elcnesh wrote:I think I may have found the source of the ConcurrentModificationExceptions... In r25449, the loop in getHiddenExtrinsicKeyword was changed from an index loop to an iteration, causing the while(true) and try...catch(IndexOutOfBoundsException) construction to no longer function. Changing the catch type to ConcurrentModificationException may very well solve it. (I'm not just doing that because I don't entirely understand what's going on, and maybe someone else does.)
I was wondering why that catch for
IndexOutOfBoundsException was there... Anyway, I think I understand what's going on now.
After looking at the stack trace posted here:
http://www.slightlymagic.net/forum/viewtopic.php?f=26&t=14791It looks like the
getHiddenExtrinsicKeyword is getting called by an event handler running on a separate thread. That means that while it is iterating through the class member
this.hiddenExtrinsicKeyword, another thread can call a function like
removeHiddenExtrinsicKeyword that alters this list.
That's why the catch for
IndexOutOfBoundsException was there previously, when it was an indexed loop. If an element was removed from the list by another thread, the size variable of the loop would not be updated, which could cause the loop index to now go out of bounds.
Using
ArrayList's iterator will have similar behaviour. The difference is that the iterator will check to see if the structure of the list has changed since the last call to
next, so it will throw a
ConcurrentModificationException immediately if the size has gone up or down.
So yes, I think that changing the catch block to catch a
ConcurrentModificationException will work. It might be kind of a sloppy solution though...
People have suggested using
ArrayList's
clone method to get around this problem. That is, instead of:
- Code: Select all
for (String s : this.hiddenExtrinsicKeyword)
Have this instead:
- Code: Select all
for (String s : this.hiddenExtrinsicKeyword.clone())
(You need a cast to
ArrayList<String>, but you get the idea.)
As I understand it, the
clone method will directly copy chunks of memory to make a shallow copy of the
ArrayList object, without using an iterator. Once the copy is made, it's now a separate list from the class member
hiddenExtrinsicKeyword, so it is guaranteed that it will not be modified by another thread. This means we can iterate through it safely.
An even more "proper" way to fix this would be to synchronize all accesses to
hiddenExtrinsicKeyword, but that's not a one-line fix.
Posted: 06 Jul 2014, 10:42