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We've got our application unit tested and there are two system tests using Robot verifying the basic functionality. There is also a bunch of requirements (acceptance tests) from business as .feature files in Gherkin that we run using Cucumber.
We started implementing them as integration tests using rest assured - everything was fine for the basic functionality. The problem is with implementing the more detailed scenarios - some things that we need to verify are not persisted in DB or returned by the endpoint.
It would be really easy to test that functionality by implementing those scenarios as unit tests but I'm not sure if that is a good practice?
I think there needs to be a back-and-forth between yourselves and the business. You'll usually end up with a couple of integration tests to prove the feature works end-to-end in a happy path/negative scenario, but you won't go through all the edge cases since that is costly (time-consuming, especially when running the integration tests). That's what the unit tests are for, covering every scenario. Convey that to the business and they might learn to trust your judgement instead of implementing each and every one of the acceptance criteria as an integration test.
It would be really easy to test that functionality by implementing those scenarios as unit tests but I'm not sure if that is a good practice?
If they're not implemented, it's definitely a good practice to do so as soon as possible, but, I'd pay attention to the test's scope. I also see no problem in using Cucumber to aid your unit testing effort, but don't fall into the mistake of making your unit tests bigger scope acceptance tests. Keep them as direct as possible.
You mention the complexity of some of these scenarios:
some things that we need to verify are not persisted in DB or returned by the endpoint
Then the code should probably be more testable/maintainable on different levels. You can achieve this with some techniques. Depending on the language you use, you could use a tool similar to mockito spy to spy object's interactions that otherwise, you have no way of checking, but, it's only wise to do so on appropriate test scope, like with Unit and Mock tests.
Also, consider that it's perfectly fine if you cover different parts of a given flow with different test levels. Ideally, you'll cover most of it with unit tests.
You can keep the most complex unit parts covered with unit tests, and still maintain the higher level acceptance scenarios business asks you to but don't do deep assertions in the tests.
I interpret your question as "Is it ok to use unit tests at the same time as you use BDD?"
My take is that these are two techniques that complement each other. If something is easier using one technique, then use that technique in that particular case.
Use whatever that supports your delivery of working software. Working software is the goal, not using a specific tool.
Assume that I'm writing an application which uses Test Driven Development.
All the samples that I find are very small examples that try to explain how tests need to be written in TDD.
When you write a test in TDD, you write a very small piece of code whose purpose is to test a single piece of code, a single method, so a unit test.
After some time, a requirement from the client is received and you need to change your original code allowing it to accept much more arguments and splitting the method into multiple methods over multiple layers.
Let's say that logging is added when a failure occurs. What do I need to test then, the logging component separately, or chained together with the original method?
This means that the original unit test is in fact becoming an integration test as I'm testing multiple components together now.
Is this something that should be avoided, or how does one solve those kind of issues if needed?
Kind regards
TDD in the real world actually uses both unit tests and integration tests. Unit tests are seen in tutorials because it's easier to understand simple examples, but real applications need some integration tests. It's typical for the first test you write to be an integration test (see bdd).
However, integration tests are slow and hard to maintain (they touch more of the system than unit tests, so they change more frequently), so it's good to have only as many integration tests as needed and do as much of your testing with unit tests as is reasonable.
When requirements on a class cause it to become larger and you refactor the class into smaller classes, its unit tests are now integration tests. Address this by writing focused unit tests for the new classes and removing most of the old tests for the original class. It may be appropriate to leave behind one or a few of the old tests as integration tests. It also may be appropriate to rewrite some of the old tests to use test doubles (stubs, mocks, etc.) for what are now instances of other classes. Coincidentally, I recently wrote an answer about the mechanics of rewriting tests when you refactor a class out of another class.
In addition to the other answers, you could have a look at the extended TDD cycle as defined in the book Growing Object-Oriented Software Guided by Tests. They are using acceptance tests to drive the inner loop of writing unit tests; depending on the situation, however, I found that you can also use integration tests to do that.
So no need to avoid them. What matters in my experience are the granularity and number of tests (less integration-, more unit tests).
