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

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 extends ThreadLocal {
     private static class ValueHolder {
         public Object value;
     }
     // Weak & Concurrent would be even the better, but Java API wont offer that :(
     private static ConcurrentMap map = new ConcurrentHashMap;
     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 groupLocal = new ThreadGroupLocal();
 groupLocal.setValue("foo");
 //...
 String foo = groupLocal.getValue();

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