static final void helpExpungeStaleExceptions(){
  final ReentrantLock lock=exceptionTableLock;
  if (lock.tryLock()) {
    try {
      expungeStaleExceptions();
    }
  finally {
      lock.unlock();
    }
  }
}

static final void helpExpungeStaleExceptions ( ) { final ReentrantLock lock = exceptionTableLock ; if ( lock . tryLock ( ) ) { try { expungeStaleExceptions ( ) ; } finally { lock . unlock ( ) ; } } }
if lock is available , poll stale refs and remove them . called from forkjoinpool when pools become quiescent .
