EasyMock Delegation: Testing Twitter... without Twitter
So you've decided the pain of maintaining your third-party unit tests have finally outweighed the pain of reworking them into something more predictable, and you've entered into the wild world of mock objects. There's no way around it, really... it was bound to happen. Look, even if your app didn't talk to a third-party service, you'd still want to prevent any possible variation in the environment in which your test is running, outside of the exact thing you're testing, making all things absolutely predictable. This way if your tests fail, you'll know exactly where to look. But why am I telling you this? You are all pros! There are many resources out there for the "hows" and "whys" of mock objects, but if you're already familiar with the concepts and libraries, this post is for you.
This is a post about how to use EasyMock for mocking Twitter, and how you can use Delegation to allow you to accomplish certain things more directly in dealing with complex implementations you'd otherwise have to do yourself.
Disclaimer: We're using EasyMock, But you might also wanna check out jMock and Mockito as other options.
So let's say you've built an app that integrates with a thirdparty API such as Twitter, and so you'll probably want to test your app against specific responses from Twitter to make sure the business logic is performing correctly. For example, let's just say every time Justin Bieber gets retweeted, your app sets off a fireworks show. Now, your unit tests want to test this behavior and you are using EasyMock to mock whatever home grown REST client you've written, returning the mock JSON response you've put together. You set your Expectations and Answers, and voila! You've mocked Twitter!
But then you realize that popular thirdparty Java clients exist for a reason, and instead of going through the process maintaining every single endpoint that Twitter provides, you decide to switch to something like Twitter4J, which makes integrating with Twitter very easy (it really does). So you figure, "I'll just Mock Twitter4J", and it should be easy, right? Wrong. Here's why:
Twitter4J's User Timeline endpoint is defined as:
ResponseList<Status> Twitter.getUserTimeline()
And this is the code you expect to write:
ResponseList<Status> list = ... // Figure out how to construct a ResponseList object with the Statuses you want. Twitter mockTwitter = createMock(Twitter.class);expect(mockTwitter.getUserTimeline(EasyMock.isA(String.class), EasyMock.isA(Paging.class))).andReturn(list);
... but you you find yourself in a rabbit hole. At this point you've just painfully realized that to create the ResponseList object with custom JSON is not exactly trivial and here's why: ResponseList is an interface, and so is Status. Are there implementations? Well, if you follow the Twitter4J source far down enough, you'll find a series of factories that translate the JSON responses from the HttpResponse objects into implementations of the return types we're working with. It's a lot of code to write! If you want to put all the control of the response in your hands, because essentially you're going to have to recreate what's being done under the hood for Twitter4J. So, I've put together some code so you can easily mock Twitter4J's getUserTimeline(), in a way that puts control back in your hands that uses some of EasyMock's tricks, without writing a lot of code. So without further ado:
Step 1: Define Your JSON Response
This is the hard part :) Creating your own custom tweets as JSON Strings is such a pain. So do yourself a favor and run a few calls against Twitter, copy/paste, escape all those quotes, and make any necessary changes via grep.. etc. So let's say in your unit test, you'll want the first Twitter UserTimeline call to returns 30 statuses from Oprah, the second call to return 20 statuses from Justin Bieber, and the last call returns nothing, your String array looks something like:
public static String OPRAH_RESPONSE = "[ { ... // 30 statuses from Oprah public static String JBIEBER_RESPONSE = "[ { ..// 20 statuses from Biebs public static String EMPTY_RESPONSE = "[]" String expectedJsonResponses = new String[]{ OPRAH_RESPONSE, JBIEBER_RESPONSE, EMPTY_RESPONSE };
And let's turn the String array into JSON:
JSONArray array = new JSONArray(twitterRawJSONResponse);
Step 2: Create A Method Which Mocks ResponseList
If you've taken a look at the Twitter4J source, you'll notice that ResultStatus is also an extension of java.util.List and there are a bunch of methods that are already defined by the List interface, and we're going to use EasyMock to delegate those particular methods by ResponseList to an implementation of List that we can more easily create and work with... an ArrayList. Creating an ArrayList, is a lot less verbose than recreating Twitter4J's ResponseList. Go ahead and check out the source, it's rather complex.
