Question

I wrote a Singleton class for getting connections. However, I cannot get connections using Singleton.

I want to use my Singleton for getting several connections and to make query to database, using the connection Singleton. I always try several ways for this, and no success.

This is my Singleton class:

import java.sql.*;

public class ConnectDB {

private static Connection connect;
private static ConnectDB instance;

private ConnectDB()
{

    try {

        Class.forName("com.mysql.jdbc.Driver");
        //connect DB
        connect = DriverManager.getConnection("jdbc:mysql://ip/database","root","password");

    }

    catch(SQLException e)
    {
        System.err.println(e.getMessage());

    }

    catch(ClassNotFoundException e)
    {

        System.err.println(e.getMessage());

    }   
}

  public static ConnectDB getInstance()
  {

      if(instance == null) {

          instance = new ConnectDB();

      }

      return instance;

  }

}

Now, I get the connections:

 public class NameClass {




public void getInfoDatabase()
{   

       Connection cnn = ConnectDB.getConnection();
       PreparedStatement ps;
       ResultSet rs;

       try {

           ps = cnn.prepareStatement("select *  from tables");

           rs = ps.executeQuery();

           while(rs.next())
           {

               String tables = rs.getString("table1");
               System.out.println(tables);
           }

Answer 1

If you want to use multiple connections efficiently you might be after a connection pool:

In software engineering, a connection pool is a cache of database
connections maintained so that the connections can be reused when
future requests to the database are required. Connection pools are
used to enhance the performance of executing commands on a database.

Having a Singleton which will return many connections defies the purpose of the Singleton, whose task is to provide the same instance whenever it is called.

I recommend you take a look at this previous SO thread where various connection pool libraries are discussed.