Metadataquoi??

Transcript

1. Android Makers 2025 - Martin Bonnin Metadaquoi??? Gérez la compatibilité

   de vos applications et bibliothèques Kotlin
2. None

3. Agenda Java Kotlin Android Gradle KMP

4. Java features static public void doStuff() { var numbers =

   new ArrayList<Integer>(); numbers.add(1); numbers.add(2); numbers.add(3); numbers.removeFirst(); numbers.forEach(System.out :: println); } Java 10 (source) Java 21 (api) Java 8 (source) Java 7 (target)

5. Main.java:1: error: cannot access Stuff import Stuff; ^ bad class

   file: lib.jar(Stuff) class file has wrong version 67.0, should be 52.0 Please remove or make sure it appears in the correct subdirectory of the classpath. Execution failed for task ':compileJava'. > Could not resolve all files for configuration ':compileClasspath'. > Could not resolve com.example:stuff:0.0.0. Required by: root project : > Dependency resolution is looking for a library compatible with JVM runtime version 8, but ‘com.example:stuff:0.0.0’ is only compatible with JVM runtime version 23 or newer. Exception in thread "main" java.lang.UnsupportedClassVersionError: Main has been compiled by a more recent version of the Java Runtime (class file version 67.0), this version of the Java Runtime only recognizes class file versions up to 52.0 Exception in thread "main" java.lang.NoSuchMethodError: java.util.ArrayList.removeFirst()Ljava/lang/Object; at Main.main(Main.java:9) Using with Java 8 💥

6. java { toolchain { languageVersion = JavaLanguageVersion.of(8) } }

7. java { toolchain { languageVersion = JavaLanguageVersion.of(8) } } java

   { sourceCompatibility = JavaVersion.VERSION_1_8 targetCompatibility = JavaVersion.VERSION_1_8 }

8. java { toolchain { languageVersion = JavaLanguageVersion.of(8) } } java

   { sourceCompatibility = JavaVersion.VERSION_1_8 targetCompatibility = JavaVersion.VERSION_1_8 } tasks.withType(JavaCompile :: class.java) { options.release = 8 } release = sourceCompatibility + targetCompatibility + apiCompatibility

9. Kotlin 2.1.20 context(stream: PrintStream) fun doStuff() { val strings =

   mutableListOf( "one", "two", Uuid.random(), ) strings.removeFirst() strings.forEach(stream :: println) } kotlin { compilerOptions { jvmTarget.set(JvmTarget.JVM_1_8) } } Kotlin 2.1 (source + target) Kotlin 2.0 (api) Java 21 (api) build.gradle.kts Kotlin 1.4 (source)

10. Kotlin 2.1.20 context(stream: PrintStream) fun doStuff() { val strings =

    mutableListOf( "one", "two", Uuid.random(), ) strings.removeFirst() strings.forEach(stream :: println) } kotlin { compilerOptions { jvmTarget.set(JvmTarget.JVM_1_8) freeCompilerArgs.add("-Xjdk-release=8") } } Kotlin 2.1 (source + target) Kotlin 2.0 (api) Java 21 (api) build.gradle.kts Kotlin 1.4 (source)

11. Kotlin 2.1.20 context(stream: PrintStream) fun doStuff() { val strings =

    mutableListOf( "one", "two", Uuid.random(), ) strings.removeFirst() strings.forEach(stream :: println) } kotlin { compilerOptions { jvmTarget.set(JvmTarget.JVM_1_8) freeCompilerArgs.add("-Xjdk-release=8") } } // This is also needed tasks.withType(JavaCompile :: class.java) { options.release = 8 } Kotlin 2.1 (source + target) Kotlin 2.0 (api) Java 21 (api) build.gradle.kts Kotlin 1.4 (source)

12. e: file: // lib.jar!/META-INF/lib.kotlin_moduleModule was compiled with an incompatible version

    of Kotlin. The binary version of its metadata is 2.1.0, expected version is 1.9.0. Compiling with Kotlin 1.9.0 💥

13. Metadata, kesako? $ javap -v -p Stuff.class Classfile Stuff Compiled

    from “stuff.kt" Last modified Apr 5, 2025; size 2214 bytes public final class StuffKt minor version: 0 major version: 52 Constant pool: #1 = Utf8 StuffKt #2 = Class #1 // StuffKt … { public static final void doStuff(java.io.PrintStream); descriptor: (Ljava/io/PrintStream;)V flags: (0x0019) ACC_PUBLIC, ACC_STATIC, ACC_FINAL Code: stack=3, locals=8, args_size=1 0: aload_0 1: ldc #9 / / String stream 3: invokestatic #15 / / Method kotlin/jvm/internal/Intrinsics.checkNotNullParameter:(Ljava/lang/ Object;Ljava/lang/String;)V } ... InnerClasses: public static final #22= #27 of #21; // Companion=class kotlin/uuid/Uuid$Companion of class kotlin/uuid/Uuid RuntimeVisibleAnnotations: 0: #77(#78=[I#79,I#80,I#81],#82=I#79,#83=I#84,#85=[s#86],#87=[s#5,s#88,s#74,s#6,s#89]) kotlin.Metadata( mv=[2,1,0] k=2 xi=48 d1=["\u0000\u000e\n\u0000\n\u0002\u0010\u0002\n\u0002\u0018\u0002\n\u0002\b\u0002\u001a\u000f\u0010\u0000\u001a\u00020\u0001R \u00020\u0002¢\u0006\u0002\u0010\u0003¨\u0006\u0004"] d2=["doStuff","","Ljava/io/PrintStream;","(Ljava/io/PrintStream;)V","kotlin-compatibility-crash-course"] ) context(stream: PrintStream) fun doStuff() { val strings = mutableListOf( "one", "two", Uuid.random(), ) strings.removeFirst() strings.forEach(stream :: println) }

