JUnit 5 Quick Start Guide and Advanced

Table of contents



Testing is important. We should all know that. They can help reproduce and fix problems in our code. They can also remind you that you shouldn’t have done something long after you forgot that you shouldn’t do that thing and a lot more.

The problem is often that developers see testing as some overhead for developing that should be done after the code is finished. That’s wrong: Testing should be a vital part of development!
That problem is something JUnit 5 can help you with. It introduced a lot of interesting, fast ways to create tests. Be it Test-Factories, Test-Extensions (that only have to be written once), Lambda-Support etc. It pushed the D.R.Y. principle a lot!

To understand this guide you should know the basics of testing and JUnit (4), but otherwise you wouldn’t be here I guess. You can also check out the best-practice-part included in this repository. It was created by request of a colleague:
Checkout JUnit best-practices included in this repository

Also JUnit 5 supports running parallel to JUnit 4 if that’s something you need.

Some headers related to code will have a code-link behind their name directing to the corresponding class in the GitHub-Repository.
For the whole source code see my GitHub-Repository:


For an IDE I recommend IntelliJ IDEA 2016.2(+) right now since it brings native support for JUnit 5.
As for dependencies:




  testCompile group: 'org.junit.jupiter', name: 'junit-jupiter-api', version: '5.0.0-M3'
  testCompile group: 'org.junit.platform', name: 'junit-platform-runner', version: '1.0.0-M3'
  testRuntime group: 'org.junit.jupiter', name: 'junit-jupiter-engine', version: '5.0.0-M3'

General changes (code)


This paragraph contains the small or general changes made in the transition from JUnit 4 to JUnit 5. Those are simple but still note worthy.

The first change is made to the most basic of things: the test and the @Test-annotation themselves. You no longer need to make the test public, however you can still not make it static or private. Also timeout and expected parameter functionality has moved elsewhere.



Other annotations have received slight changes as well, including the common @BeforeClass, @BeforeEach, their @After... equivalents, @Ignored and the lesser known @Category. All of these have been renamed and given the same treatment regarding public as @Test.


Assert and Assume classes have been renamed as well and are now called Assertions and Assumptions. Not much has changed for the naming of the methods of both classes.


New features: Basics (code)


Here I want to introduce some basics for the new features available in the new version.
There is a new pretty annotation called @DisplayName which is supposed to improve the readability of test reports, so you don’t need 40-character test-names to make clear what the test is about at a glance.



You can now also group tests with inner classes annotated with @Nested.



Assertions and Lambda-Support

Now for the probably most known and anticipated feature in JUnit 5: Lambda-Support…
JUnit 5 Assertions and Assumptions classes and its methods now provide Lambda support. This is achieved by providing methods with functional interfaces as parameters.

The most used ones are the BooleanSupplier and Supplier<String>. The first one is used for assertions and the latter one to provide a result-message. Those are however just alternatives to the older plain boolean and String.
Assertion methods like assertTrue(...) are now just overloaded with combinations of those four parameters:
(boolean | BooleanSupplier) & (String | Supplier<String>) resulting in 4 different methods. This is what most lambda-supporting methods are designed like.


A new important functional interface is Executable. It is very similar to a Runnable, however it throws a Throwable meaning you can execute assertions like assertTrue() and an AssertionError may be thrown affecting your test-result. It is used in several assertions like the new assertAll(Executable... executables) which can be also used to prevent repetition.


This new functional interface is also used in the new replacement of the old @Test-parameter expected which is called assertThrows(). It asserts whether an exception was thrown. If you need the exception-instance itself to e.g. assert the message, you can instead use expectThrows() which also has the exception as return type.



Parameter Resolver

The biggest new feature in JUnit 5 is the new Extension-API. A part of it is the ParameterResolver-Interface which is an extension of the Extension-Interface itself. The ParameterResolver-Interface provide a way for dependency injection on method level by injecting data into test-method parameters.
JUnit 5 provides two implementations by itself: TestInfo which contains some meta information and the appropriate Test-Method and Test-Class instances and TestReporter which can be used to publish test entries.
A lot more on the Extension-Api is following further below.


New features: Advanced (code)


Building upon the ParameterResolver paragraph of the last chapter let’s look at implementing your own ParameterResolver. You can also see the first visual sign of the Extension-API in the form of the @ExtendWith-Annotation. The final result is:


This is achieved by the following implementations:

The first implementation processes the String parameter className. It checks whether the parameter class is a String and throws an exception otherwise. To resolve and inject the parameter it just returns the test classes name.


The seconds implementation processes the Long parameter parameterIndex. It does basically the same but resolves the parameter by getting the index from the parameterContext.



Another big feature are the new Test-Factories. These are annotated with @TestFactory instead of @Test. Their return type is some kind of collection of DynamicTests. The class DynamicTest provides several static methods to create those. You basically have to provide test data and based on it a display name as well as some kind of Executable. In my example you can see me using the stream()-method of said class.




Here I will show you an Extension that is not based on the ParameterResolver but instead implements the TestExecutionCondition. The same thing that powers the @Disabled annotation. If we want to customize it we need out own implementation. There are about a dozen of those Extension categories. TestExecutionCondition is just one of them. Some are functional interfaces like the one we’re talking about, others like the ParameterResolver are not.
My example called @DisabledOnMonday does just that. It disables that test-method or -class on mondays. The implementation only checks for the weekday and returns an appropriate ConditionEvaluationResult resulting in the test being ignored when the weekday matches.



Again this could without problem be placed on class level.

Advanced Test-Samples (code)

Extended disabled weekdays

Let’s extend that @DisabledOnMonday annotation a bit. What if you want to choose the weekday? Creating 7 annotations is kind of overkill. A way to achieve this could be to add another annotation that contains the weekdays:


The @DisabledWeekdays annotation doesn’t do much more than hold an int array corresponding to the weekdays.


The extension looks slightly different now, since it needs to determine the weekdays from the annotation. Luckily the evaluate()-method provides the TestExtensionContext so it’s fairly easy to get those.


Extend Test-Annotation

So what if you want to save some that space occupied by all those annotations. Let’s make it all-in-one in this example:


What you basically do here is to create a new annotation and annotate that with @Test. Then you pack all you need in there like your extensions, parameter resolvers, targets, parameters, etc. The annotation @UITest above looks like this:


The extensions used do not really matter here. One extension resolves the Pane from the fxml path and the other one just prints some data. This is rather a showcase of an @Test-Extension and including utilizing the extension features of JUnit 5. If you want to see code nevertheless look into the repository.

Benchmarking Example

As for the last example right now I will showcase some benchmarking possibilities and it isn’t even that complicated. There are several extensions that can be used for that. BeforeAllCallback, BeforeTestExecutionCallback and their After...-equivalents. Each of these interfaces has a method that will be executed at some point during the tests. E.g. before each test or after etc. So by implementing those 4 interfaces in one extension we can create a class that timestamps each time a method is called and after it finished including calculating the difference. Then we just need to annotate an annotation @Benchmarked with that extension and then place that on top of a test-method or -class. Done. The final benchmarked test-method will look something like this:


The corresponding test-output:


The extension couldn’t be simpler:


Of course I could have also included @Benchmarked in a separate @BenchmarkedTest annotation that would have extended @Test as well saving that one line.

Milestone 3 Changes (code)



Closing words


Feel free to express critique and contribute to the repository :)


You can use this repository in any way you want. May it be for workshops or presentations. Just give credits. ;)


Further Reference

Official JUnit 5 User Guide
JUnit 5 GitHub
JUnit 5 Milestone plan