how to mock outbound gateways in spring integration

add a bean id to the handler as,

@Bean
    public IntegrationFlow sendSms() {
        final ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.configure(MapperFeature.ACCEPT_CASE_INSENSITIVE_PROPERTIES, true);
        final JsonObjectMapper<?, ?> mapper = new Jackson2JsonObjectMapper(objectMapper);
        return IntegrationFlows.from("sendSmsGatewayRequestChannel")
                .handle(Http.outboundGateway(smsEndpoint).expectedResponseType(String.class).httpMethod(HttpMethod.GET)
                        .uriVariableExpressions(createExpressionMap()), **e -> e.id("smsOutboundGateway")**)
                .transform(Transformers.fromJson(SmsResponseDto.class, mapper)).get();
    }

then you can mock the response by,

final ArgumentCaptor<Message<?>> messageArgumentCaptor = ArgumentCaptor.forClass(Message.class);
        final MessageHandler mockMessageHandler = MockIntegration.mockMessageHandler(messageArgumentCaptor)
                .handleNextAndReply(m -> MessageBuilder.withPayload(
                        "response")
                        .build());
        mockIntegrationContext.substituteMessageHandlerFor("smsOutboundGateway", mockMessageHandler);

Note: I am new to spring integration. so i don't know whether this is the correct way to do it. anyways, it's working :D. correct if i'm wrong. :)

Thanks Artem Bilan for the help.