Include empty layers when listing and extracting

Fixes gh-21301

spring-boot-project/spring-boot-tools/spring-boot-jarmode-layertools/src/main/java/org/springframework/boot/jarmode/layertools/IndexedLayers.java

@@ -20,16 +20,16 @@ import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.NoSuchFileException;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Iterator;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.jar.JarFile;
 import java.util.zip.ZipEntry;
 
 import org.springframework.util.Assert;
-import org.springframework.util.LinkedMultiValueMap;
-import org.springframework.util.MultiValueMap;
 import org.springframework.util.StreamUtils;
 import org.springframework.util.StringUtils;
 
@@ -41,18 +41,19 @@ import org.springframework.util.StringUtils;
  */
 class IndexedLayers implements Layers {
 
-	private MultiValueMap<String, String> layers = new LinkedMultiValueMap<>();
+	private final Map<String, List<String>> layers = new LinkedHashMap<>();
 
 	IndexedLayers(String indexFile) {
 		String[] lines = Arrays.stream(indexFile.split("\n")).map((line) -> line.replace("\r", ""))
 				.filter(StringUtils::hasText).toArray(String[]::new);
-		String layer = null;
+		List<String> contents = null;
 		for (String line : lines) {
 			if (line.startsWith("- ")) {
-				layer = line.substring(3, line.length() - 2);
+				contents = new ArrayList<>();
+				this.layers.put(line.substring(3, line.length() - 2), contents);
 			}
 			else if (line.startsWith("  - ")) {
-				this.layers.add(layer, line.substring(5, line.length() - 1));
+				contents.add(line.substring(5, line.length() - 1));
 			}
 			else {
 				throw new IllegalStateException("Layer index file is malformed");

spring-boot-project/spring-boot-tools/spring-boot-jarmode-layertools/src/test/java/org/springframework/boot/jarmode/layertools/ExtractCommandTests.java

@@ -69,10 +69,11 @@ class ExtractCommandTests {
 	@Test
 	void runExtractsLayers() throws Exception {
 		this.command.run(Collections.emptyMap(), Collections.emptyList());
-		assertThat(this.extract.list()).containsOnly("a", "b", "c");
+		assertThat(this.extract.list()).containsOnly("a", "b", "c", "d");
 		assertThat(new File(this.extract, "a/a/a.jar")).exists();
 		assertThat(new File(this.extract, "b/b/b.jar")).exists();
 		assertThat(new File(this.extract, "c/c/c.jar")).exists();
+		assertThat(new File(this.extract, "d")).isDirectory();
 	}
 
 	@Test
@@ -119,7 +120,7 @@ class ExtractCommandTests {
 
 		@Override
 		public Iterator<String> iterator() {
-			return Arrays.asList("a", "b", "c").iterator();
+			return Arrays.asList("a", "b", "c", "d").iterator();
 		}
 
 		@Override

spring-boot-project/spring-boot-tools/spring-boot-jarmode-layertools/src/test/java/org/springframework/boot/jarmode/layertools/IndexedLayersTests.java

@@ -52,7 +52,7 @@ class IndexedLayersTests {
 	@Test
 	void iteratorReturnsLayers() throws Exception {
 		IndexedLayers layers = new IndexedLayers(getIndex());
-		assertThat(layers).containsExactly("test", "application");
+		assertThat(layers).containsExactly("test", "empty", "application");
 	}
 
 	@Test

spring-boot-project/spring-boot-tools/spring-boot-jarmode-layertools/src/test/resources/org/springframework/boot/jarmode/layertools/test-layers.idx

 - "test":
   - "BOOT-INF/lib/a.jar"
   - "BOOT-INF/lib/b.jar"
+- "empty":
 - "application":
   - "BOOT-INF/classes/Demo.class"
   - "META-INF/"