code: static Class<? extends JournalManager> getJournalClass(Configuration conf,
        String uriScheme) {
      String key = "dfs.name.edits.journal-plugin" + "." + uriScheme;
      Class<? extends JournalManager> clazz = null;
      try {
        clazz = conf.getClass(key, null, JournalManager.class);
      } catch (RuntimeException re) {
        throw new IllegalArgumentException("Invalid class specified for "
            + uriScheme, re);
      }
  
      if (clazz == null) {
        LOG.warn("No class configured for " + uriScheme + ", " + key
            + " is empty");
        throw new IllegalArgumentException("No class configured for " + uriScheme);
      }
      return clazz;
    }
docs: Retrieve the implementation class for a Journal scheme.
  @param conf The configuration to retrieve the information from
  @param uriScheme The uri scheme to look up.
  @return the class of the journal implementation
  @throws IllegalArgumentException if no class is configured for uri