Prevent a jar from being repackaged more than once

Previously, Repackager would repackage a jar file as many times as
it was asked to do so. This lead to problems if a user made a mistake
when using Maven that led to the package phase being driven twice,
for example by running "mvn clean install package".

This commit updates Repackager so that a repackage call becomes a
no-op if the source jar's manifest already contains the
Spring-Boot-Version attribute which is added by repackaging.

Fixes #1251

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

@@ -107,6 +107,11 @@ public class Repackager {
 		if (libraries == null) {
 			throw new IllegalArgumentException("Libraries must not be null");
 		}
+
+		if (alreadyRepackaged()) {
+			return;
+		}
+
 		destination = destination.getAbsoluteFile();
 		File workingSource = this.source;
 		if (this.source.equals(destination)) {
@@ -132,6 +137,19 @@ public class Repackager {
 		}
 	}
 
+	private boolean alreadyRepackaged() throws IOException {
+		JarFile jarFile = new JarFile(this.source);
+
+		try {
+			Manifest manifest = jarFile.getManifest();
+			return manifest != null
+					&& manifest.getMainAttributes().getValue(BOOT_VERSION_ATTRIBUTE) != null;
+		}
+		finally {
+			jarFile.close();
+		}
+	}
+
 	private void repackage(JarFile sourceJar, File destination, Libraries libraries)
 			throws IOException {
 		final JarWriter writer = new JarWriter(destination);
@@ -208,7 +226,7 @@ public class Repackager {
 		String launcherClassName = this.layout.getLauncherClassName();
 		if (launcherClassName != null) {
 			manifest.getMainAttributes()
-					.putValue(MAIN_CLASS_ATTRIBUTE, launcherClassName);
+			.putValue(MAIN_CLASS_ATTRIBUTE, launcherClassName);
 			if (startClass == null) {
 				throw new IllegalStateException("Unable to find main class");
 			}