Publish a runtime variant that supports Java 22

In order to support Java 22, we must use spring-core 6.1.x.
spring-core 6.1.x is a multi-release jar so, in order to support Java
22, a version of Gradle that supports multi-release jars must be
used.

This commit adds a new variant to spring-boot-gradle-plugin for
modern versions of Gradle. When Gradle's plugin API version is 8.7 or
later, we use spring-core 6.1.x. spring-core 6.0.x is used at all
other times.

Closes spring-projectsgh-40074

buildSrc/src/main/java/org/springframework/boot/build/MavenPublishingConventions.java

@@ -83,7 +83,6 @@ private void customizeMavenPublication(MavenPublication publication, Project pro
 		project.getPlugins()
 			.withType(JavaPlugin.class)
 			.all((javaPlugin) -> customizeJavaMavenPublication(publication, project));
-		suppressMavenOptionalFeatureWarnings(publication);
 	}
 
 	private void customizePom(MavenPom pom, Project project) {
@@ -102,7 +101,10 @@ private void customizePom(MavenPom pom, Project project) {
 	}
 
 	private void customizeJavaMavenPublication(MavenPublication publication, Project project) {
-		addMavenOptionalFeature(project);
+		addMavenOptionalFeature(publication, project);
+		if (publication.getName().equals("pluginMaven")) {
+			return;
+		}
 		publication.versionMapping((strategy) -> strategy.usage(Usage.JAVA_API, (mappingStrategy) -> mappingStrategy
 			.fromResolutionOf(JavaPlugin.RUNTIME_CLASSPATH_CONFIGURATION_NAME)));
 		publication.versionMapping(
@@ -112,9 +114,10 @@ private void customizeJavaMavenPublication(MavenPublication publication, Project
 	/**
 	 * Add a feature that allows maven plugins to declare optional dependencies that
 	 * appear in the POM. This is required to make m2e in Eclipse happy.
+	 * @param publication the project's Maven publication
 	 * @param project the project to add the feature to
 	 */
-	private void addMavenOptionalFeature(Project project) {
+	private void addMavenOptionalFeature(MavenPublication publication, Project project) {
 		JavaPluginExtension extension = project.getExtensions().getByType(JavaPluginExtension.class);
 		extension.registerFeature("mavenOptional",
 				(feature) -> feature.usingSourceSet(extension.getSourceSets().getByName("main")));
@@ -123,6 +126,7 @@ private void addMavenOptionalFeature(Project project) {
 		javaComponent.addVariantsFromConfiguration(
 				project.getConfigurations().findByName("mavenOptionalRuntimeElements"),
 				ConfigurationVariantDetails::mapToOptional);
+		suppressMavenOptionalFeatureWarnings(publication);
 	}
 
 	private void suppressMavenOptionalFeatureWarnings(MavenPublication publication) {

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/build.gradle

@@ -15,7 +15,34 @@ configurations {
 		// Downgrade SLF4J is required for tests to run in Eclipse
 		resolutionStrategy.force("org.slf4j:slf4j-api:1.7.36")
 	}
-	all {
+	modernGradleRuntimeClasspath {
+		extendsFrom runtimeClasspath
+		canBeConsumed = false
+		canBeResolved = true
+	}
+	modernGradleRuntimeElements {
+		extendsFrom configurations.implementation, configurations.runtimeOnly
+		canBeConsumed = true
+		canBeResolved = false
+		attributes {
+			attribute(Category.CATEGORY_ATTRIBUTE, project.objects.named(Category, Category.LIBRARY))
+			attribute(Bundling.BUNDLING_ATTRIBUTE, project.objects.named(Bundling, Bundling.EXTERNAL))
+			attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 17)
+			attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, project.objects.named(LibraryElements, LibraryElements.JAR))
+			attribute(Usage.USAGE_ATTRIBUTE, project.objects.named(Usage, Usage.JAVA_RUNTIME))
+			attribute(GradlePluginApiVersion.GRADLE_PLUGIN_API_VERSION_ATTRIBUTE, project.objects.named(GradlePluginApiVersion, "8.7"))
+		}
+		outgoing.artifacts.addAll(configurations.runtimeElements.outgoing.artifacts)
+	}
+	runtimeElements {
+		attributes {
+			attribute(GradlePluginApiVersion.GRADLE_PLUGIN_API_VERSION_ATTRIBUTE, project.objects.named(GradlePluginApiVersion, "7.5"))
+		}
+	}
+	all { configuration ->
+		if (configuration.name == 'modernGradleRuntimeClasspath') {
+			return
+		}
 		resolutionStrategy {
 			eachDependency { dependency ->
 				// Downgrade Jackson as Gradle cannot cope with 2.15.0's multi-version
@@ -44,6 +71,10 @@ configurations {
 	}
 }
 
+components.java.addVariantsFromConfiguration(configurations.modernGradleRuntimeElements) {
+	mapToMavenScope("runtime")
+}
+
 dependencies {
 	asciidoctorExtensions("io.spring.asciidoctor:spring-asciidoctor-extensions-section-ids")
 
@@ -166,3 +197,18 @@ artifacts {
 toolchain {
 	maximumCompatibleJavaVersion = JavaLanguageVersion.of(20)
 }
+
+publishing {
+	publications.matching { it.name == 'pluginMaven' }.configureEach {
+		versionMapping {
+			allVariants {
+				fromResolutionOf(JavaPlugin.RUNTIME_CLASSPATH_CONFIGURATION_NAME)
+			}
+		}
+		versionMapping {
+			variant(GradlePluginApiVersion.GRADLE_PLUGIN_API_VERSION_ATTRIBUTE, project.objects.named(GradlePluginApiVersion, "8.7")) {
+				fromResolutionOf("modernGradleRuntimeClasspath")
+			}
+		}
+	}
+}