Under unfavorable circumstances method getMungedConstructor in Sun14ReflectionProvider class might return null, what leads to NullPointerException in method newInstance. It occurs when GC has cleared the WeakReference. See stack trace below: ... Caused by: java.lang.NullPointerException at com.thoughtworks.xstream.converters.reflection.Sun14ReflectionProvider.newInstance(Sun14ReflectionProvider.java:74) at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.instantiateNewInstance(AbstractReflectionConverter.java:2 79) at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.unmarshal(AbstractReflectionConverter.java:149) at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:81) ... 49 more This happens as well with current SVN release (rev 1488) Proposed fix: private Constructor getMungedConstructor(Class type) throws NoSuchMethodException { final WeakReference ref = (WeakReference)constructorCache.get(type); Constructor ctor = null; if(ref != null) { ctor = (Constructor) ref.get(); } if (ctor == null) { ctor = reflectionFactory.newConstructorForSerialization(type, Object.class.getDeclaredConstructor(new Class[0])); constructorCache.put(type, new WeakReference(ctor)); } return ctor; }