Address deprecation warning that's generated by Gradle 4.x

In Gradle 4.x, SourceSetOutput now has multiple classes directories
and getClassesDir() has been deprecated. This commit introduces the
use of reflection to use getClassesDirs() when it's available rather
than getClassesDir().

Closes gh-9559

spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/run/FindMainClassTask.java

@@ -16,11 +16,16 @@
 
 package org.springframework.boot.gradle.run;
 
+import java.io.File;
 import java.io.IOException;
+import java.lang.reflect.Method;
+import java.util.Arrays;
+import java.util.Collection;
 
 import org.gradle.api.DefaultTask;
 import org.gradle.api.Project;
 import org.gradle.api.Task;
+import org.gradle.api.file.FileCollection;
 import org.gradle.api.plugins.ApplicationPluginConvention;
 import org.gradle.api.plugins.ExtraPropertiesExtension;
 import org.gradle.api.tasks.Input;
@@ -30,6 +35,7 @@ import org.gradle.api.tasks.TaskAction;
 
 import org.springframework.boot.gradle.SpringBootPluginExtension;
 import org.springframework.boot.loader.tools.MainClassFinder;
+import org.springframework.util.ReflectionUtils;
 
 /**
  * Task to find and set the 'mainClassName' convention when it's missing by searching the
@@ -103,13 +109,13 @@ public class FindMainClassTask extends DefaultTask {
 		if (mainClass == null) {
 			// Search
 			if (this.mainClassSourceSetOutput != null) {
-				project.getLogger().debug("Looking for main in: "
-						+ this.mainClassSourceSetOutput.getClassesDir());
+				Collection<File> classesDirs = getClassesDirs(
+						this.mainClassSourceSetOutput);
+				getProject().getLogger().debug("Looking for main in: " + classesDirs);
 				try {
-					mainClass = MainClassFinder.findSingleMainClass(
-							this.mainClassSourceSetOutput.getClassesDir(),
+					mainClass = MainClassFinder.findSingleMainClass(classesDirs,
 							SPRING_BOOT_APPLICATION_CLASS_NAME);
-					project.getLogger().info("Computed main class: " + mainClass);
+					getProject().getLogger().info("Computed main class: " + mainClass);
 				}
 				catch (IOException ex) {
 					throw new IllegalStateException("Cannot find main class", ex);
@@ -132,6 +138,17 @@ public class FindMainClassTask extends DefaultTask {
 		return mainClass;
 	}
 
+	private Collection<File> getClassesDirs(SourceSetOutput sourceSetOutput) {
+		Method getClassesDirs = ReflectionUtils.findMethod(SourceSetOutput.class,
+				"getClassesDirs");
+		if (getClassesDirs == null) {
+			return Arrays.asList(sourceSetOutput.getClassesDir());
+		}
+		FileCollection classesDirs = (FileCollection) ReflectionUtils
+				.invokeMethod(getClassesDirs, sourceSetOutput);
+		return classesDirs.getFiles();
+	}
+
 	private JavaExec findRunTask(Project project) {
 		Task runTask = project.getTasks().findByName("run");
 		if (runTask instanceof JavaExec) {