What if you have no choice to call some blocking codebase from a legacy library or some legacy system. This is a way arround for the problem. Though this is not reactive but at least it's non blocking.

BlockingCallsInReactiveStreams.java

    @Test
    void callingBlockingCodeInReactiveStreams() {
        Mono.fromCallable(() -> {
            // Your blocking code here
            return new Object();
        }).subscribeOn(Schedulers.boundedElastic());

        Flux.fromIterable(Stream.of(1, 2, 3, 4, 5).map(i -> {
                    // Your blocking code here
                    return new Object();
                }).toList())
                .subscribeOn(Schedulers.boundedElastic());
    }