Protect PluginApplicationActions against absent plugin classes

Closes gh-24526

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

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -47,15 +47,8 @@ class KotlinPluginAction implements PluginApplicationAction {
 	}
 
 	@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;
-		}
+		return KotlinPluginWrapper.class;
 	}
 
 }

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

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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 +30,11 @@ interface PluginApplicationAction extends Action<Project> {
 
 	/**
 	 * The class of the {@code Plugin} that, when applied, will trigger the execution of
-	 * this action. May return {@code null} if the plugin class is not on the classpath.
-	 * @return the plugin class or {@code null}
+	 * this action.
+	 * @return the plugin class
+	 * @throws ClassNotFoundException if the plugin class cannot be found
+	 * @throws NoClassDefFoundError if an error occurs when defining the plugin class
 	 */
-	Class<? extends Plugin<? extends Project>> getPluginClass();
+	Class<? extends Plugin<? extends Project>> getPluginClass() throws ClassNotFoundException, NoClassDefFoundError;
 
 }

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

@@ -18,6 +18,7 @@ package org.springframework.boot.gradle.plugin;
 
 import java.util.Arrays;
 import java.util.List;
+import java.util.function.Consumer;
 
 import org.gradle.api.GradleException;
 import org.gradle.api.Plugin;
@@ -119,11 +120,19 @@ public class SpringBootPlugin implements Plugin<Project> {
 				new WarPluginAction(singlePublishedArtifact), new MavenPluginAction(bootArchives.getUploadTaskName()),
 				new DependencyManagementPluginAction(), new ApplicationPluginAction(), new KotlinPluginAction());
 		for (PluginApplicationAction action : actions) {
-			Class<? extends Plugin<? extends Project>> pluginClass = action.getPluginClass();
-			if (pluginClass != null) {
-				project.getPlugins().withType(pluginClass, (plugin) -> action.execute(project));
+			withPluginClassOfAction(action,
+					(pluginClass) -> project.getPlugins().withType(pluginClass, (plugin) -> action.execute(project)));
 		}
 	}
+
+	private void withPluginClassOfAction(PluginApplicationAction action,
+			Consumer<Class<? extends Plugin<? extends Project>>> consumer) {
+		try {
+			consumer.accept(action.getPluginClass());
+		}
+		catch (Throwable ex) {
+			// Plugin class unavailable. Continue.
+		}
 	}
 
 }