#2035 - Introduce MediaTypeConfigurationCustomizer.

HAL and HAL Forms now support customization of the media type-specific configuration via MediaTypeConfigurationCustomizer instances registered in the application context.

src/main/java/org/springframework/hateoas/mediatype/MediaTypeConfigurationCustomizer.java

@@ -0,0 +1,34 @@
+/*
+ * 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.mediatype;
+
+/**
+ * Callback interface to customize media type-specific configuration. Declare instances of the interface as bean methods
+ * in Spring configuration.
+ *
+ * @author Oliver Drotbohm
+ * @since 2.2
+ */
+public interface MediaTypeConfigurationCustomizer<T> {
+
+	/**
+	 * Customize the given configuration instance.
+	 *
+	 * @param configuration will never be {@literal null}.
+	 * @return must not be {@literal null}.
+	 */
+	T customize(T configuration);
+}

src/main/java/org/springframework/hateoas/mediatype/MediaTypeConfigurationFactory.java

@@ -0,0 +1,77 @@
+/*
+ * 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.mediatype;
+
+import java.util.function.Supplier;
+import java.util.stream.Stream;
+
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.util.Assert;
+
+/**
+ * Factory to provide instances of media type-specific configuration processed by
+ * {@link MediaTypeConfigurationCustomizer}s.
+ *
+ * @author Oliver Drotbohm
+ * @since 2.2
+ */
+public class MediaTypeConfigurationFactory<T, S extends MediaTypeConfigurationCustomizer<T>> {
+
+	private final Supplier<T> supplier;
+	private final Supplier<Stream<S>> customizers;
+
+	private T resolved;
+
+	/**
+	 * Creates a new {@link MediaTypeConfigurationFactory} for the given supplier of the original instance and all
+	 * {@link MediaTypeConfigurationCustomizer}s.
+	 *
+	 * @param supplier must not be {@literal null}.
+	 * @param customizers must not be {@literal null}.
+	 */
+	MediaTypeConfigurationFactory(Supplier<T> supplier, Supplier<Stream<S>> customizers) {
+
+		Assert.notNull(supplier, "Supplier must not be null!");
+		Assert.notNull(customizers, "Customizers must not be null!");
+
+		this.supplier = supplier;
+		this.customizers = customizers;
+	}
+
+	public MediaTypeConfigurationFactory(Supplier<T> supplier, ObjectProvider<S> customizers) {
+		this(supplier, () -> customizers.orderedStream());
+	}
+
+	/**
+	 * Returns the customized configuration instance.
+	 *
+	 * @return will never be {@literal null}.
+	 */
+	public T getConfiguration() {
+
+		if (resolved == null) {
+
+			var source = supplier.get();
+
+			Assert.notNull(source, "Source instance must not be null!");
+
+			this.resolved = this.customizers.get()
+					.reduce(source, (config, customizer) -> customizer.customize(config), (__, r) -> r);
+		}
+
+		return resolved;
+	}
+}

src/main/java/org/springframework/hateoas/mediatype/hal/HalMediaTypeConfiguration.java

@@ -24,6 +24,8 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.hateoas.client.LinkDiscoverer;
 import org.springframework.hateoas.config.HypermediaMappingInformation;
+import org.springframework.hateoas.mediatype.MediaTypeConfigurationCustomizer;
+import org.springframework.hateoas.mediatype.MediaTypeConfigurationFactory;
 import org.springframework.hateoas.mediatype.MessageResolver;
 import org.springframework.hateoas.server.LinkRelationProvider;
 import org.springframework.http.MediaType;
@@ -42,19 +44,19 @@ public class HalMediaTypeConfiguration implements HypermediaMappingInformation {
 
 	private final LinkRelationProvider relProvider;
 	private final ObjectProvider<CurieProvider> curieProvider;
-	private final ObjectProvider<HalConfiguration> halConfiguration;
+	private final MediaTypeConfigurationFactory<HalConfiguration, ? extends MediaTypeConfigurationCustomizer<HalConfiguration>> configurationFactory;
 	private final @Qualifier("messageResolver") MessageResolver resolver;
 	private final AutowireCapableBeanFactory beanFactory;
 
