Replace constant exceptions with inlined ones

Issue: SRP-17475

spring-webflux/src/main/java/org/springframework/web/reactive/DispatcherHandler.java

@@ -142,16 +142,23 @@ public class DispatcherHandler implements WebHandler, ApplicationContextAware {
 	@Override
 	public Mono<Void> handle(ServerWebExchange exchange) {
 		if (this.handlerMappings == null) {
-			return Mono.error(HANDLER_NOT_FOUND_EXCEPTION);
+			return createNotFoundError();
 		}
 		return Flux.fromIterable(this.handlerMappings)
 				.concatMap(mapping -> mapping.getHandler(exchange))
 				.next()
-				.switchIfEmpty(Mono.error(HANDLER_NOT_FOUND_EXCEPTION))
+				.switchIfEmpty(createNotFoundError())
 				.flatMap(handler -> invokeHandler(exchange, handler))
 				.flatMap(result -> handleResult(exchange, result));
 	}
 
+	private <R> Mono<R> createNotFoundError() {
+		return Mono.defer(() -> {
+			Exception ex = new ResponseStatusException(HttpStatus.NOT_FOUND, "No matching handler");
+			return Mono.error(ex);
+		});
+	}
+
 	private Mono<HandlerResult> invokeHandler(ServerWebExchange exchange, Object handler) {
 		if (this.handlerAdapters != null) {
 			for (HandlerAdapter handlerAdapter : this.handlerAdapters) {

spring-webflux/src/main/java/org/springframework/web/reactive/resource/ResourceWebHandler.java

@@ -88,8 +88,6 @@ public class ResourceWebHandler implements WebHandler, InitializingBean {
 
 	private static final Set<HttpMethod> SUPPORTED_METHODS = EnumSet.of(HttpMethod.GET, HttpMethod.HEAD);
 
-	private static final Exception NOT_FOUND_EXCEPTION = new ResponseStatusException(HttpStatus.NOT_FOUND);
-
 	private static final Log logger = LogFactory.getLog(ResourceWebHandler.class);
 
 
@@ -324,7 +322,7 @@ public class ResourceWebHandler implements WebHandler, InitializingBean {
 		return getResource(exchange)
 				.switchIfEmpty(Mono.defer(() -> {
 					logger.debug(exchange.getLogPrefix() + "Resource not found");
-					return Mono.error(NOT_FOUND_EXCEPTION);
+					return Mono.error(new ResponseStatusException(HttpStatus.NOT_FOUND));
 				}))
 				.flatMap(resource -> {
 					try {

spring-webflux/src/test/java/org/springframework/web/reactive/DispatcherHandlerErrorTests.java

@@ -19,6 +19,7 @@ package org.springframework.web.reactive;
 import java.time.Duration;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicReference;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -81,15 +82,19 @@ public class DispatcherHandlerErrorTests {
 	@Test
 	public void noHandler() {
 		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/does-not-exist"));
-		Mono<Void> publisher = this.dispatcherHandler.handle(exchange);
+		Mono<Void> mono = this.dispatcherHandler.handle(exchange);
 
-		StepVerifier.create(publisher)
-				.consumeErrorWith(error -> {
-					assertThat(error, instanceOf(ResponseStatusException.class));
-					assertThat(error.getMessage(),
-							is("404 NOT_FOUND \"No matching handler\""));
+		StepVerifier.create(mono)
+				.consumeErrorWith(ex -> {
+					assertThat(ex, instanceOf(ResponseStatusException.class));
+					assertThat(ex.getMessage(), is("404 NOT_FOUND \"No matching handler\""));
 				})
 				.verify();
+
+		// SPR-17475
+		AtomicReference<Throwable> exceptionRef = new AtomicReference<>();
+		StepVerifier.create(mono).consumeErrorWith(exceptionRef::set).verify();
+		StepVerifier.create(mono).consumeErrorWith(ex -> assertNotSame(exceptionRef.get(), ex)).verify();
 	}
 
 	@Test

spring-webflux/src/test/java/org/springframework/web/reactive/resource/ResourceWebHandlerTests.java

@@ -24,6 +24,7 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -274,6 +275,7 @@ public class ResourceWebHandlerTests {
 	private void testInvalidPath(String requestPath, ResourceWebHandler handler) {
 		ServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get(""));
 		setPathWithinHandlerMapping(exchange, requestPath);
+
 		StepVerifier.create(handler.handle(exchange))
 				.expectErrorSatisfies(err -> {
 					assertThat(err, instanceOf(ResponseStatusException.class));
@@ -468,11 +470,18 @@ public class ResourceWebHandlerTests {
 		MockServerHttpRequest request = MockServerHttpRequest.method(httpMethod, "").build();
 		MockServerWebExchange exchange = MockServerWebExchange.from(request);
 		setPathWithinHandlerMapping(exchange, "not-there.css");
-		StepVerifier.create(this.handler.handle(exchange))
+		Mono<Void> mono = this.handler.handle(exchange);
+
+		StepVerifier.create(mono)
 				.expectErrorSatisfies(err -> {
 					assertThat(err, instanceOf(ResponseStatusException.class));
 					assertEquals(HttpStatus.NOT_FOUND, ((ResponseStatusException) err).getStatus());
 				}).verify(TIMEOUT);
+
+		// SPR-17475
+		AtomicReference<Throwable> exceptionRef = new AtomicReference<>();
+		StepVerifier.create(mono).consumeErrorWith(exceptionRef::set).verify();
+		StepVerifier.create(mono).consumeErrorWith(ex -> assertNotSame(exceptionRef.get(), ex)).verify();
 	}
 
 	@Test