Tidak boleh memanggil 'java.sql.Connection.prepareStatement(String)' kerana 'this.conexao' adalah batal

Question

Saya cuba melakukan tindakan dalam skrin lain dari satu skrin tetapi setiap kali saya mengklik butang log masuk saya mendapat ralat ini -----> java.lang.NullPointerException: Tidak boleh memanggil "java.sql. Connection.prepareStatement(String)” kerana “this.conexao” kosong

Berikut ialah kod kelas

package br.com.mycompany.telas;

import java.sql.*;
import javax.swing.*;

public class TelaLogin extends javax.swing.JFrame {

    Connection conexao = null;
    PreparedStatement stm = null;
    ResultSet rs = null;

    public void logar() {
        String sql = "select * from usuarios where login=? and senha= ?";

        try {
            stm = conexao.prepareStatement(sql);
            stm.setString(1, txtUsuario.getText());
            stm.setString(1, txtSenha.getText());
            rs = stm.executeQuery();

            if (rs.next()) {
                TelaPrincipal principal = new TelaPrincipal();
                principal.setVisible(true);
            } else {
                JOptionPane.showMessageDialog(null, "deu ruim");
            }
        } catch (Exception e) {
            System.out.println(e);
        }
    }

    public TelaLogin() {
        initComponents();
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        jLabel1 = new javax.swing.JLabel();
        jLabel2 = new javax.swing.JLabel();
        txtUsuario = new javax.swing.JTextField();
        txtSenha = new javax.swing.JPasswordField();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jLabel1.setText("Usuario");

        jLabel2.setText("Senha");

        jButton1.setText("Login");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(146, 146, 146)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addComponent(jLabel2, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addComponent(jLabel1, javax.swing.GroupLayout.DEFAULT_SIZE, 58, Short.MAX_VALUE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addComponent(txtUsuario)
                    .addComponent(txtSenha, javax.swing.GroupLayout.DEFAULT_SIZE, 270, Short.MAX_VALUE))
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(283, Short.MAX_VALUE)
                .addComponent(jButton1)
                .addGap(258, 258, 258))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(87, 87, 87)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel1)
                    .addComponent(txtUsuario, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addGap(58, 58, 58)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel2)
                    .addComponent(txtSenha, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 53, Short.MAX_VALUE)
                .addComponent(jButton1)
                .addGap(49, 49, 49))
        );

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

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
        logar();
    }                                        

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new TelaLogin().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JPasswordField txtSenha;
    private javax.swing.JTextField txtUsuario;
    // End of variables declaration                   
}

Answer 1

Anda harus memulakan sambungan anda jika tidak ia akan sentiasa kosong.

Connection conexao = null;
String dbms = "mysql"; // or other
String serverName = "localhost"; // or the name/IP of database server
String portNumber = "3306";
String dbName = "database_name";
String userName = "userName";
String password = "password";

// in your TelaLogin constructor you can try to establish a connection
Properties connectionProps = new Properties();
connectionProps.put("user", this.userName);
connectionProps.put("password", this.password);
conexao = DriverManager.getConnection(
               "jdbc:" + this.dbms + "://" +
               this.serverName +
               ":" + this.portNumber +
               "/" + this.dbName,
               connectionProps);

Selain itu, jangan lupa untuk menambah kebergantungan pelanggan bagi pangkalan data yang diperlukan dalam perpustakaan.