-	private HalConfiguration resolvedConfiguration;
-
 	public HalMediaTypeConfiguration(LinkRelationProvider relProvider, ObjectProvider<CurieProvider> curieProvider,
-			ObjectProvider<HalConfiguration> halConfiguration, MessageResolver resolver,
-			AutowireCapableBeanFactory beanFactory) {
+			ObjectProvider<HalConfiguration> halConfiguration,
+			ObjectProvider<MediaTypeConfigurationCustomizer<HalConfiguration>> customizers,
+			MessageResolver resolver, AutowireCapableBeanFactory beanFactory) {
 
 		this.relProvider = relProvider;
 		this.curieProvider = curieProvider;
-		this.halConfiguration = halConfiguration;
+		this.configurationFactory = new MediaTypeConfigurationFactory<>(
+				() -> halConfiguration.getIfAvailable(HalConfiguration::new), customizers);
 		this.resolver = resolver;
 		this.beanFactory = beanFactory;
 	}
@@ -70,7 +72,7 @@ public class HalMediaTypeConfiguration implements HypermediaMappingInformation {
 	 */
 	@Override
 	public List<MediaType> getMediaTypes() {
-		return getResolvedConfiguration().getMediaTypes();
+		return configurationFactory.getConfiguration().getMediaTypes();
 	}
 
 	/*
@@ -80,7 +82,7 @@ public class HalMediaTypeConfiguration implements HypermediaMappingInformation {
 	@Override
 	public ObjectMapper configureObjectMapper(ObjectMapper mapper) {
 
-		HalConfiguration halConfiguration = getResolvedConfiguration();
+		HalConfiguration halConfiguration = configurationFactory.getConfiguration();
 
 		mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
 		mapper.registerModule(new Jackson2HalModule());
@@ -91,18 +93,4 @@ public class HalMediaTypeConfiguration implements HypermediaMappingInformation {
 
 		return mapper;
 	}
-
-	/**
-	 * Lookup and cache the {@link HalConfiguration} instance to be used.
-	 *
-	 * @return will never be {@literal null}.
-	 */
-	private HalConfiguration getResolvedConfiguration() {
-
-		if (resolvedConfiguration == null) {
-			this.resolvedConfiguration = halConfiguration.getIfAvailable(HalConfiguration::new);
-		}
-
-		return resolvedConfiguration;
-	}
 }

src/main/java/org/springframework/hateoas/mediatype/hal/forms/HalFormsMediaTypeConfiguration.java

@@ -24,6 +24,8 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.hateoas.client.LinkDiscoverer;
 import org.springframework.hateoas.config.HypermediaMappingInformation;
+import org.springframework.hateoas.mediatype.MediaTypeConfigurationCustomizer;
+import org.springframework.hateoas.mediatype.MediaTypeConfigurationFactory;
 import org.springframework.hateoas.mediatype.MessageResolver;
 import org.springframework.hateoas.mediatype.hal.CurieProvider;
 import org.springframework.hateoas.mediatype.hal.HalConfiguration;
@@ -45,22 +47,32 @@ class HalFormsMediaTypeConfiguration implements HypermediaMappingInformation {
 
 	private final DelegatingLinkRelationProvider relProvider;
 	private final ObjectProvider<CurieProvider> curieProvider;
-	private final ObjectProvider<HalFormsConfiguration> halFormsConfiguration;
-	private final ObjectProvider<HalConfiguration> halConfiguration;
+	private final MediaTypeConfigurationFactory<HalFormsConfiguration, ? extends MediaTypeConfigurationCustomizer<HalFormsConfiguration>> configurationFactory;
 	private final MessageResolver resolver;
 	private final AbstractAutowireCapableBeanFactory beanFactory;
 
-	private HalFormsConfiguration resolvedConfiguration;
-
 	public HalFormsMediaTypeConfiguration(DelegatingLinkRelationProvider relProvider,
-			ObjectProvider<CurieProvider> curieProvider, ObjectProvider<HalFormsConfiguration> halFormsConfiguration,
-			ObjectProvider<HalConfiguration> halConfiguration, MessageResolver resolver,
-			AbstractAutowireCapableBeanFactory beanFactory) {
+			ObjectProvider<CurieProvider> curieProvider,
+			ObjectProvider<HalConfiguration> halConfiguration,
+			ObjectProvider<MediaTypeConfigurationCustomizer<HalConfiguration>> halCustomizers,
+			ObjectProvider<HalFormsConfiguration> halFormsConfiguration,
+			ObjectProvider<MediaTypeConfigurationCustomizer<HalFormsConfiguration>> halFormsCustomizers,
+			MessageResolver resolver, AbstractAutowireCapableBeanFactory beanFactory) {
 
