Reactor 애플리케이션에서 예외가 발생하면 main 메소드로부터 예외 발생 지점까지의 호출 순서를 제대로 확인하기가 어려움
이것은 reactor의 스케쥴러가 멀티쓰레드 기반이라서 스케쥴을 분리할 때마다 멀티쓰레드가 분리되면서 호출 순서가 끊어지기 때문임
¶ 예제
다음 순서로 호출됨
public class SimpleFluxTest {
public static void main(String[] args) {
PoliteServer server = new PoliteServer(new KitchenService());
server.doingMyJob().subscribe();
}
}
public class PoliteServer {
private final KitchenService kitchen;
PoliteServer(KitchenService kitchen) {
this.kitchen = kitchen;
}
Flux<Dish> doingMyJob() {
return this.kitchen.getDishes()
.doOnNext(dish -> System.out.println("Thank you for " + dish + "!"))
.doOnError(error -> {
System.out.println("So sorry about " + error.getMessage());
error.printStackTrace();
})
.doOnComplete(() -> System.out.println("Thanks for all your hard work!"))
.map(Dish::deliver);
}
}
PoliteServier.doingMyJob()의 doOnError()에서 예외 발생 시 stacktrace를 출력하게 했음
public class KitchenService {
Flux<Dish> getDishes() {
return Flux.just(new Dish("Sesame chicken"),
new Dish("Lo mein noodles, plain"),
null,
new Dish("Sweet & sour beef"));
}
}
KitchenService.getDishes()는 Flux 스트림을 생성하는 역할만 담당함. 중간에 일부러 null을 끼워넣었음
java.lang.NullPointerException: The 1th array element was null
at reactor.core.publisher.FluxArray$ArraySubscription.fastPath(FluxArray.java:168)
at reactor.core.publisher.FluxArray$ArraySubscription.request(FluxArray.java:97)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.request(FluxPeekFuseable.java:144)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.request(FluxPeekFuseable.java:144)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.request(FluxPeekFuseable.java:144)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.request(FluxMapFuseable.java:169)
at reactor.core.publisher.LambdaSubscriber.onSubscribe(LambdaSubscriber.java:119)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onSubscribe(FluxMapFuseable.java:96)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onSubscribe(FluxPeekFuseable.java:178)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onSubscribe(FluxPeekFuseable.java:178)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onSubscribe(FluxPeekFuseable.java:178)
at reactor.core.publisher.FluxArray.subscribe(FluxArray.java:53)
at reactor.core.publisher.FluxArray.subscribe(FluxArray.java:59)
at reactor.core.publisher.Flux.subscribe(Flux.java:8469)
at reactor.core.publisher.Flux.subscribeWith(Flux.java:8642)
at reactor.core.publisher.Flux.subscribe(Flux.java:8439)
at reactor.core.publisher.Flux.subscribe(Flux.java:8363)
at reactor.core.publisher.Flux.subscribe(Flux.java:8281)
at com.terzeron.grammar.SimpleFluxTest.main(SimpleFluxTest.java:6)
main()에서 subscribe() 호출만 있을 뿐, 애플리케이션 코드 상의 호출 관계가 전혀 보이지 않음
¶ Hooks.onOperatorDebug() 활용
main()에 Hooks.onOperatorDebug()를 추가하면 자세한 호출 순서를 가진 stacktrace를 살펴볼 수 있음
public static void main(String[] args) {
Hooks.onOperatorDebug();
PoliteServer server = new PoliteServer(new KitchenService());
server.doingMyJob().subscribe();
}
다음과 같이 stacktrace가 달라짐
java.lang.NullPointerException: The 1th array element was null
at reactor.core.publisher.FluxArray$ArraySubscription.fastPath(FluxArray.java:168)
Suppressed: The stacktrace has been enhanced by Reactor, refer to additional information below:
Assembly trace from producer [reactor.core.publisher.FluxArray] :
reactor.core.publisher.Flux.just(Flux.java:1333)
com.terzeron.grammar.KitchenService.getDishes(KitchenService.java:7)
Error has been observed at the following site(s):
*______Flux.just ⇢ at com.terzeron.grammar.KitchenService.getDishes(KitchenService.java:7)
|_ Flux.doOnNext ⇢ at com.terzeron.grammar.PoliteServer.doingMyJob(PoliteServer.java:14)
Original Stack Trace:
at reactor.core.publisher.FluxArray$ArraySubscription.fastPath(FluxArray.java:168)
at reactor.core.publisher.FluxArray$ArraySubscription.request(FluxArray.java:97)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.request(FluxPeekFuseable.java:144)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.request(FluxPeekFuseable.java:144)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.request(FluxPeekFuseable.java:144)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.request(FluxMapFuseable.java:169)
at reactor.core.publisher.LambdaSubscriber.onSubscribe(LambdaSubscriber.java:119)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onSubscribe(FluxMapFuseable.java:96)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onSubscribe(FluxPeekFuseable.java:178)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onSubscribe(FluxPeekFuseable.java:178)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onSubscribe(FluxPeekFuseable.java:178)
at reactor.core.publisher.FluxArray.subscribe(FluxArray.java:53)
at reactor.core.publisher.FluxArray.subscribe(FluxArray.java:59)
at reactor.core.publisher.Flux.subscribe(Flux.java:8469)
at reactor.core.publisher.Flux.subscribeWith(Flux.java:8642)
at reactor.core.publisher.Flux.subscribe(Flux.java:8439)
at reactor.core.publisher.Flux.subscribe(Flux.java:8363)
at reactor.core.publisher.Flux.subscribe(Flux.java:8281)
at com.terzeron.grammar.SimpleFluxTest.main(SimpleFluxTest.java:9)
main()부터 doingMyJob()을 거쳐 getDishes(), Flux.just()까지 호출되고 여기서 null이 포함되어 NullPointerException이 발생했음을 확인할 수 있음
¶ 주의사항
Hooks.onOperatorDebug()가 디버깅에 도움을 줄 수 있긴 하지만, 예외 발생 시 성능 저하의 원인이 될 수 있어서 Spring Framework 개발진은 최후의 수단으로 사용할 것을 권고하고 있음
https://github.com/spring-projects/spring-framework/issues/22007