Configure Javadoc links to external sources

build.gradle

@@ -103,5 +103,11 @@ configure(moduleProjects) {
 		}
 	}
 
+	ext.javadocLinks = [
+			"https://docs.oracle.com/javase/8/docs/api/",
+			"https://docs.spring.io/spring-framework/docs/5.3.x/javadoc-api/",
+			"https://javadoc.io/doc/com.graphql-java/graphql-java/17.3/"
+	] as String[]
+
 	apply from: "${rootDir}/gradle/publishing.gradle"
 }

gradle/publishing.gradle

@@ -4,12 +4,15 @@ plugins.withType(JavaPlugin) {
 	javadoc {
 		description = "Generates project-level javadoc for use in -javadoc jar"
 
-		options.encoding = "UTF-8"
-		options.memberLevel = JavadocMemberLevel.PROTECTED
-		options.author = true
-		options.header = project.name
-		options.use = true
-		options.addStringOption("Xdoclint:none", "-quiet")
+		options {
+			encoding = "UTF-8"
+			memberLevel = JavadocMemberLevel.PROTECTED
+			author = true
+			header = project.name
+			use = true
+			links = project.ext.javadocLinks
+			addStringOption("Xdoclint:none", "-quiet")
+		}
 
 		// Suppress warnings due to cross-module @see and @link references.
 		// Note that global 'api' task does display all warnings.