public static AuditActor user(@Nonnull String username) {
        if (isNullOrEmpty(username)) {
            throw new IllegalArgumentException("username must not be null or empty");
        }
        return new AutoValue_AuditActor(URN_GRAYLOG_USER + username);
    }
@Test
    public void testUser() {
        final AuditActor actor = AuditActor.user("jane");

        assertThat(actor.urn()).isEqualTo("urn:graylog:user:jane");
    }