code: public V getService(K key) {
          ComponentContext ctx = context;
  
          if (ctx == null || key == null)
              return null;
  
          ConcurrentServiceReferenceElement<V> e = elementMap.get(key);
          if (e == null) {
              return null;
          } else {
              return e.getService(ctx);
          }
      }
docs: Retrieve the service associated with key.
  
  @param key The key associated with the requested service
  @return The service if available, null otherwise.