Be defensive about JUL calls from JAR Handler

Update nested JAR support to only obtain JUL loggers when absolutely
necessary and to defensively deal with failures.

Prior to this commit it was not possible to override
`java.util.logging.manager` to use a nested JAR as the logger
implementation.

Fixes gh-9848

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 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.
@@ -71,8 +71,6 @@ public class Handler extends URLStreamHandler {
 		rootFileCache = new SoftReference<Map<File, JarFile>>(null);
 	}
 
-	private final Logger logger = Logger.getLogger(getClass().getName());
-
 	private final JarFile jarFile;
 
 	private URLStreamHandler fallbackHandler;
@@ -105,10 +103,10 @@ public class Handler extends URLStreamHandler {
 		}
 		catch (Exception ex) {
 			if (reason instanceof IOException) {
-				this.logger.log(Level.FINEST, "Unable to open fallback handler", ex);
+				log(false, "Unable to open fallback handler", ex);
 				throw (IOException) reason;
 			}
-			this.logger.log(Level.WARNING, "Unable to open fallback handler", ex);
+			log(true, "Unable to open fallback handler", ex);
 			if (reason instanceof RuntimeException) {
 				throw (RuntimeException) reason;
 			}
@@ -116,6 +114,18 @@ public class Handler extends URLStreamHandler {
 		}
 	}
 
+	private void log(boolean warning, String message, Exception cause) {
+		try {
+			Logger.getLogger(getClass().getName())
+					.log((warning ? Level.WARNING : Level.FINEST), message, cause);
+		}
+		catch (Exception ex) {
+			if (warning) {
+				System.err.println("WARNING: " + message);
+			}
+		}
+	}
+
 	private URLStreamHandler getFallbackHandler() {
 		if (this.fallbackHandler != null) {
 			return this.fallbackHandler;