How to check if user has already been registered in the database?

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How can I check if a user is already registered? For example, when registering a user check if the login has already been saved in the database. DAO looked like this: 

public boolean validarLogin (String login) throws SQLException, ClassNotFoundException {
    boolean existe = false;
    Pessoa p =null;
    PreparedStatement st=null;
    this.conn=new ConnectionFactory().getConnectionFactory();
    String sql = ("SELECT login FROM pessoa WHERE login = ?");
    ResultSet rs = st.executeQuery();
    while(rs.next()){
        String loginBanco = rs.getString("login");
        if (loginBanco.equals("login")){
            existe = true;
            st.close();
        }
        else {
            JOptionPane.showMessageDialog(null, "Usuário" + login + "existente");
        }
    }
    return existe;
}

What do you think, is that right?

    
asked by anonymous 15.02.2018 / 20:20

2 answers

1

Use PreparedStatment when your query have "?" 

PreparedStatement statement =  this.conn.prepareStatement("SELECT login FROM pessoa WHERE login = ?");    
statement.setString(1, login);    
ResultSet rs = statement.executeQuery();

In your code, you are already using, but you are trying to execute the query on a null variable.

    
15.02.2018 / 20:34
0

Your code has some problems. First, do not mix% view logic with database logic.

Since your JOptionPane class is this question , it have serious connection management problems. I'll fix it based on what I did on this other answer here and also : 

package dao;
import java.sql.DriverManager;
import java.sql.SQLException;

public final class ParametrosDeConexao {
    private final String url;
    private final String usuario;
    private final String senha;

    public ParametrosDeConexao(String url, String usuario, String senha) {
        this.url = url;
        this.usuario = usuario;
        this.senha = senha;
    }

    public Connection conectar() throws SQLException {
        return DriverManager.getConnection(url, usuario, senha);
    }
}

With this, your code will look like the one below. Note that the ConnectionFactory field represents what your CONECTAR class was: 

private static final ParametrosDeConexao CONECTAR =
        new ParametrosDeConexao( 
                "jdbc:postgresql://localhost:5432/topografiaJava",
                "postgres",
                "1");

private static final String LOGIN_SQL =
        "SELECT login FROM pessoa WHERE login = ?";

public boolean validarLogin(String login) throws SQLException {
    try (
        Connection c = CONECTAR.conectar();
        PreparedStatement ps = c.prepareStatement(LOGIN_SQL)
    ) {
        ps.setString(1, login);
        try (ResultSet rs = st.executeQuery()) {
            return rs.next();
        }
    }
}

Notice how the ConnectionFactory method has become quite lean. To do this:

  • Remember to use try-with-resources . Unless you are working with Java 6 or earlier, NEVER stop using it with validarLogin(String) , Connection or PreparedStatement .

  • Remember to fill in the parameters of ResultSet properly (with PreparedStatement in this case.

  • The setString(1, login) will give you the return rs.next(); or true you want depending on whether or not there is false . In your case, it is unnecessary to read this result.

  • % w / w% is no longer required. See more about this here .

15.02.2018 / 20:57
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