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]

Answer 1

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

Orderlist.getNewOrder().remove(drm); 

call remove on iterator.

itr.remove()