java应用程序查询数据库返回nullpointerException

Question

我的 java 代码有问题。它应该将数据插入数据库，但它返回一个空指针异常。这是我的代码。

package client;
import java.sql.*;
import java.awt.Desktop;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.*;
import java.util.*;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.JLabel;
import javax.swing.JComboBox;
import javax.swing.JButton;
import javax.swing.JFileChooser;
import javax.swing.JOptionPane;

public class AddCon {


    private Connection conn;
private JFrame frmAddContract;
private JTextField ContractNo;
private JTextField EngagerName;
private JTextField contNo;
    private JTextField month;
    private JTextField day;
private JTextField year;
private JTextField tme;
private JTextField cladd;
private JTextField eventadd;
private JTextField textField_fp;
    private JTextField refer;


/**
 * Launch the application.
 */
public static void main(String[] args) {



        EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                AddCon window = new AddCon();
                window.frmAddContract.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

/**
 * Create the application.
 */
public AddCon() {

        try{
           //Load database driver and connect
                Class.forName("com.mysql.jdbc.Driver");
                Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/KusinaNiKambal","root","1234");
                Statement stmt = null;
                stmt = conn.createStatement();}

        catch (Exception e){
                JOptionPane.showMessageDialog(null, e);}

/**
 * Initialize the contents of the frame.
 */
    initialize();
}
private void initialize() {
    frmAddContract = new JFrame();
    frmAddContract.setResizable(false);
    frmAddContract.setTitle("Add Contract");
    frmAddContract.setBounds(100, 100, 450, 640);
    frmAddContract.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frmAddContract.getContentPane().setLayout(null);

    ContractNo = new JTextField();
    ContractNo.setBounds(38, 32, 152, 20);
    frmAddContract.getContentPane().add(ContractNo);
    ContractNo.setColumns(10);

    JLabel lblContract = new JLabel("Contract #");
    lblContract.setBounds(38, 18, 76, 14);
    frmAddContract.getContentPane().add(lblContract);

    JLabel lblEngager = new JLabel("Engager");
    lblEngager.setBounds(38, 63, 48, 14);
    frmAddContract.getContentPane().add(lblEngager);

    EngagerName = new JTextField();
    EngagerName.setBounds(38, 78, 152, 20);
    frmAddContract.getContentPane().add(EngagerName);
    EngagerName.setColumns(10);

    contNo = new JTextField();
    contNo.setBounds(38, 128, 152, 20);
    frmAddContract.getContentPane().add(contNo);
    contNo.setColumns(10);

    JLabel lblNewLabel = new JLabel("Contact #");
    lblNewLabel.setBounds(38, 109, 86, 14);
    frmAddContract.getContentPane().add(lblNewLabel);

    month = new JTextField();
    month.setBounds(38, 255, 116, 20);
    frmAddContract.getContentPane().add(month);

    day = new JTextField();
    day.setBounds(181, 255, 81, 20);
    frmAddContract.getContentPane().add(day);

    year = new JTextField();
    year.setBounds(294, 255, 86, 20);
    frmAddContract.getContentPane().add(year);
    year.setColumns(10);

    JLabel lblMonth = new JLabel("Month in number format");
    lblMonth.setBounds(38, 236, 100, 14);
    frmAddContract.getContentPane().add(lblMonth);

    JLabel lblDay = new JLabel("Day");
    lblDay.setBounds(181, 236, 48, 14);
    frmAddContract.getContentPane().add(lblDay);

    JLabel lblYear = new JLabel("Year");
    lblYear.setBounds(294, 236, 48, 14);
    frmAddContract.getContentPane().add(lblYear);

    JLabel lblEventDate = new JLabel("Event Date");
    lblEventDate.setBounds(38, 222, 76, 14);
    frmAddContract.getContentPane().add(lblEventDate);

    tme = new JTextField();
    tme.setBounds(38, 307, 59, 20);
    frmAddContract.getContentPane().add(tme);
    tme.setColumns(10);

    cladd = new JTextField();
    cladd.setBounds(38, 180, 363, 31);
    frmAddContract.getContentPane().add(cladd);
    cladd.setColumns(10);