TDD or not, the idea of an unit test is that you isolate an unit of the application and verify its code flows in isolation. An unit is typically a class and you would be looking at atleast one unit test per code branch of a method. E.g. If classA.methodA() has 3 branches, you will have 3+ unit tests for that method.
A true unit test injects the mocked/stubbed dependencies into the component, invokes the method to be tested and verifies its behavior and/or object state. Unit tests in principle should encourage you to improve the design of your source code in terms of loose coupling, separation of concern, etc. (SOLID principles).
Further, code coverage is a good measure of the quality of your unit tests, but striving for 100% isn't advised. Also, writing unit tests for every application layer is overkill; you would want to target layers that contain business logic to achieve a good return-of-investment. Lastly, do not write unit tests without having a Continuous Integration pipeline, since they tend to become stale very quickly.
On the contrary, when you start verifying two or more units as one test, it becomes an integration test since your test result is influenced by the success or failure of each unit. These tend to require more effort to setup the environment, could be flaky due to external dependencies and could be slow based on volume of transactions. These are definitely useful and you should aim for a code coverage based on your budget constraints. Integration tests should also be part of the CI/CD pipeline, but could be run less often than unit tests.
Hope this helps.
There is no real fundamental difference between a unit test and an integration test.
A low level (unit) test of one of your classes will likely also exercise, and rely on, classes provided by your runtime environment or application framework. So your unit test could also be considered as an integration test of the combination of your code with the code of the runtime environment.
With no fundamental difference between them, there is no reason to be concerned if something once labelled "unit test" is now labelled "integration test".
Im developing with Grails. Since the framework will bootstrap data and a fully flushed out spring context, I find that I write a lot of integration tests for services. Let me rephrase that: I find I write no unit tests for services, only integration tests. Is this a bad idea? The only downside I see to it is that my tests take a bit longer to run.
I do use unit testing on controllers, as in a controller I am testing the various application flows, result types, redirect logic, etc. But the majority of tests I write are integration tests. This seems a break from tradition J2EE tests, where mostly unit tests are written.
edit- to be clear, Im not writing integration tests because the code is so complicated only integration tests will do. Im writing integration tests because it is easier to test everything together, since the framework gives you so much. I do mock certain things, like if a service collaborates with the acegi authenticationService, I mock that. I also mock anytime we interact with a webservice, because you have to in order to get a test that runs without special setup.
I clearly see a trend towards more functional tests and fewer unit tests, in particular for heavily-connected components low on logic. If I implement a complicated algorithm in a particular class, I will usually write unit tests for it, but if the complexity is due to integration with other components (which is very frequently the case), unit tests are not worth the hassle.
In general, the important measure is the code coverage.
In can be advantageous to do integration tests if the global interface is more stable (less subject to change).
You may be able to use a mock framework to isolate different parts of your services for individual testing. Instead of relying on the massive Spring context, directly instantiate your service class and fill in the dependencies not directly relevant to the test with mocks. You may need a little refactoring to decouple your dependencies, but it will usually be for the best. This can lead to writing more and smaller tests instead of relying on a few large integration tests.
I am in a similar situation where many established tests are really integration tests, and it can be difficult, but not impossible, to change this going forward.
You should test as soon as possible, because you want errors to show up as soon as possible, ideally while our code is still under your own control. The later an error is discovered the higher the costs of the correction will be.
If you are exposing non trivial logic, you should test the logic of the main decision paths with unit tests, and the exposure of that logic and the interaction with external dependencies in the integration tests.
If you are a lone developer or working in a small team, sometimes it's difficult to see the distinction between unit and integration tests. If you are in an environment where multiple teams are working on a single product, the assumption is that the logic inside your code has already been verified (unit test) and now you want to see how the modules play with each other (integration test).
It is not a good pattern to load too much into the integration test, because you are deferring testing to a later phase in your project or development environment, and sooner or later you are going to find that you are discovering errors once the code has left your desk. That will mean holding up the entire and possibly the product release while you fix something you could and should have discovered earlier.
