In a long running high volume application, we have had an occurence of a NPE at com.thoughtworks.xstream.converters.reflection.Sun14ReflectionProvider.newInstance(Sun14ReflectionProvider.java:74). After looking at the source, at tiny window has been found in the 'getMungedConstructor' method involving the use of WeakReference and bad luck with the timing of the garbage collector. A suggested fix for this method follows: private Constructor getMungedConstructor(Class type) throws NoSuchMethodException { WeakReference ref = (WeakReference)constructorCache.get(type); Constructor mungedConstructor = ref != null? ref.get(): null; if (mungedConstructor == null) { Constructor javaLangObjectConstructor = Object.class.getDeclaredConstructor(new Class[0]); mungedConstructor = reflectionFactory.newConstructorForSerialization(type, javaLangObjectConstructor); ref = new WeakReference(mungedConstructor); constructorCache.put(type, ref); } return mungedConstructor; } Thank you for your attention.