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

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootTestContextBootstrapper.java

@@ -82,16 +82,6 @@ import org.springframework.util.StringUtils;
  */
 public class SpringBootTestContextBootstrapper extends DefaultTestContextBootstrapper {
 
-	private static final String[] WEB_ENVIRONMENT_CLASSES = { "jakarta.servlet.Servlet",
-			"org.springframework.web.context.ConfigurableWebApplicationContext" };
-
-	private static final String REACTIVE_WEB_ENVIRONMENT_CLASS = "org.springframework."
-			+ "web.reactive.DispatcherHandler";
-
-	private static final String MVC_WEB_ENVIRONMENT_CLASS = "org.springframework.web.servlet.DispatcherServlet";
-
-	private static final String JERSEY_WEB_ENVIRONMENT_CLASS = "org.glassfish.jersey.server.ResourceConfig";
-
 	private static final String ACTIVATE_SERVLET_LISTENER = "org.springframework.test."
 			+ "context.web.ServletTestExecutionListener.activateListener";
 
@@ -112,7 +102,7 @@ public class SpringBootTestContextBootstrapper extends DefaultTestContextBootstr
 		TestContext context = super.buildTestContext();
 		verifyConfiguration(context.getTestClass());
 		WebEnvironment webEnvironment = getWebEnvironment(context.getTestClass());
-		if (webEnvironment == WebEnvironment.MOCK && deduceWebApplicationType() == WebApplicationType.SERVLET) {
+		if (webEnvironment == WebEnvironment.MOCK && WebApplicationType.deduce() == WebApplicationType.SERVLET) {
 			context.setAttribute(ACTIVATE_SERVLET_LISTENER, true);
 		}
 		else if (webEnvironment != null && webEnvironment.isEmbedded()) {
@@ -171,21 +161,7 @@ public class SpringBootTestContextBootstrapper extends DefaultTestContextBootstr
 				TestPropertySourceUtils.convertInlinedPropertiesToMap(configuration.getPropertySourceProperties()));
 		Binder binder = new Binder(source);
 		return binder.bind("spring.main.web-application-type", Bindable.of(WebApplicationType.class))
-			.orElseGet(this::deduceWebApplicationType);
-	}
-
-	private WebApplicationType deduceWebApplicationType() {
-		if (ClassUtils.isPresent(REACTIVE_WEB_ENVIRONMENT_CLASS, null)
-				&& !ClassUtils.isPresent(MVC_WEB_ENVIRONMENT_CLASS, null)
-				&& !ClassUtils.isPresent(JERSEY_WEB_ENVIRONMENT_CLASS, null)) {
-			return WebApplicationType.REACTIVE;
-		}
-		for (String className : WEB_ENVIRONMENT_CLASSES) {
-			if (!ClassUtils.isPresent(className, null)) {
-				return WebApplicationType.NONE;
-			}
-		}
-		return WebApplicationType.SERVLET;
+			.orElseGet(WebApplicationType::deduce);
 	}
 
 	/**

core/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -275,7 +275,7 @@ public class SpringApplication {
 		this.resourceLoader = resourceLoader;
 		Assert.notNull(primarySources, "'primarySources' must not be null");
 		this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
-		this.properties.setWebApplicationType(WebApplicationType.deduceFromClasspath());
+		this.properties.setWebApplicationType(WebApplicationType.deduce());
 		this.bootstrapRegistryInitializers = new ArrayList<>(
 				getSpringFactoriesInstances(BootstrapRegistryInitializer.class));
 		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));

core/spring-boot/src/main/java/org/springframework/boot/WebApplicationType.java

@@ -21,6 +21,7 @@ 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.core.io.support.SpringFactoriesLoader;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -28,6 +29,7 @@ import org.springframework.util.ClassUtils;
  *
  * @author Andy Wilkinson
  * @author Brian Clozel
+ * @author Phillip Webb
  * @since 2.0.0
  */
 public enum WebApplicationType {
@@ -53,23 +55,28 @@ public enum WebApplicationType {
 	private static final String[] SERVLET_INDICATOR_CLASSES = { "jakarta.servlet.Servlet",
 			"org.springframework.web.context.ConfigurableWebApplicationContext" };
 
