I'm having trouble trying to list data from a mysql table

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I'm having trouble trying to list data from a mysql table

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#1 by (3 votes)
#2 by (2 votes)

3

Manufacturing Class

import java.sql.Connection;
import java.sql.DriverManager;


public class Fabrica {

    public  Connection getConexao() {
        Connection conn = null;
        try {
            Class.forName("com.mysql.jdbc.Driver");

            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/teste", "root", "123");

        } catch (Exception e) {
            e.printStackTrace();
        }
        return conn;
    }

}

AlunoDAO Class

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.logging.Level;
import java.util.logging.Logger;

public class AlunoDAO {

private Connection conn;

    public void insert(String nome, int matricula) {
        String sql = "insert into aluno values (?,?)";
        conn = new Fabrica().getConexao();

        try {

            PreparedStatement ps = conn.prepareStatement(sql);
            ps.setString(1, nome);
            ps.setInt(2, matricula);          
            ps.execute();
            ps.close();

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                conn.close();
            } catch (SQLException ex) {
               Logger.getLogger(AlunoDAO.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }

    public void select() {
        String sql = "select * from aluno";

        try {


            PreparedStatement ps = this.conn.prepareStatement(sql); //linha 41

            ResultSet result = ps.executeQuery();
            while(result.next()) {
                System.out.println(result.getString("nome"));
                System.out.println(result.getInt("matricula"));
            }              
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

MAIN Class

public class Principal {
    public static void main(String[] args) {      
        AlunoDAO dao = new AlunoDAO();


        dao.select(); //linha 10
    }
}

ERROR:   java.lang.NullPointerException

at AlunoDAO.select (AlunoDAO.java:41)

at Principal.main (Principal.java:10)

mysql java jdbc

asked by anonymous 19.09.2017 / 22:45

2 answers

2

Complementing the response of @rray, if you do not want to have to pass the connection every time you instantiate a Student you can do this:

Factory :

public class Fabrica {

    public static Connection getConexao() {
        static Connection conn = null;
        try {
            Class.forName("com.mysql.jdbc.Driver");

            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/teste", "root", "123");

        } catch (Exception e) {
            e.printStackTrace();
        }
        return conn;
    }
}

AlunoDAO (Constructor) You can then use conn in all AlunoDAO methods.

public class AlunoDAO {
  private Connection conn;
  public void AlunoDAO(){
    conn = Fabrica.getConexao();
  }

Call :

AlunoDAO dao = new AlunoDAO();

19.09.2017 / 23:03

Remove line break after each INSERT Capture content to be pasted

score 3 · Accepted Answer

The most correct would be to initialize conn in the constructor of your DAO. This code does not work because it ( conn ) is null. It is only instantiated in insert() method but not select() .

Once the connection has already been created you can remove the other points where you did it manually. Always prefer to capture the more specific exception and not more generic. I suggest reading some tag exception

see the insert () section:

public void insert(String nome, int matricula) {
    String sql = "insert into aluno values (?,?)";
    conn = new Fabrica().getConexao(); //<--Aqui, pega a conexão o que não acontece no select

Solution:

public class AlunoDAO {
  private Connection conn;
  public void AlunoDAO(Connection conn){
    conn = conn;
  }

The call should be:

AlunoDAO dao = new AlunoDAO(new Fabrica().getConexao());