protected MappingFastJsonValue getOrCreateContainer(Object body) {
        return (body instanceof MappingFastJsonValue ? (MappingFastJsonValue) body : new MappingFastJsonValue(body));
    }
Wrap the body in a {@link MappingFastJsonValue} value container (for providing
additional serialization instructions) or simply cast it if already wrapped.
src/main/java/com/alibaba/fastjson/support/spring/FastJsonpResponseBodyAdvice.java