【阿里面試題】JAVA多線程統計文件夾大小

以下爲多線程統計文件夾大小的工具類，經過測試，使用forkjointask能更好的發揮多核心cpu的性能，提升速度。這裏使用List和for 循環代替了遞歸。

   * 獲取文件夾大小
     * @param dir
     * @return
     */
    public static long getDirSize(String dir) {
        File folder = new File(dir);
        if(!folder.isDirectory()) {
            return folder.length();
        }
        return IoOperateHolder.forkjoinPool.invoke(new CalDirCommand(folder));
    }

    static class CalDirCommand extends RecursiveTask<Long> {
        private File folder;

        CalDirCommand(File folder){
            this.folder = folder;
        }

        @Override
        protected Long compute() {
            AtomicLong size = new AtomicLong(0);
            File[] files = folder.listFiles();
            if(files == null || files.length == 0) {
                return 0L;
            }
            List<ForkJoinTask<Long>> jobs = new ArrayList<>();
            for(File f : files) {
                if(!f.isDirectory()) {
                    size.addAndGet(f.length());
                } else {
                    jobs.add(new CalDirCommand(f));
                }
            }
            for(ForkJoinTask<Long> t : invokeAll(jobs)) {
                size.addAndGet(t.join());
            }
            return size.get();
        }
    }

    private static final class IoOperateHolder {
        final static ForkJoinPool forkjoinPool = new ForkJoinPool();
    }



    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        long result = getDirSize("/Users/sunhf");
       System.out.println("大小："+result+"字節， 用時："+(System.currentTimeMillis() - start)+"ms");
       //單線程 大小：263251930518
    }