Set kotlin.version when Kotlin's Gradle plugin is applied

Closes gh-11711

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/asciidoc/reacting.adoc

@@ -4,6 +4,8 @@
 When another plugin is applied the Spring Boot plugin reacts by making various changes
 to the project's configuration. This section describes those changes.
 
+
+
 [[reacting-to-other-plugins-java]]
 === Reacting to the Java plugin
 
@@ -25,6 +27,18 @@ plugin:
 
 
 
+[[reacting-to-other-plugins-kotlin]]
+=== Reacting to the Kotlin plugin
+
+When {kotlin-plugin}[Kotlin's Gradle plugin] is applied to a project, the Spring Boot
+plugin:
+
+1. Aligns the Koltin version used in Spring Boot's dependency management with the version
+   of the plugin. This is achieved by setting the `kotlin.version` property with a value
+   that matches the version of the Kotlin plugin.
+
+
+
 [[reacting-to-other-plugins-war]]
 === Reacting to the war plugin
 

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/KotlinPluginAction.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * 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
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.gradle.plugin;
+
+import org.gradle.api.Plugin;
+import org.gradle.api.Project;
+import org.gradle.api.plugins.ExtraPropertiesExtension;
+import org.jetbrains.kotlin.gradle.plugin.KotlinPluginWrapper;
+
+/**
+ * {@link PluginApplicationAction} that reacts to Kotlin's Gradle plugin being applied by
+ * configuring a {@code kotlin.version} property to align the version used for dependency
+ * management for Kotlin with the version of its plugin.
+ *
+ * @author Andy Wilkinson
+ */
+class KotlinPluginAction implements PluginApplicationAction {
+
+	@Override
+	public void execute(Project project) {
+		String kotlinVersion = project.getPlugins().getPlugin(KotlinPluginWrapper.class)
+				.getKotlinPluginVersion();
+		ExtraPropertiesExtension extraProperties = project.getExtensions()
+				.getExtraProperties();
+		if (!extraProperties.has("kotlin.version")) {
+			extraProperties.set("kotlin.version", kotlinVersion);
+		}
+	}
+
+	@Override
+	@SuppressWarnings("unchecked")
+	public Class<? extends Plugin<? extends Project>> getPluginClass() {
+		try {
+			return (Class<? extends Plugin<? extends Project>>) Class
+					.forName("org.jetbrains.kotlin.gradle.plugin.KotlinPluginWrapper");
+		}
+		catch (Throwable ex) {
+			return null;
+		}
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/PluginApplicationAction.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -30,9 +30,9 @@ interface PluginApplicationAction extends Action<Project> {
 
 	/**
 	 * The class of the {@code Plugin} that, when applied, will trigger the execution of
-	 * this action.
+	 * this action. May return {@code null} if the plugin class is not on the classpath.
 	 *
-	 * @return the plugin class
+	 * @return the plugin class or {@code null}
 	 */
 	Class<? extends Plugin<? extends Project>> getPluginClass();
 

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/SpringBootPlugin.java

@@ -108,10 +108,15 @@ public class SpringBootPlugin implements Plugin<Project> {
 				new JavaPluginAction(singlePublishedArtifact),
 				new WarPluginAction(singlePublishedArtifact),
 				new MavenPluginAction(bootArchives.getUploadTaskName()),
-				new DependencyManagementPluginAction(), new ApplicationPluginAction());
+				new DependencyManagementPluginAction(), new ApplicationPluginAction(),
+				new KotlinPluginAction());
 		for (PluginApplicationAction action : actions) {
-			project.getPlugins().withType(action.getPluginClass(),
-					(plugin) -> action.execute(project));
+			Class<? extends Plugin<? extends Project>> pluginClass = action
+					.getPluginClass();
+			if (pluginClass != null) {
+				project.getPlugins().withType(pluginClass,
+						(plugin) -> action.execute(project));
+			}
 		}
 	}