From 74a4b1a694f78bc1a04540c748cabe73c239b099 Mon Sep 17 00:00:00 2001
From: Gimin Kim <138752849+Gimini-3@users.noreply.github.com>
Date: Fri, 31 Jul 2026 23:35:55 +0900
Subject: [PATCH] Preserve async API version resolver order

Prior to this commit, the `DefaultApiVersionStrategy` reactive variant
would attempt to resolve the API version with the first resolver that
replies with a version. This contradicts the API that registers
resolvers in order.
This commit ensures that each resolver is called in order.

Signed-off-by: Gimin Kim <138752849+Gimini-3@users.noreply.github.com>
---
 .../accept/DefaultApiVersionStrategy.java     |  2 +-
 .../DefaultApiVersionStrategiesTests.java     | 33 +++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/spring-webflux/src/main/java/org/springframework/web/reactive/accept/DefaultApiVersionStrategy.java b/spring-webflux/src/main/java/org/springframework/web/reactive/accept/DefaultApiVersionStrategy.java
index 4dfdb26a088..f2adc8cd892 100644
--- a/spring-webflux/src/main/java/org/springframework/web/reactive/accept/DefaultApiVersionStrategy.java
+++ b/spring-webflux/src/main/java/org/springframework/web/reactive/accept/DefaultApiVersionStrategy.java
@@ -169,7 +169,7 @@ public class DefaultApiVersionStrategy implements ApiVersionStrategy {
 	@Override
 	public Mono<String> resolveApiVersion(ServerWebExchange exchange) {
 		return Flux.fromIterable(this.versionResolvers)
-				.flatMap(resolver -> resolver instanceof AsyncApiVersionResolver asyncResolver ?
+				.concatMap(resolver -> resolver instanceof AsyncApiVersionResolver asyncResolver ?
 						asyncResolver.resolveVersionAsync(exchange) :
 						Mono.justOrEmpty(resolver.resolveVersion(exchange)))
 				.next();
diff --git a/spring-webflux/src/test/java/org/springframework/web/reactive/accept/DefaultApiVersionStrategiesTests.java b/spring-webflux/src/test/java/org/springframework/web/reactive/accept/DefaultApiVersionStrategiesTests.java
index f8b2c7c8889..0906989e7e9 100644
--- a/spring-webflux/src/test/java/org/springframework/web/reactive/accept/DefaultApiVersionStrategiesTests.java
+++ b/spring-webflux/src/test/java/org/springframework/web/reactive/accept/DefaultApiVersionStrategiesTests.java
@@ -23,6 +23,7 @@ import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
+import reactor.test.publisher.TestPublisher;
 
 import org.springframework.web.accept.InvalidApiVersionException;
 import org.springframework.web.accept.MissingApiVersionException;
@@ -128,10 +129,42 @@ class DefaultApiVersionStrategiesTests {
 				.withMessage("versionRequired cannot be set to true if a defaultVersion is also configured");
 	}
 
+	@Test
+	void resolveVersionWithResolversInOrder() {
+		TestPublisher<String> firstResolverPublisher = TestPublisher.create();
+		ApiVersionStrategy strategy = apiVersionStrategy(List.of(
+				(AsyncApiVersionResolver) exchange -> firstResolverPublisher.flux().next(),
+				(AsyncApiVersionResolver) exchange -> Mono.just("2.0")));
+
+		StepVerifier.create(strategy.resolveApiVersion(
+					MockServerWebExchange.from(MockServerHttpRequest.get("/"))))
+				.then(() -> firstResolverPublisher.emit("1.0"))
+				.expectNext("1.0")
+				.verifyComplete();
+	}
+
+	@Test
+	void resolveVersionPropagatesErrorFromEarlierResolver() {
+		TestPublisher<String> firstResolverPublisher = TestPublisher.create();
+		ApiVersionStrategy strategy = apiVersionStrategy(List.of(
+				(AsyncApiVersionResolver) exchange -> firstResolverPublisher.flux().next(),
+				(AsyncApiVersionResolver) exchange -> Mono.just("2.0")));
+
+		StepVerifier.create(strategy.resolveApiVersion(
+					MockServerWebExchange.from(MockServerHttpRequest.get("/"))))
+				.then(() -> firstResolverPublisher.error(new IllegalStateException("test")))
+				.expectErrorMessage("test")
+				.verify();
+	}
+
 	private static DefaultApiVersionStrategy apiVersionStrategy() {
 		return apiVersionStrategy(null, false, null);
 	}
 
+	private static DefaultApiVersionStrategy apiVersionStrategy(List<ApiVersionResolver> resolvers) {
+		return new DefaultApiVersionStrategy(resolvers, parser, null, null, false, null, null);
+	}
+
 	private static DefaultApiVersionStrategy apiVersionStrategy(
 			@Nullable String defaultVersion, boolean detectSupportedVersions,
 			@Nullable Predicate<Comparable<?>> supportedVersionPredicate) {