@Override
    public AbstractHttpMessageBuilder contentJson(Object content) {
        requireNonNull(content, "content");
        checkState(publisher == null, "publisher has been set already");
        try {
            return content(MediaType.JSON, HttpData.wrap(JacksonUtil.writeValueAsBytes(content)));
        } catch (JsonProcessingException e) {
            throw new IllegalArgumentException("failed to serialize " + content, e);
        }
    }
Returns the {@link Publisher} that was set by {@link #content(MediaType, Publisher)}.
core/src/main/java/com/linecorp/armeria/common/AbstractHttpMessageBuilder.java