我有以下代码：

@GetMapping("/enrollment/{id}")
public Mono<ResponseEntity<Response>> findByEnrollment(@PathVariable("id") String enrollment){
    return Mono.just(enrollment)
               .flatMap(e -> technomechanicalService.findByEnrollmentVehicle(e)
                                                    .collectList()
                                                    .map(t -> ResponseEntity.ok()
                                                                            .body(new Response(t, HttpStatus.OK.value(), null)))
                                                    .defaultIfEmpty(new ResponseEntity<>(new Response(null, HttpStatus.BAD_REQUEST.value(), "No se encontro la placa!"), 
                                                                            HttpStatus.BAD_REQUEST)));

}

当使用mongodb中不存在的值从客户机（ Postman ）消费时，它不会转到defaultifempty。
我给你的解决方案是在Map之前添加一个过滤器。如下所示：

@GetMapping("/enrollment/{id}")
public Mono<ResponseEntity<Response>> findByEnrollment(@PathVariable("id") String enrollment){
    return Mono.just(enrollment)
               .flatMap(e -> technomechanicalService.findByEnrollmentVehicle(e)
                                                    .collectList()
                                                    .filter(t -> {
                                                        if(!t.isEmpty()) return true;
                                                        return false;
                                                    })
                                                    .map(t -> ResponseEntity.ok()
                                                                            .body(new Response(t, HttpStatus.OK.value(), null)))
                                                    .defaultIfEmpty(new ResponseEntity<>(new Response(null, HttpStatus.BAD_REQUEST.value(), null), 
                                                                            HttpStatus.BAD_REQUEST)));

}

我的问题是.defaultifempty（）会去哪里？