Although Unit Testing should be common knowledge, still a lot of people do not write effective Unit Tests or miss some basic concepts. In this post I'll touch upon some basic guidelines to write effective Unit Tests by using my own rules for writing guidelines. Of course this is subjective and corresponds to my own experience, so feel free to add your own as a comment. If you have no idea how to sell Unit Testing to your manager, you might also want to read one of my previous posts.
#1 Dependency Injection/Inversion of Control MUST be used everywhere
The biggest advantage of DI & testing is the ability to inject classes of your own choice for testing purposes. This makes it much easier to really test a single unit, with all its exceptions, and to mock (see below) certain classes.
Although frameworks such as Spring and Guice make it easier to use DI, you can apply DI perfectly without those frameworks as well by having a dedicated controller/configuration class that simply injects objects into other objects instead of letting the classes instantiate new objects themselves. When testing certain methods, you can easily inject other objects instead.
#2 Tests SHOULD be small and fast
It is a very thin line between actually testing a unit of code and simple integration tests. Certain dependencies, such as DAOs to load data from a database for example, SHOULD be mocked and injected in the actual code so that there is no dependency on a database when simply testing some business logic. This guarantees a fast execution of the test code and the fact that only a simple unit of code is being tested.
Mocking simply means mimicking behaviour of real objects in a controlled way. A very good framework for mocking is for example http://mockito.org/
#3 Unit Tests MUST NOT have side effects
A Unit Test MUST clean up any mess it makes when testing code so that the system is restored to the original state before the test was executed. This makes sure every single test is guaranteed to start from the same deterministic starting position and there are no surprises because a previously executed test has left the system in an inconsistent state.
You MUST use the setup and cleanup methods to handle these kind of things without the need of repeating the same code or polluting the actual test.
#4 Unit Tests MUST NOT depend on each other
The order in which Unit Tests are run MUST NOT be assumed because the order is non-deterministic in standard Unit Test frameworks. Even if you would not comply to item #3, there is no way that you can be sure that a certain test can depend on the result of a previous test, because you have no idea which one will be executed first.
Unit Tests can not depend on each other by design to keep things simple and clean. This is not a bug!
#5 For every bug found, a second Unit Test MUST be implemented
The whole idea behind Unit Testing is to make sure or prove that code is fit for use. If a bug is found, the code is clearly not fit for use and the code must be fixed. Once the bug is fixed, a new Unit Test MUST be written to proof that the code is now fit for use. This must be repeated for every bug found.
Each new Unit Test SHOULD also be documented with some background information about the bug and with a reference to the bug or issue tracking system when applicable.
#6 Every method implementing business logic MUST at least have one Unit Test
Getters and setters SHOULD NOT be tested since they generally do not contain any business logic, but all other methods MUST have at least one Unit Test implemented to prove that the code is actually fit for use. Whenever a bug is found, item #5 MUST be applied.
Unit Testing user interfaces is much more difficult and MAY be skipped, but at least the controller or presenter (in MVC or MVP respectively) MUST be tested. MVP has some advantages to make Unit Testing easier, but that's a whole other discussion ( see also http://www.martinfowler.com/eaaDev/uiArchs.html)
#7 Unit Tests MUST be kept in the same package as the source code
When Unit Test classes are kept in the same package, it is much easier to test all the methods (except for private methods) of a certain class.
You MUST keep the Unit Tests in another directory though to separate your Unit Tests from your actual application code. This makes releasing much easier.
#8 Unit Tests MUST be time independent
This might sound strange, but when a test is written to run only on Februari 29th or in the morning because your application behaves differently at that specific time for example, your tests are not guaranteed to reflect reality when they are executed on another time. When a job is scheduled to run on a certain time for example, you MUST NOT test the scheduled job itself (the actual code may never be executed), but you MUST test the actual business logic in the job.
For Java specific Unit Testing, be also very carefully when you are dependent on the Date or Calendar classes. There are many problems with those standard implementations (http://parleys.com/#st=5&id=100&sl=1) and pretty much any alternative, such as Joda time (http://joda-time.sourceforge.net/), is better suited, even in normal business logic outside Unit Tests!
#9 Unit Tests MUST NOT load data from a hard-coded location on a filesystem
All files MUST be put next to the actual test classes and retrieved as a stream through the classloader (ie. in Java: TestClass.class.
getClassLoader().getResource AsStream("<package>.< filename>") ).
When files need to be manipulated, you MUST copy them first to a temporary directory (ie. in Java: System.getProperty("java.io.
tmpdir")) and manipulate the files over there by injecting the reference to the resource in your actual business logic. You MUST empty the created files in this directory at the end of your test to comply with item #3.