Fix Dokka documentation for Kotlin+Java projects

buildSrc/src/main/kotlin/dokka-for-java.gradle.kts

@@ -25,6 +25,8 @@
  */
 
 import io.spine.internal.dependency.Dokka
+import io.spine.internal.gradle.dokka.onlyJavaSources
+
 import java.time.LocalDate
 import org.jetbrains.dokka.base.DokkaBase
 import org.jetbrains.dokka.base.DokkaBaseConfiguration
@@ -56,6 +58,10 @@ dependencies {
 
 tasks.withType<DokkaTask>().configureEach {
     dokkaSourceSets.configureEach {
+        sourceRoots.setFrom(
+            onlyJavaSources()
+        )
+
         skipEmptyPackages.set(true)
     }
 

buildSrc/src/main/kotlin/io/spine/internal/dependency/Dokka.kt

@@ -70,7 +70,7 @@ object Dokka {
     object SpineExtensions {
         private const val group = "io.spine.tools"
 
-        const val version = "2.0.0-SNAPSHOT.2"
+        const val version = "2.0.0-SNAPSHOT.3"
         const val lib = "${group}:spine-dokka-extensions:${version}"
     }
 }

buildSrc/src/main/kotlin/io/spine/internal/gradle/dokka/DokkaExtensions.kt

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2022, TeamDev. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Redistribution and use in source and/or binary forms, with or without
+ * modification, must retain the above copyright notice and the following
+ * disclaimer.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package io.spine.internal.gradle.dokka
+
+import java.io.File
+import java.time.LocalDate
+import org.gradle.api.file.FileCollection
+import org.jetbrains.dokka.base.DokkaBase
+import org.jetbrains.dokka.base.DokkaBaseConfiguration
+import org.jetbrains.dokka.gradle.DokkaTask
+import org.jetbrains.dokka.gradle.GradleDokkaSourceSetBuilder
+
+/**
+ * Returns only Java source roots out of all present in the source set.
+ *
+ * It is a helper method for generating documentation by Dokka only for Java code. It is helpful
+ * when both Java and Kotlin source files are present in a source set. Dokka can properly generate
+ * documentation for either Kotlin or Java depending on the configuration, but not both.
+ */
+internal fun GradleDokkaSourceSetBuilder.onlyJavaSources(): FileCollection {
+    return sourceRoots.filter(File::isJavaSourceDirectory)
+}
+
+private fun File.isJavaSourceDirectory(): Boolean {
+    return isDirectory() && name == "java"
+}