@Override
  public FileSystem newFileSystem(URI uri, Map<String, ?> env) throws IOException {
    checkArgument(
        uri.getScheme().equalsIgnoreCase(URI_SCHEME),
        "uri (%s) scheme must be '%s'",
        uri,
        URI_SCHEME);
    checkArgument(
        isValidFileSystemUri(uri), "uri (%s) may not have a path, query or fragment", uri);
    checkArgument(
        env.get(FILE_SYSTEM_KEY) instanceof FileSystem,
        "env map (%s) must contain key '%s' mapped to an instance of %s",
        env,
        FILE_SYSTEM_KEY,
        FileSystem.class);

    FileSystem fileSystem = (FileSystem) env.get(FILE_SYSTEM_KEY);
    if (fileSystems.putIfAbsent(uri, fileSystem) != null) {
      throw new FileSystemAlreadyExistsException(uri.toString());
    }
    return fileSystem;
  }
@deprecated Not intended to be called directly; this class is only for use by Java itself.
jimfs/src/main/java/com/google/common/jimfs/SystemJimfsFileSystemProvider.java