Use a dumber offlineLinks map but expose more structured information to gradle.

gradle/documentation/render-javadoc.gradle

@@ -1,7 +1,5 @@
 import org.gradle.internal.jvm.Jvm
 
-import javax.annotation.Nullable
-
 /*
  * Licensed to the Apache Software Foundation (ASF) under one or more
  * contributor license agreements.  See the NOTICE file distributed with
@@ -261,8 +259,26 @@ configure(project(':lucene:luke')) {
   project.tasks.matching { it.name == 'renderSiteJavadoc' }.configureEach { it.enabled = false }
 }
 
+class OfflineLink implements Serializable {
+  @Input
+  String url
+
+  @InputDirectory
+  @PathSensitive(PathSensitivity.RELATIVE)
+  @IgnoreEmptyDirectories
+  File location
+
+  OfflineLink(String url, File location) {
+    this.url = url
+    this.location = location
+  }
+}
+
+@CacheableTask
 class RenderJavadocTask extends DefaultTask {
   @InputFiles
+  @PathSensitive(PathSensitivity.RELATIVE)
+  @IgnoreEmptyDirectories
   @SkipWhenEmpty
   SourceDirectorySet srcDirSet;
   
@@ -284,8 +300,14 @@ class RenderJavadocTask extends DefaultTask {
   @Input
   boolean relativeProjectLinks = false
 
-  @Input
-  def offlineLinks = [:]
+  @Internal
+  Map<String, File> offlineLinks = [:]
+
+  // Computes cacheable inputs from the map in offlineLinks.
+  @Nested
+  List<OfflineLink> getCacheableOfflineLinks() {
+    return offlineLinks.collect { url, location -> new OfflineLink(url, location) }
+  }
 
   @Input
   @Optional
@@ -312,8 +334,10 @@ class RenderJavadocTask extends DefaultTask {
   final Property<String> executable = project.objects.property(String).convention(
       project.provider { Jvm.current().javadocExecutable.toString() })
 
-  @Input
-  def taskResources
+  @InputDirectory
+  @PathSensitive(PathSensitivity.RELATIVE)
+  @IgnoreEmptyDirectories
+  File taskResources
 
   /** Utility method to recursively collect all tasks with same name like this one that we depend on */
   private Set findRenderTasksInDependencies() {