Fluent assertion library simplifies writing and extending custom assertion methods. Supports multiple assertions, nullability, iterable assertions, extracting data, table assertions, and custom assertion creation.
assertk is a fluent assertion library for Kotlin inspired by AssertJ.
You might be asking, "If AssertJ already exists, why create another library?". It's true, assertk is very similar to AssertJ. But assertk is written in Kotlin so it has one major advantage: extension methods. This makes adding your own assertion methods far simpler.
See Custom Assertions below to find out how to do this.
repositories {
mavenCentral()
}
dependencies {
testImplementation("com.willowtreeapps.assertk:assertk:0.28.1")
}
assertk has full multiplatform support and it can be used in jvm, js, or native projects. For example to set it up using the multiplatform plugin:
plugins {
kotlin("multiplatform")
}
kotlin {
sourceSets {
val commonTest by getting {
dependencies {
implementation("com.willowtreeapps.assertk:assertk:0.28.1")
}
}
}
}
Simple usage is to wrap the value or property you are testing in assertThat() and call assertion methods on the
result.
You can see all built-in assertions in the docs.
Since null is a first-class concept in kotlin's type system, you need to be explicit in your assertions.
val nullString: String? = null
assertThat(nullString).hasLength(4)
will not compile, since hasLength() only makes sense on non-null values. You can chain isNotNull() to handle this.
val nullString: String? = null
assertThat(nullString).isNotNull().hasLength(4)
// -> expected to not be null
This will first ensure the string is not null before running any other checks.
You can assert multiple things on a single value by providing a lambda as the second argument. All assertions will be run even if the first one fails.
val string = "Test"
assertThat(string).all {
startsWith("L")
hasLength(3)
}
// -> The following 2 assertions failed:
// - expected to start with:<"L"> but was:<"Test">
// - expected to have length:<3> but was:<"Test"> (4)
You can wrap multiple assertions in an assertAll to ensure all of them get run, not just the first one.
assertAll {
assertThat(false).isTrue()
assertThat(true).isFalse()
}
// -> The following 2 assertions failed:
// - expected to be true
// - expected to be false
You can assert on the contents of an Iterable/List with the various contains* functions. They have different
semantics as follows:
There's a few ways you extract the data you want to assert on. While you can do this yourself before calling the assertion, these methods will add the extra context to the failure message which can be helpful.
The simplest way is with prop(). It will take a property (or function, or a name and a lambda) and return an
assertion on that property.
val person = Person(age = 22)
assertThat(person).prop(Person::age).isEqualTo(20)
// -> expected [age]:<2[0]> but was:<2[2]> (Person(age=22))
For collections, you can use index() to pull a specific index from a list, and key() to pull a specific value from
a map.
assertThat(listOf(1, 2, 3)).index(1).isEqualTo(1)
// -> expected: [[1]]:<1> but was:<2> ([1, 2, 3])
assertThat(mapOf("one" to 1, "two" to 2, "three" to 3)).key("two").isEqualTo(1)
// -> expected: [["two"]]:<1> but was:<2> ({"one"=1, "two"=2, "three"=3})
You can also extract a property from a collection using extracting().
val people = listOf(Person(name = "Sue"), Person(name = "Bob"))
assertThat(people)
.extracting(Person::name)
.containsExactly("Sue", "Bob")
If you expect an exception to be thrown, you can use assertFailure which takes a lambda.
assertFailure {
throw Exception("error")
}.hasMessage("wrong")
// -> expected [message] to be:<["wrong"]> but was:<["error"]>
If you have multiple sets of values you want to test with, you can create a table assertion.
tableOf("a", "b", "result")
.row(0, 0, 1)
.row(1, 2, 4)
.forAll { a, b, result ->
assertThat(a + b).isEqualTo(result)
}
// -> the following 2 assertions failed:
// on row:(a=<0>,b=<0>,result=<1>)
// - expected:<[1]> but was:<[0]>
// on row:(a=<1>,b=<2>,result=<4>)
// - expected:<[4]> but was:<[3]>
Up to 4 columns are supported.
One of the goals of this library is to make custom assertions easy to make. All assertions are just extension methods.
fun Assert<Person>.hasAge(expected: Int) {
prop(Person::age).isEqualTo(expected)
}
assertThat(person).hasAge(20)
// -> expected [age]:<2[0]> but was:<2[2]> (Person(age=22))
For completely custom assertions, you have a few building blocks. given will give you the actual value to assert on,
and expected() and show() will help you format your failure message.
fun Assert<Person>.hasAge(expected: Int) = given { actual ->
if (actual.age == expected) return
expected("age:${show(expected)} but was age:${show(actual.age)}")
}
assertThat(person).hasAge(20)
// -> expected age:<20> but was age:<22>
You can also build assertions that chain by using transform. This allows you to both assert on the actual value, and
return something more specific that additional assertions can be chained on.
fun Assert<Person>.hasMiddleName(): Assert<String> = transform(appendName("middleName", separator = ".")) { actual ->
if (actual.middleName != null) {
actual.middleName
} else {
expected("to not be null")
}
}
assertThat(person).hasMiddleName().isEqualTo("Lorie")
// -> expected [middleName]:to not be null
Note: this is a bit of a contrived example as you'd probably want to build this out of existing assertions instead.
fun Assert<Person>.hasMiddleName(): Assert<String> = prop(Person::middleName).isNotNull()
The general rule of thumb is to prefer building out of the existing assertions unless you can give a more meaningful error message.
Contributions are more than welcome! Please see the Contributing Guidelines and be mindful of our Code of Conduct.
Surfaced from shared tags and platforms — no rankings paid for.
import assertk.assertThat
import assertk.assertions.*
class PersonTest {
val person = Person(name = "Bob", age = 18)
fun testName() {
assertThat(person.name).isEqualTo("Alice")
// -> expected:<["Alice"]> but was:<["Bob"]>
}
fun testAge() {
assertThat(person.age, "age").isGreaterThan(20)
// -> expected [age] to be greater than:<20> but was:<18>
}
fun testNameProperty() {
assertThat(person::name).isEqualTo("Alice")
// -> expected [name]:<["Alice"]> but was:<["Bob"]>
}
}
| Assertion | Description |
|---|
| containsAtLeast | Asserts the iterable contains at least the expected elements, in any order. The collection may also contain additional elements. |
| containsSubList | Asserts that a collection contains a subset of items the same order, but may have additional elements in the list. |
| containsOnly | Asserts the iterable contains only the expected elements, in any order. Duplicate values in the expected and actual are ignored. |
| containsExactlyInAnyOrder | Asserts the iterable contains exactly the expected elements, in any order. Each value in expected must correspond to a matching value in actual, and visa-versa. |
| containsExactly | Asserts the list contains exactly the expected elements. They must be in the same order and there must not be any extra elements. |
| containsNone | Asserts the iterable does not contain any of the expected elements |