Are mocks just stubs by another name, or something more?

[An older article that I published internally to a client community, reprinted with changes to remove client references]

An opinion that I've run into among some of my clients is that mock objects are just what we've always called stubs, and that it's an old approach. It's actually a quite common perspective - one I have held myself for part of my career. In fact early mock object approaches were very much like "fake implementations", but modern mocks are different. While they can both be considered "test doubles" or "test stand-ins", stubbed out interfaces or provide expected data for the system under test, mock objects provide behavioural expectation. These terminologies can be confusing, but we can sort that out.

jMock and EasyMock are two examples of mock object frameworks which allow for the specification of behaviour. jMock uses a domain specific language (DSL) such that you code the expectations in a fairly verbal syntax (often called a fluent interface). Something along the lines of

myMock.expects(methodCall(SOME_METHOD)
.which().returns(A_VALUE))
.then().expects(methodCall(OTHER_METHOD))

... which should be vaguely like english to the initiated. EasyMock, on the other hand, uses a "proxy/record/replay" approach instead, which some find easier. The point is that they both define a set of expected interactions, rather than a first this state, then the next state only.

Martin Fowler, around the middle of his article "Mocks Aren't Stubs", after describing fakes vs. mocks approaches in more detail. He starts to use a clarifying terminology which I like:

"In the two styles of testing I've shown above, the first case uses a real warehouse object and the second case uses a mock warehouse, which of course isn't a real warehouse object. Using mocks is one way to not use a real warehouse in the test, but there are other forms of unreal objects used in testing like this.

"The vocabulary for talking about this soon gets messy - all sorts of words are used: stub, mock, fake, dummy. For this article I'm going to follow the vocabulary of Gerard Meszaros's book. It's not what everyone uses, but I think it's a good vocabulary and since it's my essay I get to pick which words to use.

"Meszaros uses the term Test Double as the generic term for any kind of pretend object used in place of a real object for testing purposes. The name comes from the notion of a Stunt Double in movies. (One of his aims was to avoid using any name that was already widely used.) Meszaros then defined four particular kinds of double:

• Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
• Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an in memory database is a good example).
• Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test. Stubs may also record information about calls, such as an email gateway stub that remembers the messages it 'sent', or maybe only how many messages it 'sent'.
• Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.

"Of these kinds of doubles, only mocks insist upon behavior verification. The other doubles can, and usually do, use state verification. Mocks actually do behave like other doubles during the exercise phase, as they need to make the SUT believe it's talking with its real collaborators - but mocks differ in the setup and the verification phases."

This use of behavioural expectations for "Test Double" objects is quite handy, especially in systems where you have components which make use of heavy service objects or (gasp) singletons which may do more than a simple "call-and-answer" method-call on the object. Deep call chains may require stronger behavioural testing. Being able to provide an object to the system under test that expects to be called a certain way and in a certain order can create a much more precise test, and reduce the amount of code you have to write in a "fake" object. Otherwise, to fully and precisely test, one ends up with severely large number of Test Dummies and ballooning "Fake Objects", which themselves could have error and often have to be tested. Having a system like jMock or EasyMock can reduce the size of your testing code, thus removing some code that could easily become out-of-sync with the system under test, and either introduce false-positive errors or become insufficient and therefore meaningless. So less code, less maintenance, less syncing problems, and the tests are able to be more precise at the same time.

Another approach is to radically re-factor your code into smaller components, each of which is substantially simpler to test. If a component does one thing and does it well, and components interact and collaborate, then you often can use simpler mock behaviours, or simple fakes without as much effort on the testing. Great resources for testable code are available here .

Mocks aren't always easily intuitive for someone used to building fakes (it wasn't for me, and it still occasionally trips me up), but once you're comfortable with the approach, it can be much crisper. This is especially true as people try to get better coverage in isolated unit tests, or who are trying to test-drive their software. The linked Fowler article is a good one, and certainly worth reading for those trying to figure out how to more meaningfully test components without having to start up external servers or simulators.

Why I hate the java.util collection library.

<rant>

It's very simple. While it was a vast improvement over Vector and Dictionary of their day, the Collections library as of Java 1.2 did what all new Java APIs from Sun seem to do... require lots of code to use simply and allow the user to do invalid things.

I won't spend a lot of time on the former, since it could be the topic of a whole other blog post, and I should preface all of this by saying that Java is my most proficient language. So this is not an anti-Java bigot speaking... just a frustrated user who wishes Sun and the community wouldn't keep heaping bad APIs on top of bad APIs.

Sorry... back to the point.

