java.lang.NullPointerException creating statement from MySQL query

Question

I am trying to check username and pwd from a MySQL DB, the method CheckFromDB() calls methods from AccessoDB class, in order to establish a connection with the DB.

I get that the problem shows in the moment I crate the statement in the AccessoDB.selectFixedQuery() method.

Here is the code I used:

   public boolean checkFromDB(){
    AccessoDB accesso = new AccessoDB();
    accesso.openConnectionToDB();
    ResultSet rs = accesso.selectFixedQuery("SELECT * FROM user WHERE nome = "+this.username+" && pwd = "+this.password);
    String uname_db = "", pwd_db = "";
    try {
        while (rs.next())
        {
            uname_db=rs.getString("nome");
            System.out.println(uname_db);
            pwd_db = rs.getString("pwd");
        }
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    accesso.closeConnectionToDB();
    boolean res = false;
    res = uname_db.equals(this.username)&& pwd_db.equals(this.password);
    return res;
}

Method selectFixedQuery in class AccessoDB:

    public ResultSet selectFixedQuery(String p_sqlQuery) {

    // istantiate a resultSet
    ResultSet rs = null;
    Statement st = null;
    try {

        // create statement associated to the query
        st = dbConnection.createStatement();

        // esegue la query
        rs = st.executeQuery(p_sqlQuery); 

    } catch (SQLException e) {
        Logger.println("ANALISI DELL'ERRORE");
        Logger.println(e.getMessage());
        Logger.println("FINE ANALISI DELL'ERRORE");
    }

    // ritorna il risultato
    return rs;
} // end method

    public void openConnectionToDB() {

    try {
        // allocazione del driver (antico)
        Class.forName(dbDriverName).newInstance();

        // creazione della connessione
        dbConnection = DriverManager.getConnection(connectionString, 
                                                    dbUsername, 
                                                    dbPassword);                    
    }
    // serve solo se si usa il Driver
    catch (ClassNotFoundException cnfe) {
        Logger.println("Driver not found!");
        Logger.println(cnfe.getMessage());
    } 
    catch (SQLException sqle) {
        Logger.println("Conessione al DB non riuscita!");
        Logger.println(sqle.getMessage());
    }
    catch (Exception e) {
        // gestire errore insolito
        Logger.println(e.getMessage());
    }
} // end method

I added openConnectionToDB(), sorry what is stack trace? Error message? — Argentina, Apr 18 '16 at 09:22
once u caught the exception, you can prince the stack trace as below. e.printStackTrace(); — Janath, Apr 18 '16 at 09:26
also you can print and see below variable has proper values. this.username , this.password — Janath, Apr 18 '16 at 09:34
ok done, no problems with this.username and this.password, and it prints nothing with e.printStackTrace(), i've put it in both try/catch blocks — Argentina, Apr 18 '16 at 09:43

KP. · Accepted Answer · 2016-04-18T09:27:19.337

0

openConnectionToDB() should return dbConnection object, right now your connection object is lost in the method calls. Store it and pass it back to your selectFixedQuery() method

edited Apr 18 '16 at 09:27

answered Apr 18 '16 at 09:23

KP.

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1
12

nope. because it use the same accesso object. – Janath Apr 18 '16 at 09:26
he probably used a global class variable not showed in the code – Gianni B. Apr 18 '16 at 09:26

java.lang.NullPointerException creating statement from MySQL query

1 Answers1