This is what EasyMock refers to as Delegation. EasyMock describes delegation as:
" <Delegation allows mock objects> to delegate the call to a concrete implementation of the mocked interface that will then provide the answer. The pros are that the arguments found in EasyMock.getCurrentArguments() for IAnswer are now passed to the method of the concrete implementation. This is refactoring safe. The cons are that you have to provide an implementation which is kind of doing a mock manually... Which is what you try to avoid by using EasyMock. It can also be painful if the interface has many methods."
... however in this case, it works so beautifully! Â Instead of redefining a behavior from scratch, you can just delegate to a similar object that already has the behavior defined, except your delegate is completely in your control.
Another scenario in which you would want to use Delegation could be around a database cursor, for example MongoDB's DBCursor. Â It implements the Iterator interface. Â If you were mocking some database behavior and you had a pre-constructed list of objects you wanted work with, instead of trying to mock the behavior of DBCursor.next() and manually plot out what each subsequent next() would return (because remember, mock objects is a recording that needs to be replayed in a certain order), you can just delegate the behavior to a more easily constructed type that you can work with. Â Â
Let's see it in action and create an ArrayList object and populate it with some tweets!
final List<Status> list = new ArrayList<Status>(); if (array.length() > 0) { for (int x = 0; x < array.length(); x++) { JSONObject obj = (JSONObject) array.get(x); Status status = DataObjectFactory.createStatus(obj.toString()); list.add(status); } }
Now we're going to create a mock of the ResponseList interface and delegate the methods being used by Twitter4J over to our List that we just created:
ResponseList<Status> responseList = EasyMock.createMock(ResponseList.class);
Now you can use Delegation! Since you're mocking the ResponseList and you'll probably want to treat your ResponseList as a regular ArrayList to iterate through your results, all you have to do is simply delegate those method calls to your list object:
EasyMock.expect(responseList.toArray(EasyMock.isA(Status[].class))).andDelegateTo(list).anyTimes();
Your list contains the custom Status objects you've generated from your JSON, and they've been loaded into a list, so all you'd need to do is just delegate the work to ArrayList. Â This way, Â you don't have to write custom code for every single Twitter scenario you want to test. Simply load the JSON into objects, parse them into Status objects, put them in a collection and delegate ResponseList's behavior to that collection.
Let's put all this code into a method called: buildTwitterServiceStatusResponseList(String jsonResponse)
Remember this from above?
String expectedJsonResponses = new String[]{ OPRAH_RESPONSE, JBIEBER_RESPONSE, EMPTY_RESPONSE };
Let's pass those into the above method and create a mock Twitter4J:
Twitter twitter = EasyMock.createMock(Twitter.class); IExpectationSetters<ResponseList<Status>> expectation = EasyMock.expect(twitter.getUserTimeline(EasyMock.isA(String.class), EasyMock.isA(Paging.class))); for (String resp : expectedJsonResponses) { expectation.andReturn(buildTwitterServiceStatusResponseList(resp)).times(1); } EasyMock.replay(twitter);
That's all there really is to it. I've put a gist together where you can see the code in it's entirety.
Remember:Â If your code uses other methods for ResponseList, you'll have to create an expectation for those method calls that you can either Answer, Return, or DelegateTo... Â or EasyMock will fail.
In summary, delegation in EasyMock is useful, if you care about the results that are coming back from your mocked service, and constructing a delegate is trivial compared to actually working with the objects themselves.
At TRAACKR our team spends a LOT of time thinking and implementing the ins and outs, as well as the dos and don'ts of interfacing with third party API's and so we hope to provide engineers with some of the tools of the trade that will hopefully make your lives a little easier.
