Revert errorhandler order in RouterFunctionBuilder

Prior to this commit, error handlers in the WebMvc.fn and WebFlux.fn
router function builders had to be registered in an unintuitive, reverse
order, due to the filter chain composition model used.
This commit reverses the error handler order, so that more specific
error handlers can come before generic ones.

Closes gh-25541

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RouterFunctionBuilder.java

@@ -23,6 +23,7 @@ import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.function.Supplier;
+import java.util.stream.Stream;
 
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -43,6 +44,8 @@ class RouterFunctionBuilder implements RouterFunctions.Builder {
 
 	private final List<HandlerFilterFunction<ServerResponse, ServerResponse>> filterFunctions = new ArrayList<>();
 
+	private final List<HandlerFilterFunction<ServerResponse, ServerResponse>> errorHandlers = new ArrayList<>();
+
 
 	@Override
 	public RouterFunctions.Builder add(RouterFunction<ServerResponse> routerFunction) {
@@ -310,8 +313,9 @@ class RouterFunctionBuilder implements RouterFunctions.Builder {
 		Assert.notNull(predicate, "Predicate must not be null");
 		Assert.notNull(responseProvider, "ResponseProvider must not be null");
 
-		return filter((request, next) -> next.handle(request)
+		this.errorHandlers.add(0, (request, next) -> next.handle(request)
 				.onErrorResume(predicate, t -> responseProvider.apply(t, request)));
+		return this;
 	}
 
 	@Override
@@ -321,8 +325,9 @@ class RouterFunctionBuilder implements RouterFunctions.Builder {
 		Assert.notNull(exceptionType, "ExceptionType must not be null");
 		Assert.notNull(responseProvider, "ResponseProvider must not be null");
 
-		return filter((request, next) -> next.handle(request)
+		this.errorHandlers.add(0, (request, next) -> next.handle(request)
 				.onErrorResume(exceptionType, t -> responseProvider.apply(t, request)));
+		return this;
 	}
 
 	@Override
@@ -332,12 +337,12 @@ class RouterFunctionBuilder implements RouterFunctions.Builder {
 		}
 		RouterFunction<ServerResponse> result = new BuiltRouterFunction(this.routerFunctions);
 
-		if (this.filterFunctions.isEmpty()) {
+		if (this.filterFunctions.isEmpty() && this.errorHandlers.isEmpty()) {
 			return result;
 		}
 		else {
 			HandlerFilterFunction<ServerResponse, ServerResponse> filter =
-					this.filterFunctions.stream()
+					Stream.concat(this.filterFunctions.stream(), this.errorHandlers.stream())
 							.reduce(HandlerFilterFunction::andThen)
 							.orElseThrow(IllegalStateException::new);
 

spring-webflux/src/test/java/org/springframework/web/reactive/function/server/RouterFunctionBuilderTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.function.server;
 
+import java.io.IOException;
 import java.util.Collections;
 import java.util.concurrent.atomic.AtomicInteger;
 
@@ -210,4 +211,26 @@ public class RouterFunctionBuilderTests {
 				.verifyComplete();
 	}
 
+	@Test
+	public void multipleOnErrors() {
+		RouterFunction<ServerResponse> route = RouterFunctions.route()
+				.GET("/error", request -> Mono.error(new IOException()))
+				.onError(IOException.class, (t, r) -> ServerResponse.status(200).build())
+				.onError(Exception.class, (t, r) -> ServerResponse.status(201).build())
+				.build();
+
+		MockServerHttpRequest mockRequest = MockServerHttpRequest.get("https://example.com/error").build();
+		ServerRequest serverRequest = new DefaultServerRequest(MockServerWebExchange.from(mockRequest), Collections.emptyList());
+
+		Mono<HttpStatus> responseStatus = route.route(serverRequest)
+				.flatMap(handlerFunction -> handlerFunction.handle(serverRequest))
+				.map(ServerResponse::statusCode);
+
+		StepVerifier.create(responseStatus)
+				.assertNext(status -> assertThat(status).isEqualTo(HttpStatus.OK))
+				.verifyComplete();
+
+	}
+
+
 }