Question

I am looking at this piece of code. This constructor delegates to the native method “System.arraycopy”

Is it Thread safe? And by that I mean can it ever throw a ConcurrentModificationException?

public Collection<Object> getConnections(Collection<Object> someCollection) {
    return new ArrayList<Object>(someCollection);
}

Does it make any difference if the collection being copied is ThreadSafe eg a CopyOnWriteArrayList?

public Collection<Object> getConnections(CopyOnWriteArrayList<Object> someCollection) {
    return new ArrayList<Object>(someCollection);
}

Edit:
I am aware that ThreadSafe != ConcurrentModificationException. I am trying to take a snapshot of data at a point in time. Therefore if another Thread writes to someCollection midway thru the copy I dont care if the result has the new object or not. I just dont want it to throw a ConcurrentModificationException or worse

Answer 1

Your question is whether you can safely get a snapshot of a collection that might be undergoing concurrent modification by another thread using new ArrayList<Foo>(thatCollection). The answer is: as long as thatCollection itself is thread-safe, yes. So if it’s a CopyOnWriteArrayList, synchronizedList or Vector, If it’s not thread-safe, for example if it’s another ArrayList, you’re not fine. (What will happen could be worse than a ConcurrentModificationException.)

The reason is that the ArrayList constructor makes only a single atomic call to the other collection — to its toArray method. So it essentially enjoys whatever thread-safety guarantees that method itself has. It was not always implemented like this, but is now for just this reason. We do the same thing in Guava with ImmutableList.copyOf.