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;
+	}
+
 }