Let me start from definition:
Unit Test is a software verification and validation method in which a programmer tests if individual units of source code are fit for use
Integration testing is the activity of software testing in which individual software modules are combined and tested as a group.
Although they serve different purposes very often these terms are mixed up. Developers refer to automated integration tests as unit tests. Also some argue which one is better which seems to me as a wrong question at all.
I would like to ask development community to share their opinions on why automated integration tests cannot replace classic unit tests.
Here are my own observations:
Integration tests can not be used with TDD approach
Integration tests are slow and can not be executed very often
In most cases integration tests do not indicate the source of the problem
it's more difficult to create test environment with integration tests
it's more difficult to ensure high coverage (e.g. simulating special cases, unexpected failures etc)
Integration tests can not be used with Interaction based testing
Integration tests move moment of discovering defect further (from paxdiablo)
EDIT: Just to clarify once again: the question is not about whether to use integration or unit testing and not about which one is more useful. Basically I want to collect arguments to the development teams which write ONLY integration tests and consider them as unit tests.
Any test which involve components from different layers is considered as integration test. This is to compare to unit test where isolation is the main goal.
Thank you,
Andrey
Integration tests tell you whether it's working. Unit tests tell you what isn't working. So long as everything is working, you "don't need" the unit tests - but once something is wrong, it's very nice to have the unit test point you directly to the problem. As you say, they serve different purposes; it's good to have both.
To directly address your subject: integration tests aren't a problem, aren't the problem. Using them instead of unit tests is.
There have been studies(a) that show that the cost of fixing a bug becomes higher as you move away from the point where the bug was introduced.
For example, it will generally cost you relatively little to fix a bug in software you haven't even pushed up to source control yet. It's your time and not much of it, I'd warrant (assuming you're any good at your job).
Contrast that with how much it costs to fix when the customer (or all your customers) find that problem. Many level of people get involved and new software has to be built in a hurry and pushed out to the field.
That's the extreme comparison. But even the difference between unit and integration tests can be apparent. Code that fails unit testing mostly affects only the single developer (unless other developers/testers/etc are waiting on it, of course). However, once your code becomes involved in integration testing, a defect can begin holding up other people on your team.
We wouldn't dream of replacing our unit tests with integration tests since:
Our unit tests are automated as well so, other than initial set-up, the cost of running them is small.
They form the beginning of the integration tests. All unit tests are rerun in the integration phase to check that the integration itself hasn't broken anything, and then there are the extra tests that have been added by the integration team.
(a) See, for example, http://slideshare.net/Vamsipothuri/defect-prevention, slide # 5, or search the net for Defect prevention : Reducing costs and enhancing quality. Th graph from the chart is duplicated below in case it ever becomes hard to find on the net:
I find integration tests markedly superior to unit tests. If I unit test my code, I'm only testing what it does versus my understanding of what it should do. That only catches implementation errors. But often a much bigger problem is errors of understanding. Integration tests catch both.
In addition, there is a dramatic cost difference; if you're making intensive use of unit tests, it's not uncommon for them to outweigh all the rest of your code put together. And they need to be maintained, just like the rest of the code does. Integration tests are vastly cheaper -- and in most cases, you already need them anyway.
There are rare cases where it might be necessary to use unit tests, e.g. for internal error handling paths that can't be triggered if the rest of the system is working correctly, but most of the time, integration tests alone give better results for far lower cost.
Integration tests are slow.
Integration tests may break different
reasons (it is not focused and
isolated). Therefore you need more
debugging on failures.
Combination of
scenarios are to big for integration
test when it is not unit tested.
Mostly I do unit tests and 10 times less integration tests (configuration, queries).
In many cases you need both. Your observations are right on track as far as I'm concerned with respect to using integration tests as unit tests, but they don't mean that integration tests are not valuable or needed, just that they serve a different purpose. One could equally argue that unit tests can't replace integration tests, precisely because they remove the dependencies between objects and they don't exercise the real environment. Both are correct.
It's all about reducing the iteration time.
With unit tests, you can write a line of code and verify it in a minute or so. With integration tests, it usually takes significantly longer (and the cost increases as the project grows).
Both are clearly useful, as both will detect issues that the other fails to detect.
