; 


String url = "jdbc:mysql://localhost:3306/"; 
String user = "root"; 
String password = "root"; 


public PassF(){ 
    super("Passenger Details"); 
    setBounds(100, 100, 792, 631); 
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 

    setLayout(null); 

    JPanel panel = new JPanel(); 
    panel.setBounds(0, 0, 831, 78); 
    add(panel); 

    JLabel l1 = new JLabel(); 

    l1.setIcon(new ImageIcon("C:\\Users\\Yetin\\workspace\\Assignment\\src\\passenger.png")); 
    panel.setBackground(Color.WHITE); 
    panel.add(l1); 

    JLabel lblFirstName = new JLabel("First Name "); 
    lblFirstName.setForeground(new Color(25, 25, 112)); 
    lblFirstName.setFont(new Font("Candara", Font.BOLD, 15)); 
    lblFirstName.setBounds(34, 132, 95, 23); 
    add(lblFirstName); 

    JLabel lblLastName = new JLabel("Last Name "); 
    lblLastName.setForeground(new Color(25, 25, 112)); 
    lblLastName.setFont(new Font("Candara", Font.BOLD, 15)); 
    lblLastName.setBounds(316, 132, 80, 23); 
    add(lblLastName); 

    fname = new JTextField(); 
    setBounds(239, 132, 126, 22); 
    add(fname); 
    fname.setColumns(10); 

    lname = new JTextField(); 
    lname.setBounds(432, 132, 126, 22); 
    add(lname); 
    lname.setColumns(10); 

    JLabel lblAddress = new JLabel("Address"); 
    lblAddress.setForeground(new Color(25, 25, 112)); 
    lblAddress.setFont(new Font("Candara", Font.BOLD, 15)); 
    lblAddress.setBounds(34, 177, 95, 23); 
    add(lblAddress); 

    address = new JTextField(); 
    address.setBounds(139, 178, 126, 56); 
    add(address); 
    address.setColumns(10); 

    JLabel lblReservationId = new JLabel("Reservation ID"); 
    lblReservationId.setForeground(new Color(25, 25, 112)); 
    lblReservationId.setFont(new Font("Candara", Font.BOLD, 15)); 
    lblReservationId.setBounds(316, 173, 107, 31); 
    add(lblReservationId); 

    resId = new JTextField(); 
    resId.setBounds(432, 178, 126, 22); 
    add(resId); 
    resId.setColumns(10); 

    JLabel origin= new JLabel("Origin"); 
    origin.setForeground(new Color(25, 25, 112)); 
    origin.setFont(new Font("Candara", Font.BOLD, 15)); 
    origin.setBounds(34, 273, 95, 23); 
    add(origin); 

    JLabel lblDestination = new JLabel("Destination"); 
    lblDestination.setForeground(new Color(25, 25, 112)); 
    lblDestination.setFont(new Font("Candara", Font.BOLD, 15)); 
    lblDestination.setBounds(34, 316, 95, 23); 
    add(lblDestination); 

    ori = new JComboBox(); 
    ori.setModel(new DefaultComboBoxModel(new String[] {"Afghanistan", "Albania", "Algeria", "Andorra", "Angola", "Bangladesh", "Barbados", "Belarus", "Belgium", "Brazil", "China", "Egypt", "France", "Germany", "Hong Kong", "India", "Maldives", "Madagascar", "Mauritius", "Nigeria ", "Russia", "Seychelles", "Singapore ", "United Arab Emirates"})); 
    ori.setBounds(139, 273, 126, 22); 
    add(ori); 

    des = new JComboBox(); 
    des.setModel(new DefaultComboBoxModel(new String[] {"Afghanistan", "Albania", "Algeria", "Andorra", "Angola", "Bangladesh", "Barbados", "Belarus", "Belgium", "Brazil", "China", "Egypt", "France", "Germany", "Hong Kong", "India", "Maldives", "Madagascar", "Mauritius", "Nigeria ", "Russia", "Seychelles", "Singapore ", "United Arab Emirates"})); 
    des.setBounds(139, 317, 126, 22); 
    add(des); 

    JLabel clas = new JLabel(); 
    clas.setForeground(new Color(25, 25, 112)); 
    clas.setBounds(316, 231, 246, 132); 
    clas.setBorder(BorderFactory.createTitledBorder(null, "Class", TitledBorder.LEFT, TitledBorder.TOP, new Font("Candara", Font.BOLD, 15), new Color(25, 25, 112))); 
    add(clas); 

    ButtonGroup group = new ButtonGroup(); 

    premium = new JRadioButton("Premium"); 
    premium.setBounds(338, 257, 109, 23); 
    premium.setForeground(new Color(25, 25, 112)); 
    premium.setFont(new Font("Candara", Font.BOLD, 15)); 
    group.add(premium); 
    add(premium); 

    economy = new JRadioButton("Economy"); 
    economy.setBounds(338, 291, 109, 23); 
    add(economy); 
    economy.setForeground(new Color(25, 25, 112)); 
    economy.setFont(new Font("Candara", Font.BOLD, 15)); 
    group.add(economy); 
    add(economy); 

    business = new JRadioButton("Business"); 
    business.setBounds(338, 324, 109, 23); 
    business.setForeground(new Color(25, 25, 112)); 
    business.setFont(new Font("Candara", Font.BOLD, 15)); 
    group.add(business); 
    add(business); 

