code: @Override
      public String getExternalIdentifier() {
          try {
              LOGGER.debug("getExternalIdentifier for property {} ", property().getName());
              return property().getValue().getString();
          } catch (final RepositoryException e) {
              throw new RepositoryRuntimeException(e);
          }
      }
docs: /*
  (non-Javadoc)
  @see org.fcrepo.kernel.api.utils.CacheEntry#getExternalIdentifier()