Mono<Void>.flatMap is not getting invoked when it returns Mono<Something>

Mono<Void>.flatMap is not getting invoked when it returns Mono<Something>

Mono<Void> doesn't produce a value because Void is not an object. Basically, it's the same as an empty Mono; it just completes without any actual element.

As per docs: https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Mono.html

Mono<Void> should be used for Publisher that just completes without any value.

So a flatMap, as well as any other method that is supposed to process a value, would never be called after Mono.just(music).then().flatMap.

But if you just want to update and return the value you already have, then you can do:

return filePart.transferTo(file).then(Mono.defer({
    music.setFilePath(file.getAbsolutePath());
    music.setMeta(new Metadata());
    log.info(String.valueOf(music));
    // assuming .save returns a publisher
    return this.musicRepository.save(music);
});