docs: @Nullable
    @SuppressWarnings("LiteralClassName")
    private static Throwable initJndi() {
      if (GrpcUtil.IS_RESTRICTED_APPENGINE) {
        return new UnsupportedOperationException(
            "Currently running in an AppEngine restricted environment");
      }
      try {
        Class.forName("javax.naming.directory.InitialDirContext");
        Class.forName("com.sun.jndi.dns.DnsContextFactory");
      } catch (ClassNotFoundException e) {
        return e;
      } catch (RuntimeException e) {
        return e;
      } catch (Error e) {
        return e;
      }
      return null;
    }