public void setFileSystem(FileSystem fileSystem) {
    // allowed to not be JimfsFileSystem for testing purposes only
    checkState(this.fileSystem == null, "may not set fileSystem twice");
    this.fileSystem = checkNotNull(fileSystem);
  }
Sets the file system to use for created paths.
jimfs/src/main/java/com/google/common/jimfs/PathService.java