CountDownLatch

Java CountDownLatch class that
implements barrier synchronization.

A barrier is a synchronization technique
that halts the
progress of one or more threads at a
particular point.

Barriers are commonly used in several ways.

One way is to defer the start of
current computations until after an object
has been initialized.

Another common use is to allow one thread
to wait until a group
of other concurrent threads are done with
their processing before it can continue.