Java Null指针异常与MySQL

Question

我已经完成了与mysql的数据库连接以使其日食。 执行代码时，出现空指针异常错误。 当我单击“添加”按钮以在数据库中添加新记录时，会发生这种情况。 请帮忙

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

所以我通过阅读您的代码发现了..您的问题在这里..

 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");
        }

您不会抛出异常，因此可以将其吞下。 st为空。 更改为

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

那么您将得到确切的原因或jdbc连接存在的问题。 可能缺少广口瓶，错误的路径或身份验证问题..也可能有其他问题，但您需要首先知道原因。
编辑：我认为它可能正在打印“连接错误”。如果连接建立失败，也没有必要继续进行PreparedStatement。

Answer 2

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