无法同步（java.util.ConcurrentModificationException）

Question

I have the following Java code:

private Object guiUpdateLock = new Object();

public void updateLinkBar(SortedSet<Arbitrage> arbitrages) {
    synchronized (guiUpdateLock) {
        System.out.println("start");
        for (Arbitrage arbitrage : arbitrages) {
          //do some GUI stuff
        }
        System.out.println("end");
    }
}

updateLinkBar() is called from many threads, and occasionally I get java.util.ConcurrentModificationException in "for" cycle. But I can't understand why since I'm making a lock on object which obviously doesn't work because I can see two "start" in a row in the output.

Thank you in advance.

Answer 1

Locks must protect objects and not segments of code.

In your case you accept an arbitrary collection, acquire your private lock, and operate on the collection. Meanwhile the rest of your code may, in other threads, do whatever it wants with the collection and it doesn't have to contend for your private lock to do it.

You must significantly redesign your code such that all access to the collection in question is covered by the same lock.

Answer 2

Without your complete code I have to resort to guessing, but the most likely case is that the two threads are using different guiUpdateLog -Objects to synchronize on. My further guessing would be that they are using different instances of the class that contains the guiUpdateLock - and since it is not static there will be different Object -instances as well.