public static void addImplClass(Class<? extends VFS> clazz) {
    if (clazz != null) {
      USER_IMPLEMENTATIONS.add(clazz);
    }
  }
Adds the specified class to the list of {@link VFS} implementations. Classes added in this
manner are tried in the order they are added and before any of the built-in implementations.

@param clazz The {@link VFS} implementation class to add.
src/main/java/org/apache/ibatis/io/VFS.java