Add classpath index support for exploded archives

Update the `Repackager` class so that an additional `classpath.idx` file
is written into the jar that provides the original order of the
classpath. The `JarLauncher` class now uses this file when running as
an exploded archive to ensure that the classpath order is the same as
when running from the far jar.

Closes gh-9128

Co-authored-by: Phillip Webb <pwebb@pivotal.io>

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

@@ -17,6 +17,7 @@
 package org.springframework.boot.loader.tools;
 
 import java.io.BufferedInputStream;
+import java.io.BufferedWriter;
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.File;
@@ -27,13 +28,16 @@ import java.io.FilterInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.io.OutputStreamWriter;
 import java.net.URL;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.attribute.PosixFilePermission;
 import java.util.Arrays;
 import java.util.Enumeration;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Set;
 import java.util.jar.JarEntry;
 import java.util.jar.JarFile;
@@ -52,6 +56,7 @@ import org.apache.commons.compress.archivers.zip.UnixStat;
  *
  * @author Phillip Webb
  * @author Andy Wilkinson
+ * @author Madhura Bhave
  * @since 1.0.0
  */
 public class JarWriter implements LoaderClassesWriter, AutoCloseable {
@@ -190,6 +195,28 @@ public class JarWriter implements LoaderClassesWriter, AutoCloseable {
 		}
 	}
 
+	/**
+	 * Write a simple index file containing the specified UTF-8 lines.
+	 * @param location the location of the index file
+	 * @param lines the lines to write
+	 * @throws IOException if the write fails
+	 * @since 2.3.0
+	 */
+	public void writeIndexFile(String location, List<String> lines) throws IOException {
+		if (location != null) {
+			JarArchiveEntry entry = new JarArchiveEntry(location);
+			writeEntry(entry, (outputStream) -> {
+				BufferedWriter writer = new BufferedWriter(
+						new OutputStreamWriter(outputStream, StandardCharsets.UTF_8));
+				for (String line : lines) {
+					writer.write(line);
+					writer.write("\n");
+				}
+				writer.flush();
+			});
+		}
+	}
+
 	private long getNestedLibraryTime(File file) {
 		try {
 			try (JarFile jarFile = new JarFile(file)) {

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

@@ -28,6 +28,7 @@ import java.util.Map;
  * @author Phillip Webb
  * @author Dave Syer
  * @author Andy Wilkinson
+ * @author Madhura Bhave
  * @since 1.0.0
  */
 public final class Layouts {
@@ -88,6 +89,11 @@ public final class Layouts {
 			return "BOOT-INF/classes/";
 		}
 
+		@Override
+		public String getClasspathIndexFileLocation() {
+			return "BOOT-INF/classpath.idx";
+		}
+
 		@Override
 		public boolean isExecutable() {
 			return true;

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

@@ -45,6 +45,7 @@ import org.springframework.util.StringUtils;
  * @author Phillip Webb
  * @author Andy Wilkinson
  * @author Stephane Nicoll
+ * @author Madhura Bhave
  * @since 1.0.0
  */
 public class Repackager {
@@ -59,6 +60,8 @@ public class Repackager {
 
 	private static final String BOOT_LIB_ATTRIBUTE = "Spring-Boot-Lib";
 
+	private static final String BOOT_CLASSPATH_INDEX_ATTRIBUTE = "Spring-Boot-Classpath-Index";
+
 	private static final byte[] ZIP_FILE_HEADER = new byte[] { 'P', 'K', 3, 4 };
 
 	private static final long FIND_WARNING_TIMEOUT = TimeUnit.SECONDS.toMillis(10);
@@ -336,6 +339,7 @@ public class Repackager {
 	private void addBootBootAttributesForRepackagingLayout(Attributes attributes, RepackagingLayout layout) {
 		attributes.putValue(BOOT_CLASSES_ATTRIBUTE, layout.getRepackagedClassesLocation());
 		putIfHasLength(attributes, BOOT_LIB_ATTRIBUTE, this.layout.getLibraryLocation("", LibraryScope.COMPILE));
+		putIfHasLength(attributes, BOOT_CLASSPATH_INDEX_ATTRIBUTE, layout.getClasspathIndexFileLocation());
 	}
 
 	private void addBootBootAttributesForPlainLayout(Attributes attributes, Layout layout) {
@@ -473,6 +477,10 @@ public class Repackager {
 				writer.writeNestedLibrary(entry.getKey().substring(0, entry.getKey().lastIndexOf('/') + 1),
 						entry.getValue());
 			}
+			if (Repackager.this.layout instanceof RepackagingLayout) {
+				String location = ((RepackagingLayout) (Repackager.this.layout)).getClasspathIndexFileLocation();
+				writer.writeIndexFile(location, new ArrayList<>(this.libraryEntryNames.keySet()));
+			}
 		}
 