OTOH, from a "pure" TDD approach, unit tests aren't tests, they're specifications of functionality. Integration tests, OTOH, really do "test" in the more traditional sense of the word.
Integration testing generally happens after unit testing. I'm not sure what value there is in testing interactions between units that have not themselves been tested.
There's no sense in testing how the gears of a machine turn together if the gears might be broken.
The two types of tests are different. Unit tests, in my opinion are not a alternative to integration tests. Mainly because integration tests are usually context specific. You may well have a scenario where a unit test fails and your integration doesn't and vice versa. If you implement incorrect business logic in a class that utilizes many other components, you would want your integration tests to highlight these, your unit tests are oblivious to this.I understand that integration testing is quick and easy. I would argue you rely on your unit tests each time you make a change to your code-base and having a list of greens would give you more confidence that you have not broken any expected behavior at the individual class level. Unit tests give you a test against a single class is doing what it was designed to do. Integration tests test that a number of classes working together do what you expect them to do for that particular collaboration instance. That is the whole idea of OO development: individual classes that encapsulate particular logic, which allows for reuse.
I think coverage is the main issue.
A unit test of a specific small component such as a method or at most a class is supposed to test that component in every legal scenario (of course, one abstracts equivalence classes but every major one should be covered). As a result, a change that breaks the established specification should be caught at this point.
In most cases, an integration uses only a subset of the possible scenarios for each subunit, so it is possible for malfunctioning units to still produce a program that initially integrates well.
It is typically difficult to achieve maximal coverage on the integration testing for all the reasons you specified below. Without unit tests, it is more likely that a change to a unit that essentially operates it in a new scenario would not be caught and might be missed in the integration testing. Even if it is not missed, pinpointing the problem may be extremely difficult.
I am not sure that most developers refer to unit tests as integration tests. My impression is that most developers understand the differences, which does not mean they practice either.
A unit test is written to test a method on a class. If that class depends on any kind of external resource or behavior, you should mock them, to ensure you test just your single class. There should be no external resources in a unit test.
An integration test is a higher level of granularity, and as you stated, you should test multiple components to check if they work together as expected. You need both integration tests and unit tests for most projects. But it is important they are kept separate and the difference is understood.
Unit tests, in my opinion, are more difficult for people to grasp. It requires a good knowledge of OO principles (fundamentally based on one class one responsibility). If you are able to test all your classes in isolation, chances are you have a well design solution which is maintainable, flexible and extendable.
When you check-in, your build server should only run unit tests and
they should be done in a few seconds, not minutes or hours.
Integration tests should be ran overnight or manually as needed.
Unit tests focus on testing an individual component and do not rely on external dependencies. They are commonly used with mocks or stubs.
Integration tests involve multiple components and may rely on external dependencies.
I think both are valuable and neither one can replace the other in the job they do. I do see a lot of integration tests masquerading as unit tests though having dependencies and taking a long time to run. They should function separately and as part of a continuous integration system.
Integration tests do often find things that unit tests do not though...
Integration tests let you check that whole use cases of your application work.
Unit tests check that low-level logic in your application is correct.
Integration tests are more useful for managers to feel safer about the state of the project (but useful for developers too!).
Unit tests are more useful for developers writing and changing application logic.
And of course, use them both to achieve best results.
It is a bad idea to "use integration tests instead of unit tests" because it means you aren't appreciating that they are testing different things, and of course passing and failing tests will give you different information. They make up sort of a ying and yang of testing as they approach it from either side.
Integration tests take an approach that simulates how a user would interact with the application. These will cut down on the need for as much manual testing, and passing tests will can tell you that you app is good to go on multiple platforms. A failing test will tell you that something is broken but often doesn't give you a whole lot of information about what's wrong with the underlying code.
Unit tests should be focusing on making sure the inputs and outputs of your function are what you expect them to be in all cases. Passing units tests can mean that your functions are working according to spec (assuming you have tests for all situations). However, all your functions working properly in isolation doesn't necessarily mean that everything will work perfectly when it's deployed. A failing unit test will give you detailed, specific information about why it's failing which should in theory make it easier to debug.
