@Override
    public Optional<Object> invoke(Function<InvokerContext, Object> invokeFunc, Function<Throwable, Object> exFunc,
            String serviceName) {
        return invoke(invokeFunc, exFunc, serviceName, getRetry(null));
    }
@Test
    public void invokeWithNoInstances() {
        Object exResult = new Object();
        final Function<InvokerContext, Object> invokerFunc = invokerContext -> null;
        final Function<Throwable, Object> exFunc = ex -> exResult;
        Optional<Object> invoke = retryService.invoke(invokerFunc, exFunc, serviceName);
        Assert.assertFalse(invoke.isPresent());
    }