AsyncServlet5.java

@Resource UserTransaction ut;

@Override
protected void doGet(HttpServletRequest request, final HttpServletResponse response) throws ServletException, IOException {

    ut.begin();	
...
    CompletableFuture<String> cf = new CompletableFuture<>();
    service.foo(cf, pw);
...
    cf.whenCompleteAsync((s, t)->{
		...
        ut.commit(); // => exception: "BaseTransaction.commit - ARJUNA016074: no transaction!"
    });
}