Java - multithreaded copy-on-write

Question

Is it possible to add software enforced copy-on-write for multithreaded applications in Java? By this I mean threads having a reference to the same object, but when one thread attempts to modify it, the object pointed to is copied and the reference is adjusted to point to that copy.

Answer 1

Yes. Lazy copying is easy to implement, but you would generally have to do it yourself.

Answer 2

The only implementation I know is the

java.util.concurrent.CopyOnWriteArrayList

see

http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/CopyOnWriteArrayList.html

and the related Set class

java.util.concurrent.CopyOnWriteArraySet

and finally

org.apache.mina.util.CopyOnWriteMap

but it depends from your need.

Answer 3

If your question is,

is it possible to enforce copy-on-write behavior across the board for an entire Java runtime

then the answer is,

No, there is no such general capability in Java.

Actually, I think the closest you can possibly get to that goal is using Clojure. All its default data structures are copy-on-write internally, and on the outside they are simply immutable objects.

The references you talk about are called, surprisingly, refs and they support full in-memory transactions. A simpler kind of a reference is atom, which fits your description 100%.

The whole Core API is devoted to elegant and epressive manipulation of these structures in a thread-safe, lock-free manner.