问链式doOnNext用于返回Flux的方法，但在直接调用Flux对象时不起作用。

EN

@Controller
public class ProblemController {
    @PostMapping("/upload")
    public Mono<ResponseEntity<String>> upload(ServerWebExchange exchange) {
        Flux<DataBuffer> body = exchange.getRequest().getBody()
                // This prints!
                .doOnNext(db -> System.out.println("Bytes A: " + db.readableByteCount()));

        // Does not print anything...
        body.doOnNext(db -> System.out.println("Bytes B: " + db.readableByteCount()));

        return body
                .next()
                .map(db -> String.format("Uploaded %d bytes", db.readableByteCount()))
                .map(s -> new ResponseEntity<String>(s, HttpStatus.ACCEPTED));
    }
}

curl --location --request POST 'http://localhost:8080/upload' \
--form 'file=@"/path/to/file.txt"'

        Flux<DataBuffer> body = exchange.getRequest().getBody()
                // This prints!
                .doOnNext(db -> System.out.println("Bytes A: " + db.readableByteCount()))
                .doOnNext(db -> System.out.println("Bytes X: " + db.readableByteCount()));