Support custom fat jar layouts

Allow support for custom Lyout implementations with both the Maven
and Gradle plugin. Implementations of `LayoutFactory` can now be
specified to allow customization of the layout. In addition a
layout may now implement `CustomLoaderLayout` if it wishes to
write custom loader classes.

See gh-7263

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

@@ -26,6 +26,7 @@ import org.gradle.api.plugins.JavaPlugin;
 
 import org.springframework.boot.gradle.buildinfo.BuildInfo;
 import org.springframework.boot.loader.tools.Layout;
+import org.springframework.boot.loader.tools.LayoutFactory;
 import org.springframework.boot.loader.tools.Layouts;
 
 /**
@@ -90,6 +91,12 @@ public class SpringBootPluginExtension {
 	 */
 	LayoutType layout;
 
+	/**
+	 * The layout factory that will be used when no explicit layout is specified.
+	 * Alternative layouts can be provided by 3rd parties.
+	 */
+	LayoutFactory layoutFactory;
+
 	/**
 	 * Libraries that must be unpacked from fat jars in order to run. Use Strings in the
 	 * form {@literal groupId:artifactId}.
@@ -196,6 +203,14 @@ public class SpringBootPluginExtension {
 		this.layout = layout;
 	}
 
+	public LayoutFactory getLayoutFactory() {
+		return this.layoutFactory;
+	}
+
+	public void setLayoutFactory(LayoutFactory layoutFactory) {
+		this.layoutFactory = layoutFactory;
+	}
+
 	public Set<String> getRequiresUnpack() {
 		return this.requiresUnpack;
 	}

spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/repackage/RepackageTask.java

@@ -213,7 +213,8 @@ public class RepackageTask extends DefaultTask {
 				copy(file, outputFile);
 				file = outputFile;
 			}
-			Repackager repackager = new Repackager(file);
+			Repackager repackager = new Repackager(file,
+					this.extension.getLayoutFactory());
 			repackager.addMainClassTimeoutWarningListener(
 					new LoggingMainClassTimeoutWarningListener());
 			setMainClass(repackager);