code: public T getDefaultExtension() {
          getExtensionClasses();
          if (null == cachedDefaultName || cachedDefaultName.length() == 0 || "true".equals(cachedDefaultName)) {
              return null;
          }
          return getExtension(cachedDefaultName);
      }
docs: 返回缺省的扩展，如果没有设置则返回<code>null</code>