 	}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,4 +31,15 @@ public interface RepackagingLayout extends Layout {
 	 */
 	String getRepackagedClassesLocation();
 
+	/**
+	 * Returns the location of the classpath index file that should be written or
+	 * {@code null} if not index is required. The result should include the filename and
+	 * is relative to the root of the jar.
+	 * @return the classpath index file location
+	 * @since 2.3.0
+	 */
+	default String getClasspathIndexFileLocation() {
+		return null;
+	}
+
 }

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

@@ -18,11 +18,14 @@ package org.springframework.boot.loader.tools;
 
 import java.io.ByteArrayInputStream;
 import java.io.File;
+import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.attribute.PosixFilePermission;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Calendar;
 import java.util.Enumeration;
 import java.util.List;
@@ -45,6 +48,7 @@ import org.zeroturnaround.zip.ZipUtil;
 import org.springframework.boot.loader.tools.sample.ClassWithMainMethod;
 import org.springframework.boot.loader.tools.sample.ClassWithoutMainMethod;
 import org.springframework.util.FileCopyUtils;
+import org.springframework.util.StreamUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
@@ -59,6 +63,7 @@ import static org.mockito.Mockito.mock;
  *
  * @author Phillip Webb
  * @author Andy Wilkinson
+ * @author Madhura Bhave
  */
 class RepackagerTests {
 
@@ -299,6 +304,34 @@ class RepackagerTests {
 		assertThat(entry.getComment()).hasSize(47);
 	}
 
+	@Test
+	void index() throws Exception {
+		TestJarFile libJar1 = new TestJarFile(this.tempDir);
+		libJar1.addClass("a/b/C.class", ClassWithoutMainMethod.class, JAN_1_1985);
+		File libJarFile1 = libJar1.getFile();
+		TestJarFile libJar2 = new TestJarFile(this.tempDir);
+		libJar2.addClass("a/b/C.class", ClassWithoutMainMethod.class, JAN_1_1985);
+		File libJarFile2 = libJar2.getFile();
+		TestJarFile libJar3 = new TestJarFile(this.tempDir);
+		libJar3.addClass("a/b/C.class", ClassWithoutMainMethod.class, JAN_1_1985);
+		File libJarFile3 = libJar3.getFile();
+		this.testJarFile.addClass("a/b/C.class", ClassWithMainMethod.class);
+		File file = this.testJarFile.getFile();
+		Repackager repackager = new Repackager(file);
+		repackager.repackage((callback) -> {
+			callback.library(new Library(libJarFile1, LibraryScope.COMPILE));
+			callback.library(new Library(libJarFile2, LibraryScope.COMPILE));
+			callback.library(new Library(libJarFile3, LibraryScope.COMPILE));
+		});
+		assertThat(hasEntry(file, "BOOT-INF/classpath.idx")).isTrue();
+		ZipUtil.unpack(file, new File(file.getParent()));
+		FileInputStream inputStream = new FileInputStream(new File(file.getParent() + "/BOOT-INF/classpath.idx"));
+		String index = StreamUtils.copyToString(inputStream, StandardCharsets.UTF_8);
+		String[] libraries = index.split("\\r?\\n");
+		assertThat(Arrays.asList(libraries)).contains("BOOT-INF/lib/" + libJarFile1.getName(),
+				"BOOT-INF/lib/" + libJarFile2.getName(), "BOOT-INF/lib/" + libJarFile3.getName());
+	}
+
 	@Test
 	void duplicateLibraries() throws Exception {
 		TestJarFile libJar = new TestJarFile(this.tempDir);