In the end I believe a combination of both unit and integration tests will yield the quickest a most bug-free software. You could choose to use one and not the other, but I avoid using the phrase "instead of".
How I see integration testing & unit testing:
Unit Testing: Test small things in isolation with low level details including but not limited to 'method conditions', checks, loops, defaulting, calculations etc.
Integration testing: Test wider scope which involves number of components, which can impact the behaviour of other things when married together. Integration tests should cover end to end integration & behaviours. The purpose of integration tests should be to prove systems/components work fine when integrated together.
(I think) What is referred here by OP as integration tests are leaning more to scenario level tests.
But where do we draw the line between unit -> integration -> scenario?
What I often see is developers writing a feature and then when unit testing it mocking away every other piece of code this feature uses/consumes and only test their own feature-code because they think someone else tested that so it should be fine. This helps code coverage but can harm the application in general.
In theory the small isolation of Unit Test should cover a lot since everything is tested in its own scope. But such tests are flawed and do not see the complete picture.
A good Unit test should try to mock as least as possible. Mocking API and persistency would be something for example. Even if the application itself does not use IOC (Inversion Of Control) it should be easy to spin up some objects for a test without mocking if every developer working on the project does it as well it gets even easier. Then the test are useful. These kind of tests have an integration character to them aren't as easy to write but help you find design flaws of your code. If it is not easy to test then adapt your code to make it easy to test. (TDD)
Pros
Fast issue identification
Helps even before a PR merge
Simple to implement and maintain
Providing a lot of data for code quality checking (e.g. coverage etc.)
Allows TDD (Test Driven Development)
Cons
Misses scenario integration errors
Succumbs to developer blindness in their own code(happens to all of us)
A good integration test would be executed for complete end to end scenarios and even check persistency and APIs which the unit test could not cover so you might know where to look first when those fail.
Pros:
Test close to real world e2e scenario
Finds Issues that developers did not think about
Very helpful in microservices architectures
Cons:
Most of the time slow
Need often a rather complex setup
Environment (persistency and api) pollution issues (needs cleanup steps)
Mostly not feasible to be used on PR's (Pull Requests)
TLDR: You need both you cant replace one with the other! The question is how to design such tests to get the best from both. And not just have them to show good statistics to the management.
Recently we have been adding automated tests to our existing java applications.
What we have
The majority of these tests are integration tests, which may cover a stack of calls like:-
HTTP post into a servlet
The servlet validates the request and calls the business layer
The business layer does a bunch of stuff via hibernate etc and updates some database tables
The servlet generates some XML, runs this through XSLT to produce response HTML.
We then verify that the servlet responded with the correct XML and that the correct rows exist in the database (our development Oracle instance). These rows are then deleted.
We also have a few smaller unit tests which check single method calls.
These tests are all run as part of our nightly (or adhoc) builds.
The Question
This seems good because we are checking the boundaries of our system: servlet request/response on one end and database on the other. If these work, then we are free to refactor or mess with anything inbetween and have some confidence that the servlet under test continues to work.
What problems are we likely to run into with this approach?
I can't see how adding a bunch more unit tests on individual classes would help. Wouldn't that make it harder to refactor as it's much more likely we will need to throw away and re-write tests?
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)
long pow2(int p); // returns 2^p for 0 <= p <= 30
Your test and your spec look essentially the same (this is sort of pseudo-xUnit for illustration):
assertEqual(1073741824,pow2(30);
assertEqual(1, pow2(0));
assertException(domainError, pow2(-1));
assertException(domainError, pow2(31));
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 than sizeof(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 of pow2(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.
You are asking pros and cons of two different things (what are the pros and cons of riding a horse vs riding a motorcycle?)
Of course both are "automated tests" (~riding) but that doesn't mean that they are alternative (you don't ride a horse for hundreds of miles, and you don't ride a motorcycle in closed-to-vehicle muddy places)
Unit Tests test the smallest unit of the code, usually a method. Each unit test is closely tied to the method it is testing, and if it's well written it's tied (almost) only with that.
