public class DataSource{
	private DataSource _data_source;
	
	private DataSource{}
 
	public static DataSource getDataSource(){
		//to make this threadsafe, synchronize should be added
		if(_data_source==null){
			//assuming rdbms has been set in the System properties.
			String rdbms = System.getProperty("rdbms");
 
			if(rdbms.equals("oracle"){
				_data_source = new OracleDataSource();
			}else if(rdbms.equals("mySQL"){
				_data_source = new MySQLDataSource();
			}else if(rdbms.equals("MSSQLServer"){
				_data_source = new MSSQLServerDataSource();
			}else{
				throw new IllegalStateException("rdbms was not found in system properties");
			}
		}
 
		return _data_source;
	}
}