Set timestamps of nested jars

Update JarWriter to set the entry timestamp for nested jars. The
timestamp is set to same time as the first (non directory) entry in
the nested jar.

This change should make it easier to created layered docker images.

Fixes gh-2807

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2013 the original author or authors.
+ * Copyright 2012-2015 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.
@@ -162,6 +162,7 @@ public class JarWriter {
 			throws IOException {
 		File file = library.getFile();
 		JarEntry entry = new JarEntry(destination + library.getName());
+		entry.setTime(getNestedLibraryTime(file));
 		if (library.isUnpackRequired()) {
 			entry.setComment("UNPACK:" + FileUtils.sha1Hash(file));
 		}
@@ -169,6 +170,28 @@ public class JarWriter {
 		writeEntry(entry, new InputStreamEntryWriter(new FileInputStream(file), true));
 	}
 
+	private long getNestedLibraryTime(File file) {
+		try {
+			JarFile jarFile = new JarFile(file);
+			try {
+				Enumeration<JarEntry> entries = jarFile.entries();
+				while (entries.hasMoreElements()) {
+					JarEntry entry = entries.nextElement();
+					if (!entry.isDirectory()) {
+						return entry.getTime();
+					}
+				}
+			}
+			finally {
+				jarFile.close();
+			}
+		}
+		catch (Exception ex) {
+			// Ignore and just use the source file timestamp
+		}
+		return file.lastModified();
+	}
+
 	/**
 	 * Write the required spring-boot-loader classes to the JAR.
 	 * @throws IOException