Concurrent modification exception in ArrayList and Concurrent HashMap

Question

Following is my data structure to store data and a method for updating data.

ConcurrentHashMap<String, MyOrder> myOrder = new ConcurrentHashMap<String, MyOrder>();

public void updateOrder(MyData DR) {
      MyOrder Orderlist = myOrder.get(DR.keyID);
      Orderlist.getCanceled().add(DR);
      if (Orderlist.getCanceled().size() == 10) {
          Orderlist.getCanceled().remove(0);
      }

      /* remove order when cancelled */
      Iterator<MyData> itr = Orderlist.getNewOrder().iterator();
      MyData drm = null;
      while (itr.hasNext()) {
          drm = itr.next();
          if (drm.ClOrdID.equalsIgnoreCase(DR.ClOrdID)) {
              Orderlist.getNewOrder().remove(drm); 
              getTable().get(drm.InsKey).getCompOrder().remove(drm);
              break;
          }
      }
}

I have below method which is accessed all the time every second via polling to display some data in a web page retrieving via above concurrent hash map which is also updated with data time to time and updated when orders get removed as well.

/* accessed for data display every second */
public List<MyOrder> getMyDataList(String keyID) {
    List<MyOrder> orderList = new ArrayList<MyOrder>();
    try {
        if (myOrder.containsKey(keyID)) {
            orderList.add(myOrder.get(keyID));
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return orderList;
}

My question is when the methods are executed concurrently for while, it gives the following concurrent modification exception, How do I overcome this? What is the best solution to apply for this?. exception points to above method "getMyDataList"

