Use fast exceptions in hasMoreElements in LaunchedURLClassLoader

When nested jars are being used, hasMoreElements requires opening a
connection for an entry in every nested jar. If that entry doesn't
exist, a FileNotFoundException is thrown to indicate that a particular
jar doesn't contain the requested entry. This exception is used to
indicate the lack of an entry and is then swallowed, i.e. its stack
trace is of no importance. This means that the performance of
hasMoreElements can be improved by switching on fast exceptions while
it's being called. When fast exceptions are switched on a general
purpose pre-initialized FileNotFoundException is thrown rather than
creating a new FileNotFoundException instance each time.

In certain situations, the use of fast exceptions as described above
can improve performance fairly significantly. The JRE's default SAAJ
implementation uses META-INF/services-based discovery for _every_
request that's handled by Spring Web Services. Each discovery attempt
results in hasMoreElements being called making its performance
critical to throughput.

See gh-3640

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2014 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.
@@ -33,6 +33,7 @@ import org.springframework.boot.loader.jar.JarFile;
  *
  * @author Phillip Webb
  * @author Dave Syer
+ * @author Andy Wilkinson
  */
 public class LaunchedURLClassLoader extends URLClassLoader {
 
@@ -107,9 +108,15 @@ public class LaunchedURLClassLoader extends URLClassLoader {
 
 			@Override
 			public boolean hasMoreElements() {
+				try {
+					Handler.setUseFastConnectionExceptions(true);
 					return rootResources.hasMoreElements()
 							|| localResources.hasMoreElements();
 				}
+				finally {
+					Handler.setUseFastConnectionExceptions(false);
+				}
+			}
 
 			@Override
 			public URL nextElement() {