NPE in Mahout TopItems#getTopUsers

Question

I'm using the mahout version 0.9. I have got NPE at demo server with following stacktrace.

HTTP Status 500 - Request processing failed; nested exception is java.lang.NullPointerException
type Exception report
message Request processing failed; nested exception is java.lang.NullPointerException
description The server encountered an internal error that prevented it from fulfilling this request.
exception 
org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerException
 org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:973)
 org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
 org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
root cause 
java.lang.NullPointerException
 org.apache.mahout.cf.taste.impl.recommender.TopItems.getTopUsers(TopItems.java:119)
 org.apache.mahout.cf.taste.impl.neighborhood.NearestNUserNeighborhood.getUserNeighborhood(NearestNUserNeighborhood.java:94)
 org.apache.mahout.cf.taste.impl.recommender.GenericUserBasedRecommender.recommend(GenericUserBasedRecommender.java:87)
 org.apache.mahout.cf.taste.impl.recommender.AbstractRecommender.recommend(AbstractRecommender.java:63)
 .....recommendationengine.service.RecommendationService.findRecommendedProductsForOpportunityByMahout(RecommendationService.java:69)
 .....recommendationengine.controller.RecommendationController.recommendedForOpportunity(RecommendationController.java:54)
 sun.reflect.GeneratedMethodAccessor49.invoke(Unknown Source)
 sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
 java.lang.reflect.Method.invoke(Method.java:491)
 org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:215)
 org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132)
 org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104)
 org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:749)
 org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:690)
 org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:83)
 org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:945)
 org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:876)
 org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961)
 org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
 org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
note The full stack trace of the root cause is available in the Apache Tomcat/7.0.40 logs.
Apache Tomcat/7.0.40

After restart the server for debugging problem not reproduced. I have download the source code of Mahout 0.9 and can't understand how it's possible take NPE at TopItems.java:119, one possible case it if topItems Queue is empty but it's impossible if I'm correct understand the code in method. Could any one help me with this NPE?

Thank you guys.

Update NPE is possible if howMany param is 0, but I'm using 5. Will research it....

Update 2 howMany param can't be less than 1 because it's checked in constructor of NearestNUserNeighborhood. I have no ideas more... :(

Answer 1

It was hasted on Heroku and was going to sleep some times. I think it was reproduced after resume app.
Question can be closed.