They are great to guide the design of new code and the refactoring of existing code. They are great to spot problems long before the system is ready for integration tests. Note that I wrote guide and all the Test Driven Development is about this word.
It does not make any sense to have manual Unit Tests.
What about refactoring, which seems to be your main concern? If you are refactoring just the implementation (content) of a method, but not its existence or "external behavior", the Unit Test is still valid and incredibly useful (you cannot imagine how much useful until you try).
If you are refactoring more aggressively, changing methods existence or behavior, then yes, you need to write a new Unit Test for each new method, and possibly throw away the old one. But writing the Unit Test, especially if you write it before the code itself, will help to clarify the design (i.e. what the method should do, and what it shouldn't) without being confused by the implementation details (i.e. how the method should do the thing that it needs to do).
Automated Integration Tests test the biggest unit of the code, usually the entire application.
They are great to test use cases which you don't want to test by hand. But you can also have manual Integration Tests, and they are as effective (only less convenient).
Starting a new project today, it does not make any sense not to have Unit Tests, but I'd say that for an existing project like yours it does not make too much sense to write them for everything you already have and it's working.
In your case, I'd rather use a "middle ground" approach writing:
smaller Integration Tests which only test the sections you are going to refactor. If you are refactoring the whole thing, then you can use your current Integration Tests, but if you are refactoring only -say- the XML generation, it does not make any sense to require the presence of the database, so I'd write a simple and small XML Integration Test.
a bunch of Unit Tests for the new code you are going to write. As I already wrote above, Unit Tests will be ready as soon as you "mess with anything in between", making sure that your "mess" is going somewhere.
In fact your Integration Test will only make sure that your "mess" is not working (because at the beginning it will not work, right?) but it will not give you any clue on
why it is not working
if your debugging of the "mess" is really fixing something
if your debugging of the "mess" is breaking something else
Integration Tests will only give the confirmation at the end if the whole change was successful (and the answer will be "no" for a long time). The Integration Tests will not give you any help during the refactoring itself, which will make it harder and possibly frustrating. You need Unit Tests for that.
I agree with Charlie about Integration-level tests corresponding more to user actions and the correctness of the system as a whole. I do think there is alot more value to Unit Tests than just localizing failures more tightly though. Unit tests provide two main values over integration tests:
1) Writing unit tests is as much an act of design as testing. If you practice Test Driven Development/Behavior Driven Development the act of writing the unit tests helps you design exactly what you code should do. It helps you write higher quality code (since being loosely coupled helps with testing) and it helps you write just enough code to make your tests pass (since your tests are in effect your specification).
2) The second value of unit tests is that if they are properly written they are very very fast. If I make a change to a class in your project can I run all the corresponding tests to see if I broke anything? How do I know which tests to run? And how long will they take? I can guarantee it will be longer than well written unit tests. You should be able to run all of you unit tests in a couple of minutes at the most.
Just a few examples from personal experience:
Unit Tests:
(+) Keeps testing close to the relevant code
(+) Relatively easy to test all code paths
(+) Easy to see if someone inadvertently changes the behavior of a method
(-) Much harder to write for UI components than for non-GUI
Integration Tests:
(+) It's nice to have nuts and bolts in a project, but integration testing makes sure they fit each other
(-) Harder to localize source of errors
(-) Harder to tests all (or even all critical) code paths
Ideally both are necessary.
Examples:
Unit test: Make sure that input index >= 0 and < length of array. What happens when outside bounds? Should method throw exception or return null?
Integration test: What does the user see when a negative inventory value is input?
The second affects both the UI and the back end. Both sides could work perfectly, and you could still get the wrong answer, because the error condition between the two isn't well-defined.
The best part about Unit testing we've found is that it makes devs go from code->test->think to think->test->code. If a dev has to write the test first, [s]he tends to think more about what could go wrong up front.
To answer your last question, since unit tests live so close to the code and force the dev to think more up front, in practice we've found that we don't tend to refactor the code as much, so less code gets moved around - so tossing and writing new tests constantly doesn't appear to be an issue.
The question has a philisophical part for sure, but also points to pragmatic considerations.
