Testing Your Codemod

Testing

If you want to write tests for your codemod (highly recommended, you're changing peoples' code!), let's show how.

First, we add the test dependency.

// could instead use any of the codeql, sonar, or pmd plugins
implementation("io.codemodder:codemodder-testutils:$VERSION")

This type adds a mixin called CodemodTestMixin.

This mixin reads annotation data from a JUnit test class and performs all the logic for validating the codemod based on convention. It provides the two basic following features:

verification of expected code changes
verification of dependencies injected (if any)

Here's the test for our ReadLinesCodemod:

@Metadata(
    codemodType = ReadLinesCodemod.class,
    testResourceDir = "readlines",
    dependencies = {})
final class ReadLinesCodemodTest implements CodemodTestMixin {}

That's it!

This mixin expects a directory called readlines in the test classpath (e.g., src/test/resources/readlines). This directory can have multiple file pairs that follow a name pattern like ArbitraryTestName.java.before and ArbitraryTestName.java.after. For each of these pairs, the codemod will be run on the .before file, and the output should match the .after file. These files end up being your test cases, and should be thorough.

You can also specify if you expect a codemod to be injected as a dependency into the project manifest when a change is made.

Testing​

Testing