 		this.relProvider = relProvider;
 		this.curieProvider = curieProvider;
-		this.halFormsConfiguration = halFormsConfiguration;
-		this.halConfiguration = halConfiguration;
+
+		Supplier<HalFormsConfiguration> defaultConfig = () -> {
+
+			MediaTypeConfigurationFactory<HalConfiguration, ?> customizedHalConfiguration = new MediaTypeConfigurationFactory<>(
+					() -> halConfiguration.getIfAvailable(HalConfiguration::new), halCustomizers);
+
+			return new HalFormsConfiguration(
+					customizedHalConfiguration.getConfiguration());
+		};
+
+		this.configurationFactory = new MediaTypeConfigurationFactory<>(
+				() -> halFormsConfiguration.getIfAvailable(defaultConfig), halFormsCustomizers);
 		this.resolver = resolver;
 		this.beanFactory = beanFactory;
 	}
@@ -73,7 +85,7 @@ class HalFormsMediaTypeConfiguration implements HypermediaMappingInformation {
 	@Bean
 	HalFormsTemplatePropertyWriter halFormsTemplatePropertyWriter() {
 
-		HalFormsConfiguration configuration = getResolvedConfiguration();
+		HalFormsConfiguration configuration = configurationFactory.getConfiguration();
 		HalFormsTemplateBuilder builder = new HalFormsTemplateBuilder(configuration, resolver);
 
 		return new HalFormsTemplatePropertyWriter(builder);
@@ -86,7 +98,7 @@ class HalFormsMediaTypeConfiguration implements HypermediaMappingInformation {
 	@Override
 	public ObjectMapper configureObjectMapper(ObjectMapper mapper) {
 
-		HalFormsConfiguration halFormsConfig = getResolvedConfiguration();
+		HalFormsConfiguration halFormsConfig = configurationFactory.getConfiguration();
 		CurieProvider provider = curieProvider.getIfAvailable(() -> CurieProvider.NONE);
 
 		mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
@@ -105,18 +117,15 @@ class HalFormsMediaTypeConfiguration implements HypermediaMappingInformation {
 	 */
 	@Override
 	public List<MediaType> getMediaTypes() {
-		return getResolvedConfiguration().getMediaTypes();
+		return configurationFactory.getConfiguration().getMediaTypes();
 	}
 
+	/**
+	 * For testing purposes.
+	 *
+	 * @return
+	 */
 	HalFormsConfiguration getResolvedConfiguration() {
-
-		Supplier<HalFormsConfiguration> defaultConfig = () -> new HalFormsConfiguration(
-				halConfiguration.getIfAvailable(HalConfiguration::new));
-
-		if (resolvedConfiguration == null) {
-			this.resolvedConfiguration = halFormsConfiguration.getIfAvailable(defaultConfig);
-		}
-
-		return resolvedConfiguration;
+		return configurationFactory.getConfiguration();
 	}
 }

src/test/java/org/springframework/hateoas/mediatype/MediaTypeConfigurationFactoryUnitTests.java

@@ -0,0 +1,60 @@
+/*
+ * 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.mediatype;
+
+import static org.assertj.core.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.*;
+import static org.mockito.Mockito.*;
+
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+/**
+ * @author Oliver Drotbohm
+ */
+@ExtendWith(MockitoExtension.class)
+class MediaTypeConfigurationFactoryUnitTests {
+
+	@Mock MediaTypeConfigurationCustomizer<Object> first, second;
+
+	@Test // GH-2035
+	void invokesCustomizers() {
+
+		var source = new Object();
+		var afterFirst = new Object();
+		var afterSecond = new Object();
+
+		doReturn(afterFirst).when(first).customize(source);
+		doReturn(afterSecond).when(second).customize(afterFirst);
+
+		var factory = new MediaTypeConfigurationFactory<>(() -> source, () -> Stream.of(first, second));
+
+		assertThat(factory.getConfiguration()).isSameAs(afterSecond);
+
+		verify(first, times(1)).customize(source);
+		verify(second, times(1)).customize(afterFirst);
+
+		assertThat(factory.getConfiguration()).isSameAs(afterSecond);
+
+		// Does not re-process source instance
+		verify(first, times(1)).customize(any());
+		verify(second, times(1)).customize(any());
+	}
+}