code: public synchronized ExecutorService getDefaultThreadPool() {
      if (defaultThreadPool == null) {
        defaultThreadPool =
            Executors.newCachedThreadPool(
                new ThreadFactoryBuilder()
                    .setDaemon(true)
                    .setNameFormat("JimfsFileSystem-" + uri.getHost() + "-defaultThreadPool-%s")
                    .build());
  
        // ensure thread pool is closed when file system is closed
        fileStore
            .state()
            .register(
                new Closeable() {
                  @Override
                  public void close() {
                    defaultThreadPool.shutdown();
                  }
                });
      }
      return defaultThreadPool;
    }
docs: Returns a default thread pool to use for asynchronous file channels when users do not provide
  an executor themselves. (This is required by the spec of newAsynchronousFileChannel in
  FileSystemProvider.)