Why this code does not trigger ConcurrentModificationException?

Question

I'm modifying same list from multiple threads, shouldn't it trigger ConcurrentModificationException while iterating list?

What could be done to trigger this exception?

public class ConcurrentTest {

    static List<String> list = setupList();

    public static List<String> setupList() {
        System.out.println("setup predefined list");

        List<String> l = new ArrayList();
        for(int i = 0; i < 50;i++) {
            l.add("test" + i);
        }

        return l;
    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(50);

        for(int i = 0; i < 50; i++) {
            executorService.submit( () -> {     
                list.add("key1");       

                Thread currentThread = Thread.currentThread();
                System.out.println( Thread.currentThread().getName() + ", " + list.size() );

                for(String val: list) {
                    try {
                        Thread.sleep(25);
                    }
                    catch(Exception e) {

                    }
                }

            });
        }

        executorService.shutdown();
    }
}

score 1 · Answer 1 · answered Nov 19 '19 at 08:59

1

The ConcurrentModificationException triggers when you modify a list while iterating on it. In your code qhen you iterate on the list, you only sleep the thread and do not modify it.This would trigger the exception:

for(String s: list) {
    list.add("something");
}

answered Nov 19 '19 at 08:59

PauMAVA

1,163
14
23

This is not true. The *same* list is being modified from other threads – Thiyagu Nov 19 '19 at 09:28

score 1 · Answer 2 · answered Nov 19 '19 at 09:21

Have you check whether below block throwing exception or not.

try {
    list.add("key1");       

    Thread currentThread = Thread.currentThread();
    System.out.println( Thread.currentThread().getName() + ", " + list.size() );

    for(String val: list) {
        try {
            Thread.sleep(25);
        }
        catch(Exception e) {

        }
    }
}catch (Exception e) {
    e.printStackTrace();
}

Please wrap around try...catch block and print() error message to check its throwing error or not.

Thiyagu · Accepted Answer · 2019-11-19T09:31:59.547

Your code does (and can) produce a ConcurrentModificationException. You are not catching it to print it.

With the below, we can see that it indeed throws many ConcurrentModificationException.

try {
    for (String val : list) {
        try {
          Thread.sleep(25);
        } catch (Exception e) {
        }
    }
} catch (Exception e) {
    e.printStackTrace();
}

NOTE: From the javadoc,

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw {@code ConcurrentModificationException} on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs

Also see: java.util.ConcurrentModificationException not thrown when expected

Or, you could get the returned Future and collect them in a list. By doing that, you will get the exception when calling get on (atleast) one of the futures.

Why this code does not trigger ConcurrentModificationException?

3 Answers3