Doubts when applying a DAO method

Question

Well I have 2 classes the User class and the Request class

my user class is like this:

public class Usuario {
    private static Usuario instance;
    private Long id;
    private String login;
    private String senha;

    public Usuario(Long id, String login, String senha){
    this.id=id;
    this.login=login;
    this.senha=senha;
    }

    public Usuario(String login, String senha){
    this.login=login;
    this.senha=senha;
    }
    public Usuario(Long id){
    this.id = id;
    }
   public static Usuario getInstance() {
        if (instance == null)
            instance = new Usuario();
        return instance;
    }

I created a constructor to store the ID, first doubt how I could apply this to my login validation method to save the ID: my validation method:

public void validaLogin(){
    UsuarioDAO dao = new UsuarioDAO();
    List<Usuario> usuarios = dao.getList();
    Usuario usuario = Usuario.getInstance();

    for(int x = 0; x< usuarios.size(); x++){
        if(jLogin.getText().equals(usuarios.get(x).getLogin()) && jSenha.getText().equals(usuarios.get(x).getSenha())){
            Principal pr = new Principal ();
            x = usuarios.size();
            fecha();
            try {
                pr.start(new Stage());
            } catch (Exception ex) {
                Logger.getLogger(Login.class.getName()).log(Level.SEVERE, null, ex);
            }
        } else{
            if(x == usuarios.size()-1){
                Alert al = new Alert(Alert.AlertType.ERROR);
                al.setHeaderText("Login Invalido");
                al.show();
            }
        }
    }
}

the User:

public List<Usuario> getList(){
   List<Usuario> usuarios = new ArrayList<>();
   String sql = "SELECT * FROM usuario";
    try {
        PreparedStatement stmt = con.prepareStatement(sql);
        ResultSet rs = stmt.executeQuery();
        while(rs.next()){
            Usuario u = new Usuario();
            u.setId(rs.getLong("id_usuario"));
            u.setLogin(rs.getString("login"));
            u.setSenha(rs.getString("senha"));
            usuarios.add(u);
        }
        stmt.close();
        rs.close();
        con.close();  
    } catch (SQLException ex) {
        return null;
    }
    return usuarios;
}

I wanted you to keep the id on all the Trials that are opened as I would get it?

Answer 1

Usually the login validation is done in the database, it is not a good practice to store in your memory a list of users with login and password not encrypted.

I’d do it this way:

String sql = "SELECT id_usuario, nome FROM usuario where login = ? and senha = ?";
try {
    PreparedStatement stmt = con.prepareStatement(sql);

    ps.setString(1, login);
    ps.setString(2, criptografar(senha)); // método qualquer de criptografia

    ResultSet rs = stmt.executeQuery();
    if (rs.next()){
        Usuario usuarioLogado = new Usuario(rs.getLong("id_usuario"));
        usuarioLogado.setNome(rs.getString("nome")); // vai exibir mesmo o login?
        return usuarioLogado;
    }
    stmt.close();
    rs.close();
    con.close();  
} catch (SQLException ex) {
    return null;
}
return null;

If this information is to be on all screens it may be the case to use a global variable in the main class. (Main.userLogado.getID())