List ConcurrentModificationException

Question

I have the following code

public void saveProjects(List<Project> proj) throws DatabaseException {
    for (Project listItems: proj) { // error here

        insertProjects(listItems);
    }
}

private void insertProjects(Project prj) throws DatabaseException {
    commitObjects(prj);
}

When I am executing the above, I am getting the following exception at for (Project listItems: proj) {

java.util.ConcurrentModificationException at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:449) at java.util.AbstractList$Itr.next(AbstractList.java:420)

How can I resolve this issue with usage of next or with iterator?

Edit 1

Code snippet where I am calling saveProjects

projectList.add(proj);
  for (Project persist: projectList) {
       persist.setProjectId("K7890");
       persist.setName(fileName);

          myDAO.saveProjects(projectList);

     }
  projectList.clear();

Not sure of the reasons for the down-votes. It's a legitimate and complete question. 1+ — Hovercraft Full Of Eels, Oct 27 '13 at 19:59
What does `commitObjects(prj)` do? Does it somehow modify the `List proj`? — Hovercraft Full Of Eels, Oct 27 '13 at 20:01
@HovercraftFullOfEels I have included commitObjects method code snippet. — Jacob, Oct 27 '13 at 20:05
@MaximShoustin `callablestatement` is database connection type. — Jacob, Oct 27 '13 at 20:10
That's strange, doesn't look like you modify the list somewhere. Can you try to wrap the list with `Collections.unmodifiableList(...)` before iterating over it? Are you calling something concurrently? — Ortwin Angermeier, Oct 27 '13 at 20:16
Why you ran twice over `projectList`. 1st loop `for (Project persist: projectList) {` and after you run again into `saveProjects` — Maxim Shoustin, Oct 27 '13 at 20:29
@MaximShoustin Does that causing the exception? If so what is the best approach for this? — Jacob, Oct 27 '13 at 20:31
Some more of your stack trace showing where in the code in the code those lines are could be useful. — Peter Lawrey, Oct 27 '13 at 21:01

Maxim Shoustin · Accepted Answer · 2013-10-27T21:52:45.400

6

From code

for (Project persist: projectList) { 
     persist.setProjectId("K7890");
     persist.setName(fileName);

      myDAO.saveProjects(projectList); 
 }

projectList.clear(); // <-- clear might cause to this Exception

Reference

Why do you get a ConcurrentModificationException when using an iterator?

The java.util Collection classes are fail-fast, which means that if one thread changes a collection while another thread is traversing it through with an iterator the iterator.hasNext() or iterator.next() call will throw ConcurrentModificationException.

Even the synchronized collection wrapper classes SynchronizedMap and SynchronizedList are only conditionally thread-safe, which means all individual operations are thread-safe but compound operations where flow of control depends on the results of previous operations may be subject to threading issues. (List myList = Collections.synchronizedList (myList) ! It may not work here)

Solutions for multi-thread access situation

Solution 1: You can convert your list to an array with list.toArray() and iterate on the array. This approach is not recommended if the list is large.

Solution 2: You can lock the entire list while iterating by wrapping your code within a synchronized block. This approach adversely affects scalability of your application if it is highly concurrent.

Solution 3: You can use the ConcurrentHashMap and CopyOnWriteArrayList classes, which provide much better scalability and the iterator returned by ConcurrentHashMap.iterator() will not throw ConcurrentModificationException while preserving thread-safety.

Solution for single-thread access situation

Use:

it.remove();

It removes the current object via the Iterator it which has a reference to your underlying collection list.

Avoid:

list.remove(myObject);

edited Oct 27 '13 at 21:52

answered Oct 27 '13 at 19:57

Maxim Shoustin

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Maxim, If I remove for loop from second, then it stops saving records. – Jacob Oct 27 '13 at 20:38
If you commit `insertProjects(listItems);` you still get this error? – Maxim Shoustin Oct 27 '13 at 20:41
You mean directly calling `insertProjects(listItems);`? – Jacob Oct 27 '13 at 20:44
toggle comment like //insertProjects(listItems); Maybe connection does something. Im not familar with Data access logic – Maxim Shoustin Oct 27 '13 at 20:45
When I comment `insertProjects(listItems);` then there no exceptions. – Jacob Oct 27 '13 at 20:51
BTW, try to comment `projectList.clear();` Take a look on this post: http://stackoverflow.com/questions/17899738/java-concurrentmodificationexception-when-clearing-list-and-arraylist – Maxim Shoustin Oct 27 '13 at 20:52
Sorry, even after commenting out `insertProjects(listItems);` it does throw exception. – Jacob Oct 27 '13 at 20:53
Yes by removing `projectList.clear()` solves the problem. I have a quetion though, do we need to clear the list after it's use? – Jacob Oct 27 '13 at 21:03

Ortwin Angermeier · Answer 2 · 2013-10-27T20:25:29.407

1

Looks strange, my guess is that you modify the list (List<Project> proj) in another thread while iterating over it?

Because you do not alter the list in any way in the code you have given us.

You can try calling the saveProjects method with a copy of the proj list.

edited Oct 27 '13 at 20:25

answered Oct 27 '13 at 20:20

Ortwin Angermeier

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That's a comment at best (as you have done already), not an answer. – Jeroen Vannevel Oct 27 '13 at 20:22

List ConcurrentModificationException

2 Answers2