Properly fix Java 8 API compatibility (#2218)

buildSrc/src/main/kotlin/Java9Modularity.kt

@@ -3,12 +3,16 @@
  */
 
 import org.gradle.api.*
+import org.gradle.api.file.*
+import org.gradle.api.provider.*
+import org.gradle.api.tasks.*
 import org.gradle.api.tasks.bundling.*
 import org.gradle.api.tasks.compile.*
+import org.gradle.jvm.toolchain.*
 import org.gradle.kotlin.dsl.*
+import org.gradle.process.*
 import org.jetbrains.kotlin.gradle.dsl.*
 import org.jetbrains.kotlin.gradle.plugin.mpp.*
-import org.jetbrains.kotlin.gradle.plugin.mpp.pm20.*
 import org.jetbrains.kotlin.gradle.plugin.mpp.pm20.util.*
 import org.jetbrains.kotlin.gradle.targets.jvm.*
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
@@ -46,19 +50,17 @@ object Java9Modularity {
 
                 kotlin.sourceSets.create(sourceSetName) {
                     val sourceFile = this.kotlin.find { it.name == "module-info.java" }
-                    val targetFile = compileKotlinTask.destinationDirectory.file("../module-info.class").get().asFile
+                    val targetDirectory = compileKotlinTask.destinationDirectory.map {
+                        it.dir("../${it.asFile.name}Module")
+                    }
 
                     // only configure the compilation if necessary
                     if (sourceFile != null) {
-                        // the default source set depends on this new source set
-                        defaultSourceSet.dependsOn(this)
-
                         // register a new compile module task
-                        val compileModuleTask = registerCompileModuleTask(compileModuleTaskName, compileKotlinTask, sourceFile, targetFile)
+                        val compileModuleTask = registerCompileModuleTask(compileModuleTaskName, compileKotlinTask, sourceFile, targetDirectory)
 
                         // add the resulting module descriptor to this target's artifact
-                        artifactTask.dependsOn(compileModuleTask)
-                        artifactTask.from(targetFile) {
+                        artifactTask.from(compileModuleTask) {
                             if (multiRelease) {
                                 into("META-INF/versions/9/")
                             }
@@ -73,39 +75,43 @@ object Java9Modularity {
         }
     }
 
-    private fun Project.registerCompileModuleTask(taskName: String, compileTask: KotlinCompile, sourceFile: File, targetFile: File) =
+    private fun Project.registerCompileModuleTask(taskName: String, compileTask: KotlinCompile, sourceFile: File, targetDirectory: Provider<out Directory>) =
         tasks.register(taskName, JavaCompile::class) {
-            // Also add the module-info.java source file to the Kotlin compile task;
-            // the Kotlin compiler will parse and check module dependencies,
-            // but it currently won't compile to a module-info.class file.
-            compileTask.source(sourceFile)
-
-
             // Configure the module compile task.
-            dependsOn(compileTask)
             source(sourceFile)
-            outputs.file(targetFile)
             classpath = files()
-            destinationDirectory.set(compileTask.destinationDirectory)
-            sourceCompatibility = JavaVersion.VERSION_1_9.toString()
-            targetCompatibility = JavaVersion.VERSION_1_9.toString()
+            destinationDirectory.set(targetDirectory)
+            javaCompiler.set(
+                this@registerCompileModuleTask.the<JavaToolchainService>().compilerFor {
+                    languageVersion.set(JavaLanguageVersion.of(11))
+                }
+            )
+            options.release.set(9)
+
+            options.compilerArgumentProviders.add(object : CommandLineArgumentProvider {
+                @get:CompileClasspath
+                val compileClasspath = compileTask.libraries
 
-            doFirst {
-                // Provide the module path to the compiler instead of using a classpath.
-                // The module path should be the same as the classpath of the compiler.
-                options.compilerArgs = listOf(
-                    "--release", "9",
-                    "--module-path", compileTask.libraries.asPath,
+                @get:CompileClasspath
+                val compiledClasses = compileTask.destinationDirectory
+
+                @get:Input
+                val moduleName = sourceFile
+                    .readLines()
+                    .single { it.contains("module ") }
+                    .substringAfter("module ")
+                    .substringBefore(' ')
+                    .trim()
+
+                override fun asArguments() = mutableListOf(
+                    // Provide the module path to the compiler instead of using a classpath.
+                    // The module path should be the same as the classpath of the compiler.
+                    "--module-path",
+                    compileClasspath.asPath,
+                    "--patch-module",
+                    "$moduleName=${compiledClasses.get()}",
                     "-Xlint:-requires-transitive-automatic"
                 )
-            }
-
-            doLast {
-                // Move the compiled file out of the Kotlin compile task's destination dir,
-                // so it won't disturb Gradle's caching mechanisms.
-                val compiledFile = destinationDirectory.file(targetFile.name).get().asFile
-                targetFile.parentFile.mkdirs()
-                compiledFile.renameTo(targetFile)
-            }
+            })
         }
 }

formats/json/jvmMain/src/kotlinx/serialization/json/internal/CharsetReader.kt

@@ -20,20 +20,7 @@ internal class CharsetReader(
             .onMalformedInput(CodingErrorAction.REPLACE)
             .onUnmappableCharacter(CodingErrorAction.REPLACE)
         byteBuffer = ByteBuffer.wrap(ByteArrayPool8k.take())
-        // An explicit cast is needed here due to an API change in Java 9, see #2218.
-        //
-        // In Java 8 and earlier, the `flip` method was final in `Buffer`, and returned a `Buffer`.
-        // In Java 9 and later, the method was opened, and `ByteFuffer` overrides it, returning a `ByteBuffer`.
-        //
-        // You could observe this by decompiling this call with `javap`:
-        // Compiled with Java 8 it produces `INVOKEVIRTUAL java/nio/ByteBuffer.flip ()Ljava/nio/Buffer;`
-        // Compiled with Java 9+ it produces `INVOKEVIRTUAL java/nio/ByteBuffer.flip ()Ljava/nio/ByteBuffer;`
-        //
-        // This causes a `NoSuchMethodError` when running a class, compiled with a newer Java version, on Java 8.
-        //
-        // To mitigate that, `--bootclasspath` / `--release` options were introduced in `javac`, but there are no
-        // counterparts for these options in `kotlinc`, so an explicit cast is required.
-        (byteBuffer as Buffer).flip() // Make empty
+        byteBuffer.flip() // Make empty
     }
 
     @Suppress("NAME_SHADOWING")
@@ -105,7 +92,7 @@ internal class CharsetReader(
             if (bytesRead < 0) return bytesRead
             byteBuffer.position(position + bytesRead)
         } finally {
-            (byteBuffer as Buffer).flip() // see the `init` block in this class for the reasoning behind the cast
+            byteBuffer.flip()
         }
         return byteBuffer.remaining()
     }

gradle/configure-source-sets.gradle

@@ -2,14 +2,15 @@
  * Copyright 2017-2022 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(8))
+    }
+}
+
 kotlin {
     jvm {
         withJava()
-        configure([compilations.main, compilations.test]) {
-            kotlinOptions {
-                jvmTarget = '1.8'
-            }
-        }
     }
 
     js {

settings.gradle

@@ -2,6 +2,10 @@
  * Copyright 2017-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
+plugins {
+    id 'org.gradle.toolchains.foojay-resolver-convention' version '0.5.0'
+}
+
 rootProject.name = 'kotlinx-serialization'
 
 include ':kotlinx-serialization-core'