14. Kotlin Metadata https://kotlinlang.org/api/core/kotlin-stdlib/kotlin/-metadata/ https://github.com/JetBrains/kotlin/blob/master/core/metadata/src/metadata.proto

15. kotlin { compilerOptions { languageVersion.set(KotlinVersion.KOTLIN_1_9) apiVersion.set(KotlinVersion.KOTLIN_1_9) } coreLibrariesVersion = "1.9.0"

    } languageVersion & apiVersion •languageVersion: metadata version •apiVersion: stdlib version at compile time •coreLibrariesVersion: kotlin dependencies versions (stdlib, reflect, etc..)

16. kotlin { compilerOptions { languageVersion.set(KotlinVersion.KOTLIN_2_0) apiVersion.set(KotlinVersion.KOTLIN_1_9) } coreLibrariesVersion = "1.9.0"

    } languageVersion & apiVersion kotlinc 1.9.0 understands! •kotlinc has n+1 best effort forward compatibility •kotlinc has forever backward compatibility •kotlin-stdlib has forever backward compatibility

17. Android • art != openJDK • .dex != .class JVM

    javac rt.jar art D8 android.jar

18. android { kotlinOptions { jvmTarget = "8" } compileOptions {

    // also needed targetCompatibility = JavaVersion.VERSION_1_8 sourceCompatibility = JavaVersion.VERSION_1_8 } } jvmTarget

19. android { kotlinOptions { jvmTarget = "17" } compileOptions {

    // also needed targetCompatibility = JavaVersion.VERSION_17 sourceCompatibility = JavaVersion.VERSION_17 } } Same dex code jvmTarget 🤷

20. android { kotlinOptions { jvmTarget = "17" } compileOptions {

    // make Gradle happy targetCompatibility = JavaVersion.VERSION_17 sourceCompatibility = JavaVersion.VERSION_17 } compileSdk = 36 defaultConfig { minSdk = 21 } } compileSdk and minSdk

21. The importance of Lint

22. Gradle publishing { "formatVersion": "1.1", "component": { "group": "com.android.tools.build", "module":

    "gradle", "version": "8.9.1", "attributes": { "org.gradle.status": "release" } }, "createdBy": { "gradle": { "version": "8.11.1" } }, "variants": [ { "name": "apiElements", "attributes": { "org.gradle.category": "library", "org.gradle.dependency.bundling": "external", "org.gradle.jvm.environment": "standard-jvm", "org.gradle.jvm.version": 11, "org.gradle.libraryelements": "jar", "org.gradle.usage": "java-api", "org.jetbrains.kotlin.platform.type": "jvm" }, https://dl.google.com/android/maven2/com/android/tools/build/gradle/8.9.1/gradle-8.9.1.module kotlin { compilerOptions { jvmTarget.set(JvmTarget.JVM_11) freeCompilerArgs.add("-Xjdk-release=11") } } // This is also needed to make Gradle happy tasks.withType(JavaCompile :: class.java) { options.release = 11 }

23. https://github.com/gradle/gradle/issues/16345

24. Gradle embedded Kotlin plugins { id("org.jetbrains.kotlin.jvm").version("2.1.20") } kotlin { compilerOptions

    { // embeddedKotlinVersion = 2.0.21 KotlinVersion.fromVersion(embeddedKotlinVersion.substringBeforeLast(".")).let { languageVersion.set(it) apiVersion.set(it) } } coreLibrariesVersion = embeddedKotlinVersion }

25. KMP

26. (Simplified) Decision tree https://mbonnin.net/compatibility

27. •Use Java release. •kotlinc has n+1 forward compatibility. •Android jvmTarget

    doesn’t matter. •Isolate your Gradle tasks. •KMP is hard but getting better. Takeaways Use latest tooling with compatibility flags!

28. Merci !!! https://github.com/GradleUp/compat-patrouille

29. Merci !!! https://github.com/GradleUp/compat-patrouille/ 🐶 https://kotlinlang.org/docs/compatibility-modes.html https://kotlinlang.org/docs/kotlin-evolution-principles.html https://jakewharton.com/kotlins-jdk-release-compatibility-flag/ https://jakewharton.com/gradle-toolchains-are-rarely-a-good-idea/ https://blog.alllex.me/posts/2023-11-03-liberal-library-tooling/ https://www.liutikas.net/2025/01/10/Conservative-Librarian.html

    https://source.android.com/docs/core/runtime/dex-format#dex-file-magic https://github.com/JetBrains/kotlin/blob/master/core/metadata/src/metadata.proto https://youtrack.jetbrains.com/issue/KT-68792/Bump-KLIB-ABI-version-in-2.1 https://youtrack.jetbrains.com/issue/KT-55808/Support-metadata-version-checks-for-klibs-in-the-compiler https://youtrack.jetbrains.com/issue/KT-42293/Native-provide-binary-compatibility-between-incremental-releases https://youtrack.jetbrains.com/issue/KT-71375/Prevent-Kotlins-removeFirst-and-removeLast-from-causing-crashes- on-Android-14-and-below-after-upgrading-to-Android-API-Level-35 https://youtrack.jetbrains.com/issue/KT-58998/Require-stdlib-version-consistent-with-api-version https://mbonnin.net/2021-11-17_kotlin-compatibility-quicksheet/ https://mbonnin.net/compatibility