Binary compatibility validator

The tool allows to dump binary API of a Kotlin library that is public in sense of Kotlin visibilities and ensures that the public binary API wasn't changed in a way that make this change binary incompatible.

Setup
What constitutes the public API
- Classes
- Members
What makes an incompatible change to the public binary API
- Class changes
- Class member changes

Setup

Binary compatibility validator is a Gradle plugin that is added to the build.gradle in the following way:

ext.validator_version = "0.2.4"

buildscript {
    dependencies {
        classpath "org.jetbrains.kotlinx:binary-compatibility-validator:$validator_version"
    }
}
                    
apply plugin: 'binary-compatibility-validator'

It is enough to apply the plugin only to the root configuration file; all sub-projects will be configured automatically.

Tasks

The plugin provides two tasks:

apiDump — builds the project and dumps its public API in project api subfolder. API is dumped in a human-readable format. If API dump already exists, it will be overwritten.
apiCheck — builds the project and checks that project's public API is the same as golden value in project api subfolder. This task is automatically inserted into check pipeline, so both build and check tasks will start checking public API upon their execution.

Optional parameters

Binary compatibility validator can be additionally configured with the following DSL:

apiValidation {
    /**
     * Packages that are excluded from public API dumps even if they
     * contain public API. 
     */
    ignoredPackages += ["kotlinx.coroutines.internal"]

    /**
     * Sub-projects that are excluded from API validation 
     */
    ignoredProjects += ["benchmarks", "examples"]
    
    /**
     * Set of annotations that exclude API from being public.
     * Typically, it is all kinds of `@InternalApi` annotations that mark 
     * effectively private API that cannot be actually private for technical reasons.
     */
    nonPublicMarkers += ["my.package.MyInternalApiAnnotation"]

    /**
     * Flag to programmatically disable compatibility validator
     */
    validationDisabled = true
}

Workflow

When starting to validate your library public API, we recommend the following workflow:

Preparation phase (one-time action):
- As the first step, apply the plugin, configure it and execute apiDump.
- Validate your public API manually.
- Commit .api files to your VCS.
- At this moment, default check task will validate public API along with test run and will fail the build if API differs.
Regular workflow
- When doing code changes that do not imply any changes in public API, no additional actions should be performed. check task on your CI will validate everything.
- When doing code changes that imply changes in public API, whether it is a new API or adjustments in existing one, check task will start to fail. apiDump should be executed manually, the resulting diff in .api file should be verified: only signatures you expected to change should be changed.
- Commit the resulting .api diff along with code changes.

What constitutes the public API

Classes

A class is considered to be effectively public if all of the following conditions are met:

it has public or protected JVM access (ACC_PUBLIC or ACC_PROTECTED)
it has one of the following visibilities in Kotlin:
- no visibility (means no Kotlin declaration corresponds to this compiled class)
- public
- protected
- internal, only in case if the class is annotated with PublishedApi
it isn't a local class
it isn't a synthetic class with mappings for when tableswitches ($WhenMappings)
it contains at least one effectively public member, in case if the class corresponds to a kotlin file with top-level members or a multifile facade
in case if the class is a member in another class, it is contained in the effectively public class
in case if the class is a protected member in another class, it is contained in the non-final class

Members

A member of the class (i.e. a field or a method) is considered to be effectively public if all of the following conditions are met:

it has public or protected JVM access (ACC_PUBLIC or ACC_PROTECTED)
it has one of the following visibilities in Kotlin:
- no visibility (means no Kotlin declaration corresponds to this class member)
- public
- protected
- internal, only in case if the class is annotated with PublishedApi
Note that Kotlin visibility of a field exposed by lateinit property is the visibility of its setter.
in case if the member is protected, it is contained in non-final class
it isn't a synthetic access method for a private field

What makes an incompatible change to the public binary API

Class changes

For a class a binary incompatible change is:

changing the full class name (including package and containing classes)
changing the superclass, so that the class no longer has the previous superclass in the inheritance chain
changing the set of implemented interfaces so that the class no longer implements interfaces it had implemented before
changing one of the following access flags:
- ACC_PUBLIC, ACC_PROTECTED, ACC_PRIVATE — lessening the class visibility
- ACC_FINAL — making non-final class final
- ACC_ABSTRACT — making non-abstract class abstract
- ACC_INTERFACE — changing class to interface and vice versa
- ACC_ANNOTATION — changing annotation to interface and vice versa

Class member changes

For a class member a binary incompatible change is:

changing its name
changing its descriptor (erased return type and parameter types for methods); this includes changing field to method and vice versa
changing one of the following access flags:
- ACC_PUBLIC, ACC_PROTECTED, ACC_PRIVATE — lessening the member visibility
- ACC_FINAL — making non-final field or method final
- ACC_ABSTRACT — making non-abstract method abstract
- ACC_STATIC — changing instance member to static and vice versa

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