Create a new layer for loader classes

Create a dedicated layer that is used to hold the launcher support
classes. The layer sits between `dependencies` and
`snapshot-dependencies` so that the layer is sensible for both
SNAPSHOT and RELEASE versions of Spring Boot

Closes gh-20529

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

@@ -100,7 +100,7 @@ class BootJarTests extends AbstractBootArchiveTests<TestBootJar> {
 	@Test
 	void whenJarIsLayeredThenLayersIndexIsPresentAndListsLayersInOrder() throws IOException {
 		try (JarFile jarFile = new JarFile(createLayeredJar())) {
-			assertThat(entryLines(jarFile, "BOOT-INF/layers.idx")).containsExactly("dependencies",
+			assertThat(entryLines(jarFile, "BOOT-INF/layers.idx")).containsExactly("dependencies", "spring-boot-loader",
 					"snapshot-dependencies", "application");
 		}
 	}

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/ImplicitLayerResolver.java

@@ -24,8 +24,13 @@ package org.springframework.boot.loader.tools;
  */
 class ImplicitLayerResolver extends StandardLayers {
 
+	private static final String SPRING_BOOT_LOADER_PREFIX = "org/springframework/boot/loader/";
+
 	@Override
 	public Layer getLayer(String name) {
+		if (name.startsWith(SPRING_BOOT_LOADER_PREFIX)) {
+			return SPRING_BOOT_LOADER;
+		}
 		return APPLICATION;
 	}
 

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/StandardLayers.java

@@ -41,6 +41,11 @@ public abstract class StandardLayers implements Layers {
 	 */
 	public static final Layer DEPENDENCIES = new Layer("dependencies");
 
+	/**
+	 * The spring boot loader layer.
+	 */
+	public static final Layer SPRING_BOOT_LOADER = new Layer("spring-boot-loader");
+
 	/**
 	 * The snapshot dependencies layer.
 	 */
@@ -55,6 +60,7 @@ public abstract class StandardLayers implements Layers {
 	static {
 		List<Layer> layers = new ArrayList<>();
 		layers.add(DEPENDENCIES);
+		layers.add(SPRING_BOOT_LOADER);
 		layers.add(SNAPSHOT_DEPENDENCIES);
 		layers.add(APPLICATION);
 		LAYERS = Collections.unmodifiableList(layers);

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/java/org/springframework/boot/loader/tools/ImplicitLayerResolverTests.java

@@ -34,8 +34,8 @@ class ImplicitLayerResolverTests {
 
 	@Test
 	void iteratorReturnsLayers() {
-		assertThat(this.layers).containsExactly(StandardLayers.DEPENDENCIES, StandardLayers.SNAPSHOT_DEPENDENCIES,
-				StandardLayers.APPLICATION);
+		assertThat(this.layers).containsExactly(StandardLayers.DEPENDENCIES, StandardLayers.SPRING_BOOT_LOADER,
+				StandardLayers.SNAPSHOT_DEPENDENCIES, StandardLayers.APPLICATION);
 	}
 
 	@Test