public Builder registerContentHandler(Type type, ContentHandlerWithResult.Factory<?> handler) {
      this.handlerFactories.put(checkNotNull(type, "type"), checkNotNull(handler, "handler"));
      return this;
    }
Will call {@link ContentHandlerWithResult.Factory#create()} on {@code handler} for each
content stream. The {@code handler} is expected to have a generic parameter of {@code type}.
sax/src/main/java/feign/sax/SAXDecoder.java