code: protected void prepareInternal(Map conf, String overrideBase, Configuration hadoopConf) {
          this.conf = conf;
          if (overrideBase == null) {
              overrideBase = (String)conf.get(Config.BLOBSTORE_DIR);
          }
          if (overrideBase == null) {
              throw new RuntimeException("You must specify a blobstore directory for HDFS to use!");
          }
          LOG.debug("directory is: {}", overrideBase);
  //        try {
  //            // if a HDFS keytab/principal have been supplied login, otherwise assume they are
  //            // logged in already or running insecure HDFS.
  //            String principal = (String) conf.get(Config.BLOBSTORE_HDFS_PRINCIPAL);
  //            String keyTab = (String) conf.get(Config.BLOBSTORE_HDFS_KEYTAB);
  //
  //            if (principal != null && keyTab != null) {
  //                UserGroupInformation.loginUserFromKeytab(principal, keyTab);
  //            } else {
  //                if (principal == null && keyTab != null) {
  //                    throw new RuntimeException("You must specify an HDFS principal to go with the keytab!");
  //
  //                } else {
  //                    if (principal != null && keyTab == null) {
  //                        throw new RuntimeException("You must specify HDFS keytab go with the principal!");
  //                    }
  //                }
  //            }
  //        } catch (IOException e) {
  //            throw new RuntimeException("Error logging in from keytab!", e);
  //        }
          Path baseDir = new Path(overrideBase, BASE_BLOBS_DIR_NAME);
          try {
              if (hadoopConf != null) {
                  _hbs = new HdfsBlobStoreImpl(baseDir, conf, hadoopConf);
              } else {
                  _hbs = new HdfsBlobStoreImpl(baseDir, conf);
              }
          } catch (IOException e) {
              throw new RuntimeException(e);
          }
  //        _localSubject = getHadoopUser();
      }
docs: Allow a Hadoop Configuration to be passed for testing. If it's null then the hadoop configs
  must be in your classpath.