Test driven design used as the means to become a better developer has its merits, but it is not required for that. Many a good programmer exists who never wrote a unit test. The best reason for unit tests is the power they give you when refactoring, especially when many people are changing the source at the same time. Spotting bugs on checkin is also a huge time-saver for a project (consider moving to a CI model and build on checkin instead of nightly). So if you write a unit test, either before or after you written the code it tests, you are sure at that moment about the new code you've written. It is what can happen to that code later that the unit test ensures against - and that can be significant. Unit tests can stop bugs before tehy get to QA, thereby speeding up your projects.
Integration tests stress the interfaces between elements in your stack, if done correctly. In my experience, integration is the most unpredictable part of a project. Getting individual pieces to work tends not to be that hard, but putting everything together can be very difficult because of the types of bugs that can emerge at this step. In many cases, projects are late because of what happens in integration. Some of the errors encountered in this step are found in interfaces that have been broken by some change made on one side that was not communicated to the other side. Another source of integration errors are in configurations discovered in dev but forgotten by the time the app goes to QA. Integration tests can help reduce both types dramatically.
The importance of each test type can be debated, but what will be of most importance to you is the application of either type to your particular situation. Is the app in question being developed by a small group of people or many different groups? Do you have one repository for everything, or many repos each for a particular component of the app? If you have the latter, then you will have challenges with inter compatability of different versions of different components.
Each test type is designed to expose the problems of different levels of integration in the development phase to save time. Unit tests drive the integration of the output many developers operating on one repository. Integration tests (poorly named) drive the integration of components in the stack - components often written by separate teams. The class of problems exposed by integration tests are typically more time-consuming to fix.
So pragmatically, it really boils down to where you most need speed in your own org/process.
The thing that distinguishes Unit tests and Integration tests is the number of parts required for the test to run.
Unit tests (theoretically) require very (or no) other parts to run.
Integration tests (theoretically) require lots (or all) other parts to run.
Integration tests test behaviour AND the infrastructure. Unit tests generally only test behaviour.
So, unit tests are good for testing some stuff, integration tests for other stuff.
So, why unit test?
For instance, it is very hard to test boundary conditions when integration testing. Example: a back end function expects a positive integer or 0, the front end does not allow entry of a negative integer, how do you ensure that the back end function behaves correctly when you pass a negative integer to it? Maybe the correct behaviour is to throw an exception. This is very hard to do with an integration test.
So, for this, you need a unit test (of the function).
Also, unit tests help eliminate problems found during integration tests. In your example above, there are a lot of points of failure for a single HTTP call:
the call from the HTTP client
the servlet validation
the call from the servlet to the business layer
the business layer validation
the database read (hibernate)
the data transformation by the business layer
the database write (hibernate)
the data transformation -> XML
the XSLT transformation -> HTML
the transmission of the HTML -> client
For your integration tests to work, you need ALL of these processes to work correctly. For a Unit test of the servlet validation, you need only one. The servlet validation (which can be independent of everything else). A problem in one layer becomes easier to track down.
You need both Unit tests AND integration tests.
Unit tests execute methods in a class to verify proper input/output without testing the class in the larger context of your application. You might use mocks to simulate dependent classes -- you're doing black box testing of the class as a stand alone entity. Unit tests should be runnable from a developer workstation without any external service or software requirements.
Integration tests will include other components of your application and third party software (your Oracle dev database, for example, or Selenium tests for a webapp). These tests might still be very fast and run as part of a continuous build, but because they inject additional dependencies they also risk injecting new bugs that cause problems for your code but are not caused by your code. Preferably, integration tests are also where you inject real/recorded data and assert that the application stack as a whole is behaving as expected given those inputs.
The question comes down to what kind of bugs you're looking to find and how quickly you hope to find them. Unit tests help to reduce the number of "simple" mistakes while integration tests help you ferret out architectural and integration issues, hopefully simulating the effects of Murphy's Law on your application as a whole.
Joel Spolsky has written very interesting article about unit-testing (it was dialog between Joel and some other guy).
