The key difference, to me, is that integration tests reveal if a feature is working or is broken, since they stress the code in a scenario close to reality. They invoke one or more software methods or features and test if they act as expected.
On the opposite, a Unit test testing a single method relies on the (often wrong) assumption that the rest of the software is correctly working, because it explicitly mocks every dependency.
Hence, when a unit test for a method implementing some feature is green, it does not mean the feature is working.
Say you have a method somewhere like this:
public SomeResults DoSomething(someInput) {
var someResult = [Do your job with someInput];
Log.TrackTheFactYouDidYourJob();
return someResults;
}
DoSomething is very important to your customer: it's a feature, the only thing that matters. That's why you usually write a Cucumber specification asserting it: you wish to verify and communicate the feature is working or not.
Feature: To be able to do something
In order to do something
As someone
I want the system to do this thing
Scenario: A sample one
Given this situation
When I do something
Then what I get is what I was expecting for
No doubt: if the test passes, you can assert you are delivering a working feature. This is what you can call Business Value.
If you want to write a unit test for DoSomething you should pretend (using some mocks) that the rest of the classes and methods are working (that is: that, all dependencies the method is using are correctly working) and assert your method is working.
In practice, you do something like:
public SomeResults DoSomething(someInput) {
var someResult = [Do your job with someInput];
FakeAlwaysWorkingLog.TrackTheFactYouDidYourJob(); // Using a mock Log
return someResults;
}
You can do this with Dependency Injection, or some Factory Method or any Mock Framework or just extending the class under test.
Suppose there's a bug in Log.DoSomething().
Fortunately, the Gherkin spec will find it and your end-to-end tests will fail.
The feature won't work, because Log is broken, not because [Do your job with someInput] is not doing its job. And, by the way, [Do your job with someInput] is the sole responsibility for that method.
Also, suppose Log is used in 100 other features, in 100 other methods of 100 other classes.
Yep, 100 features will fail. But, fortunately, 100 end-to-end tests are failing as well and revealing the problem. And, yes: they are telling the truth.
It's very useful information: I know I have a broken product. It's also very confusing information: it tells me nothing about where the problem is. It communicates me the symptom, not the root cause.
Yet, DoSomething's unit test is green, because it's using a fake Log, built to never break. And, yes: it's clearly lying. It's communicating a broken feature is working. How can it be useful?
(If DoSomething()'s unit test fails, be sure: [Do your job with someInput] has some bugs.)
Suppose this is a system with a broken class:
A single bug will break several features, and several integration tests will fail.
On the other hand, the same bug will break just one unit test.
Now, compare the two scenarios.
The same bug will break just one unit test.
- All your features using the broken
Log are red
- All your unit tests are green, only the unit test for
Log is red
Actually, unit tests for all modules using a broken feature are green because, by using mocks, they removed dependencies. In other words, they run in an ideal, completely fictional world. And this is the only way to isolate bugs and seek them. Unit testing means mocking. If you aren't mocking, you aren't unit testing.
The difference
Integration tests tell what's not working. But they are of no use in guessing where the problem could be.
Unit tests are the sole tests that tell you where exactly the bug is. To draw this information, they must run the method in a mocked environment, where all other dependencies are supposed to correctly work.
That's why I think that your sentence "Or is it just a unit test that spans 2 classes" is somehow displaced. A unit test should never span 2 classes.
This reply is basically a summary of what I wrote here: Unit tests lie, that's why I love them.
Best Answer
Unit tests localize failures more tightly. Integration-level tests more closely correspond to user requirements and so are better predictor of delivery success. Neither of them is much good unless built and maintained, but both of them are very valuable if properly used.
(more...)
The thing with units tests is that no integration level test can exercise all the code as much as a good set of unit tests can. Yes, that can mean that you have to refactor the tests somewhat, but in general your tests shouldn't depend on the internals so much. So, lets say for example that you have a single function to get a power of two. You describe it (as a formal methods guy, I'd claim you specify it)
Your test and your spec look essentially the same (this is sort of pseudo-xUnit for illustration):
Now your implementation can be a for loop with a multiple, and you can come along later and change that to a shift.
If you change the implementation so that, say, it's returning 16 bits (remember that
sizeof(long)is only guaranteed to be no less thansizeof(short)) then this tests will fail quickly. An integration-level test should probably fail, but not certainly, and it's just as likely as not to fail somewhere far downstream of the computation ofpow2(28).The point is that they really test for diferent situations. If you could build sufficiently details and extensive integration tests, you might be able to get the same level of coverage and degree of fine-grained testing, but it's probably hard to do at best, and the exponential state-space explosion will defeat you. By partitioning the state space using unit tests, the number of tests you need grows much less than exponentially.