#973 - Explicit configuration options for web stacks.

@EnableHypermediaSupport now exposes a "stacks" attribute to limit the enablement of hypermedia functionality to a certain web stack.

src/main/asciidoc/configuration.adoc

@@ -13,3 +13,18 @@ To let the `RepresentationModel` subtypes be rendered according to the specifica
 * By default, it enables `@EnableEntityLinks` (see <<fundamentals.obtaining-links.entity-links>>) and automatically picks up `EntityLinks` implementations and bundles them into a `DelegatingEntityLinks` instance that you can autowire.
 * It automatically picks up all `RelProvider` implementations in the `ApplicationContext` and bundles them into a `DelegatingRelProvider` that you can autowire. It registers providers to consider `@Relation` on domain types as well as Spring MVC controllers. If the https://github.com/atteo/evo-inflector[EVO inflector] is on the classpath, collection `rel` values are derived by using the pluralizing algorithm implemented in the library (see <<spis.rel-provider>>).
 
+[[configuration.at-enable.stacks]]
+=== Explicitly enabling support for dedicated web stacks
+
+By default, `@EnableHypermediaSupport` will reflectively detect the web application stack you're using and hook into the Spring components registered for those to enable support for hypermedia representations.
+However, there are situations in which you'd only explicitly want to activate support for a particular stack.
+E.g. if your Spring WebMVC based application uses WebFlux' `WebClient` to make outgoing requests and that one is not supposed to work with hypermedia elements, you can restrict the functionality to be enabled by explicitly declaring WebMvc in the configuration:
+
+.Explicitly activating hypermedia support for a particular web stack
+====
+[source, java]
+----
+@EnableHypermediaSupport(…, stacks = WebStack.WEBMVC)
+class MyHypermediaConfiguration { … }
+----
+====

src/main/java/org/springframework/hateoas/config/EnableHypermediaSupport.java

@@ -26,7 +26,9 @@ import java.util.List;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.annotation.Import;
 import org.springframework.hateoas.MediaTypes;
+import org.springframework.hateoas.support.WebStack;
 import org.springframework.http.MediaType;
+import org.springframework.web.reactive.function.client.WebClient;
 
 /**
  * Activates hypermedia support in the {@link ApplicationContext}. Will register infrastructure beans to support all
@@ -49,6 +51,15 @@ public @interface EnableHypermediaSupport {
 	 */
 	HypermediaType[] type();
 
+	/**
+	 * Configures which {@link WebStack}s we're supposed to enable support for. By default we're activating it for all
+	 * available ones if they happen to be in use. Configure this explicitly in case you're using WebFlux components like
+	 * {@link WebClient} but don't want to use hypermedia operations with it.
+	 *
+	 * @return
+	 */
+	WebStack[] stacks() default { WebStack.WEBMVC, WebStack.WEBFLUX };
+
 	/**
 	 * Hypermedia representation types supported.
 	 *

src/main/java/org/springframework/hateoas/config/WebStackImportSelector.java

@@ -15,8 +15,10 @@
  */
 package org.springframework.hateoas.config;
 
-import java.util.ArrayList;
-import java.util.List;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
 
 import org.springframework.context.annotation.ImportSelector;
 import org.springframework.core.type.AnnotationMetadata;
@@ -29,23 +31,44 @@ import org.springframework.hateoas.support.WebStack;
  */
 class WebStackImportSelector implements ImportSelector {
 
+	private static final String WEB_STACK_MISSING = "At least one web stack has to be selected in @EnableHypermediaSupport on %s!";
+
+	static Map<WebStack, String> CONFIGS;
+
+	static {
+
+		Map<WebStack, String> configs = new HashMap<>();
+
+		configs.put(WebStack.WEBMVC, "org.springframework.hateoas.config.WebMvcHateoasConfiguration");
+		configs.put(WebStack.WEBFLUX, "org.springframework.hateoas.config.WebFluxHateoasConfiguration");
+
+		CONFIGS = Collections.unmodifiableMap(configs);
+	}
+
 	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.context.annotation.ImportSelector#selectImports(org.springframework.core.type.AnnotationMetadata)
 	 */
 	@Override
-	public String[] selectImports(AnnotationMetadata importingClassMetadata) {
+	public String[] selectImports(AnnotationMetadata metadata) {
 
