jqwik

jqwik is a property-based testing library for Java and the JVM. It implements the QuickCheck style of testing, generating large numbers of random inputs to validate properties and relationships of code under test. It integrates with JUnit 5, enabling tests to be discovered and run using standard build tools and IDEs.

A test in jqwik is a property annotated with @Property, and inputs can be declared with the @ForAll annotation on method parameters. The framework uses Arbitrary<T> generators to create values; developers can supply custom generators via @Provide methods that return Arbitrary<T> for their types.

Built-in and composable arbitraries: jqwik provides generators for primitive types, strings, collections, optionals, maps, and more, and supports composing them with functional combinators to build complex inputs. It also supports shrinking to simplify failing counterexamples, aiming to produce the smallest input that still reproduces a failure.

Execution and results: When a property runs, jqwik executes it against many generated inputs. If a counterexample is found, jqwik shrinks it to a minimal example and reports the seed used for reproduction, aiding debugging. Tests remain deterministic when a seed is fixed, facilitating reproducibility.

Open source and ecosystem: jqwik is open-source software available on its repository. It is actively maintained with documentation and examples, and it can be used with Maven or Gradle to run through the standard JUnit 5 test lifecycle.