来自地图的同步集还是来自同步地图的集？

Question

I need a concurrent weak hash set without using Guava collections. Which one is correct? Any side effects?

private Set<Session> subscribers1 = Collections.newSetFromMap(
        Collections.synchronizedMap(new WeakHashMap<Session, Boolean>())
);
private Set<Session> subscribers2 = Collections.synchronizedSet(
        Collections.newSetFromMap(new WeakHashMap<Session, Boolean>())
);

Answer 1

If you look at the implementations, it's almost the same.

Collections.newSetFromMap creates a new SetFromMap extends AbstracSet instance. The SetFromMap class is basically a wrapper around a map object. But it does inherit the addAll method from the AbstracSet . This means that in the first option:

private Set<Session> subscribers1 = Collections.newSetFromMap(
        Collections.synchronizedMap(new WeakHashMap<Session, Boolean>())
);

not all operations will be synchronized (namely the addAll method - though it uses the synchronized add method under the hood). Due to this fact I'd recommend choosing the second option:

private Set<Session> subscribers2 = Collections.synchronizedSet(
        Collections.newSetFromMap(new WeakHashMap<Session, Boolean>())
);