Professor X: For someone who hates mutants... you certainly keep some strange company.
William Stryker: Oh, they serve their purpose... as long as they can be controlled.

For an introduction to mutation testing and Stryker's features, see stryker-mutator.io. Looking for mutation testing in JavaScript or .NET?

For the quickstart, see our website's quickstart.

Stryker4s is a mutation testing framework for Scala. It allows you to test your tests by temporarily inserting bugs. Depending on your project setup, there are multiple ways to get started with Stryker4s.

Stryker4s provides a sbt plugin for easy use within sbt projects. To install the plugin, add the following line to plugins.sbt :

addSbtPlugin("io.stryker-mutator" % "sbt-stryker4s" % stryker4sVersion)

After adding the plugin, Stryker4s can be used by running sbt stryker in the root of your project.

Multi-module projects are fully supported. Run Stryker on the desired module by running sbt <module-name>/stryker.

The Maven plugin can be added as follows in pom.xml under <plugins> :

<plugin>
    <groupId>io.stryker-mutator</groupId>
    <artifactId>stryker4s-maven-plugin</artifactId>
    <version>${stryker4s.version}</version>
</plugin>

You can then run Stryker4s with the command mvn stryker4s:run. Note that this is different than the command for the sbt plugin.

Stryker4s provides a plugin for the Mill build tool. Import the plugin in your build.mill and mix the Stryker4sModule trait into the ScalaModule you want to mutation test :

//| mvnDeps:
//| - io.stryker-mutator:::mill-stryker4s:<version>
package build
import mill.*, scalalib.*
import stryker4s.mill.Stryker4sModule

object foo extends ScalaModule, Stryker4sModule {
  // ...
  object test extends ScalaTests {
    // ...
  }
}

You can then run Stryker4s with ./mill foo.stryker, where foo is the name of your module. The tests of the module are run via its first child test module (e.g. object test extends ScalaTests); override strykerTestModule to point it elsewhere.

Configuration is read from stryker-prefixed overrides on the module (e.g. def strykerConcurrency = Some(4)), from stryker4s.conf, or from command-line arguments (e.g. ./mill foo.stryker --thresholds.break 80).

We also publish SNAPSHOT versions of each commit on master. To use a pre-release, add the following setting to your build.sbt and plugins.sbt:

resolvers += Resolver.sonatypeCentralSnapshots

Then replace the Stryker4s version with this version: .

See the configuration page for all the configuration options available in Stryker4s.

Stryker4s supports a variety of mutators, which can be found here. Do you have a suggestion for a (new) mutator? Feel free to create an issue!

An always up-to-date reference is also available in the MutantMatcher source.

See the releases page for all the latest changes made.

Want to contribute? That's great! Please have a look at our contributing guide.