Java Connection Pool implementation

Question

can you look at my connection pool if it is a possible way to implement it?

public class ConnectionPool {
    private static List<DBConnection> pool = null;
    private static int available = 0;
    private ConnectionPool() {}

    public static DBConnection getConnection() {
        if (pool == null) {
             pool = new ArrayList<DBConnection>();
             for (int i = 0; i < 3; i++) {
                 try {
                    pool.add(new DBConnection());
                    available++;
                } catch (SQLException e) {
                    e.printStackTrace();
                }
             }
        }
        if (pool.size() > 0) {
            available--;
            return pool.remove(available);
        } else {
            return null;
        }
    }

    public static void returnConnection(DBConnection c) {
        pool.add(c);
        available++;
    }
}

I'm using only one array and the client should ask the connection pool for a connection use it and then return it to the connection pool.

  Connection connection = ConnectionPool.getConnection();
  connection.execute("insert into users values('"+user.getUsername()+"', '"+user.getPassword()+"', '"+user.getGroup()+"', '"+banned+"', '"+broker+admin+sharehodler+company+"')");      
  ConnectionPool.returnConnection(connection);
  connection = null;

Please I need feedback on this implementation. Thank you

Answer 1

There are some points that make this implementation very problematic.

• Thread safety. What if several threads work with the pool? You are not locking the list on read/write access.
• Static maximum pool size of 3 connections, also you immediately create all of them, whether they are needed or not. A common strategy is to create a bunch of connections and create some more when needed, until the allowed/configured maximum is reached.
• You only have static methods. It should be possible to have several pools, meaning you need instances of ConnectionPool.
• No way to pass host+dbname+user+password to the connections that are created.
• You don't deal with 'broken' connections - you may have to re-create a connection if an existing one screwed up. This is far more relevant than I thought before I started using pools.
• Use config values instead of static values, see point #2
• Lastly: sure, it's interesting to write this stuff yourself - but if you need a pool for a project, pick an existing one, such as c3p0 or the tomcat connection pool.

I'm sure there's more to point out, but unless these are fixed, there's no use in continuing.