Server EndPoint for WebSocket Java API中的易变变量？

Question

Looking at other examples across the web, when a server endpoint is created by means of Java API for WebSocket, the shared variables among all EndPoint instances are defined as static, but not volatile. I would like to know the reason.

As I understand, the clients opening websockets will create a new thread in the server. So to be thread-safe, shouldn't be defined as volatile the shared variables among EndPoint instances?

ie Instead of:

@ServerEndpoint("/endpoint")
public class WebsocketEndPoint {
   private static Set<WebsocketEndPoint> endPoint = new CopyOnWriteArraySet<WebsocketEndPoint> ();

   @OnOpen
   public void onOpen(Session session, String message) {
...

is not better this code?:

@ServerEndpoint("/endpoint")
public class WebsocketEndPoint {
   private static volatile Set<WebsocketEndPoint> endPoint = new CopyOnWriteArraySet<WebsocketEndPoint> ();

   @OnOpen
   public void onOpen(Session session, String message) {
...

Answer 1

You need to use the volatile keyword in cases in which a variable is modified by some thread, but may be read by a different one (and there aren't any other synchronization mechanisms in place).

So the question is whether there is a scenario in which some thread changes the Set to a different one, and a second thread tries to access it. For example, if some thread does this: endPoint = new HashSet<>() , while a different thread does this: endPoint.get(...) , then yes, the Set should be volatile .

However if the different threads use the same Set instance, then the endPoint reference doesn't change, and is "effectively final". In that case, there's no need to add the volatile keyword.

So the short answer is "it depends", since we need to look at complete example to see whether the Set should be volatile. I do want to point out one thing though - the use of an CopyOnWriteArraySet seems a bit odd since modifications to the set would only create a modified copy in the context of the modifying thread (that's what copy-on-write means).