@Override
  public FileSystem newFileSystem(URI uri, Map<String, ?> env) throws IOException {
    throw new UnsupportedOperationException(
        "This method should not be called directly;"
            + "use an overload of Jimfs.newFileSystem() to create a FileSystem.");
  }
Returns the singleton instance of this provider.
jimfs/src/main/java/com/google/common/jimfs/JimfsFileSystemProvider.java