The main idea was that unit tests is very good thing but only if you use them in "limited" quantity. Joel doesn't recommend to achive state when 100% of your code is under testcases.
The problem with unit tests is that when you want to change architecture of your application you'll have to change all corresponding unit tests. And it'll take very much time (maybe even more time than the refactoring itself). And after all that work only few tests will fail.
So, write tests only for code that really can make some troubles.
How I use unit tests: I don't like TDD so I first write code then I test it (using console or browser) just to be sure that this code do nessecary work. And only after that I add "tricky" tests - 50% of them fail after first testing.
It works and it doesn't take much time.
We have 4 different types of tests in our project:
Unit tests with mocking where necessary
DB tests that act similar to unit tests but touch db & clean up afterwards
Our logic is exposed through REST, so we have tests that do HTTP
Webapp tests using WatiN that actually use IE instance and go over major functionality
I like unit tests. They run really fast (100-1000x faster than #4 tests). They are type safe, so refactoring is quite easy (with good IDE).
Main problem is how much work is required to do them properly. You have to mock everything: Db access, network access, other components. You have to decorate unmockable classes, getting a zillion mostly useless classes. You have to use DI so that your components are not tightly coupled and therefore not testable (note that using DI is not actually a downside :)
I like tests #2. They do use the database and will report database errors, constraint violations and invalid columns. I think we get valuable testing using this.
#3 and especially #4 are more problematic. They require some subset of production environment on build server. You have to build, deploy and have the app running. You have to have a clean DB every time. But in the end, it pays off. Watin tests require constant work, but you also get constant testing. We run tests on every commit and it is very easy to see when we break something.
So, back to your question. Unit tests are fast (which is very important, build time should be less than, say, 10 minutes) and the are easy to refactor. Much easier than rewriting whole watin thing if your design changes. If you use a nice editor with good find usages command (e.g. IDEA or VS.NET + Resharper), you can always find where your code is being tested.
With REST/HTTP tests, you get a good a good validation that your system actually works. But tests are slow to run, so it is hard to have a complete validation at this level. I assume your methods accept multiple parametres or possibly XML input. To check each node in XML or each parameter, it would take tens or hundreds of calls. You can do that with unit tests, but you cannot do that with REST calls, when each can take a big fraction of a second.
Our unit tests check special boundary conditions far more often than #3 tests. They (#3) check that main functionality is working and that's it. This seems to work pretty well for us.
As many have mentioned, integration tests will tell you whether your system works, and unit tests will tell you where it doesn't. Strictly from a testing perspective, these two kinds of tests complement each other.
I can't see how adding a bunch more
unit tests on individual classes would
help. Wouldn't that make it harder to
refactor as it's much more likely we
will need to throw away and re-write
tests?
No. It will make refactoring easier and better, and make it clearer to see what refactorings are appropriate and relevant. This is why we say that TDD is about design, not about testing. It's quite common for me to write a test for one method and in figuring out how to express what that method's result should be to come up with a very simple implementation in terms of some other method of the class under test. That implementation frequently finds its way into the class under test. Simpler, more solid implementations, cleaner boundaries, smaller methods: TDD - unit tests, specifically - lead you in this direction, and integration tests do not. They're both important, both useful, but they serve different purposes.
Yes, you may find yourself modifying and deleting unit tests on occasion to accommodate refactorings; that's fine, but it's not hard. And having those unit tests - and going through the experience of writing them - gives you better insight into your code, and better design.
Although the setup you described sounds good, unit testing also offers something important. Unit testing offers fine levels of granularity. With loose coupling and dependency injection, you can pretty much test every important case. You can be sure that the units are robust; you can scrutinise individual methods with scores of inputs or interesting things that don't necessarily occur during your integration tests.
E.g. if you want to deterministically see how a class will handle some sort of failure that would require a tricky setup (e.g. network exception when retrieving something from a server) you can easily write your own test double network connection class, inject it and tell it to throw an exception whenever you feel like it. You can then make sure that the class under test gracefully handles the exception and carries on in a valid state.
You might be interested in this question and the related answers too. There you can find my addition to the answers that were already given here.