Fix packager code to generate layer index file

Update `Packager` to create the layer index file when repackaging.

Closes gh-19767

Author: philwebb

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

@@ -159,6 +159,7 @@ protected final void write(JarFile sourceJar, Libraries libraries, AbstractJarWr
 		writeLoaderClasses(writer);
 		writer.writeEntries(sourceJar, getEntityTransformer(), writeableLibraries);
 		writeableLibraries.write(writer);
+		writeLayerIndex(writer);
 	}
 
 	private void writeLoaderClasses(AbstractJarWriter writer) throws IOException {
@@ -171,6 +172,17 @@ else if (layout.isExecutable()) {
 		}
 	}
 
+	private void writeLayerIndex(AbstractJarWriter writer) throws IOException {
+		if (this.layers != null && getLayout() instanceof LayeredLayout) {
+			String location = ((LayeredLayout) this.layout).getLayersIndexFileLocation();
+			if (StringUtils.hasLength(location)) {
+				List<String> layerNames = new ArrayList<>();
+				this.layers.forEach((layer) -> layerNames.add(layer.toString()));
+				writer.writeIndexFile(location, layerNames);
+			}
+		}
+	}
+
 	private EntryTransformer getEntityTransformer() {
 		if (getLayout() instanceof RepackagingLayout) {
 			return new RepackagingEntryTransformer((RepackagingLayout) getLayout(), this.layers);
@@ -304,8 +316,7 @@ else if (layout instanceof RepackagingLayout) {
 	}
 
 	private void addBootBootAttributesForLayeredLayout(Attributes attributes, LayeredLayout layout) {
-		String layersIndexFileLocation = layout.getLayersIndexFileLocation();
-		putIfHasLength(attributes, BOOT_LAYERS_INDEX_ATTRIBUTE, layersIndexFileLocation);
+		putIfHasLength(attributes, BOOT_LAYERS_INDEX_ATTRIBUTE, layout.getLayersIndexFileLocation());
 		putIfHasLength(attributes, BOOT_CLASSPATH_INDEX_ATTRIBUTE, layout.getClasspathIndexFileLocation());
 	}
 

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

@@ -261,13 +261,20 @@ void layeredLayout() throws Exception {
 			callback.library(new Library(libJarFile3, LibraryScope.COMPILE));
 		});
 		assertThat(hasPackagedEntry("BOOT-INF/classpath.idx")).isTrue();
-		String index = getPackagedEntryContent("BOOT-INF/classpath.idx");
-		String[] libraries = index.split("\\n");
-		List<String> expected = new ArrayList<>();
-		expected.add("BOOT-INF/layers/0001/lib/" + libJarFile1.getName());
-		expected.add("BOOT-INF/layers/0002/lib/" + libJarFile2.getName());
-		expected.add("BOOT-INF/layers/0003/lib/" + libJarFile3.getName());
-		assertThat(Arrays.asList(libraries)).containsExactly(expected.toArray(new String[0]));
+		String classpathIndex = getPackagedEntryContent("BOOT-INF/classpath.idx");
+		List<String> expectedJars = new ArrayList<>();
+		expectedJars.add("BOOT-INF/layers/0001/lib/" + libJarFile1.getName());
+		expectedJars.add("BOOT-INF/layers/0002/lib/" + libJarFile2.getName());
+		expectedJars.add("BOOT-INF/layers/0003/lib/" + libJarFile3.getName());
+		assertThat(Arrays.asList(classpathIndex.split("\\n"))).containsExactly(expectedJars.toArray(new String[0]));
+		assertThat(hasPackagedEntry("BOOT-INF/layers.idx")).isTrue();
+		String layersIndex = getPackagedEntryContent("BOOT-INF/layers.idx");
+		List<String> expectedLayers = new ArrayList<>();
+		expectedLayers.add("default");
+		expectedLayers.add("0001");
+		expectedLayers.add("0002");
+		expectedLayers.add("0003");
+		assertThat(Arrays.asList(layersIndex.split("\\n"))).containsExactly(expectedLayers.toArray(new String[0]));
 	}
 
 	@Test