Spring Integration - passing json parameters to HttpRequestExecutingMessageHandler

I want to pass json parameters in the URL of rest API with SI - HttpRequestExecutingMessageHandler.

This is the Controller ---

@RestController
@RequestMapping("/api")
public class NotificationController {

    @Autowired
    NotificationGateway notificationGateway;
    
    
    @PostMapping
    @RequestMapping("/sendmsg")
    public String sendMsg(@RequestBody SMSData smsData) {
        return notificationGateway.sendMsg(smsData);
    }
}

This is the Gateway ---

@MessagingGateway
public interface NotificationGateway {


    @Gateway(requestChannel = "httpOutRequest")
    public String sendMsg(SMSData smsData);
}

This is the Service ---

@Configuration
public class HTTPConfig {

    private final Logger log = LoggerFactory.getLogger(HTTPConfig.class);

    @Bean
    @ServiceActivator(inputChannel = "httpOutRequest")
    public MessageHandler outbound() {
        log.info("Inside HTTPConfig :: outbound() invoked ");
        HttpRequestExecutingMessageHandler handler = new HttpRequestExecutingMessageHandler(
                "http://localhost:9090/Gateway/rest?msg={msg}&mobileNo={mobileNo}", restTemplate());
        handler.setHttpMethod(HttpMethod.GET);
        handler.setExpectedResponseType(String.class);
        handler.setExpectReply(true);

        ExpressionParser expressionParser = new SpelExpressionParser();
        handler.setUriVariableExpressions(
                Collections.singletonMap("msg", expressionParser.parseExpression("headers['msg']")));
        handler.setUriVariableExpressions(
                Collections.singletonMap("mobileNo", expressionParser.parseExpression("headers['mobileNo']")));
        return handler;
    }

    @Bean
    public RestTemplateBuilder restTemplateBuilder() {
        log.info("Inside HTTPConfig :: restTemplateBuilder() invoked ");
        return new RestTemplateBuilder();
    }

    @Bean
    public RestTemplate restTemplate() {
        log.info("Inside HTTPConfig :: restTemplate() invoked ");
        RestTemplate restTemplate = new RestTemplate();
        return restTemplate;
    }

}

This is the DTO---

public class SMSData {

    public String msg;
    public String mobileNo; 
    
}

Please help with how to pass parameters in outbound() JSON --- {"msg" : "This is my message", "mobileNo" : "00123456987"}

Your contract is this public String sendMsg(SMSData smsData);, but then you try to pass into request params from headers: headers['msg'] and headers['mobileNo']. If you say that your SMSData contains all the info, then you need to make those expressions against payload: payload['msg'] and payload['mobileNo'], respectively.