java.util.concurrent - Waiting for a hierarchy of tasks to complete -

this abstraction of actual problem, hope it's accurate enough explain things.

i'm processing file hierarchy, , i'm processing files asynchronously using java threadpoolexecutor finite number of threads , unbounded queue. starting @ particular directory, add task queue each file within directory. shutdown , await completion.

the tasks despatched using executor.execute(runnable).

the problem when directory contains further directories, 1 of these tasks may spawn further tasks, , these tasks not being executed because @ top level threadpoolexecutor has been shut down.

so question is, in top level thread, how await completion of whole hierarchy of tasks, recognising haven't started yet?

i did abstraction of problem. if described, walk whole hierarchy in orginal parent thread , fire off tasks there. in real problem can't that: it's essential feature of problem spawned child task submits further tasks.

in top level thread can use that:

    countdownlatch latch = new countdownlatch(n);      executor.execute(new worker(latch, sometask));     .     .     .     n      latch.await(); 

a countdownlatch initialized n can used make 1 thread wait until n threads have completed action, or action has been completed n times. http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/countdownlatch.html