Synchronize on class loading lock in RestartClassLoader

Closes gh-12171

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/RestartClassLoader.java

 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -142,6 +142,7 @@ public class RestartClassLoader extends URLClassLoader implements SmartClassLoad
 		if (file != null && file.getKind() == Kind.DELETED) {
 			throw new ClassNotFoundException(name);
 		}
+		synchronized (getClassLoadingLock(name)) {
 			Class<?> loadedClass = findLoadedClass(name);
 			if (loadedClass == null) {
 				try {
@@ -156,6 +157,7 @@ public class RestartClassLoader extends URLClassLoader implements SmartClassLoad
 			}
 			return loadedClass;
 		}
+	}
 
 	@Override
 	protected Class<?> findClass(final String name) throws ClassNotFoundException {