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

@@ -0,0 +1,130 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.loader;
+
+import java.io.BufferedReader;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.net.MalformedURLException;
+import java.net.URISyntaxException;
+import java.net.URL;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Objects;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+/**
+ * A class path index file that provides ordering information for JARs.
+ *
+ * @author Madhura Bhave
+ * @author Phillip Webb
+ */
+final class ClassPathIndexFile {
+
+	private final File root;
+
+	private final List<String> lines;
+
+	private final Set<String> folders;
+
+	private ClassPathIndexFile(File root, List<String> lines) {
+		this.root = root;
+		this.lines = lines;
+		this.folders = this.lines.stream().map(this::getFolder).filter(Objects::nonNull).collect(Collectors.toSet());
+	}
+
+	private String getFolder(String name) {
+		int lastSlash = name.lastIndexOf("/");
+		return (lastSlash != -1) ? name.substring(0, lastSlash) : null;
+	}
+
+	int size() {
+		return this.lines.size();
+	}
+
+	boolean containsFolder(String name) {
+		if (name == null || name.isEmpty()) {
+			return false;
+		}
+		if (name.endsWith("/")) {
+			return containsFolder(name.substring(0, name.length() - 1));
+		}
+		return this.folders.contains(name);
+	}
+
+	List<URL> getUrls() {
+		return Collections.unmodifiableList(this.lines.stream().map(this::asUrl).collect(Collectors.toList()));
+	}
+
+	private URL asUrl(String line) {
+		try {
+			return new File(this.root, line).toURI().toURL();
+		}
+		catch (MalformedURLException ex) {
+			throw new IllegalStateException(ex);
+		}
+	}
+
+	static ClassPathIndexFile loadIfPossible(URL root, String location) throws IOException {
+		return loadIfPossible(asFile(root), location);
+	}
+
+	private static ClassPathIndexFile loadIfPossible(File root, String location) throws IOException {
+		return loadIfPossible(root, new File(root, location));
+	}
+
+	private static ClassPathIndexFile loadIfPossible(File root, File indexFile) throws IOException {
+		if (indexFile.exists() && indexFile.isFile()) {
+			try (InputStream inputStream = new FileInputStream(indexFile)) {
+				return new ClassPathIndexFile(root, loadLines(inputStream));
+			}
+		}
+		return null;
+	}
+
+	private static List<String> loadLines(InputStream inputStream) throws IOException {
+		List<String> lines = new ArrayList<>();
+		BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8));
+		String line = reader.readLine();
+		while (line != null) {
+			if (!line.trim().isEmpty()) {
+				lines.add(line);
+			}
+			line = reader.readLine();
+		}
+		return Collections.unmodifiableList(lines);
+	}
+
+	private static File asFile(URL url) {
+		if (!"file".equals(url.getProtocol())) {
+			throw new IllegalArgumentException("URL does not reference a file");
+		}
+		try {
+			return new File(url.toURI());
+		}
+		catch (URISyntaxException ex) {
+			return new File(url.getPath());
+		}
+	}
+
+}

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

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.loader;
 
+import java.io.IOException;
+import java.net.URL;
 import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
@@ -28,17 +30,23 @@ import org.springframework.boot.loader.archive.Archive;
  *
  * @author Phillip Webb
  * @author Andy Wilkinson
+ * @author Madhura Bhave
  * @since 1.0.0
  */
 public abstract class ExecutableArchiveLauncher extends Launcher {
 
 	private static final String START_CLASS_ATTRIBUTE = "Start-Class";
 
+	protected static final String BOOT_CLASSPATH_INDEX_ATTRIBUTE = "Spring-Boot-Classpath-Index";
+
 	private final Archive archive;
 
+	private final ClassPathIndexFile classPathIndex;
+
 	public ExecutableArchiveLauncher() {
 		try {
 			this.archive = createArchive();
+			this.classPathIndex = getClassPathIndex(this.archive);
 		}
 		catch (Exception ex) {
 			throw new IllegalStateException(ex);
@@ -48,12 +56,17 @@ public abstract class ExecutableArchiveLauncher extends Launcher {
 	protected ExecutableArchiveLauncher(Archive archive) {
 		try {
 			this.archive = archive;
+			this.classPathIndex = getClassPathIndex(this.archive);
 		}
 		catch (Exception ex) {
 			throw new IllegalStateException(ex);
 		}
 	}
 
