Mocking Httpcalls always return NullPointerException - scala

Question

I'm trying to mock Http calls for unit test.
To do that I have done the following, I have created a RequestMock case class:
case class RequestMock() {
def sendRequest(httpRequest: HttpRequest)(implicit actorSystem: ActorSystem): Future[HttpResponse] = {
Http().singleRequest(httpRequest)
}
}
and in my service, I have written the following piece of code :
case class Service(requestHandler: RequestMock) {
....
for {
response <- {
requestHandler.sendRequest(
HttpRequest(
method = HttpMethods.GET,
uri = "http://database:9000"
)
)
} yield {
response
}
}
For the unit test, I'm trying to mock HttpCalls, to do that, I have done the following :
def test_2 = mock[RequestMock]
And for defining the mock behaviour I have done the following
when(test_2.sendRequest(
HttpRequest(
method = HttpMethods.GET,
uri = "http://database:9000")
)).thenReturn{
Future(
HttpResponse(
StatusCodes.OK,
entity = HttpEntity(ContentTypes.`text/plain(UTF-8)`,"connection established"))
But, when I execute unit tests, I always get the following error:
java.lang.NullPointerException
Does anyone know how I can solve this issue ?

Answer 1

A couple of problems.
First of all, test_2 should be a val, not a def.
With def like you have it, you get a different instance every time you access it. So, you define the stub on one instance, but then create your Service with a different one, that does not have sendRequest defined, so returns null by default, and that causes your NPE.
The next problem, that you will probably encounter after you fix this one is that you are not defining all of the behavior.
when(test_2.sendRequest(
HttpRequest(
method = HttpMethods.GET,
uri = "http://database:9000")
))
Only creates a stub for a method call with this specific parameter value. So, if your tests try to make a POST for example or hit a different endpoint, you'll get an NPE again.
Even if you only ever use one request, it is better to define the stub for any argument, to avoid weird NPE failures if the code happens to send a different one (you are writing a test, so should not just assume automatically, that the code will always do what you expect - you would not need the test in the first place if that was the case):
when(test2.sendRequest(any)),thenReturn(Future.successful(...))
(Note Future.successful above – that's the correct way to create Future that is immediately satisfied, what you are doing makes it run on a thread ... not a big deal in your case, but still icky).
Then, after the test code is run, you can check that the parameter value passed to the sendRequest was actually what you expect:
verify(test2)
.sendRequest(HttpRequest(method = HttpMethods.GET, uri = "http://database:9000"))