06. May 2022
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Kotlin Assertion Libraries - Kluent

Looking for a neat Kotlin assertion library? We got a blog series looking at several Kotlin assertion libraries for you! -> Kluent
Kluent Assertion Kotlin

Kotlin Assertions Kluent

IntroductionAssertJStriktAssertKKoTest AssertionsAtriumKluentConclusion

Looking at Kluent

The library Kluent markets itself as “Fluent Assertion Library for Kotlin“. In short it provides fluency and infix notations. However it has a hard dependency on Junit4 to make error assertions work properly. Additionally kotlin-test comes as a compile dependency.


Similarly to many other 3rd party libraries it is sufficent to add the dependency for Kluent. The current version is not directly obvious from the github repository. Therefore it can only be determined by looking into Maven Central or reading the CHANGELOG.md.



Syntax and Error messages

With Kluent you get the option for fluent or infix usage and provides backtick versions. Autocompletion always starts with should.

Fluent Style

The fluent interface acts comparable to AssertJ’s and allows the chaining of assertions. It will fail the test fast on the first error in the chain. Important to note is the constraint on the asserted type. E.g you can’t check the length of the String.


Softassertions in Kluent have the same general style as AssertJ.

Corresponding error message

This implementation wins over the Kotlin variant of AssertJ SoftAssertions.

Assertion of Exceptions

Kluents syntax for exception assertion is quite descriptive. It uses invoking and coInvoking as keywords. The latter is support for suspend functions.

Equality vs Equivalency

With the shouldBeEqualTo objects will be compared by hashCode/equals like usual. Kluent offers additional ways to compare objects. Comparable to AssertJs field comparisons the shouldBeEquivalentTo compares two objects by their properties. This also works for collections.

Additionally there are several helper functions which allow you to configure how you want to compare.  For example in-/excluding, ordering and recursion. Unfortunately this feature comes with the caveat of being @Opt(ExperimentalStdlibApi::class).

Nullability and Narrowing

There is no inherent narrowing supplied by Kluent. The only available narrowing is the casting from nullable objects to the non-nullabe types.
That means for a nullable types we have to narrow it down to the actual type before we can verify specifically against the underlying type, but it is still possible to assert e.g. String? against String?.

Supported Types and Diversity of Assertions

With Kluent you get mostly the basic assertions.  It covers general basics, numericals, charsequence and collections. Additionally it supports some asserts for java.time.LocalDate/LocalTime/LocalDateTime as well as java.io.File.

Custom Assertions

There is a generic should function by which Kluent allows you to define you own assertions.

You can either return false or throw an exception to fail the assertion.

Of Note

Kluent also wrapped MockitoKotlin until version 1.64 but dropped it since then. If you like a wrapper for mockitokotlin2 you can use the old Mocking.kt and MockingBacktick.kt files as reference (Open Source Licensing applies).

Active Development

The last release was in July 2021. At the time of writing there are 12 Issues and 4 open PRs. Considering the library aims to simply provide basic fluent assertion support there are not as many possible additional useful functionalities.

Quality of Documentation

The documentation is not really fancy but covers examples for every functionality. Unfortunately the version and build status are outdated. You have to actively search for the current version and the Changelog.

IntroductionAssertJStriktAssertKKoTest AssertionsAtriumKluentConclusion

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