+	protected ClassPathIndexFile getClassPathIndex(Archive archive) throws IOException {
+		return null;
+	}
+
 	@Override
 	protected String getMainClass() throws Exception {
 		Manifest manifest = this.archive.getManifest();
@@ -67,15 +80,42 @@ public abstract class ExecutableArchiveLauncher extends Launcher {
 		return mainClass;
 	}
 
+	@Override
+	protected ClassLoader createClassLoader(Iterator<Archive> archives) throws Exception {
+		List<URL> urls = new ArrayList<>(guessClassPathSize());
+		while (archives.hasNext()) {
+			urls.add(archives.next().getUrl());
+		}
+		if (this.classPathIndex != null) {
+			urls.addAll(this.classPathIndex.getUrls());
+		}
+		return super.createClassLoader(urls.toArray(new URL[0]));
+	}
+
+	private int guessClassPathSize() {
+		if (this.classPathIndex != null) {
+			return this.classPathIndex.size() + 10;
+		}
+		return 50;
+	}
+
 	@Override
 	protected Iterator<Archive> getClassPathArchivesIterator() throws Exception {
-		Iterator<Archive> archives = this.archive.getNestedArchives(this::isSearchCandidate, this::isNestedArchive);
+		Archive.EntryFilter searchFilter = (entry) -> isSearchCandidate(entry) && !isFolderIndexed(entry);
+		Iterator<Archive> archives = this.archive.getNestedArchives(searchFilter, this::isNestedArchive);
 		if (isPostProcessingClassPathArchives()) {
 			archives = applyClassPathArchivePostProcessing(archives);
 		}
 		return archives;
 	}
 
+	private boolean isFolderIndexed(Archive.Entry entry) {
+		if (this.classPathIndex != null) {
+			return this.classPathIndex.containsFolder(entry.getName());
+		}
+		return false;
+	}
+
 	private Iterator<Archive> applyClassPathArchivePostProcessing(Iterator<Archive> archives) throws Exception {
 		List<Archive> list = new ArrayList<>();
 		while (archives.hasNext()) {

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,8 +16,13 @@
 
 package org.springframework.boot.loader;
 
+import java.io.IOException;
+import java.util.jar.Attributes;
+import java.util.jar.Manifest;
+
 import org.springframework.boot.loader.archive.Archive;
 import org.springframework.boot.loader.archive.Archive.EntryFilter;
+import org.springframework.boot.loader.archive.ExplodedArchive;
 
 /**
  * {@link Launcher} for JAR based archives. This launcher assumes that dependency jars are
@@ -26,10 +31,13 @@ import org.springframework.boot.loader.archive.Archive.EntryFilter;
  *
  * @author Phillip Webb
  * @author Andy Wilkinson
+ * @author Madhura Bhave
  * @since 1.0.0
  */
 public class JarLauncher extends ExecutableArchiveLauncher {
 
+	private static final String DEFAULT_CLASSPATH_INDEX_LOCATION = "BOOT-INF/classpath.idx";
+
 	static final EntryFilter NESTED_ARCHIVE_ENTRY_FILTER = (entry) -> {
 		if (entry.isDirectory()) {
 			return entry.getName().equals("BOOT-INF/classes/");
@@ -44,6 +52,23 @@ public class JarLauncher extends ExecutableArchiveLauncher {
 		super(archive);
 	}
 
+	@Override
+	protected ClassPathIndexFile getClassPathIndex(Archive archive) throws IOException {
+		// Only needed for exploded archives, regular ones already have a defined order
+		if (archive instanceof ExplodedArchive) {
+			String location = getClassPathIndexFileLocation(archive);
+			return ClassPathIndexFile.loadIfPossible(archive.getUrl(), location);
+		}
+		return super.getClassPathIndex(archive);
+	}
+
+	private String getClassPathIndexFileLocation(Archive archive) throws IOException {
+		Manifest manifest = archive.getManifest();
+		Attributes attributes = (manifest != null) ? manifest.getMainAttributes() : null;
+		String location = (attributes != null) ? attributes.getValue(BOOT_CLASSPATH_INDEX_ATTRIBUTE) : null;
+		return (location != null) ? location : DEFAULT_CLASSPATH_INDEX_LOCATION;
+	}
+
 	@Override
 	protected boolean isPostProcessingClassPathArchives() {
 		return false;

