ConcurrentModificationException in synchronized collection

Question

I am using SynchronizedCollection.containsAll problem is facing is when i ever i run the following code i am getting ConcurrentModification Exception.

From my understanding code should terminate without any exception.

public class Main {

    public static void main(String[] args) {
        List l1 = new LinkedList(), l2 = new LinkedList();
        for (int i = 0; i < 100000; i++) {
            l1.add("" + i);
        }
        for (int i = 0; i < 100000; i++) {
            l2.add("" + i);
        }
        // reverse to make the search take a little longer
        Collections.reverse(l2);
        final List sl1 = Collections.synchronizedList(l1);
        final List sl2 = Collections.synchronizedList(l2);

        new Thread() {
            public void run() {
                // synchronized (sl2) {
                sl1.containsAll(sl2);
                // }
            }
        }.start();
        new Thread() {
            public void run() {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                }
                sl2.add("3");
            }
        }.start();
    }

}

Can some one help me understand why i am getting this exception.

instead of Thread.sleep(100); change it to Thread.sleep(1000); and you will see — Youcef LAIDANI
– Youcef LAIDANI, Commented Jun 9, 2017 at 9:22

assylias · Accepted Answer · 2017-06-09 09:23:28Z

2

As per the documentation of Collections::synchronizedList

It is imperative that the user manually synchronize on the returned list when iterating over it.

In your example, when you run sl.containsAll(sl2), you iterate over sl2 without synchronizing on sl2. That's admittedly an implementation detail of the containsAll method but it is clearly indicated in the javadoc:

This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.

You can fix the problem by synchronizing on sl2 (i.e. uncomment the code that you have commented out).

answered Jun 9, 2017 at 9:23

assylias

330k84 gold badges680 silver badges806 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Joker · Accepted Answer · 2017-06-09 09:23:46Z

0

I checked the code, i think exception is intended but if current code

public boolean containsAll(Collection<?> coll) {
synchronized(mutex) {return c.containsAll(coll);}
        }

is changed to....

public boolean containsAll(Collection<?> coll) {
synchronized(mutex) {
               synchronized(coll) {
              return c.containsAll(coll);}}
        }

Then the problem will be fixed.

answered Jun 9, 2017 at 9:23

Joker

11.2k4 gold badges62 silver badges128 bronze badges

1 Comment

assylias Over a year ago

That could easily lead to deadlocks and there is no reason to assume that coll is a synchronized collection.

Collectives™ on Stack Overflow

ConcurrentModificationException in synchronized collection

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related