TestMethod로 정의된 메서드는 웹 서비스 콜아웃을 지원하지 않습니다.
오류가 발생한 예
@isTest
private class PostTest {
    @isTest
    static void test() {
        Test.startTest();
        Service.execute();
        Test.stopTest();
    }
}public with sharing class Service {
  @future(callout=true)
  public static void execute() {
    ServiceC sc = new ServiceC();
    sc.post();
   }
}public with sharing class ServiceC {
  private HttpResponse post() {
    HttpRequest request = new HttpRequest();
    (생략)
    HttpResponse response = new Http().send(request);
    return response;
  }
}오류 해결 예
@isTest
private class PostTest {
    @isTest
    static void test() {
        Test.startTest();
        Test.setMock(HttpCalloutMock.class, new ServiceHttpCalloutMock());
        Service.execute();
        Test.stopTest();
    }
}@isTest
public class ServiceHttpCalloutMock implements HttpCalloutMock {
  public HTTPResponse respond(HTTPRequest req) {
    HttpResponse res = new HttpResponse();
    res.setStatusCode(200);
    res.setBody('test');
    return res;
  }
}