code: public AlertChannelCache alertChannels(long policyId)
      {
          AlertChannelCache cache = channels.get(policyId);
          if(cache == null)
              channels.put(policyId, cache = new AlertChannelCache(policyId));
          return cache;
      }
docs: Returns the cache of alert channels for the given policy, creating one if it doesn't exist .
  @param policyId The id of the policy for the cache of alert channels
  @return The cache of alert channels for the given policy