code: protected Object _getCacheManager() {
          net.sf.ehcache.CacheManager cacheManager = net.sf.ehcache.CacheManager.getInstance();
          if (cacheManager != null)
              return cacheManager;
          return net.sf.ehcache.CacheManager.newInstance();
      }
docs: 返回值不能是CacheManager,因为要考虑没有加ehcache的情况