public void start(BrokerService service) {
        handlers.values().forEach(handler -> handler.start(service));
    }
@Test
    public void testStart() {
        BrokerService service = mock(BrokerService.class);
        handlers.start(service);
        verify(handler1, times(1)).start(same(service));
        verify(handler2, times(1)).start(same(service));
    }