Makes available the permit for the given thread, if it
was not already available. If the thread was blocked on
{@code park} then it will unblock. Otherwise, its next call
to {@code park} is guaranteed not to block. This operation
is not guaranteed to have any effect at all if the given
thread has not been started.
@param thread the thread to unpark, or {@code null}, in which case
this operation has no effect
Makes available the permit for the given thread, if it was not already available. If the thread was blocked on {@code park} then it will unblock. Otherwise, its next call to {@code park} is guaranteed not to block. This operation is not guaranteed to have any effect at all if the given thread has not been started.
@param thread the thread to unpark, or {@code null}, in which case this operation has no effect