java.util.ConcurrentModificationException的解決辦法
項目需求:
比較兩個list集合裡面的元素,根據某些規則,删除裡面某個集合的元素,本想以為比較簡單,但是還是碰到了一些問題。本次使用的是foreach來實作的。之前使用的是Iterator疊代來實作,不報此異常。 |
使用Iterator來疊代的代碼如下:
- public String findByEvent() throws Exception {
- try {
- eventName = new String(eventName.getBytes("iso-8859-1"), "UTF-8");
- listGroupInfo = this.groupInfoService.findByEvent(eventName);
- Iterator<GroupInfo> iter = listGroupInfo.iterator();
- if (listGroupInfo.size() != 0) {
- GroupInfo groupInfo = iter.next();
- for (int i = 0; i < listGroupInfo.size() - 1; i++) {
- for (int j = listGroupInfo.size() - 1; j > i; j--) {
- if (listGroupInfo.get(j).getGroupNumber().equals(
- listGroupInfo.get(i).getGroupNumber())) {
- listGroupInfo.remove(j);
- }
- }
- }
- }
- } catch (Exception e) {
- // TODO: handle exception
- e.printStackTrace();
- return INPUT;
- }
- return SUCCESS;
- }
上面的代碼運作正常!
本次使用的源代碼如下:
- public String findUnit() throws Exception {
- try {
- Map session = ActionContext.getContext().getSession();
- String ids = session.get("userId").toString();
- id = Long.parseLong(ids);
- listAthlete = this.athleteService.findByUserId(id);
- listUnitInfo = this.unitInfoService.findByUserId(id);
- for(UnitInfo unitInfo :listUnitInfo){
- for(Athlete athlete:listAthlete){
- if(athlete.getUnitGroup().getUnitInfo().getName().equals(unitInfo.getName())){
- System.out.println("移除:"+unitInfo.getName());
- listAthlete.remove(athlete);
- listUnitInfo.remove(unitInfo);
- }
- }
- }
- } catch (Exception e) {
- e.printStackTrace();
- System.out.print(e);
- return INPUT;
- }
- return SUCCESS;
- }
使用的是foreach,因為大家都知道使用foreach為我們帶來了極大的友善,是以考慮使用foreach也是正常的思路。但是,使用的時候,卻産生了如下的異常。
java.util.ConcurrentModificationException
- java.util.ConcurrentModificationException
- at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
- at java.util.AbstractList$Itr.next(AbstractList.java:343)
- at com.yaxing.action.AthleteAction.findUnit(AthleteAction.java:398)
- at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
- at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
- at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
- at java.lang.reflect.Method.invoke(Method.java:597)
- at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:452)
- at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:291)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:254)
- at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
- at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:263)
- at org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
- at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:133)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207)
- at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207)
- at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:190)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:94)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:243)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:270)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:145)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:171)
- at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:176)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:190)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:187)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at com.yaxing.util.competitionInterceptor.intercept(competitionInterceptor.java:57)
- at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
- at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52)
- at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:498)
- at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
- at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
- at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
- at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
- at org.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:198)
- at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
- at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
- at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
- at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
- at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
- at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
- at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
- at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
- at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
- at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
- at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
- at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
- at java.lang.Thread.run(Thread.java:619)
我們可以檢視此異常JDK文檔的說法
- public class ConcurrentModificationExceptionextends RuntimeException當方法檢測到對象的并發修改,但不允許這種修改時,抛出此異常。
- 例如,某個線程在 Collection 上進行疊代時,通常不允許另一個線性修改該 Collection。通常在這些情況下,疊代的結果是不确定的。如果檢測到這種行為,一些疊代器實作(包括 JRE 提供的所有通用 collection 實作)可能選擇抛出此異常。執行該操作的疊代器稱為快速失敗 疊代器,因為疊代器很快就完全失敗,而不會冒着在将來某個時間任意發生不确定行為的風險。
- 注意,此異常不會始終指出對象已經由不同 線程并發修改。如果單線程發出違反對象協定的方法調用序列,則該對象可能抛出此異常。例如,如果線程使用快速失敗疊代器在 collection 上疊代時直接修改該 collection,則疊代器将抛出此異常。
- 注意,疊代器的快速失敗行為無法得到保證,因為一般來說,不可能對是否出現不同步并發修改做出任何硬性保證。快速失敗操作會盡最大努力抛出 ConcurrentModificationException。是以,為提高此類操作的正确性而編寫一個依賴于此異常的程式是錯誤的做法,正确做法是:ConcurrentModificationException 應該僅用于檢測 bug。
Java中的foreach實際上使用的是還是Iterator進行處理的。我們在foreach時,從list中删除了一個元素,這導緻了Iterator抛出了ConcurrentModificationException。
- public String findUnit() throws Exception {
- try {
- Map session = ActionContext.getContext().getSession();
- String ids = session.get("userId").toString();
- id = Long.parseLong(ids);
- listAthlete = this.athleteService.findByUserId(id);
- listUnitInfo = this.unitInfoService.findByUserId(id);
- for (Iterator<Athlete> it = listAthlete.iterator();it.hasNext();){
- Athlete athlete=it.next();
- for (Iterator<UnitInfo> it2 = listUnitInfo.iterator();it2.hasNext();){
- UnitInfo unitInfo =it2.next();
- if(athlete.getUnitGroup().getUnitInfo().getName().equals(unitInfo.getName())){
- System.out.println("移除:"+unitInfo.getName());
- it2.remove();
- }
- }
- }
- } catch (Exception e) {
- e.printStackTrace();
- System.out.print(e);
- return INPUT;
- }
- return SUCCESS;
- }