Upgrade to Kotlin 2.1.0

Spring Framework 7.0 will use a Kotlin 2 baseline, using the latest 2.x
release at the time of the release.

This commit upgrades Kotlin to 2.1.0, and Kotlin Serialization and
Coroutines accordingly.

Closes gh-33629

Author: sdeleuze

buildSrc/build.gradle

@@ -20,7 +20,6 @@ ext {
 dependencies {
 	checkstyle "io.spring.javaformat:spring-javaformat-checkstyle:${javaFormatVersion}"
 	implementation "org.jetbrains.kotlin:kotlin-gradle-plugin:${kotlinVersion}"
-	implementation "org.jetbrains.kotlin:kotlin-compiler-embeddable:${kotlinVersion}"
 	implementation "org.gradle:test-retry-gradle-plugin:1.5.6"
 	implementation "io.spring.javaformat:spring-javaformat-gradle-plugin:${javaFormatVersion}"
 	implementation "io.spring.nohttp:nohttp-gradle:0.0.11"

buildSrc/src/main/java/org/springframework/build/KotlinConventions.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,15 +16,14 @@
 
 package org.springframework.build;
 
-import java.util.ArrayList;
-import java.util.List;
-
 import org.gradle.api.Project;
-import org.jetbrains.kotlin.gradle.dsl.KotlinJvmOptions;
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget;
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion;
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile;
 
 /**
  * @author Brian Clozel
+ * @author Sebastien Deleuze
  */
 public class KotlinConventions {
 
@@ -34,15 +33,14 @@ void apply(Project project) {
 	}
 
 	private void configure(KotlinCompile compile) {
-		KotlinJvmOptions kotlinOptions = compile.getKotlinOptions();
-		kotlinOptions.setApiVersion("1.7");
-		kotlinOptions.setLanguageVersion("1.7");
-		kotlinOptions.setJvmTarget("17");
-		kotlinOptions.setJavaParameters(true);
-		kotlinOptions.setAllWarningsAsErrors(true);
-		List<String> freeCompilerArgs = new ArrayList<>(compile.getKotlinOptions().getFreeCompilerArgs());
-		freeCompilerArgs.addAll(List.of("-Xsuppress-version-warnings", "-Xjsr305=strict", "-opt-in=kotlin.RequiresOptIn"));
-		compile.getKotlinOptions().setFreeCompilerArgs(freeCompilerArgs);
+		compile.compilerOptions(options -> {
+			options.getApiVersion().set(KotlinVersion.KOTLIN_2_1);
+			options.getLanguageVersion().set(KotlinVersion.KOTLIN_2_1);
+			options.getJvmTarget().set(JvmTarget.JVM_17);
+			options.getJavaParameters().set(true);
+			options.getAllWarningsAsErrors().set(true);
+			options.getFreeCompilerArgs().addAll("-Xsuppress-version-warnings", "-Xjsr305=strict", "-opt-in=kotlin.RequiresOptIn");
+		});
 	}
 
 }

framework-docs/modules/ROOT/pages/languages/kotlin/requirements.adoc

@@ -2,7 +2,7 @@
 = Requirements
 :page-section-summary-toc: 1
 
-Spring Framework supports Kotlin 1.7+ and requires
+Spring Framework supports Kotlin 2.1+ and requires
 https://search.maven.org/artifact/org.jetbrains.kotlin/kotlin-stdlib[`kotlin-stdlib`]
 and https://search.maven.org/artifact/org.jetbrains.kotlin/kotlin-reflect[`kotlin-reflect`]
 to be present on the classpath. They are provided by default if you bootstrap a Kotlin project on

framework-platform/framework-platform.gradle

@@ -18,8 +18,8 @@ dependencies {
 	api(platform("org.assertj:assertj-bom:3.26.3"))
 	api(platform("org.eclipse.jetty:jetty-bom:12.0.15"))
 	api(platform("org.eclipse.jetty.ee10:jetty-ee10-bom:12.0.15"))
-	api(platform("org.jetbrains.kotlinx:kotlinx-coroutines-bom:1.8.1"))
-	api(platform("org.jetbrains.kotlinx:kotlinx-serialization-bom:1.6.3"))
+	api(platform("org.jetbrains.kotlinx:kotlinx-coroutines-bom:1.9.0"))
+	api(platform("org.jetbrains.kotlinx:kotlinx-serialization-bom:1.7.3"))
 	api(platform("org.junit:junit-bom:5.11.3"))
 	api(platform("org.mockito:mockito-bom:5.14.2"))
 

gradle.properties

@@ -4,7 +4,7 @@ org.gradle.caching=true
 org.gradle.jvmargs=-Xmx2048m
 org.gradle.parallel=true
 
-kotlinVersion=1.9.25
+kotlinVersion=2.1.0
 
 kotlin.jvm.target.validation.mode=ignore
 kotlin.stdlib.default.dependency=false

spring-context/spring-context.gradle

@@ -47,6 +47,7 @@ dependencies {
 	testImplementation("org.awaitility:awaitility")
 	testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-core")
 	testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-reactor")
+	testImplementation("io.projectreactor:reactor-test")
 	testImplementation("io.reactivex.rxjava3:rxjava")
 	testImplementation('io.micrometer:context-propagation')
 	testImplementation("io.micrometer:micrometer-observation-test")