SessionRegistryImpl should use computeIfAbsent

[jzheaux]

Java 8 introduced computeIfAbsent on ConcurrentHashMap:

If the specified key is not already associated with a value,
attempts to compute its value using the given mapping function
and enters it into this map unless {@code null}. The entire
method invocation is performed atomically, so the function is
applied at most once per key.

Since SessionRegistryImpl uses ConcurrentHashMap and since it uses it as <String, List<?>>, it seems like an ideal circumstance to make the update to principals atomic.

Instead of:

Set<String> sessionsUsedByPrincipal = principals.get(principal);

if (sessionsUsedByPrincipal == null) {
	sessionsUsedByPrincipal = new CopyOnWriteArraySet<>();
	Set<String> prevSessionsUsedByPrincipal = principals.putIfAbsent(principal,
			sessionsUsedByPrincipal);
	if (prevSessionsUsedByPrincipal != null) {
		sessionsUsedByPrincipal = prevSessionsUsedByPrincipal;
	}
}

sessionsUsedByPrincipal.add(sessionId);

It could do:

principals.computeIfAbsent(principal, key -> new CopyOnWriteArraySet<>())
	.add(sessionId);

[rmartinus]

I can work on this as my first contribution to spring security

[rwinch]

@rmartinus Thanks the issue is yours! If you need any help, please let us know.

[rmartinus]

@jzheaux @rwinch please review my PR when you get a chance