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    private static final boolean useCommonPool =
        (ForkJoinPool.getCommonPoolParallelism() > 1);

    /**
     * Default executor -- ForkJoinPool.commonPool() unless it cannot
     * support parallelism.
     */
    private static final Executor asyncPool = useCommonPool ?
        ForkJoinPool.commonPool() : new ThreadPerTaskExecutor();

    /** Fallback if ForkJoinPool.commonPool() cannot support parallelism */
    static final class ThreadPerTaskExecutor implements Executor {
        public void execute(Runnable r) { new Thread(r).start(); }
    }

    /**
     * Null-checks user executor argument, and translates uses of
     * commonPool to asyncPool in case parallelism disabled.
     */
    static Executor screenExecutor(Executor e) {
        if (!useCommonPool && e == ForkJoinPool.commonPool())
            return asyncPool;
        if (e == null) throw new NullPointerException();
        return e;
    }