遍历列表但未修改列表时发生ConcurrentModificationException

Question

I am running my application for load testing using JMeter and getting ConcurrentModificationException in a scenario where (I think) its not supposed to come. This occurred only for few cases and not all.

I am having an ArrayList which is a class variable in which elements are added through a method. Later on in a different method which is invoked after the list is prepared, iteration is going on using stream and a local arraylist is getting prepared using some filters as follows

...
//class variable getting elements from a separate method
List<String> myGlobalList = new ArrayList(); 
...

public String returnValue(String _value){
  List myLocalList = new ArrayList();
  myGlobalList.stream().filter(ele -> {
    ele = ele.replaceAll("\\[(.*?)\\]", "$1");

    String[] strArr = ele.split(",");
    for(String sArr : strArr) {
      if(sArr.contains(_value)){
        System.out.println("sArr ---- > "+sArr);
        myLocalList.add(sArr);
        return true ;
      }
    }
    return false;
  }).findFirst();

  return myLocalList.size()>0 ? myLocalList.get(0): null;
}

The findFirst() method is throwing ConcurrentModificationException only under load (not for every request).

Can't figure out why it's happening. Am I missing any key? How can this code be modified to avoid this exception in all cases?

Update:

Below is a snippet from the stacktrace for the exception

java.util.ConcurrentModificationException
at java.util.ArrayList$ArrayListSpliterator.tryAdvance(ArrayList.java:1353)
at java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:126)
at java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:498)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:485)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
at java.util.stream.FindOps$FindOp.evaluateSequential(FindOps.java:152)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.findFirst(ReferencePipeline.java:464)
at com.pkg.MyClass.returnValue(MyClass.java:501)

Answer 1

Don't you need to get the first String in list? why not get it without using List:

  public String returnValue(String _value){   
  String findFirst = myGlobalList.stream().filter(ele -> {
    ele = ele.replaceAll("\\[(.*?)\\]", "$1");

    String[] strArr = ele.split(",");
    for(String sArr : strArr) {
      if(sArr.contains(_value)){
        System.out.println("sArr ---- > "+sArr);
        //myLocalList.add(sArr);
        return true ;
      }
    }
    return false;
  }).findFirst().get();

  return findFirst;
}

EDIT

With Optional check:

        Optional<String> findFirst = myGlobalList.stream().filter(ele -> {
        ele = ele.replaceAll("\\[(.*?)\\]", "$1");

        String[] strArr = ele.split(",");
        for (String sArr : strArr) {
            if (sArr.contains(_value)) {
                System.out.println("sArr ---- > " + sArr);
                // myLocalList.add(sArr);
                return true;
            }
        }
        return false;
    }).findFirst();
    return findFirst.isPresent() ? findFirst.get() : null;

EDIT 2

findFirst is throwing the exception, because original list have been changed, try collecting your results to new list using collect and then get the first value:

 return myGlobalList.stream().filter(ele -> {
        ele = ele.replaceAll("\\[(.*?)\\]", "$1");

        String[] strArr = ele.split(",");
        for (String sArr : strArr) {
            if (sArr.contains(_value)) {
                //System.out.println("sArr ---- > " + sArr);
                // myLocalList.add(sArr);
                return true;
            }
        }
        return false;
    }).collect(Collectors.toList()).get(0);