How to use mocks in controller tests

Even since I started to write tests for my Grails application I couldn’t find many articles on using mocks. Everyone is talking about tests and TDD but if you search for it there isn’t many articles.

Today I want to share with you a test with mocks for a simple and complete scenario. I have a simple application that can fetch Twitter tweets and present it to user. I use REST service and I use GET to fetch tweets by id like this: http://api.twitter.com/1/statuses/show/236024636775735296.json. You can copy and paste it into your browser to see a result.

My application uses Grails 2.1 with spock-0.6 for tests. I have TwitterReaderService that fetches tweets by id, then I parse a response into my Tweet class.


class TwitterReaderService {
Tweet readTweet(String id) throws TwitterError {
try {
String jsonBody = callTwitter(id)
Tweet parsedTweet = parseBody(jsonBody)
return parsedTweet
} catch (Throwable t) {
throw new TwitterError(t)
}
}

private String callTwitter(String id) {
// TODO: implementation
}

private Tweet parseBody(String jsonBody) {
// TODO: implementation
}
}

class Tweet {
String id
String userId
String username
String text
Date createdAt
}

class TwitterError extends RuntimeException {}

TwitterController plays main part here. Users call show action along with id of a tweet. This action is my subject under test. I’ve implemented some basic functionality. It’s easier to focus on it while writing tests.


class TwitterController {
def twitterReaderService

def index() {
}

def show() {
Tweet tweet = twitterReaderService.readTweet(params.id)
if (tweet == null) {
flash.message = 'Tweet not found'
redirect(action: 'index')
return
}

[tweet: tweet]
}
}

Let’s start writing a test from scratch. Most important thing here is that I use mock for my TwitterReaderService. I do not construct new TwitterReaderService(), because in this test I test only TwitterController. I am not interested in injected service. I know how this service is supposed to work and I am not interested in internals. So before every test I inject a twitterReaderServiceMock into controller:


import grails.test.mixin.TestFor
import spock.lang.Specification

@TestFor(TwitterController)
class TwitterControllerSpec extends Specification {
TwitterReaderService twitterReaderServiceMock = Mock(TwitterReaderService)

def setup() {
controller.twitterReaderService = twitterReaderServiceMock
}
}

Now it’s time to think what scenarios I need to test. This line from TwitterReaderService is the most important:


Tweet readTweet(String id) throws TwitterError

You must think of this method like a black box right now. You know nothing of internals from controller’s point of view. You’re only interested what can be returned for you:

  • a TwitterError can be thrown
  • null can be returned
  • Tweet instance can be returned

This list is your test blueprint. Now answer a simple question for each element: “What do I want my controller to do in this situation?” and you have plan test:

  • show action should redirect to index if TwitterError is thrown and inform about error
  • show action should redirect to index and inform if tweet is not found
  • show action should show found tweet

That was easy and straightforward! And now is the best part: we use twitterReaderServiceMock to mock each of these three scenarios!

In Spock there is a good documentation about interaction with mocks. You declare what methods are called, how many times, what parameters are given and what should be returned. Remember a black box? Mock is your black box with detailed instruction, e.g.: I expect you that if receive exactly one call to readTweet with parameter ‘1’ then you should throw me a TwitterError. Rephrase this sentence out loud and look at this:


1 * twitterReaderServiceMock.readTweet('1') >> { throw new TwitterError() }

This is a valid interaction definition on mock! It’s that easy! Here is a complete test that fails for now:


import grails.test.mixin.TestFor
import spock.lang.Specification

@TestFor(TwitterController)
class TwitterControllerSpec extends Specification {
TwitterReaderService twitterReaderServiceMock = Mock(TwitterReaderService)

def setup() {
controller.twitterReaderService = twitterReaderServiceMock
}

def "show should redirect to index if TwitterError is thrown"() {
given:
controller.params.id = '1'
when:
controller.show()
then:
1 * twitterReaderServiceMock.readTweet('1') >> { throw new TwitterError() }
0 * _._
flash.message == 'There was an error on fetching your tweet'
response.redirectUrl == '/twitter/index'
}
}


| Failure: show should redirect to index if TwitterError is thrown(pl.refaktor.twitter.TwitterControllerSpec)
| pl.refaktor.twitter.TwitterError
at pl.refaktor.twitter.TwitterControllerSpec.show should redirect to index if TwitterError is thrown_closure1(TwitterControllerSpec.groovy:29)

You may notice 0 * _._ notation. It says: I don’t want any other mocks or any other methods called. Fail this test if something is called! It’s a good practice to ensure that there are no more interactions than you want.

Ok, now I need to implement controller logic to handle TwitterError.


class TwitterController {

def twitterReaderService

def index() {
}

def show() {
Tweet tweet

try {
tweet = twitterReaderService.readTweet(params.id)
} catch (TwitterError e) {
log.error(e)
flash.message = 'There was an error on fetching your tweet'
redirect(action: 'index')
return
}

[tweet: tweet]
}
}

My tests passes! We have two scenarios left. Rule stays the same: TwitterReaderService returns something and we test against it. So this line is the heart of each test, change only returned values after >>:


1 * twitterReaderServiceMock.readTweet('1') >> { throw new TwitterError() }

Here is a complete test for three scenarios and controller that passes it.


import grails.test.mixin.TestFor
import spock.lang.Specification

@TestFor(TwitterController)
class TwitterControllerSpec extends Specification {

TwitterReaderService twitterReaderServiceMock = Mock(TwitterReaderService)

def setup() {
controller.twitterReaderService = twitterReaderServiceMock
}

def "show should redirect to index if TwitterError is thrown"() {
given:
controller.params.id = '1'
when:
controller.show()
then:
1 * twitterReaderServiceMock.readTweet('1') >> { throw new TwitterError() }
0 * _._
flash.message == 'There was an error on fetching your tweet'
response.redirectUrl == '/twitter/index'
}

def "show should inform about not found tweet"() {
given:
controller.params.id = '1'
when:
controller.show()
then:
1 * twitterReaderServiceMock.readTweet('1') >> null
0 * _._
flash.message == 'Tweet not found'
response.redirectUrl == '/twitter/index'
}


def "show should show found tweet"() {
given:
controller.params.id = '1'
when:
controller.show()
then:
1 * twitterReaderServiceMock.readTweet('1') >> new Tweet()
0 * _._
flash.message == null
response.status == 200
}
}


class TwitterController {

def twitterReaderService

def index() {
}

def show() {
Tweet tweet

try {
tweet = twitterReaderService.readTweet(params.id)
} catch (TwitterError e) {
log.error(e)
flash.message = 'There was an error on fetching your tweet'
redirect(action: 'index')
return
}

if (tweet == null) {
flash.message = 'Tweet not found'
redirect(action: 'index')
return
}

[tweet: tweet]
}
}

The most important thing here is that we’ve tested controller-service interaction without logic implementation in service! That’s why mock technique is so useful. It decouples your dependencies and let you focus on exactly one subject under test. Happy testing!