how to stop reactor operation chain based on validation of data in the middle

Question

In a reactor operation chain, it first queries another server for response, and then based on response fields, it continues reactor Mono chain or stops. Before using reactor, I often use if else to control whether it continues. However, the following code would throw NPE for map(any -> null). I can put body of the next map into if else, but it is not a reactor chain in that way. So how to stop reactor operation chain based on validation of data? Basically, I wish it could be Mono.empty when map(any -> null)

    private Mono<MyResponse> post() {
        return client.post()
                .uri("/somepath")
                .contentType(MediaType.APPLICATION_JSON)
                .accept(MediaType.APPLICATION_JSON)
                .body(new MyRequest(), MyRequest.class)
                .retrieve()
                .bodyToMono(MyResponse.class);
    }


Slice<MyEntity> allBy = myRepository.findAll(pageable);
Flux.fromIterable(allBy.getContent())
                    .flatMap(entity -> {
                        return this.post()
                                .map(response-> {
                                    String img = response.getImg();
                                    if (img ==null || img.equals("")) {
                                        return null;
                                    } else {
                                        return new AnotherVo();
                                    }
                                })
                                .map(anotherVo-> {
                                    entity.setField(anotherVo.getField());
                                    myRepository.save(entity);
                                    return 1;
                                });
                    }).reduce(Integer::sum)
                    .block();

Answer 1

Maybe what you are looking for is to use filter operation based on some attribute. E.g.:

.flatMap(entity -> {
                    return this.post()
                            .filter(response-> response.getImg() != null && !response.getImg().equals(""))
                            .map(any -> {
                                var anotherVo = new AnotherVo();
                                entity.setField(anotherVo.getField());
                                myRepository.save(entity);
                                return 1;
                            });
                })

Note that the map will only be executed if the filter emits some non-empty signal. If you want to add a fallback value, just use .switchIfEmpty(value)