-	private static final String WEBMVC_INDICATOR_CLASS = "org.springframework.web.servlet.DispatcherServlet";
-
-	private static final String WEBFLUX_INDICATOR_CLASS = "org.springframework.web.reactive.DispatcherHandler";
-
-	private static final String JERSEY_INDICATOR_CLASS = "org.glassfish.jersey.servlet.ServletContainer";
-
-	static WebApplicationType deduceFromClasspath() {
-		if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
-				&& !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
-			return WebApplicationType.REACTIVE;
-		}
-		for (String className : SERVLET_INDICATOR_CLASSES) {
-			if (!ClassUtils.isPresent(className, null)) {
-				return WebApplicationType.NONE;
+	/**
+	 * Deduce the {@link WebApplicationType} from the current classpath.
+	 * @return the deduced web application
+	 * @since 4.0.1
+	 */
+	public static WebApplicationType deduce() {
+		for (Deducer deducer : SpringFactoriesLoader.forDefaultResourceLocation().load(Deducer.class)) {
+			WebApplicationType deduced = deducer.deduceWebApplicationType();
+			if (deduced != null) {
+				return deduced;
 			}
 		}
-		return WebApplicationType.SERVLET;
+		return isServletApplication() ? WebApplicationType.SERVLET : WebApplicationType.NONE;
+	}
+
+	private static boolean isServletApplication() {
+		for (String servletIndicatorClass : SERVLET_INDICATOR_CLASSES) {
+			if (!ClassUtils.isPresent(servletIndicatorClass, null)) {
+				return false;
+			}
+		}
+		return true;
 	}
 
 	static class WebApplicationTypeRuntimeHints implements RuntimeHintsRegistrar {
@@ -79,9 +86,6 @@ public enum WebApplicationType {
 			for (String servletIndicatorClass : SERVLET_INDICATOR_CLASSES) {
 				registerTypeIfPresent(servletIndicatorClass, classLoader, hints);
 			}
-			registerTypeIfPresent(JERSEY_INDICATOR_CLASS, classLoader, hints);
-			registerTypeIfPresent(WEBFLUX_INDICATOR_CLASS, classLoader, hints);
-			registerTypeIfPresent(WEBMVC_INDICATOR_CLASS, classLoader, hints);
 		}
 
 		private void registerTypeIfPresent(String typeName, @Nullable ClassLoader classLoader, RuntimeHints hints) {
@@ -92,4 +96,21 @@ public enum WebApplicationType {
 
 	}
 
+	/**
+	 * Strategy that may be implemented by a module that can deduce the
+	 * {@link WebApplicationType}.
+	 *
+	 * @since 4.0.1
+	 */
+	@FunctionalInterface
+	public interface Deducer {
+
+		/**
+		 * Deduce the web application type.
+		 * @return the deduced web application type or {@code null}
+		 */
+		@Nullable WebApplicationType deduceWebApplicationType();
+
+	}
+
 }

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

module/spring-boot-webmvc/src/main/java/org/springframework/boot/webmvc/WebMvcWebApplicationTypeDeducer.java

@@ -0,0 +1,69 @@
+/*
+ * 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.webmvc;
+
+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 MVC applications use
+ * {@link WebApplicationType#SERVLET}.
+ *
+ * @author Phillip Webb
+ */
+@Order(10) // Ordered after WebFlux
+class WebMvcWebApplicationTypeDeducer implements WebApplicationType.Deducer {
+
+	private static final String[] INDICATOR_CLASSES = { "jakarta.servlet.Servlet",
+			"org.springframework.web.servlet.DispatcherServlet",
+			"org.springframework.web.context.ConfigurableWebApplicationContext" };
+
+	@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.SERVLET;
+	}
+
+	static class WebMvcWebApplicationTypeDeducerRuntimeHints 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-webmvc/src/main/java/org/springframework/boot/webmvc/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 Web MVC.
+ */
+@NullMarked
+package org.springframework.boot.webmvc;
+
+import org.jspecify.annotations.NullMarked;

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

@@ -1,3 +1,7 @@
 # Template Availability Providers
 org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
 org.springframework.boot.webmvc.autoconfigure.JspTemplateAvailabilityProvider
+
+# WebApplicationType Deducers
+org.springframework.boot.WebApplicationType$Deducer=\
+org.springframework.boot.webmvc.WebMvcWebApplicationTypeDeducer

