how to request data from our spring boot micro-service to external server for a given id's? - spring-boot

Question

Currently I have created 2 microservices and and getting the data from one service to another using RestTemplate.
Microservice -1:
import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;
import java.util.ArrayList;
import java.util.List;
#RestController
public class StringDataController {
List<String> stringList = new ArrayList<>();
#RequestMapping(value = "/securities/list", method = RequestMethod.GET, produces = MediaType.APPLICATION_JSON_VALUE)
public List<String> sendStringData(){
stringList.add("12345");
stringList.add("23435");
stringList.add("23436");
return stringList;
}
}
Microservice-2:
import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;
import java.util.List;
#RestController
#EnableAutoConfiguration
public class ExternalRequestController {
#Value ("${sampleMS1.uri}")
String sampleMS1URI;
#RequestMapping(value="/listdata", method = RequestMethod.GET, produces = MediaType.APPLICATION_JSON_VALUE)
public void receiveStringFromAnotherMS(){
List<String> list = null;
list = new RestTemplate().getForObject(sampleMS1URI,List.class);
System.out.println(list.toString());
System.out.println("-->"+list);
}
}
Now, I have to send the List(String)(list of ID's) data to some external server and response should get Map(K,V) ==> key as a String and Value as a Double.
Note: The external server is not handling by us, so we can only request the data with List of id's and then they should sending the response with price data of specific id's.
Can anyone suggest me a way to do it ?
I am new to Spring & Spring boot.
Thank you!