@Override
protected RoutesBuilder createRouteBuilder() {
    return new RouteBuilder() {
        @Override
        public void configure() {
            from("direct:start").routeId("myRoute")
                    .to("direct:send")
                    .end();

            from("direct:send")
                    .log("message")
                    .to("cxfrs:http://localhost:" + port1
                        + "/rest/helloservice/sayHello?synchronous=false"); // setting to 'true' resolves the issue

            restConfiguration()
                    .port(port1);

            rest("/rest/helloservice")
                    .post("/sayHello").routeId("rest-GET-say-hi")
                    .to("direct:sayHi");

            from("direct:sayHi")
                    .routeId("mock-GET-say-hi")
                    .log("example")
                    .to("mock:end");
}};