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;
  }
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.)
jimfs/src/main/java/com/google/common/jimfs/JimfsFileSystem.java