How do I build a mono from a callable that would return a mono?

Question

I have this code that would call reactive Spring Redis Template to store some data I am stuck at the point of the switchIfEmpty where I want to create a new entry and store it and return the result.

    public Mono<SecretKey> secretKey(String jti, int expiresInSeconds) {

        final var urlDecoder = Base64.getUrlDecoder();
        var ops = redisTemplate.opsForValue();
        return ops.getAndExpire(simpleAuthServiceProperties + ":::symmetricKeys:::"+ jti, Duration.ofSeconds(expiresInSeconds))
                .switchIfEmpty(Mono.fromCallable(() -> {
                    final var secretKey = keyGenerator.generateKey();
                    final var encoded = secretKey.getEncoded();
                    final var e = Base64.getUrlEncoder().encodeToString(encoded);
                    var x = ops.setIfAbsent(simpleAuthServiceProperties + ":::symmetricKeys:::"+ jti, e, Duration.ofSeconds(expiresInSeconds))
                            .flatMap(result -> {
                                if (result) {
                                    return Mono.just(e);
                                } else {
                                    return Mono.error(IllegalStateException::new);
                                }
                            });
                    return x;
                }))
                .map(urlDecoder::decode)
                .map(bytes->
                    new SecretKeySpec(bytes, "AES")
                );
    }

I got to the point of x which gives me a Mono<String> but I need the fromCallable to return a String

I am thinking if I just change switchIfEmpty to something else...

Answer 1

As you can see at the docs Mono.fromCallable() accepts Callable<? extends T> Callable<? extends T> , not Callable<? extends Mono<? extends T>> Callable<? extends Mono<? extends T>> Callable<? extends Mono<? extends T>> , so it is not a surprise that you get Mono<Mono<...>> as result.

You can try Mono.defer() instead of Mono.fromCallable().