Immutability - it's all backwards... what's with that?

The big issue I have with the Collections API are about allowing the user to do wrong things. This amounts to Sun having inverted Mutability vs. Immutability in the class heirarchy. Immutability, in any language that wants to guarantee semantics, ease concurrency and resource contention, and otherwise clean things up should have immutability as a default. Something set shouldn't be volatile or mutable unless specified as such, and the semantics should enforce this. But with the collections API, we have immutability as an optional characteristic of Collections. To use a simplistic example, you can do this:

Collection c = new ArrayList();
c.add(foo);
c.add(bar);

Collection has an add() method. This means that, by definition, Collection is mutable. "But wait!" you cry, you can obtain an immutable version of this collection by calling Collections.immutableCollection(c);. Sure. At which point you have something that conforms to the contract of Collection, but which may throw exceptions if part of that contract is relied upon. In other words, you should not have access to methods that allow you to break the contract. To allow this is to have written a bad contract with ambiguity. Now, to properly guard against the possibility of stray immutable collections, you may be forced to check immutability before executing the contract (the add() method) or you may have to guard against the exceptions with try-catch logic and exception handling. You can see some of this in the concurrent library's implementations of concurrent collections. It's not bad, but could be simpler with an immutable collection interface.

Additionally, consider how hard it is to create anonymous one-off implementations of Collection. You have to implement not only size, contains(), iterator(), but all the storage logic. If you're wrapping an existing data structure and merely wanted a read-only view on the structure, you are forced to implement all that extra API in your code purely to satisfy the optional contract provided in the Collection definition.

The point here is that an immutable Collection is a sub-set of the functionality of a MutableCollection. That should be obvious, but apparently wasn't to Sun. Consider had Sun used the model used in NeXTSTEP (and now Apple's Cocoa) APIs. Collection would have been defined as (simplified):

public interface Collection<T> extends Iterable<T> {
    public Iterator<T> getIterator();
    public int getSize();
    public boolean isEmpty();
    public boolean contains(T object);
    public boolean containsAll(Collection<T> object);
    public T[] toArray(T[]);
}

and

public interface MutableCollection<T> extends Collection<T> {
    public boolean add(T object);
    public boolean addAll(Collection<T> objects);
    public boolean remove(T object);
    public boolean removeAll(Collection<T> objects);
    public boolean retainAll(Collection<T> objects);
    public void clear();
}

This would, ultimately, mean that a Collection instance, typed as a Collection would not have any mutable methods available to invoke, let alone that would need guarding against stray immutable invocations. There would then be two strategy for guaranteeing immutability. One... cast the stupid thing as Collection, and onothing that has access to the cast can get at the MutableCollections methods (except by explicit reflection). Alternately, add a "getImmutableCopy()" method to the MutableCollection interface that creates a shallow copy that is NOT an implementor of MutableCollection... merely of Collection. Then you have a safe "snapshot" of the mutable object that can be freely passed around without worry that something else will modify it.

Ok, why is this such a big deal? It's about having code that means what it says. If I have to guess about the run-time state, or more concrete type of an instance to know if it's OK to invoke one of its methods or not, then I'm working with implicit contract, and that's murky territory. Java, by making an ImmutableCollection a special implementation of Collection, has inverted the hierarchy of contract, and exposed methods that are not truly available for all implementors. Optional interfaces are fine, but you don't expose the optional interface above where it's true. It's a basic piece of encapsulation that the Java folks just seemed to forget.

Now, this is a decade too late, this little rant. Truth is, I made it when I worked at Sun, but was quite the junior contracting peon, and had no real voice. Now, I have a blog, and am free to whine and be annoyed in public. :) But I hope to make a more general point here about contract. Optional contracts (APIs) need to be handled very carefully, and in a way such that an unfamiliar programmer can understand what you meant from how the contract reads. Look at your interfaces from the perspective that it should not offer what it (potentially) cannot satisfy. Polymorphism doesn't require "kitchen-sinkism" in an API. Just careful, thought-out layers.

The issues of testability also arise here, insofar as a class that has to implement optional APIs must, therefore, have more tests to satisfy what are, in essence, boilerplate code. If I made a quick-and-dirty implementation of Collection as a wrapper around an already existing, immutable data structure, then I have to implement all of those methods and test them, when in fact, half of them will throw an exception. This means (to me) that they probably shouldn't even exist. Code with a lot of boilerplate code (lots of extra getters and setters, or over-wrought interfaces) tend to be hard to test, and one of my big annoyances in life (these days) is testing boilerplate code. Ick.

</rant>