Are there thread group-local variables in Java?

Question

I am looking for a class similar to ThreadLocal which would work on thread groups instead of threads.

If there is not such a class (in some open source library) how

Answer 1

The last time i looked at the implementation (well a few years ago) Thread Locals were implemented as a simple hash table indexed by the thread id. Nothing fancy and miles away from the efficiency of C++.

You can do the same and use the thread group object as a key for your own thread group locals hash table.

Answer 2

I would store a value holder in a thread local and initialize it to the same value holder for all threads of the same group.

 public class ThreadGroupLocal<T> extends ThreadLocal<ValueHolder> {
     private static class ValueHolder {
         public Object value;
     }
     // Weak & Concurrent would be even the better, but Java API wont offer that :(
     private static ConcurrentMap<ThreadGroup, ValueHolder> map = new ConcurrentHashMap<ThreadGroup, ValueHolder>;
     private static ValueHolder valueHolderForThread(Thread t) {
         map.putIfAbsent(t.getThreadGroup(), new ValueHolder());
         return map.get(t.getThreadGroup());
     }
     @Override 
     protected ValueHolder initialValue() {
         return valueHolderForThread(Thread.currentThread());
     }
     public T getValue() { (T) get().value; }
     public void setValue(T value) { get().value = value; }
 }

and then use

 ThreadGroupLocal<String> groupLocal = new ThreadGroupLocal<String>();
 groupLocal.setValue("foo");
 //...
 String foo = groupLocal.getValue();

That does (expect for the initialization) perform exactly like a thread local.

Answer 3

ThreadGroup is rarely used, so there isn't platform support.

Using [Weak/Identity/Concurrent]HashMap<ThreadGroup,T> will just about work, if not very fast. You really want the map to be all weak, identity and concurrent but with the Java library you only get to choose one, currently.

To improve performance, note that Threads do not change ThreadGroup. Therefore, cache the value with a ThreadLocal (override initialValue). ThreadLocal has good performance (a couple dozen cycles per get).