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 would you implement it? Some better idea than having thread groups in WeakHashMap?

I am implementing a debugging framework tunable in run-time with various parameters in global, per-thread and per-thread group contexts. As a very simple example, you can have a reporting statement:

debug.log( category, message);

and specify that the log entry with that specific category will be displayed only when called by a thread in group of threads servicing network requests.

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<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.