Polish Gradle layer configuration DSL

This commit modifies the DSL for custom layer configuration in the
Gradle plugin to avoid duplication of terms that could be confusing.

Fixes gh-20563

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-layered-custom.gradle

@@ -10,7 +10,7 @@ bootJar {
 // tag::layered[]
 bootJar {
 	layers {
-		layers "dependencies", "snapshot-dependencies", "resources", "application"
+		layersOrder "dependencies", "snapshot-dependencies", "resources", "application"
 		libraries {
 			layerContent("snapshot-dependencies") {
 				coordinates {

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-layered-custom.gradle.kts

@@ -8,8 +8,7 @@ plugins {
 // tag::layered[]
 tasks.getByName<BootJar>("bootJar") {
 	layers {
-		includeLayerTools = false
-		layers("dependencies", "snapshot-dependencies", "resources", "application")
+		layersOrder("dependencies", "snapshot-dependencies", "resources", "application")
 		libraries {
 			layerContent("snapshot-dependencies") {
 				coordinates {

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

@@ -208,11 +208,11 @@ public class BootJar extends Jar implements BootArchive {
 			return;
 		}
 
-		if (this.layerConfiguration.getLayers() == null || this.layerConfiguration.getLayers().isEmpty()) {
+		if (this.layerConfiguration.getLayersOrder() == null || this.layerConfiguration.getLayersOrder().isEmpty()) {
 			this.layers = Layers.IMPLICIT;
 		}
 		else {
-			List<Layer> customLayers = this.layerConfiguration.getLayers().stream().map(Layer::new)
+			List<Layer> customLayers = this.layerConfiguration.getLayersOrder().stream().map(Layer::new)
 					.collect(Collectors.toList());
 			this.layers = new CustomLayers(customLayers, this.layerConfiguration.getClasses(),
 					this.layerConfiguration.getLibraries());

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

@@ -44,7 +44,7 @@ public class LayerConfiguration {
 
 	private boolean includeLayerTools = true;
 
-	private List<String> layerNames = new ArrayList<>();
+	private List<String> layersOrder = new ArrayList<>();
 
 	private List<ResourceStrategy> resourceStrategies = new ArrayList<>();
 
@@ -66,16 +66,16 @@ public class LayerConfiguration {
 	}
 
 	@Input
-	public List<String> getLayers() {
-		return this.layerNames;
+	public List<String> getLayersOrder() {
+		return this.layersOrder;
 	}
 
-	public void layers(String... layers) {
-		this.layerNames = Arrays.asList(layers);
+	public void layersOrder(String... layers) {
+		this.layersOrder = Arrays.asList(layers);
 	}
 
-	public void layers(List<String> layers) {
-		this.layerNames = layers;
+	public void layersOrder(List<String> layers) {
+		this.layersOrder = layers;
 	}
 
 	@Input
@@ -84,10 +84,12 @@ public class LayerConfiguration {
 	}
 
 	public void classes(ResourceStrategy... resourceStrategies) {
+		assertLayersOrderConfigured();
 		this.resourceStrategies = Arrays.asList(resourceStrategies);
 	}
 
 	public void classes(Action<LayerConfiguration> config) {
+		assertLayersOrderConfigured();
 		this.strategySpec = StrategySpec.forResources();
 		config.execute(this);
 	}
@@ -98,14 +100,20 @@ public class LayerConfiguration {
 	}
 
 	public void libraries(LibraryStrategy... strategies) {
+		assertLayersOrderConfigured();
 		this.libraryStrategies = Arrays.asList(strategies);
 	}
 
 	public void libraries(Action<LayerConfiguration> configure) {
+		assertLayersOrderConfigured();
 		this.strategySpec = StrategySpec.forLibraries();
 		configure.execute(this);
 	}
 
+	private void assertLayersOrderConfigured() {
+		Assert.state(!this.layersOrder.isEmpty(), "'layersOrder' must be configured before filters can be applied.");
+	}
+
 	public void layerContent(String layerName, Action<LayerConfiguration> config) {
 		this.strategySpec.newStrategy();
 		config.execute(this);

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

@@ -122,7 +122,7 @@ class BootJarTests extends AbstractBootArchiveTests<BootJar> {
 	@Test
 	void whenJarIsLayeredWithCustomStrategiesThenContentsAreMovedToLayerDirectories() throws IOException {
 		File jar = createLayeredJar((configuration) -> {
-			configuration.layers("my-deps", "my-internal-deps", "my-snapshot-deps", "resources", "application");
+			configuration.layersOrder("my-deps", "my-internal-deps", "my-snapshot-deps", "resources", "application");
 			configuration.libraries(createLibraryStrategy("my-snapshot-deps", "com.example:*:*.SNAPSHOT"),
 					createLibraryStrategy("my-internal-deps", "com.example:*:*"),
 					createLibraryStrategy("my-deps", "*:*"));

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

@@ -6,7 +6,7 @@ plugins {
 bootJar {
 	mainClassName = 'com.example.Application'
 	layers {
-		layers "dependencies", "commons-dependencies", "snapshot-dependencies", "static", "app"
+		layersOrder "dependencies", "commons-dependencies", "snapshot-dependencies", "static", "app"
 		libraries {
 			layerContent("snapshot-dependencies") { coordinates { include "*:*:*SNAPSHOT" } }
 			layerContent("commons-dependencies") { coordinates { include "org.apache.commons:*" } }