@Override
    public AbstractHttpMessageBuilder content(Publisher<? extends HttpData> publisher) {
        requireNonNull(publisher, "publisher");
        checkState(content == null, "content has been set already");
        this.publisher = publisher;
        return this;
    }
Returns the {@link Publisher} that was set by {@link #content(MediaType, Publisher)}.
core/src/main/java/com/linecorp/armeria/common/AbstractHttpMessageBuilder.java