    JLabel lblJourneyDate = new JLabel("Journey Date"); 
    lblJourneyDate.setForeground(new Color(25, 25, 112)); 
    lblJourneyDate.setFont(new Font("Candara", Font.BOLD, 15)); 
    lblJourneyDate.setBounds(34, 90, 95, 22); 
    add(lblJourneyDate); 



    btnAdd = new JButton(); 
    btnAdd.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent e) { 
      insert(); 
     } 
    }); 

    btnAdd.setIcon(new ImageIcon("C:\\Users\\Yetin\\workspace\\Assignment\\src\\Icons\\add.jpg")); 
    btnAdd.setForeground(new Color(34, 139, 34)); 
    btnAdd.setFont(new Font("Copperplate Gothic Bold", Font.BOLD, 15)); 
    btnAdd.setBounds(647, 143, 85, 31); 
    add(btnAdd); 


    JButton btnSave = new JButton(); 
    btnSave.setIcon(new ImageIcon("C:\\Users\\Yetin\\workspace\\Assignment\\src\\Icons\\clear.jpg")); 
    btnSave.setForeground(new Color(34, 139, 34)); 
    btnSave.setFont(new Font("Copperplate Gothic Bold", Font.BOLD, 15)); 
    btnSave.setBounds(647, 203, 85, 31); 
    add(btnSave); 

    JButton btnUpdate = new JButton(); 
    btnUpdate.setIcon(new ImageIcon("C:\\Users\\Yetin\\workspace\\Assignment\\src\\Icons\\update.jpg")); 
    btnUpdate.setForeground(new Color(34, 139, 34)); 
    btnUpdate.setFont(new Font("Copperplate Gothic Bold", Font.BOLD, 15)); 
    btnUpdate.setBounds(647, 257, 85, 31); 
    add(btnUpdate); 

    JButton btnDelete = new JButton(""); 
    btnDelete.setIcon(new ImageIcon("C:\\Users\\Yetin\\workspace\\Assignment\\src\\Icons\\delete.jpg")); 
    btnDelete.setForeground(new Color(34, 139, 34)); 
    btnDelete.setFont(new Font("Copperplate Gothic Bold", Font.BOLD, 15)); 
    btnDelete.setBounds(647, 317, 80, 31); 
    add(btnDelete); 

    dt = new JDateChooser(); 
    dt.setBounds(139, 89, 126, 23); 
    add(dt); 


    JLabel l2 = new JLabel(); 
    l2.setBounds(0, 75, 831, 547); 
    add(l2); 
    l2.setIcon(new ImageIcon("C:\\Users\\Yetin\\workspace\\Assignment\\src\\world.jpg")); 




} 




    public void insert(){ 
     try{ 
     Class.forName("com.mysql.jdbc.Driver").newInstance(); 
     con = DriverManager.getConnection(url, user, password); 

     System.out.println("Database connection succesful to: " + url); 

     }catch(Exception e){ 
      System.out.println("Error in connecting"); 
     } 


     try{ 
     PreparedStatement stmt = con.prepareStatement("insert into passenger" 
        + " (ReservID, JourneyDate, FirstName, LastName, Address, ClassType, Origin, Destination)" 
        + " values (?, ?, ?, ?, ?, ?, ?, ?"); 

      //set parameters 

      stmt.setString(1, resId.getText()); 
      stmt.setString(2, dt.getDateFormatString()); 
      stmt.setString(3, fname.getText()); 
      stmt.setString(4, lname.getText()); 
      stmt.setString(5, address.getText()); 
      stmt.setString(6, premium.getText()); 
      stmt.setString(7, ori.getSelectedItem().toString()); 
      stmt.setString(8, des.getSelectedItem().toString()); 

      //execute SQL 

      stmt.executeUpdate(); 

     }catch(Exception e){ 
       System.out.println("ERROR"); 
      } 
    } 
} 

Answer 1

所以我通過閱讀你的代碼..你的問題是在這裏發現..

try{ 
     Class.forName("com.mysql.jdbc.Driver").newInstance(); 
     con = DriverManager.getConnection(url, user, password); 

     System.out.println("Database connection succesful to: " + url); 

     }catch(Exception e){ 
      System.out.println("Error in connecting"); 
     } 

所以它吞噬你不重新拋出你的異常。 st然後是空的。將其更改爲

catch(Exception exp){ 
    throw new Exception(exp); 
} 

然後您將得到確切的原因或jdbc連接存在的問題。可能遺漏了罐子，錯誤的路徑或認證問題..也許還有其他事情，但你需要先知道原因..
編輯： 我認爲它可能是打印「連接錯誤」.. 也沒有意義如果連接無法建立，繼續使用PreparedStatement。

Answer 2

ImageIcon icon=new ImageIcon(getClass().getResource("path Name of image")); 
    buttonobj.setIcon(icon);