java dbcp2 multithreaded connection access

Question

I'm pretty familiar using connection pooling on tomcat & have used it for years without problem. However at the moment I'm working on a main method application that needs to run simultaneous threads for performance reasons, and those threads each need to access the same database. I've gotten my code to work if I strip out database code altogether & just use arrays for test purposes (e.g. multithreading works) however as soon as I add back in database connections, the first thread takes the lock and the other threads don't run at all. Have played with c3p0, and dbcp2; currently working with dbcp2. Thanks! There's tons of documentation out there, but not many code samples that seem specific to my use case. Here's a sample app:

import java.sql.*;
import org.apache.commons.dbcp2.ConnectionFactory;
import org.apache.commons.dbcp2.DriverManagerConnectionFactory;
import org.apache.commons.dbcp2.PoolableConnection;
import org.apache.commons.dbcp2.PoolableConnectionFactory;
import org.apache.commons.dbcp2.PoolingDataSource;
import org.apache.commons.dbcp2.PoolingDriver;
import org.apache.commons.dbcp2.Utils;
import org.apache.commons.pool2.ObjectPool;
import org.apache.commons.pool2.impl.GenericObjectPool;

public class SandboxApp {

    private static BasicDataSource dataSource;

    public static BasicDataSource getDataSource() {

        if (dataSource == null) {

            BasicDataSource ds = new BasicDataSource();

            ds.setUrl("jdbc:mysql://localhost:3306/my-db");
            ds.setUsername("root");
            ds.setPassword("");

            ds.setDriverClassName("org.mariadb.jdbc.Driver");

            ds.setInitialSize(3);
            ds.setMaxTotal(25);
            ds.setMinIdle(0);
            ds.setMaxIdle(8);
            ds.setMaxOpenPreparedStatements(100);

            dataSource = ds;
        }
        return dataSource;
    }

    public static void main(String [] args) throws Exception{   
       for(int i=0; i<11; i++){//spawn 11 threads & get each thread to process 600k sql rows at the same time
              new Thread("" + (i*600000)){

                public void run(){
                    System.out.println("Thread: " + getName() + " running");//prints correctly for all threads

                    Connection con = null;
                    PreparedStatement pstmt = null;
                    ResultSet rs = null;
                try {           
                    con = SandboxApp.getDataSource().getConnection();

                    pstmt = con.prepareStatement("select something from some_table limit "+getName()+",600000");
                    rs=pstmt.executeQuery();
                    while(rs.next()){  
                        System.out.println("Doing stuff for thread "+getName());//this only prints for getName() == 0
                            //give the other threads a turn...
                            try {
                                Thread.sleep(10);
                            }
                            catch(InterruptedException ex) {
                            }
                    }

                  } catch (Exception e) {
                    e.printStackTrace();
                  }finally{
                      try {pstmt.close();} catch (SQLException e) {}
                      try { con.close(); } catch(Exception e) {}
                      try { rs.close(); } catch(Exception e) {}
                  }
                }
              }.start();
        }
    }
}

Answer 1

@user207421 was right, that the getDataSource() method should be synchronized & of course I'd already tried this; however this still didn't solve for my problem of thread "0" not letting the other threads take a turn.

I stripped back everything from my code, all other libraries etc.. until I got it to work, and then started building it back up again to find the breaking point. It seems that the main determining factor is the size of the ResultSet. I tried adding in extra thread.sleep time in various places however the only thing that has worked is breaking down the queries to request smaller ResultSets.

600k result sets, only 1 thread will run, 1k ResultSets and 4 threads will run. With ResultSets containing only 100 rows, all 11 threads will run. Note, I was testing this on a 16 CPU system with 8GB of memory allocated to the JVM (aws m5.4xlarge), so hardware resources shouldn't have been a contributing factor. So I guess I'll just have to break my code into smaller chunks.

When I was initially looking into this I was surprised as to the lack of a specific code samples for this specific problem (irrespective of ResultSet size & number of threads), so I'm just posting here what finally worked for me for the sake of a complete code sample:

    import java.sql.*;
    import org.apache.commons.dbcp2.BasicDataSource;    

    public class SandboxApp {

        private static BasicDataSource dataSource;

        public static synchronized BasicDataSource getDataSource() {

            if (dataSource == null) {

                BasicDataSource ds = new BasicDataSource();

                ds.setUrl("jdbc:mysql://localhost:3306/my-db");
                ds.setUsername("root");
                ds.setPassword("");

                ds.setDriverClassName("org.mariadb.jdbc.Driver");

                ds.setInitialSize(3);
                ds.setMaxTotal(25);
                ds.setMinIdle(0);
                ds.setMaxIdle(8);
                ds.setMaxOpenPreparedStatements(100);

                dataSource = ds;
            }
            return dataSource;
        }

        public static void main(String [] args) throws Exception{   
           for(int i=0; i<11; i++){//spawn 11 threads & get each thread to process 100 sql rows at the same time
                  new Thread("" + (i*100)){

                    public void run(){
                        System.out.println("Thread: " + getName() + " running");

                        Connection con = null;
                        PreparedStatement pstmt = null;
                        ResultSet rs = null;
                    try {           
                        con = SandboxApp.getDataSource().getConnection();

                        pstmt = con.prepareStatement("select something from some_table limit "+getName()+",100");
                        rs=pstmt.executeQuery();
                        while(rs.next()){  
                            System.out.println("Doing stuff for thread "+getName());//With smaller ResultSet, this works fine for all 11 threads
                                //give the other threads a turn...
                                try {
                                    Thread.sleep(10);
                                }
                                catch(InterruptedException ex) {
                                }
                        }

                      } catch (Exception e) {
                        e.printStackTrace();
                      }finally{
                          try {pstmt.close();} catch (SQLException e) {}
                          try { con.close(); } catch(Exception e) {}
                          try { rs.close(); } catch(Exception e) {}
                      }
                    }
                  }.start();
            }
        }
    }