ConcurrentModificationException while calling size() on a sublist

Question

I am trying to save a bunch of SQL transaction. I am in a context of ESB routes, transferring from a SQL source to a SQL target, and the order of the SQL transactions is not guaranteed, so you can have a SQL update before the object was inserted.

Due to the architecture, I'm saving those SQL transactions 1000 by 1000 (I'm using a messageQueue). So some of these can fail, and I re-route them in order to retry or reject them. To improve efficiency, I'm willing to improve the older system, where if the 1000 fail, you save 1 by 1, to implements dichotomia (if the save fail, you split the list and try again), via recursivity. I am also tracking an attribute of my objects, thanks to another list (objectsNo) for further operations.

However I am getting a ConcurrentModificationException when in my first recursivity, when calling objectsList.size(). How can I avoid it ? I'm also opened, and would be very thankful to any solutions which would provide another way than dichotomia to improve efficiency (and would by such bypass my issue).

Suppressed: java.util.ConcurrentModificationException: null at java.util.ArrayList$SubList.checkForComodification(ArrayList.java:1231) at java.util.ArrayList$SubList.size(ArrayList.java:1040) at fr.company.project.esbname.mariadb.MariaDbDatabase.saveObjectWithDichotomie(MariaDbDatabase.java:398) at fr.company.project.esbname.mariadb.MariaDbDatabase.saveObjectWithDichotomie(MariaDbDatabase.java:404) at fr.company.project.esbname.mariadb.MariaDbDatabase.saveObject(MariaDbDatabase.java:350) at sun.reflect.GeneratedMethodAccessor324.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.apache.camel.component.bean.MethodInfo.invoke(MethodInfo.java:472) at org.apache.camel.component.bean.MethodInfo$1.doProceed(MethodInfo.java:291) at org.apache.camel.component.bean.MethodInfo$1.proceed(MethodInfo.java:264) at org.apache.camel.component.bean.BeanProcessor.process(BeanProcessor.java:178) at org.apache.camel.management.InstrumentationProcessor.process(InstrumentationProcessor.java:77) at org.apache.camel.processor.RedeliveryErrorHandler.process(RedeliveryErrorHandler.java:541) ... 22 common frames omitted

I tried to understand, but there should not be any mistake. Even if I used recursivity, it stays single-threaded. I considered that the issue could be with hibernate (some requests from the save which failed could stay in the cache, and lock modification), but the issue is with the size, which is on a sublist of the original list.

    private List<String> saveObjectWithDichotomie(List<Object> objects,
                                                            List<String> objectsNo,
                                                            Exchange exchange) throws JsonProcessingException {
    try {
        objectRepository.save(objects);

        return objectsNo;
    } catch (DataIntegrityViolationException e) {
        if (objects.size() == 1) {
            objectsNo.clear();
            errorProcessor.sendErrorToRejets(objects.get(0), exchange, e);

            return objectsNo;
        } else {
            List<Object> objectsFirstHalf = objects.subList(0, objects.size()/2);
            List<Object> objectsSecondHalf = objects.subList(objects.size()/2, objects.size());

            List<String> objectsNoFirstHalf = objectsNo.subList(0, objectsNo.size()/2);
            List<String> objectsNoSecondHalf = objectsNo.subList(objectsNo.size()/2, objectsNo.size());

            objectsNo.clear();

            objectsNo.addAll(
                    saveObjectWithDichotomie(objects, objectsNoFirstHalf, exchange)
            );
            objectsNo.addAll(
                    saveObjectWithDichotomie(objects, objectsNoSecondHalf, exchange)
            );

            return objectsNo;
        }
    }
}

Answer 1

If you would read the documentation of sublist is clearly says:

The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa.

That being the reason for your Exception (no need for multiple threads for this to happen). Thus when you create a new List, create it via:

List<Object> objectsFirstHalf = new ArrayList<>(objects.subList(0, objects.size()/2));