Consider modules when deducing WebApplicationType

Introduce a strategy to `WebApplicationType` to allow modules to
implement deduction logic.

Prior to this commit, modules played no part in deducing the
`WebApplicationType`. This meant that a user with `spring-webflux`
for client purposes would deduce `REACTIVE` despite no
`spring-boot-webflux` module being present.

The following deduction logic order is now implemented:

1) If the `spring-boot-webmvc` module is being used and Spring MVC
   classes are found then `SERVLET` is used.

2) If the `spring-boot-webflux` module is being used and Spring WebFlux
   classes are found then `REACTIVE` is used.

3) If `spring-web` is found and servlet classes are available then
   `SERVLET` is used.

4) If none of the above are satisfied, `NONE` is used.

This commit also updates `SpringBootTestContextBootstrapper` to use
the same deduction logic.

Fixes gh-48517

module/spring-boot-webflux/src/main/java/org/springframework/boot/webflux/WebFluxWebApplicationTypeDeducer.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2012-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.webflux;
+
+import org.jspecify.annotations.Nullable;
+
+import org.springframework.aot.hint.RuntimeHints;
+import org.springframework.aot.hint.RuntimeHintsRegistrar;
+import org.springframework.aot.hint.TypeReference;
+import org.springframework.boot.WebApplicationType;
+import org.springframework.core.annotation.Order;
+import org.springframework.util.ClassUtils;
+
+/**
+ * {@link WebApplicationType} deducer to ensure Spring WebFlux applications use
+ * {@link WebApplicationType#REACTIVE}.
+ *
+ * @author Phillip Webb
+ */
+@Order(20) // Ordered after MVC
+class WebFluxWebApplicationTypeDeducer implements WebApplicationType.Deducer {
+
+	private static final String[] INDICATOR_CLASSES = { "reactor.core.publisher.Mono",
+			"org.springframework.web.reactive.DispatcherHandler" };
+
+	@Override
+	public @Nullable WebApplicationType deduceWebApplicationType() {
+		// Guard in case the classic module is being used and dependencies are excluded
+		for (String indicatorClass : INDICATOR_CLASSES) {
+			if (!ClassUtils.isPresent(indicatorClass, null)) {
+				return null;
+			}
+		}
+		return WebApplicationType.REACTIVE;
+	}
+
+	static class WebFluxWebApplicationTypeDeducerRuntimeHints implements RuntimeHintsRegistrar {
+
+		@Override
+		public void registerHints(RuntimeHints hints, @Nullable ClassLoader classLoader) {
+			for (String servletIndicatorClass : INDICATOR_CLASSES) {
+				registerTypeIfPresent(servletIndicatorClass, classLoader, hints);
+			}
+		}
+
+		private void registerTypeIfPresent(String typeName, @Nullable ClassLoader classLoader, RuntimeHints hints) {
+			if (ClassUtils.isPresent(typeName, classLoader)) {
+				hints.reflection().registerType(TypeReference.of(typeName));
+			}
+		}
+
+	}
+
+}

module/spring-boot-webflux/src/main/java/org/springframework/boot/webflux/package-info.java

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2012-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Core integration between Spring Boot and Spring WebFlux.
+ */
+@NullMarked
+package org.springframework.boot.webflux;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-webflux/src/main/resources/META-INF/spring.factories

@@ -0,0 +1,3 @@
+# WebApplicationType Deducers
+org.springframework.boot.WebApplicationType$Deducer=\
+org.springframework.boot.webflux.WebFluxWebApplicationTypeDeducer

module/spring-boot-webflux/src/main/resources/META-INF/spring/aot.factories

@@ -0,0 +1,2 @@
+org.springframework.aot.hint.RuntimeHintsRegistrar=\
+org.springframework.boot.webflux.WebFluxWebApplicationTypeDeducer$WebFluxWebApplicationTypeDeducerRuntimeHints