The five best and coolest new features of JUnit 5!


You’ve heard of JUnit 5. It sounds pretty cool. So, maybe you want to move to JUnit 5, but you’re just not sure it’s all it will live up to the hype, right?

In this post, I’ll show you five of the best new features and capabilities of JUnit 5. When you’re done with this list, you’ll definitely want to make the move.

Feature #1 – Nested Unit Tests

Have you ever needed to group related JUnit tests together, and wrote a JUnit TestSuite to do it? Not because you really needed a test suite, really, but because that was the only way to do it? Were you wishing the whole time you could keep the tests in the same class? If so, the @Nested annotation will be welcome relief!

The new @Nested annotation feature of JUnit 5 lets you create cleaner unit test classes by using arbitrarily nested inner classes as additional units of test composition within a single test class. This allows you to keep the tests together, while at the same time allowing you to initialize them differently to simulate different runtime conditions.

Unit testing your code allows you to test all of the scenarios your code might encounter, but what good is that if you can’t make sense of the unit test reports? The ability to nest related test methods within the same test class, coupled with the new @DisplayName annotation, gives your unit tests a visual makeover. Your unit test reports never looked so good!

Feature #2 – JUnit Extensions

The JUnit 5 Extension Model follows the JUnit 5 team’s core principle of “Prefer extensions over features” and defines a number of extension points for which default implementations have been provided, but for which you may create Extensions to override the default behavior. These Extensions are callback interfaces that you implement and then register with the JUnit 5 framework, and when that extension point is reached when running a unit test, your code is invoked.

You might never need to write a custom extension, but the cool thing is, if you do, you use the same API that tool vendors use. In order to support JUnit, tool vendors often use its internal APIs, which limits what the JUnit maintainers can do in evolving the framework (it’s a catch-22). The Extension Model will allow JUnit to evolve by providing a consistent, public API for you, and tool vendors alike.

Plus, all public API methods are annotated with @API and values like:

  • Experimental – the feature is new and the JUnit team welcomes feedback,
  • Internal – the feature is internal to JUnit and you and I should steer clear of using it, and
  • Maintained – the feature is good (will not be changed in a non-backward-compatible way) until at least the next minor release).

Extensions indeed! JUnit 5 is so flexible they should code name it JUnit Yoga. Seriously.

Feature #3 – Conditions

Speaking of Extensions, JUnit 5 provides two extension points – ContainerExecutionCondition and TestExecutionCondition – that comprise the conditional test execution API. Oh, sure JUnit 4 let us @Ignore a test class or method, and JUnit 5 provides this behavior through the @Disabled annotation.

But if you want, you can create your own custom extension of one of both of the conditional test execution API methods to inspect the currently running test and decide on-the-fly, whether or not to run a test class (via ContainerExecutionCondition) or method (via TestExecutionCondition).

Wait, there’s more! JUnit 5 even allows you to deactivate conditions, like @Disabled (e.g., broken) unit tests, so you can do things like run them anyway to see if they’re still broken. Wow.

Feature #4 – Lambda Support

You love lambdas, right? I mean, who doesn’t? The ability to write compact code is very handy.

And now that feature is baked into JUnit 5 through the use of a few new FunctionalInterfaces like Executable. Methods like Assertions.assertAll() take Executable... as a varargs parameter for which you can substitute a lambda expression.

Other methods in Assertions and Assumptions use FunctionalInterfaces like Supplier<String>, which let you use lambda expressions to provide messages, and BooleanSupplier to provide a boolean result, respectively.

Feature #5 – Parameterized Tests

Writing unit tests can get tedious, especially if you need to run the same unit test with different inputs. In this case, you pretty much have to write multiple unit test methods, or multiple blocks within a single method (which can leave a smell to your unit test).  Have you ever wanted to make your unit tests a little more dynamic? What if you could setup a unit test method to run multiple times with different input values each time?

With parameterized tests, JUnit 5 allows you to specify one or more sources that supply your unit test method with the parameter values it needs to run its tests. The source generates the argument values that JUnit 5 then passes to your unit test method, one-at-a-time, until all argument values in the source are exhausted. Here are just a few examples of some of the out-of-the-box sources provided by JUnit 5:

  • @ValueSource – lets you specify an array of primitive literals that will be passed one-at-a-time to the test method.
  • @MethodSource – lets you specify one or more static, no-arg methods in the test class that returns a Stream, Iterable, Iterator, or array of argument values for your test method.
  • @CsvSource – lets you specify a list of literals that are passed to the test method (broken into tuples based on the number of arguments in your test method).
  • @ArgumentSource – lets you create your own custom arguments provider.


There are other cool features too, like:

If you are considering making the move to JUnit 5, I hope I’ve swayed you a little further. With cool features like this, why would you not make the switch?

Learn more about JUnit 5 and Java 9

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2 comments on"The five coolest new features of JUnit 5"

  1. These look like a lot of great features! I look forward to using them regularly when JUnit 5 is out for stable release.

    • Hi Benjamin, yes, the features of JUnit 5 are really awesome. And I hear your concern about the frequent release cycles, though the JUnit team are very careful to make backward compatible changes, and when not, to announce what breaking changes a release contains. I’ve noticed the releases go in streaks. Sometimes a couple of months goes by where it’s pretty quite, then there’s a frenzy of releases every week (like what is happening now). I highly recommend you just jump in and start using JUnit 5. You won’t regret it! Thanks for your comment, and thanks for reading! –jsp

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