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