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-loader-tools/src/main/java/org/springframework/boot/loader/tools/MainClassFinder.java

@@ -25,6 +25,7 @@ import java.io.InputStream;
 import java.util.ArrayDeque;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.Deque;
@@ -117,6 +118,44 @@ public abstract class MainClassFinder {
 		return callback.getMainClassName();
 	}
 
+	/**
+	 * Find a single main class from the given {@code rootFolders}. A main class annotated
+	 * with an annotation with the given {@code annotationName} will be preferred over a
+	 * main class with no such annotation.
+	 * @param rootFolders the root folders to search
+	 * @param annotationName the name of the annotation that may be present on the main
+	 * class
+	 * @return the main class or {@code null}
+	 * @throws IOException if a root folder cannot be read
+	 * @since 1.5.5
+	 */
+	public static String findSingleMainClass(Collection<File> rootFolders,
+			String annotationName) throws IOException {
+		SingleMainClassCallback callback = new SingleMainClassCallback(annotationName);
+		doWithMainClasses(rootFolders, callback);
+		return callback.getMainClassName();
+	}
+
+	/**
+	 * Perform the given callback operation on all main classes from the given root
+	 * folder.
+	 * @param <T> the result type
+	 * @param rootFolders the root folders
+	 * @param callback the callback
+	 * @return the first callback result or {@code null}
+	 * @throws IOException in case of I/O errors
+	 */
+	static <T> T doWithMainClasses(Collection<File> rootFolders,
+			MainClassCallback<T> callback) throws IOException {
+		for (File rootFolder : rootFolders) {
+			T result = doWithMainClasses(rootFolder, callback);
+			if (result != null) {
+				return result;
+			}
+		}
+		return null;
+	}
+
 	/**
 	 * Perform the given callback operation on all main classes from the given root
 	 * folder.