code: public static Driver getInstance(String instanceName) {
          if (instanceName == null) {
              instanceName = DEFAULT_INSTANCE_NAME;
          }
          if (instances.getInstances().isEmpty()) {
              throw new ConfigurationException("Driver has not been configured and driver.properties file was not found");
          }
          Driver instance = instances.getInstances().get(instanceName);
          if (instance == null) {
              throw new ConfigurationException("No configuration properties found for factory : " + instanceName);
          }
          return instance;
      }
docs: Retrieves the default instance of this class that is configured according to the properties file
  (driver.properties).
  
  @param instanceName
  The name of the instance (corresponding to the prefix in the driver.properties file)
  
  @return the named instance