-		List<String> imports = new ArrayList<>();
+		Map<String, Object> attributes = metadata.getAnnotationAttributes(EnableHypermediaSupport.class.getName());
 
-		if (WebStack.WEBMVC.isAvailable()) {
-			imports.add("org.springframework.hateoas.config.WebMvcHateoasConfiguration");
+		// Configuration class imported but not through @EnableHypermediaSupport
+
+		if (attributes == null) {
+			return new String[0];
 		}
 
-		if (WebStack.WEBFLUX.isAvailable()) {
-			imports.add("org.springframework.hateoas.config.WebFluxHateoasConfiguration");
+		WebStack[] stacks = (WebStack[]) attributes.get("stacks");
+
+		if (stacks.length == 0) {
+			throw new IllegalStateException(String.format(WEB_STACK_MISSING, metadata.getClassName()));
 		}
 
-		return imports.toArray(new String[0]);
+		return Arrays.stream(stacks) //
+				.filter(WebStack::isAvailable) //
+				.map(CONFIGS::get) //
+				.toArray(String[]::new);
 	}
 }

src/test/java/org/springframework/hateoas/config/WebStackImportSelectorUnitTest.java

@@ -0,0 +1,81 @@
+/*
+ * Copyright 2019 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
+ *
+ *      http://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.hateoas.config;
+
+import static org.assertj.core.api.Assertions.*;
+
+import org.junit.Test;
+import org.springframework.core.type.AnnotationMetadata;
+import org.springframework.core.type.StandardAnnotationMetadata;
+import org.springframework.hateoas.config.EnableHypermediaSupport.HypermediaType;
+import org.springframework.hateoas.support.WebStack;
+
+/**
+ * Unit tests for {@link WebStackImportSelector}.
+ *
+ * @author Oliver Drotbohm
+ */
+public class WebStackImportSelectorUnitTest {
+
+	WebStackImportSelector selector = new WebStackImportSelector();
+
+	@Test // #973
+	public void activatesAllWebStacksByDefault() {
+
+		AnnotationMetadata metadata = new StandardAnnotationMetadata(DefaultHypermedia.class);
+
+		assertThat(selector.selectImports(metadata))
+				.containsExactlyInAnyOrderElementsOf(WebStackImportSelector.CONFIGS.values());
+	}
+
+	@Test // #973
+	public void activatesWebMvcOnlyifConfigured() {
+
+		AnnotationMetadata metadata = new StandardAnnotationMetadata(WebMvcHypermedia.class);
+
+		assertThat(selector.selectImports(metadata)).containsExactly(WebStackImportSelector.CONFIGS.get(WebStack.WEBMVC));
+	}
+
+	@Test // #973
+	public void activatesWebFluxOnlyIfConfigured() {
+
+		AnnotationMetadata metadata = new StandardAnnotationMetadata(WebFluxHypermedia.class);
+
+		assertThat(selector.selectImports(metadata)).containsExactly(WebStackImportSelector.CONFIGS.get(WebStack.WEBFLUX));
+	}
+
+	@Test // #973
+	public void rejectsNoStacksSelected() {
+
+		AnnotationMetadata metadata = new StandardAnnotationMetadata(NoStacksHypermedia.class);
+
+		assertThatExceptionOfType(IllegalStateException.class) //
+				.isThrownBy(() -> selector.selectImports(metadata)) //
+				.withMessageContaining(NoStacksHypermedia.class.getName());
+	}
+
+	@EnableHypermediaSupport(type = HypermediaType.HAL)
+	static class DefaultHypermedia {}
+
+	@EnableHypermediaSupport(type = HypermediaType.HAL, stacks = WebStack.WEBMVC)
+	static class WebMvcHypermedia {}
+
+	@EnableHypermediaSupport(type = HypermediaType.HAL, stacks = WebStack.WEBFLUX)
+	static class WebFluxHypermedia {}
+
+	@EnableHypermediaSupport(type = HypermediaType.HAL, stacks = {})
+	static class NoStacksHypermedia {}
+}