code: public ApplicationInstanceCache applicationInstances(long applicationHostId)
      {
          ApplicationInstanceCache cache = applicationInstances.get(applicationHostId);
          if(cache == null)
              applicationInstances.put(applicationHostId, cache = new ApplicationInstanceCache(applicationHostId));
          return cache;
      }
docs: Returns the cache of application instances for the given application host, creating one if it doesn't exist .
  @param applicationHostId The id of the application host for the cache of application instances
  @return The cache of application instances for the given application host