Add a template variables contribution SPI

src/main/java/org/springframework/hateoas/server/core/AdditionalUriHandler.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2023 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.hateoas.server.core;
+
+import org.springframework.hateoas.TemplateVariables;
+import org.springframework.web.util.UriComponents;
+import org.springframework.web.util.UriComponentsBuilder;
+
+/**
+ * @author Réda Housni Alaoui
+ */
+public interface AdditionalUriHandler {
+
+	UriComponentsBuilder apply(UriComponentsBuilder uriComponentsBuilder, MethodInvocation methodInvocation);
+
+	TemplateVariables apply(TemplateVariables templateVariables, UriComponents uriComponents, MethodInvocation methodInvocation);
+}

src/main/java/org/springframework/hateoas/server/core/WebHandler.java

@@ -24,7 +24,6 @@
 import java.util.*;
 import java.util.Map.Entry;
 import java.util.concurrent.ConcurrentHashMap;
-import java.util.function.BiFunction;
 import java.util.function.Function;
 import java.util.function.Supplier;
 import java.util.stream.Collectors;
@@ -73,12 +72,11 @@ public interface PreparedWebHandler<T extends LinkBuilder> {
 
 	public static <T extends LinkBuilder> PreparedWebHandler<T> linkTo(Object invocationValue,
 			LinkBuilderCreator<T> creator) {
-		return linkTo(invocationValue, creator,
-				(BiFunction<UriComponentsBuilder, MethodInvocation, UriComponentsBuilder>) null);
+		return linkTo(invocationValue, creator, null);
 	}
 
 	public static <T extends LinkBuilder> T linkTo(Object invocationValue, LinkBuilderCreator<T> creator,
-			@Nullable BiFunction<UriComponentsBuilder, MethodInvocation, UriComponentsBuilder> additionalUriHandler,
+			@Nullable AdditionalUriHandler additionalUriHandler,
 			Function<UriMapping, UriComponentsBuilder> finisher, Supplier<ConversionService> conversionService) {
 
 		return linkTo(invocationValue, creator, additionalUriHandler).conclude(finisher,
@@ -87,7 +85,7 @@ public static <T extends LinkBuilder> T linkTo(Object invocationValue, LinkBuild
 
 	private static <T extends LinkBuilder> PreparedWebHandler<T> linkTo(Object invocationValue,
 			LinkBuilderCreator<T> creator,
-			@Nullable BiFunction<UriComponentsBuilder, MethodInvocation, UriComponentsBuilder> additionalUriHandler) {
+			@Nullable AdditionalUriHandler additionalUriHandler) {
 
 		Assert.isInstanceOf(LastInvocationAware.class, invocationValue);
 
@@ -177,7 +175,9 @@ private static <T extends LinkBuilder> PreparedWebHandler<T> linkTo(Object invoc
 					? builder.buildAndExpand(values) //
 					: additionalUriHandler.apply(builder, invocation).buildAndExpand(values);
 
-			TemplateVariables variables = NONE;
+			TemplateVariables variables = additionalUriHandler == null
+					? NONE
+					: additionalUriHandler.apply(NONE, components, invocation);
 
 			for (String parameter : optionalEmptyParameters) {
 

src/main/java/org/springframework/hateoas/server/mvc/UriComponentsContributor.java

@@ -16,9 +16,11 @@
 package org.springframework.hateoas.server.mvc;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.hateoas.TemplateVariables;
 import org.springframework.hateoas.server.MethodLinkBuilderFactory;
 import org.springframework.lang.Nullable;
 import org.springframework.web.method.support.HandlerMethodArgumentResolver;
+import org.springframework.web.util.UriComponents;
 import org.springframework.web.util.UriComponentsBuilder;
 
 /**
@@ -28,6 +30,7 @@
  *
  * @see MethodLinkBuilderFactory#linkTo(Object)
  * @author Oliver Gierke
+ * @author Réda Housni Alaoui
  */
 public interface UriComponentsContributor {
 
@@ -47,4 +50,15 @@ public interface UriComponentsContributor {
 	 * @param value can be {@literal null}.
 	 */
 	void enhance(UriComponentsBuilder builder, @Nullable MethodParameter parameter, @Nullable Object value);
+
+	/**
+	 * Enhance the given {@link TemplateVariables}
+	 *
+	 * @param templateVariables will never be {@literal null}.
+	 * @param uriComponents will never be {@literal null}.
+	 * @param parameter will never be {@literal null}.
+	 */
+	default TemplateVariables enhance(TemplateVariables templateVariables, UriComponents uriComponents, MethodParameter parameter){
+		return templateVariables;
+	}
 }

src/main/java/org/springframework/hateoas/server/mvc/WebMvcLinkBuilderFactory.java

