Preserve order of onStatus handlers

Closes gh-23880

Author: rstoyanchev

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java

@@ -419,21 +419,28 @@ private static class DefaultResponseSpec implements ResponseSpec {
 
 		private static final IntPredicate STATUS_CODE_ERROR = value -> value >= 400;
 
+		private static final StatusHandler DEFAULT_STATUS_HANDLER =
+				new StatusHandler(STATUS_CODE_ERROR, ClientResponse::createException);
+
+
 		private final Mono<ClientResponse> responseMono;
 
 		private final Supplier<HttpRequest> requestSupplier;
 
 		private final List<StatusHandler> statusHandlers = new ArrayList<>(1);
 
+
 		DefaultResponseSpec(Mono<ClientResponse> responseMono, Supplier<HttpRequest> requestSupplier) {
 			this.responseMono = responseMono;
 			this.requestSupplier = requestSupplier;
-			this.statusHandlers.add(new StatusHandler(STATUS_CODE_ERROR, ClientResponse::createException));
+			this.statusHandlers.add(DEFAULT_STATUS_HANDLER);
 		}
 
+
 		@Override
 		public ResponseSpec onStatus(Predicate<HttpStatus> statusPredicate,
 				Function<ClientResponse, Mono<? extends Throwable>> exceptionFunction) {
+
 			return onRawStatus(toIntPredicate(statusPredicate), exceptionFunction);
 		}
 
@@ -450,7 +457,8 @@ public ResponseSpec onRawStatus(IntPredicate statusCodePredicate,
 
 			Assert.notNull(statusCodePredicate, "IntPredicate must not be null");
 			Assert.notNull(exceptionFunction, "Function must not be null");
-			this.statusHandlers.add(0, new StatusHandler(statusCodePredicate, exceptionFunction));
+			int index = this.statusHandlers.size() - 1;  // Default handler always last
+			this.statusHandlers.add(index, new StatusHandler(statusCodePredicate, exceptionFunction));
 			return this;
 		}
 

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultWebClientTests.java

@@ -20,6 +20,7 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.function.Predicate;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -33,6 +34,7 @@
 
 import org.springframework.core.NamedThreadLocal;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -309,6 +311,48 @@ public void shouldApplyFiltersAtSubscription() {
 		assertThat(request.headers().getFirst("Custom")).isEqualTo("value");
 	}
 
+	@Test // gh-23880
+	public void onStatusHandlersOrderIsPreserved() {
+
+		ClientResponse response = ClientResponse.create(HttpStatus.BAD_REQUEST).build();
+		given(exchangeFunction.exchange(any())).willReturn(Mono.just(response));
+
+		Mono<Void> result = this.builder.build().get()
+				.uri("/path")
+				.retrieve()
+				.onStatus(HttpStatus::is4xxClientError, resp -> Mono.error(new IllegalStateException("1")))
+				.onStatus(HttpStatus::is4xxClientError, resp -> Mono.error(new IllegalStateException("2")))
+				.bodyToMono(Void.class);
+
+		StepVerifier.create(result).expectErrorMessage("1").verify();
+	}
+
+	@Test // gh-23880
+	@SuppressWarnings("unchecked")
+	public void onStatusHandlersDefaultHandlerIsLast() {
+
+		ClientResponse response = ClientResponse.create(HttpStatus.BAD_REQUEST).build();
+		given(exchangeFunction.exchange(any())).willReturn(Mono.just(response));
+
+		Predicate<HttpStatus> predicate1 = mock(Predicate.class);
+		Predicate<HttpStatus> predicate2 = mock(Predicate.class);
+
+		given(predicate1.test(HttpStatus.BAD_REQUEST)).willReturn(false);
+		given(predicate2.test(HttpStatus.BAD_REQUEST)).willReturn(false);
+
+		Mono<Void> result = this.builder.build().get()
+				.uri("/path")
+				.retrieve()
+				.onStatus(predicate1, resp -> Mono.error(new IllegalStateException()))
+				.onStatus(predicate2, resp -> Mono.error(new IllegalStateException()))
+				.bodyToMono(Void.class);
+
+		StepVerifier.create(result).expectError(WebClientResponseException.class).verify();
+
+		verify(predicate1).test(HttpStatus.BAD_REQUEST);
+		verify(predicate2).test(HttpStatus.BAD_REQUEST);
+	}
+
 	private ClientRequest verifyAndGetRequest() {
 		ClientRequest request = this.captor.getValue();
 		verify(this.exchangeFunction).exchange(request);