„Java.sql.Connection.prepareStatement(String)' kann nicht aufgerufen werden, da „this.conexao' null ist

Question

Ich habe versucht, von einem Bildschirm aus Aktionen in einem anderen Bildschirm auszuführen, aber wenn ich auf die Anmeldeschaltfläche klicke, erhalte ich diese Fehlermeldung -----> java.lang.NullPointerException: „java.sql. Connection.prepareStatement(String)“ kann nicht aufgerufen werden. weil „this.conexao“ leer ist

Das Folgende ist der Klassencode

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

您应该初始化您的连接，否则它将始终为空。

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);

另外，不要忘记在库中添加所需数据库的客户端依赖项。