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

@@ -20,8 +20,11 @@ import java.io.ByteArrayOutputStream;
 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
+import java.io.OutputStreamWriter;
+import java.io.Writer;
 import java.net.MalformedURLException;
 import java.net.URL;
+import java.nio.charset.StandardCharsets;
 import java.util.Enumeration;
 import java.util.LinkedHashSet;
 import java.util.List;
@@ -41,6 +44,7 @@ import org.springframework.util.FileCopyUtils;
  * Base class for testing {@link ExecutableArchiveLauncher} implementations.
  *
  * @author Andy Wilkinson
+ * @author Madhura Bhave
  */
 public abstract class AbstractExecutableArchiveLauncherTests {
 
@@ -48,11 +52,25 @@ public abstract class AbstractExecutableArchiveLauncherTests {
 	File tempDir;
 
 	protected File createJarArchive(String name, String entryPrefix) throws IOException {
+		return createJarArchive(name, entryPrefix, false);
+	}
+
+	@SuppressWarnings("resource")
+	protected File createJarArchive(String name, String entryPrefix, boolean indexed) throws IOException {
 		File archive = new File(this.tempDir, name);
 		JarOutputStream jarOutputStream = new JarOutputStream(new FileOutputStream(archive));
 		jarOutputStream.putNextEntry(new JarEntry(entryPrefix + "/"));
 		jarOutputStream.putNextEntry(new JarEntry(entryPrefix + "/classes/"));
 		jarOutputStream.putNextEntry(new JarEntry(entryPrefix + "/lib/"));
+		if (indexed) {
+			JarEntry indexEntry = new JarEntry(entryPrefix + "/classpath.idx");
+			jarOutputStream.putNextEntry(indexEntry);
+			Writer writer = new OutputStreamWriter(jarOutputStream, StandardCharsets.UTF_8);
+			writer.write("BOOT-INF/lib/foo.jar\n");
+			writer.write("BOOT-INF/lib/bar.jar\n");
+			writer.write("BOOT-INF/lib/baz.jar\n");
+			writer.flush();
+		}
 		addNestedJars(entryPrefix, "/lib/foo.jar", jarOutputStream);
 		addNestedJars(entryPrefix, "/lib/bar.jar", jarOutputStream);
 		addNestedJars(entryPrefix, "/lib/baz.jar", jarOutputStream);