    JLabel lblAddress = new JLabel("Client Address");
    lblAddress.setBounds(38, 159, 98, 14);
    frmAddContract.getContentPane().add(lblAddress);

    JLabel lblTime = new JLabel("Time");
    lblTime.setBounds(38, 286, 60, 14);
    frmAddContract.getContentPane().add(lblTime);



    eventadd = new JTextField();
    eventadd.setBounds(38, 358, 373, 50);
    frmAddContract.getContentPane().add(eventadd);
    eventadd.setColumns(10);

    JLabel lblEventAddress = new JLabel("Event Address");
    lblEventAddress.setBounds(38, 338, 116, 14);
    frmAddContract.getContentPane().add(lblEventAddress);

    textField_fp = new JTextField();
    textField_fp.setBounds(38, 454, 276, 20);
    frmAddContract.getContentPane().add(textField_fp);
    textField_fp.setColumns(10);

    JLabel lblFilepathOfContract = new JLabel("Filepath of  Contract - the PDF file");
    lblFilepathOfContract.setBounds(38, 431, 116, 14);
    frmAddContract.getContentPane().add(lblFilepathOfContract);

    JButton btnBrowse = new JButton("Browse...");
    btnBrowse.setBounds(324, 453, 89, 23);
    frmAddContract.getContentPane().add(btnBrowse);
            btnBrowse.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
                        final JFileChooser fc = new JFileChooser();
                        int returnVal = fc.showOpenDialog(null);
                        String filepath = fc.getSelectedFile().getAbsolutePath();
                        textField_fp.setText(filepath);
                    }
    });

    refer = new JTextField();
    refer.setBounds(38, 510, 276, 20);
    frmAddContract.getContentPane().add(refer);
    refer.setColumns(10);

    JLabel lblEncodedBy = new JLabel("Referred By");
    lblEncodedBy.setBounds(38, 485, 116, 14);
    frmAddContract.getContentPane().add(lblEncodedBy);




    JButton btnAdd = new JButton("Add");
    btnAdd.setBounds(101, 560, 89, 23);
    frmAddContract.getContentPane().add(btnAdd);
            btnAdd.addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent ae) {

            try{

                String cntr = ContractNo.getText();
                String en = EngagerName.getText();
                String cont = contNo.getText();
                String mo = month.getText();
                String d = day.getText();
                String yr = year.getText();
                String dte = yr + "-" + mo + "-" + d;
                String cla = cladd.getText();
                String tm = tme.getText() + ":00";
                String evadd = eventadd.getText();
                String filepath = textField_fp.getText();
                String ref = refer.getText();


                String SQL = "insert into cis "+
                    "values ('"+cntr+"','"+en+"','"+
                    cont+"','"+cla+"' '"+dte+"','"+tm+"','"+
                    evadd+"','"+filepath+"','"+ref+")";
                PreparedStatement stmt = conn.prepareStatement(SQL);
                ResultSet rs = stmt.executeQuery();

            }
            catch(Exception e){
                JOptionPane.showMessageDialog(null, e);
            }
                }


            });

    JButton btnCancel = new JButton("Cancel");
    btnCancel.setBounds(225, 560, 89, 23);
    frmAddContract.getContentPane().add(btnCancel);
             btnCancel.addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent ae) {
                     frmAddContract.setVisible(false);
                }



    });
}
}

抱歉，我在java编程方面并不是很有经验，这是我第一次编写查询数据库的代码。

Answer 1

您永远不会初始化您的 conn 实例变量。这将导致这一行(至少)出现 NullPointerException:

PreparedStatement stmt = conn.prepareStatement(SQL);
                         ^------------ NEVER INITIALIZED

<小时/>

public class AddCon {
    private Connection conn;

...

// constructor
public AddCon() {
    try{
        //Load database driver and connect
        Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection(<your parameters>);
        ^----------- MISTAKE
...

在上面的代码中，您创建了一个连接，但没有将其分配给实例变量，而是使用了局部变量。将该行替换为:

this.conn = DriverManager.getConnection(<your parameters>);

编辑:请始终发布堆栈跟踪，以防出现异常!