Faces Servlet]] Servlet.service() for servlet Faces Servlet threw exception: java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(Unknown Source) [:1.6.0_22]
at java.util.AbstractList$Itr.next(Unknown Source) [:1.6.0_22]
at com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter.serialize(DefaultTypeAdapters.java:637) [:]
at com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter.serialize(DefaultTypeAdapters.java:624) [:]
at com.google.gson.JsonSerializationVisitor.findAndInvokeCustomSerializer(JsonSerializationVisitor.java:184) [:]
at com.google.gson.JsonSerializationVisitor.visitFieldUsingCustomHandler(JsonSerializationVisitor.java:204) [:]
at com.google.gson.ReflectingFieldNavigator.visitFieldsReflectively(ReflectingFieldNavigator.java:63) [:]
at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:120) [:]
at com.google.gson.JsonSerializationContextDefault.serialize(JsonSerializationContextDefault.java:62) [:]
at com.google.gson.JsonSerializationContextDefault.serialize(JsonSerializationContextDefault.java:53) [:]
at com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter.serialize(DefaultTypeAdapters.java:643) [:]
at com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter.serialize(DefaultTypeAdapters.java:624) [:]
at com.google.gson.JsonSerializationVisitor.findAndInvokeCustomSerializer(JsonSerializationVisitor.java:184) [:]
at com.google.gson.JsonSerializationVisitor.visitUsingCustomHandler(JsonSerializationVisitor.java:160) [:]
at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:101) [:]
at com.google.gson.JsonSerializationContextDefault.serialize(JsonSerializationContextDefault.java:62) [:]
at com.google.gson.JsonSerializationContextDefault.serialize(JsonSerializationContextDefault.java:53) [:]
at com.google.gson.Gson.toJsonTree(Gson.java:220) [:]
at com.google.gson.Gson.toJson(Gson.java:260) [:]
at com.google.gson.Gson.toJson(Gson.java:240) [:]
at com.company.jsf.order.MyBean.myOrdersData(MyBean.java:488) [:]
at sun.reflect.GeneratedMethodAccessor342.invoke(Unknown Source) [:1.6.0_22]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) [:1.6.0_22]
at java.lang.reflect.Method.invoke(Unknown Source) [:1.6.0_22]
at org.apache.el.parser.AstValue.invoke(AstValue.java:196) [:6.0.0.Final]
at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:276) [:6.0.0.Final]
at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:102) [:2.0.3-FCS]
at com.sun.faces.facelets.tag.jsf.core.DeclarativeSystemEventListener.processEvent(EventHandler.java:124) [:2.0.3-FCS]
at javax.faces.component.UIComponent$ComponentSystemEventListenerAdapter.processEvent(UIComponent.java:2378) [:2.0.3-FCS]
at javax.faces.event.SystemEvent.processListener(SystemEvent.java:102) [:2.0.3-FCS]
at com.sun.faces.application.ApplicationImpl.processListeners(ApplicationImpl.java:2040) [:2.0.3-FCS]
at com.sun.faces.application.ApplicationImpl.invokeComponentListenersFor(ApplicationImpl.java:1988) [:2.0.3-FCS]
at com.sun.faces.application.ApplicationImpl.publishEvent(ApplicationImpl.java:284) [:2.0.3-FCS]
at com.sun.faces.application.ApplicationImpl.publishEvent(ApplicationImpl.java:242) [:2.0.3-FCS]
at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:104) [:2.0.3-FCS]
at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:97) [:2.0.3-FCS]
at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:135) [:2.0.3-FCS]
at javax.faces.webapp.FacesServlet.service(FacesServlet.java:309) [:2.0.3-FCS]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:324) [:6.0.0.Final]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:242) [:6.0.0.Final]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:343) [:]
at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:109) [:]
at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:83) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:97) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:100) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:78) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:54) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:35) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.authentication.www.BasicAuthenticationFilter.doFilter(BasicAuthenticationFilter.java:177) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:187) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:105) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:79) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.session.ConcurrentSessionFilter.doFilter(ConcurrentSessionFilter.java:109) [:]
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:355) [:]
at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:149) [:]
at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:237) [:3.0.0.RELEASE]
at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:167) [:3.0.0.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:274) [:6.0.0.Final]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:242) [:6.0.0.Final]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275) [:6.0.0.Final]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191) [:6.0.0.Final]
at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:181) [:6.0.0.Final]
at org.jboss.modcluster.catalina.CatalinaContext$RequestListenerValve.event(CatalinaContext.java:285) [:1.1.0.Final]
at org.jboss.modcluster.catalina.CatalinaContext$RequestListenerValve.invoke(CatalinaContext.java:261) [:1.1.0.Final]
at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:88) [:6.0.0.Final]
at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.invoke(SecurityContextEstablishmentValve.java:100) [:6.0.0.Final]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127) [:6.0.0.Final]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) [:6.0.0.Final]
at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:158) [:6.0.0.Final]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) [:6.0.0.Final]
at org.jboss.web.tomcat.service.request.ActiveRequestResponseCacheValve.invoke(ActiveRequestResponseCacheValve.java:53) [:6.0.0.Final]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:362) [:6.0.0.Final]
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:877) [:6.0.0.Final]
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:654) [:6.0.0.Final]
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:951) [:6.0.0.Final]
at java.lang.Thread.run(Unknown Source) [:1.6.0_22]

This problem is not JSF, Java-EE, Spring nor jQuery related. — Luiggi Mendoza
– Luiggi Mendoza, Commented Aug 13, 2012 at 18:34
Java 6 update 22 is fairly old. Can you try updating it to a recent version of Java 6. — Peter Lawrey
– Peter Lawrey, Commented Aug 13, 2012 at 18:40

kosa · Accepted Answer · 2012-08-13 18:28:29Z

6

Dont call remove on list while iterating, which may results in concurrent-modification exception.

Orderlist.getNewOrder().remove(drm);

call remove on iterator.

itr.remove()

answered Aug 13, 2012 at 18:28

kosa

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5 Comments

Luiggi Mendoza Over a year ago

Also, the ConcurrentHashMap just saves the states of the objects from being accesed concurrently, it doesn't provide synchronization of the inner lists inside it. The code that removes the object must be synchronized.

Swarne27 Over a year ago

will it affect the performance if synchronized? how should I do it in considering those factors as well?

kosa Over a year ago

@Swarnajith: Make sure all add/remove calls on map are inside either Synchronized method (or) synch block.

Swarne27 Over a year ago

I added synchronized to updateOrder method like this "public synchronized void updateOrder(MyData DR){" and added itr.remove as mentioned but still i'm getting the exception

Swarne27 Over a year ago

this solution works for the method i have specified. I had to fix so many methods like these and also I added synchronization to all the methods as suggested by @Nambari

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