Polish Gradle plugin after layering being enabled by default

Closes gh-20983

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootJar.java

@@ -31,7 +31,6 @@ import org.gradle.api.plugins.JavaPluginConvention;
 import org.gradle.api.specs.Spec;
 import org.gradle.api.tasks.Internal;
 import org.gradle.api.tasks.Nested;
-import org.gradle.api.tasks.Optional;
 import org.gradle.api.tasks.SourceSet;
 import org.gradle.api.tasks.bundling.Jar;
 
@@ -168,12 +167,11 @@ public class BootJar extends Jar implements BootArchive {
 	}
 
 	/**
-	 * Returns the spec that describes the layers in a layerd jar.
-	 * @return the spec for the layers or {@code null}.
+	 * Returns the spec that describes the layers in a layered jar.
+	 * @return the spec for the layers
 	 * @since 2.3.0
 	 */
 	@Nested
-	@Optional
 	public LayeredSpec getLayered() {
 		return this.layered;
 	}
@@ -181,19 +179,19 @@ public class BootJar extends Jar implements BootArchive {
 	/**
 	 * Configures the jar to be layered using the default layering.
 	 * @since 2.3.0
+	 * @deprecated since 2.4.0 as layering as now enabled by default.
 	 */
+	@Deprecated
 	public void layered() {
-		enableLayeringIfNecessary();
 	}
 
 	/**
-	 * Configures the jar to be layered, customizing the layers using the given
-	 * {@code action}.
+	 * Configures the jar's layering using the given {@code action}.
 	 * @param action the action to apply
 	 * @since 2.3.0
 	 */
 	public void layered(Action<LayeredSpec> action) {
-		action.execute(enableLayeringIfNecessary());
+		action.execute(this.layered);
 	}
 
 	@Override
@@ -290,13 +288,6 @@ public class BootJar extends Jar implements BootArchive {
 		return launchScript;
 	}
 
-	private LayeredSpec enableLayeringIfNecessary() {
-		if (this.layered == null) {
-			this.layered = new LayeredSpec();
-		}
-		return this.layered;
-	}
-
 	/**
 	 * Syntactic sugar that makes {@link CopySpec#into} calls a little easier to read.
 	 * @param <T> the result type

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/tasks/bundling/BootJarTests.java

@@ -261,16 +261,12 @@ class BootJarTests extends AbstractBootArchiveTests<TestBootJar> {
 	}
 
 	private File createLayeredJar() throws IOException {
-		return createLayeredJar(null);
+		return createLayeredJar((spec) -> {
+		});
 	}
 
 	private File createLayeredJar(Action<LayeredSpec> action) throws IOException {
-		if (action != null) {
-			getTask().layered(action);
-		}
-		else {
-			getTask().layered();
-		}
+		getTask().layered(action);
 		addContent();
 		executeTask();
 		return getTask().getArchiveFile().get().getAsFile();

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootJarIntegrationTests-implicitLayers.gradle

@@ -5,7 +5,6 @@ plugins {
 
 bootJar {
 	mainClassName = 'com.example.Application'
-	layered()
 }
 
 repositories {

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootJarIntegrationTests-layersWithCustomSourceSet.gradle

@@ -9,7 +9,6 @@ sourceSets {
 
 bootJar {
 	mainClassName = 'com.example.Application'
-	layered()
 }
 
 repositories {