docs: private CacheKeyTO getCacheKey(CacheAopProxyChain pjp, Object[] arguments, ExCache exCache, Object result) {
          Object target = pjp.getTarget();
          String methodName = pjp.getMethod().getName();
          String keyExpression = exCache.key();
          if (null == keyExpression || keyExpression.trim().length() == 0) {
              return null;
          }
          String hfieldExpression = exCache.hfield();
          return getCacheKey(target, methodName, arguments, keyExpression, hfieldExpression, result, true);
      }