code: @PerformanceSensitive
      public Class<?> getCallerClass(final String fqcn, final String pkg) {
          boolean next = false;
          Class<?> clazz;
          for (int i = 2; null != (clazz = getCallerClass(i)); i++) {
              if (fqcn.equals(clazz.getName())) {
                  next = true;
                  continue;
              }
              if (next && clazz.getName().startsWith(pkg)) {
                  return clazz;
              }
          }
          // TODO: return Object.class
          return null;
      }
docs: migrated from Log4jLoggerFactory