code: public ParallelTaskBuilder prepareHttpHead(String url) {
          reinitIfClosed();
          ParallelTaskBuilder cb = new ParallelTaskBuilder();
          cb.getHttpMeta().setHttpMethod(HttpMethod.HEAD);
          cb.getHttpMeta().setRequestUrlPostfix(url);
          return cb;
      }
docs: Prepare a parallel HTTP HEAD Task.
  
  @param url
  the UrlPostfix: e.g. in http://localhost:8080/index.html.,the url is "/index.html"
  @return the parallel task builder