code: public static void registerLogger(final Class<? extends LoggerFactory> factoryClass) {
          if (loggerFactory == null) {
              FACTORIES.add(0, factoryClass.getName());
          } else {
              throw new IllegalStateException("LoggerImplFactory must be registered before logging is initialized.");
          }
      }
docs: Register a LoggerFactory; last one registered is used.
  
  @param factoryClass the factory class