@@ -31,8 +31,11 @@
 import org.springframework.core.convert.ConversionService;
 import org.springframework.format.support.DefaultFormattingConversionService;
 import org.springframework.hateoas.Link;
+import org.springframework.hateoas.TemplateVariables;
 import org.springframework.hateoas.server.MethodLinkBuilderFactory;
+import org.springframework.hateoas.server.core.AdditionalUriHandler;
 import org.springframework.hateoas.server.core.LinkBuilderSupport;
+import org.springframework.hateoas.server.core.MethodInvocation;
 import org.springframework.hateoas.server.core.MethodParameters;
 import org.springframework.hateoas.server.core.SpringAffordanceBuilder;
 import org.springframework.hateoas.server.core.UriMapping;
@@ -44,6 +47,7 @@
 import org.springframework.web.context.support.WebApplicationContextUtils;
 import org.springframework.web.servlet.mvc.condition.NameValueExpression;
 import org.springframework.web.servlet.mvc.condition.ParamsRequestCondition;
+import org.springframework.web.util.UriComponents;
 import org.springframework.web.util.UriComponentsBuilder;
 
 /**
@@ -150,8 +154,20 @@ public WebMvcLinkBuilder linkTo(Object invocationValue) {
 		Function<UriMapping, UriComponentsBuilder> builderFactory = mapping -> UriComponentsBuilderFactory
 				.forMapping(mapping);
 
-		return WebHandler.linkTo(invocationValue, WebMvcLinkBuilder::new, (builder, invocation) -> {
+		return WebHandler.linkTo(invocationValue, WebMvcLinkBuilder::new,
+				new UriComponentsContributorsAdditionalUriHandler(uriComponentsContributors), builderFactory, getConversionService());
+	}
+
+	private static class UriComponentsContributorsAdditionalUriHandler implements AdditionalUriHandler {
+
+		private final List<UriComponentsContributor> uriComponentsContributors;
+
+		private UriComponentsContributorsAdditionalUriHandler(List<UriComponentsContributor> uriComponentsContributors) {
+			this.uriComponentsContributors = uriComponentsContributors;
+		}
 
+		@Override
+		public UriComponentsBuilder apply(UriComponentsBuilder builder, MethodInvocation invocation) {
 			String[] primaryParams = SpringAffordanceBuilder.DISCOVERER.getParams(invocation.getMethod());
 
 			if (primaryParams.length > 0) {
@@ -189,8 +205,23 @@ public WebMvcLinkBuilder linkTo(Object invocationValue) {
 			}
 
 			return builder;
+		}
+
+		@Override
+		public TemplateVariables apply(TemplateVariables templateVariables, UriComponents uriComponents, MethodInvocation invocation) {
+			MethodParameters parameters = MethodParameters.of(invocation.getMethod());
+
+			for (MethodParameter parameter : parameters.getParameters()) {
+
+				for (UriComponentsContributor contributor : uriComponentsContributors) {
+					if (contributor.supportsParameter(parameter)) {
+						templateVariables = contributor.enhance(templateVariables, uriComponents, parameter);
+					}
+				}
+			}
 
-		}, builderFactory, getConversionService());
+			return templateVariables;
+		}
 	}
 
 	private static Supplier<ConversionService> getConversionService() {

src/test/java/org/springframework/hateoas/server/mvc/WebMvcLinkBuilderFactoryUnitTest.java

@@ -31,6 +31,8 @@
 import org.springframework.format.annotation.DateTimeFormat.ISO;
 import org.springframework.hateoas.IanaLinkRelations;
 import org.springframework.hateoas.Link;
+import org.springframework.hateoas.TemplateVariable;
+import org.springframework.hateoas.TemplateVariables;
 import org.springframework.hateoas.TestUtils;
 import org.springframework.hateoas.server.mvc.WebMvcLinkBuilderUnitTest.ControllerWithMethods;
 import org.springframework.hateoas.server.mvc.WebMvcLinkBuilderUnitTest.PersonControllerImpl;
@@ -41,6 +43,7 @@
 import org.springframework.web.bind.annotation.PathVariable;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.util.UriComponents;
 import org.springframework.web.util.UriComponentsBuilder;
 
 /**
@@ -184,6 +187,17 @@ void linksToMethodWithPrimaryParam() {
 		assertThat(link.getHref()).endsWith("/something/foo?a=1&b=2");
 	}
 
+	@Test
+	void appliesTemplateVariableIfContributorConfigured() {
+
+		WebMvcLinkBuilderFactory factory = new WebMvcLinkBuilderFactory();
+		factory.setUriComponentsContributors(Collections.singletonList(new SampleUriComponentsContributor()));
+
+		Link link = factory.linkTo(methodOn(SampleController.class).sampleMethod(1L, null)).withSelfRel();
+		assertPointsToMockServer(link);
+		assertThat(link.getHref()).endsWith("/sample/1{?foo}");
+	}
+
 	interface SampleController {
 
 		@RequestMapping("/sample/{id}")
@@ -208,8 +222,19 @@ public boolean supportsParameter(MethodParameter parameter) {
 
 		@Override
 		public void enhance(UriComponentsBuilder builder, MethodParameter parameter, Object value) {
+			if (value == null) {
+				return;
+			}
 			builder.queryParam("foo", ((SpecialType) value).parameterValue);
 		}
+
+		@Override
+		public TemplateVariables enhance(TemplateVariables templateVariables, UriComponents uriComponents, MethodParameter parameter) {
+			if (uriComponents.getQueryParams().containsKey("foo")) {
+				return templateVariables;
+			}
+			return templateVariables.concat(TemplateVariable.requestParameter("foo"));
+		}
 	}
 
 	static class SpecialType {