Collections.unmodifiableList上的同步

Question

I have a question. I think i know the answer but for some reason i prefer to ask for it here.

So here is the scenario:

I have an Object which has a list as a field. Then i have a method that returns the list as an unmodifiableList.

The Oject class has other methods that add elements to the list.

So lets imagine a case where one thread is iterating throught the unmodifiable list and another thread that is adding elements to the list using the Object class method.

How do i make this thread safe? If i synchronize the unmodifiableList and the list itselft will it make it thread safe? After all they are two different object where the unmodifiableList has a field which is the naked list itselft.

Answer 1

You need to make the "naked" list synchronized:

private List<Foo> list = Collections.synchronizedList(new ArrayList<Foo>());

But beware: that will only make sure the list internal state is coherent. As soon as you iterate on the list, you can't prevent a modification to the list to happen between two calls to the list iterator. So nothing will prevent a ConcurrentModificationException to happen in that case. To prevent that, you should not return any reference (even an indirect one) to the list. All modifications and iterations to the list should be encapsulated in your class, and properly synchronized.

Answer 2

You can return an unmodifiable-clone of original list to the caller. The disadvantage is that the caller may end up with a "stale" version of the list. However, by this way you achieve safe iterations. In concurrent world, it is OK to return last successfully updated data to the caller.

public List<Thing> getThings() {
  List<Thing> copytOfThings = new ArrayList<>();
  copyOfThings.addAll(_things); //original list items.
  return Collections.unmodifiableList(copyOfThings);
}

Answer 3

There are a couple of ways you could do this:

1. Return a copy of the list, rather than an unmodifiable view of it
2. Rather than using the iterator, use List.get(int)