Link to Java APIs from Kotlin API documentation

Closes gh-40694

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

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.build;
 
+import java.net.URI;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -23,6 +24,7 @@ import dev.adamko.dokkatoo.DokkatooExtension;
 import dev.adamko.dokkatoo.formats.DokkatooHtmlPlugin;
 import org.gradle.api.Project;
 import org.gradle.api.tasks.SourceSet;
+import org.gradle.api.tasks.SourceSetContainer;
 import org.jetbrains.kotlin.gradle.dsl.KotlinJvmOptions;
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile;
 
@@ -67,9 +69,24 @@ class KotlinConventions {
 	private void configureDokkatoo(Project project) {
 		project.getPlugins().apply(DokkatooHtmlPlugin.class);
 		DokkatooExtension dokkatoo = project.getExtensions().getByType(DokkatooExtension.class);
-		dokkatoo.getDokkatooSourceSets()
-			.named(SourceSet.MAIN_SOURCE_SET_NAME)
-			.configure((spec) -> spec.getSourceRoots().setFrom(project.file("src/main/kotlin")));
+		dokkatoo.getDokkatooSourceSets().named(SourceSet.MAIN_SOURCE_SET_NAME).configure((sourceSet) -> {
+			sourceSet.getSourceRoots().setFrom(project.file("src/main/kotlin"));
+			sourceSet.getClasspath()
+				.from(project.getExtensions()
+					.getByType(SourceSetContainer.class)
+					.getByName(SourceSet.MAIN_SOURCE_SET_NAME)
+					.getOutput());
+			sourceSet.getExternalDocumentationLinks().create("spring-boot-javadoc", (link) -> {
+				link.getUrl().set(URI.create("https://docs.spring.io/spring-boot/api/java/"));
+				link.getPackageListUrl().set(URI.create("https://docs.spring.io/spring-boot/api/java/element-list"));
+			});
+			sourceSet.getExternalDocumentationLinks().create("spring-framework-javadoc", (link) -> {
+				String url = "https://docs.spring.io/spring-framework/docs/%s/javadoc-api/"
+					.formatted(project.property("springFrameworkVersion"));
+				link.getUrl().set(URI.create(url));
+				link.getPackageListUrl().set(URI.create(url + "/element-list"));
+			});
+		});
 	}
 
 }