code: public static void configure(String xmlConfig, File voltroot) {
          try {
              Class<?> loggerClz = Class.forName("org.voltcore.logging.VoltLog4jLogger");
              assert(loggerClz != null);
              Method configureMethod = loggerClz.getMethod("configure", String.class, File.class);
              configureMethod.invoke(null, xmlConfig, voltroot);
          } catch (Exception e) {}
      }
docs: Static method to change the Log4j config globally. This fails
  if you're not using Log4j for now.
  @param xmlConfig The text of a Log4j config file.
  @param voltroot The VoltDB root path