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

@@ -0,0 +1,123 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.loader;
+
+import java.io.File;
+import java.io.IOException;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.nio.file.Files;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+
+/**
+ * Tests for {@link ClassPathIndexFile}.
+ *
+ * @author Madhura Bhave
+ * @author Phillip Webb
+ */
+class ClassPathIndexFileTests {
+
+	@TempDir
+	File temp;
+
+	@Test
+	void loadIfPossibleWhenRootIsNotFileReturnsNull() throws IOException {
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> ClassPathIndexFile.loadIfPossible(new URL("https://example.com/file"), "test.idx"))
+				.withMessage("URL does not reference a file");
+	}
+
+	@Test
+	void loadIfPossibleWhenRootDoesNotExistReturnsNull() throws Exception {
+		File root = new File(this.temp, "missing");
+		assertThat(ClassPathIndexFile.loadIfPossible(root.toURI().toURL(), "test.idx")).isNull();
+	}
+
+	@Test
+	void loadIfPossibleWhenRootIsFolderThrowsException() throws Exception {
+		File root = new File(this.temp, "folder");
+		root.mkdirs();
+		assertThat(ClassPathIndexFile.loadIfPossible(root.toURI().toURL(), "test.idx")).isNull();
+	}
+
+	@Test
+	void loadIfPossibleReturnsInstance() throws Exception {
+		ClassPathIndexFile indexFile = copyAndLoadTestIndexFile();
+		assertThat(indexFile).isNotNull();
+	}
+
+	@Test
+	void sizeReturnsNumberOfLines() throws Exception {
+		ClassPathIndexFile indexFile = copyAndLoadTestIndexFile();
+		assertThat(indexFile.size()).isEqualTo(5);
+	}
+
+	@Test
+	void containsFolderWhenFolderIsPresentReturnsTrue() throws Exception {
+		ClassPathIndexFile indexFile = copyAndLoadTestIndexFile();
+		assertThat(indexFile.containsFolder("BOOT-INF/layers/one/lib")).isTrue();
+		assertThat(indexFile.containsFolder("BOOT-INF/layers/one/lib/")).isTrue();
+		assertThat(indexFile.containsFolder("BOOT-INF/layers/two/lib")).isTrue();
+	}
+
+	@Test
+	void containsFolderWhenFolderIsMissingReturnsFalse() throws Exception {
+		ClassPathIndexFile indexFile = copyAndLoadTestIndexFile();
+		assertThat(indexFile.containsFolder("BOOT-INF/layers/nope/lib/")).isFalse();
+	}
+
+	@Test
+	void getUrlsReturnsUrls() throws Exception {
+		ClassPathIndexFile indexFile = copyAndLoadTestIndexFile();
+		List<URL> urls = indexFile.getUrls();
+		List<File> expected = new ArrayList<>();
+		expected.add(new File(this.temp, "BOOT-INF/layers/one/lib/a.jar"));
+		expected.add(new File(this.temp, "BOOT-INF/layers/one/lib/b.jar"));
+		expected.add(new File(this.temp, "BOOT-INF/layers/one/lib/c.jar"));
+		expected.add(new File(this.temp, "BOOT-INF/layers/two/lib/d.jar"));
+		expected.add(new File(this.temp, "BOOT-INF/layers/two/lib/e.jar"));
+		assertThat(urls).containsExactly(expected.stream().map(this::toUrl).toArray(URL[]::new));
+	}
+
+	private URL toUrl(File file) {
+		try {
+			return file.toURI().toURL();
+		}
+		catch (MalformedURLException ex) {
+			throw new IllegalStateException(ex);
+		}
+	}
+
+	private ClassPathIndexFile copyAndLoadTestIndexFile() throws IOException, MalformedURLException {
+		copyTestIndexFile();
+		ClassPathIndexFile indexFile = ClassPathIndexFile.loadIfPossible(this.temp.toURI().toURL(), "test.idx");
+		return indexFile;
+	}
+
+	private void copyTestIndexFile() throws IOException {
+		Files.copy(getClass().getResourceAsStream("classpath-index-file.idx"),
+				new File(this.temp, "test.idx").toPath());
+	}
+
+}

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

@@ -18,7 +18,9 @@ package org.springframework.boot.loader;
 
 import java.io.File;
 import java.net.URL;
+import java.net.URLClassLoader;
 import java.util.ArrayList;
+import java.util.Iterator;
 import java.util.List;
 
 import org.junit.jupiter.api.Test;
@@ -33,6 +35,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * Tests for {@link JarLauncher}.
  *
  * @author Andy Wilkinson
+ * @author Madhura Bhave
  */
 class JarLauncherTests extends AbstractExecutableArchiveLauncherTests {
 
@@ -67,6 +70,16 @@ class JarLauncherTests extends AbstractExecutableArchiveLauncherTests {
 		}
 	}
 
+	@Test
+	void explodedJarShouldPreserveClasspathOrderWhenIndexPresent() throws Exception {
+		File explodedRoot = explode(createJarArchive("archive.jar", "BOOT-INF", true));
+		JarLauncher launcher = new JarLauncher(new ExplodedArchive(explodedRoot, true));
+		Iterator<Archive> archives = launcher.getClassPathArchivesIterator();
+		URLClassLoader classLoader = (URLClassLoader) launcher.createClassLoader(archives);
+		URL[] urls = classLoader.getURLs();
+		assertThat(urls).containsExactly(getExpectedFileUrls(explodedRoot));
+	}
+
 	protected final URL[] getExpectedFileUrls(File explodedRoot) {
 		return getExpectedFiles(explodedRoot).stream().map(this::toUrl).toArray(URL[]::new);
 	}

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/resources/org/springframework/boot/loader/classpath-index-file.idx

@@ -0,0 +1,5 @@
+BOOT-INF/layers/one/lib/a.jar
+BOOT-INF/layers/one/lib/b.jar
+BOOT-INF/layers/one/lib/c.jar
+BOOT-INF/layers/two/lib/d.jar
+BOOT-INF/layers/two/lib/e.jar