Trying to query an sql statement from a java program.

[rand Kirby]

I am trying to execute a query statement from my java program, but whenever I run it I get a nullpointerexeption on the Statement stmt = DBConnect.con.createStatement();. Does anyone know how to fix this?

public static void QueryDatabase() throws SQLException {
        /*Creates a query to the database by establishing the connection created earlier
         * then adds all elements in the database to a hashmap that is used in the gui
         */
        
        Statement stmt = DBConnect.con.createStatement();
        String query = "SELECT username, password FROM jgarofol_maxkirb.users;";
        ResultSet rs = stmt.executeQuery(query);
  
       while (rs.next()) {
            String user_name = rs.getString("username");
            String pass_word = rs.getString("password");
            hmap.put(user_name, pass_word);
        }
        if (stmt != null) { stmt.close(); }
    }
  
public static void main(String[] args)
    {
        try {
            QueryDatabase();
            System.out.print("sql query works");
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
  
Exception in thread "main" java.lang.NullPointerException
    at test.database_interact.QueryDatabase(database_interact.java:16)
    at test.database_interact.main(database_interact.java:31)

[Stephan van Hulst]

Well, I don't know what DBConnect is, but either it, or DBConnect.con is null. I can't really tell you more unless you say what DBConnect is.

[rand Kirby]

This is DBConnect

public class DBConnect {

	static Connection con = null;
	static HashMap<String, String> hmap = new HashMap<String, String>();
	
	[code=java]public static void main(String[] args)
	{
		//Creates initial connection to the mySQL server
		String url = "jdbc:mysql://***.***.**:1111/******";
		String user = "root";
		String password = "root";
		
		try {
			con = DriverManager.getConnection(url, user, password);
			System.out.print("Connecteddddddd");
		} catch (SQLException e) {
			e.printStackTrace();
			System.out.println( e.getMessage( ) );
		}
	}

[/code]

[Stephan van Hulst]

Well there you go. You initialize con to null, and never set it to anything else. Remember that only one main() method is run when you start your application.

This is not a good design though. You shouldn't have one static connection that is accessible by the entire application. Instead, classes that need to interact with a database should have a constructor that takes a DataSource. The class can then open a new or pooled connection at the moment they need it:

public final class UserRepository {

  private final DataSource dataSource;

  public UserRepository(DataSource dataSource) {
    if (dataSource == null)
      throw new IllegalArgumentException();

    this.dataSource = dataSource;
  }

  public Map<String, User> getUsers() {
    try (
      Connection connection = dataSource.getConnection();
      Statement statement = connection.createStatement();
      ResultSet resultSet = statement.executeQuery("SELECT userName FROM Users");
    ) {
      Map<String, User> users = new LinkedHashMap<>();

      while (resultSet.next()) {
        String userName = resultSet.getString("userName");
        users.put(userName, new User(userName));
      }

      return users;
    }

    catch (SQLException ex) {
      throw new RepositoryException("Could not retrieve the users.", ex);
    }
  }
}

Note also that you probably shouldn't get passwords hashes out of your database until you're going to authenticate a specific user.