Set<UserId> userIds = request.getUsers();
String msgCollId = request.getParameter("msgCollId");
List<String> messageIds = request.getListParameter("messageIds");
CollectionOptions options = new CollectionOptions(request);
HandlerPreconditions.requireNotEmpty(userIds, "No userId specified");
HandlerPreconditions.requireSingular(userIds, "Multiple userIds not supported");
UserId user = request.getUsers().iterator().next();