public List<File> listSnapshots() {
   File snapshotsDir = SnapshotUtil.getSnapshotDirectory(this);
   List<File> files =
       new ArrayList<File>(FileUtils.listFiles(snapshotsDir, new String[] {"zip"}, false));
   Collections.sort(
       files,
       new Comparator<File>() {
         public int compare(File o1, File o2) {
           return -o1.compareTo(o2);
         }
       });
   return files;
 }
 public File snapshot() {
   return SnapshotUtil.createSnapshot(this);
 }