feat: add IntelliJ IDEA formatter

[simschla]

This PR is based on @IlyasYOY work in #2020 🙏

It adds IntelliJ IDEA as a formatter for both gradle- and maven-plugin.

The implementation uses IntelliJ IDEA's formatting cli behind the scenes as intended by IntelliJ IDEA.

To make sure the IntelliJ IDEA instance launched by spotless is not interfering with a already running instance, we specify separated paths for idea.config.path and idea.system.path using a custom idea.properties file (see com.diffplug.spotless.generic.IdeaStep.State#createIdeaPropertiesFile).

For development, I've introduced a way to override the idea binary in a testenv.properties file since I do not find it feasible to add IDEA to my path just for this case.

I've tried adding a CI-step to let IDEA-tests run in CI. This part is untested and I'll work on it when the implementation is deemed feasible.

[Copilot]

Pull Request Overview

Introduces support for using IntelliJ IDEA’s command-line formatter in Spotless, including both Maven and Gradle plugins, test harness enhancements, and CI integration.

• Add IdeaStep implementation and test environment overrides for custom IDEA binary and code style settings.
• Integrate IDEA formatter into Maven (<idea>) and Gradle (idea()) DSLs, plus documentation and changelogs.
• Enhance test infrastructure (@IdeaTest, TestEnvVars, CI matrix) to run IDEA-based formatting tests.

Reviewed Changes

Copilot reviewed 24 out of 26 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
testlib/src/test/java/com/diffplug/spotless/generic/IdeaStepTest.java	Adds unit tests for IdeaStep behavior
testlib/src/main/resources/java/idea/full.dirty.java	Dirty Java sample for IDEA formatting tests
testlib/src/main/resources/java/idea/full.clean.java	Clean Java sample for IDEA formatting tests
testlib/src/main/java/com/diffplug/spotless/tag/IdeaTest.java	Introduces @IdeaTest JUnit tag
testlib/src/main/java/com/diffplug/spotless/ResourceHarness.java	Adds hasDifferentContent / notSameAsResource helpers
plugin-maven/src/test/java/com/diffplug/spotless/maven/generic/IdeaTest.java	Maven integration test for IDEA formatter
plugin-maven/src/test/java/com/diffplug/spotless/maven/MavenRunner.java	Extend MavenRunner to propagate system properties
plugin-maven/src/test/java/com/diffplug/spotless/maven/MavenIntegrationHarness.java	Propagate spotless* properties into Maven test runs
plugin-maven/src/main/java/com/diffplug/spotless/maven/generic/Idea.java	Implements Maven FormatterStepFactory for IDEA
plugin-maven/src/main/java/com/diffplug/spotless/maven/FormatterFactory.java	Registers IDEA step in Maven formatter factory
plugin-maven/build.gradle	Pass spotlessProjectDir system property for IDEA tests
plugin-maven/README.md	Document IDEA support in Maven plugin
plugin-maven/CHANGES.md	Changelog entry for new IDEA support
plugin-gradle/src/test/java/com/diffplug/gradle/spotless/JavaIdeaTest.java	Gradle integration test for IDEA formatter
plugin-gradle/src/main/java/com/diffplug/gradle/spotless/JavaExtension.java	Minor comment fix
plugin-gradle/src/main/java/com/diffplug/gradle/spotless/FormatExtension.java	Adds idea() DSL and IdeaConfig for IDEA formatting
plugin-gradle/README.md	Document IDEA support in Gradle plugin
plugin-gradle/CHANGES.md	Changelog entry for new IDEA support
lib/src/main/java/com/diffplug/spotless/generic/TestEnvVars.java	Implements loading of testenv.properties for overrides
lib/src/main/java/com/diffplug/spotless/generic/IdeaStep.java	Core implementation of the IDEA formatting step
gradle/special-tests.gradle	Include idea in special test suite
INTELLIJ_IDEA_SCREENSHOTS.md	Add docs for extracting IDEA code style settings
.github/workflows/ci.yml	Add CI matrix entry and steps to run IDEA-based tests

Comments suppressed due to low confidence (3)

plugin-maven/README.md:41

• [nitpick] Remove the stray // TODO add idea comment in the Languages section header; the documentation already includes IDEA, so clean up the header to avoid confusion.

- **Languages** // TODO add idea

testlib/src/test/java/com/diffplug/spotless/generic/IdeaStepTest.java:82

• [nitpick] The test method name formattingsWithOutDefaultDoesNothing has inconsistent casing and a typo (WithOut); consider renaming it to formattingWithoutDefaultsDoesNothing for clarity.

	void formattingsWithOutDefaultDoesNothing() throws Exception {

testlib/src/test/java/com/diffplug/spotless/generic/IdeaStepTest.java:115

• [nitpick] Consider adding tests for custom binaryPath and codeStyleSettingsPath builder options to verify that overrides are applied correctly.

}

[Pankraz76]

amazing feat thanks.

• How will it effect the code later on?
  • Can we apply specific inspections as well?
  • Or does it just apply the default format settings imposed by idea?

[simschla]

amazing feat thanks.

• How will it effect the code later on?

  • Can we apply specific inspections as well?
  • Or does it just apply the default format settings imposed by idea?

The implementation uses IntelliJ's command line interface for formatting. So no, there is no support for inspections atm.

[nedtwigg]

Amazing! This LGTM. If you're happy with it too then press merge and I'll cut a release this week.

[simschla]

Awesome! I'll go ahead and merge it then 😄

[nedtwigg]

Released in plugin-gradle 7.1.0 and plugin-maven 2.45.0.