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

@@ -0,0 +1,2 @@
+org.springframework.aot.hint.RuntimeHintsRegistrar=\
+org.springframework.boot.webmvc.WebMvcWebApplicationTypeDeducer$WebMvcWebApplicationTypeDeducerRuntimeHints

smoke-test/spring-boot-smoke-test-web-application-type/build.gradle

@@ -25,6 +25,7 @@ dependencies {
 	implementation(project(":starter:spring-boot-starter-webflux"))
 
 	testImplementation(project(":starter:spring-boot-starter-test"))
+	testImplementation(project(":test-support:spring-boot-test-support"))
 }
 
 tasks.named("compileTestJava") {

smoke-test/spring-boot-smoke-test-web-application-type/src/test/java/smoketest/webapplicationtype/WebApplicationTypeIntegrationTests.java

@@ -0,0 +1,92 @@
+/*
+ * 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 smoketest.webapplicationtype;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.WebApplicationType;
+import org.springframework.boot.testsupport.classpath.ClassPathExclusions;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Integration tests for {@link WebApplicationType} deducers.
+ *
+ * @author Phillip Webb
+ */
+class WebApplicationTypeIntegrationTests {
+
+	@Test
+	@ClassPathExclusions("spring-boot-webflux*")
+	void deduceWhenNoWebFluxModuleAndWebMvcModule() {
+		assertThat(WebApplicationType.deduce()).isEqualTo(WebApplicationType.SERVLET);
+	}
+
+	@Test
+	@ClassPathExclusions({ "spring-boot-webflux*", "spring-web-*" })
+	void deduceWhenNoWebFluxModuleAndNoSpringWebAndWebMvcModule() {
+		assertThat(WebApplicationType.deduce()).isEqualTo(WebApplicationType.NONE);
+	}
+
+	@Test
+	@ClassPathExclusions("spring-boot-webmvc*")
+	void deduceWhenNoWebMvcModuleAndWebFluxModule() {
+		assertThat(WebApplicationType.deduce()).isEqualTo(WebApplicationType.REACTIVE);
+	}
+
+	@Test
+	@ClassPathExclusions({ "spring-boot-webmvc*", "spring-webflux-*" })
+	void deduceWhenNoWebMvcModuleAndNoWebFluxAndWebFluxModule() {
+		assertThat(WebApplicationType.deduce()).isEqualTo(WebApplicationType.SERVLET);
+	}
+
+	@Test
+	@ClassPathExclusions({ "spring-boot-webmvc*", "spring-webflux-*", "spring-web-*" })
+	void deduceWhenNoWebMvcModuleAndNoWebFluxAndNoWebAndWebFluxModule() {
+		assertThat(WebApplicationType.deduce()).isEqualTo(WebApplicationType.NONE);
+	}
+
+	@Test
+	void deduceWhenWebMvcAndWebFlux() {
+		assertThat(WebApplicationType.deduce()).isEqualTo(WebApplicationType.SERVLET);
+	}
+
+	@Test
+	@ClassPathExclusions("spring-webmvc-*")
+	void deduceWhenNoWebMvc() {
+		assertThat(WebApplicationType.deduce()).isEqualTo(WebApplicationType.REACTIVE);
+	}
+
+	@Test
+	@ClassPathExclusions({ "spring-webmvc-*", "spring-webflux-*" })
+	void deduceWhenNoWebMvcAndNoWebFlux() {
+		assertThat(WebApplicationType.deduce()).isEqualTo(WebApplicationType.SERVLET);
+	}
+
+	@Test
+	@ClassPathExclusions({ "spring-web-*" })
+	void deduceWhenNoWeb() {
+		assertThat(WebApplicationType.deduce()).isEqualTo(WebApplicationType.NONE);
+	}
+
+	@Test
+	@ClassPathExclusions(packages = "jakarta.servlet", files = "spring-webflux-*")
+	void deduceWhenNoServletOrWebFlux() {
+		assertThat(WebApplicationType.deduce()).isEqualTo(WebApplicationType.NONE);
+	}
+
+}