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 just before "return (Constructor) ref.get();". 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