Allow registration of RSocket metadata extractors

Prior to this commit, customizing the extraction of RSocket metadata
from frames would require developers to override the default
`MetadataExtractor` while configuring `RSocketStrategies`.
This touches on many infrastructure parts, whereas the goal is just to
configure an extra metadata entry extractor using already configured
codecs.

This commit adds a way to register metadata entry extractors on the
`RSocketStrategies` builder with a `Consumer`-based API.

Closes gh-23645

spring-messaging/src/main/java/org/springframework/messaging/rsocket/DefaultMetadataExtractor.java

@@ -49,7 +49,7 @@ import org.springframework.util.MimeType;
  * @author Rossen Stoyanchev
  * @since 5.2
  */
-public class DefaultMetadataExtractor implements MetadataExtractor {
+public class DefaultMetadataExtractor implements MetadataExtractor, MetadataExtractorRegistry {
 
 	private final List<Decoder<?>> decoders;
 
@@ -78,59 +78,14 @@ public class DefaultMetadataExtractor implements MetadataExtractor {
 		return this.decoders;
 	}
 
-
-	/**
-	 * Decode metadata entries with the given {@link MimeType} to the specified
-	 * target class, and store the decoded value in the output map under the
-	 * given name.
-	 * @param mimeType the mime type of metadata entries to extract
-	 * @param targetType the target value type to decode to
-	 * @param name assign a name for the decoded value; if not provided, then
-	 * the mime type is used as the key
-	 */
-	public void metadataToExtract(MimeType mimeType, Class<?> targetType, @Nullable String name) {
-		String key = name != null ? name : mimeType.toString();
-		metadataToExtract(mimeType, targetType, (value, map) -> map.put(key, value));
-	}
-
-	/**
-	 * Variant of {@link #metadataToExtract(MimeType, Class, String)} that accepts
-	 * {@link ParameterizedTypeReference} instead of {@link Class} for
-	 * specifying a target type with generic parameters.
-	 * @param mimeType the mime type of metadata entries to extract
-	 * @param targetType the target value type to decode to
-	 */
-	public void metadataToExtract(
-			MimeType mimeType, ParameterizedTypeReference<?> targetType, @Nullable String name) {
-
-		String key = name != null ? name : mimeType.toString();
-		metadataToExtract(mimeType, targetType, (value, map) -> map.put(key, value));
-	}
-
-	/**
-	 * Variant of {@link #metadataToExtract(MimeType, Class, String)} that allows
-	 * custom logic to be used to map the decoded value to any number of values
-	 * in the output map.
-	 * @param mimeType the mime type of metadata entries to extract
-	 * @param targetType the target value type to decode to
-	 * @param mapper custom logic to add the decoded value to the output map
-	 * @param <T> the target value type
-	 */
+	@Override
 	public <T> void metadataToExtract(
 			MimeType mimeType, Class<T> targetType, BiConsumer<T, Map<String, Object>> mapper) {
 
 		registerMetadata(mimeType, ResolvableType.forClass(targetType), mapper);
 	}
 
-	/**
-	 * Variant of {@link #metadataToExtract(MimeType, Class, BiConsumer)} that
-	 * accepts {@link ParameterizedTypeReference} instead of {@link Class} for
-	 * specifying a target type with generic parameters.
-	 * @param mimeType the mime type of metadata entries to extract
-	 * @param type the target value type to decode to
-	 * @param mapper custom logic to add the decoded value to the output map
-	 * @param <T> the target value type
-	 */
+	@Override
 	public <T> void metadataToExtract(
 			MimeType mimeType, ParameterizedTypeReference<T> type, BiConsumer<T, Map<String, Object>> mapper) {
 

spring-messaging/src/main/java/org/springframework/messaging/rsocket/DefaultRSocketStrategies.java

@@ -46,6 +46,7 @@ import org.springframework.util.SimpleRouteMatcher;
  * Default implementation of {@link RSocketStrategies}.
  *
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  * @since 5.2
  */
 final class DefaultRSocketStrategies implements RSocketStrategies {
@@ -128,6 +129,8 @@ final class DefaultRSocketStrategies implements RSocketStrategies {
 		@Nullable
 		private MetadataExtractor metadataExtractor;
 
+		private final List<Consumer<MetadataExtractorRegistry>> metadataExtractors = new ArrayList<>();
+
 		DefaultRSocketStrategiesBuilder() {
 			this.encoders.add(CharSequenceEncoder.allMimeTypes());
 			this.encoders.add(new ByteBufferEncoder());
@@ -200,6 +203,12 @@ final class DefaultRSocketStrategies implements RSocketStrategies {
 			return this;
 		}
 
+		@Override
+		public Builder metadataExtractors(Consumer<MetadataExtractorRegistry> consumer) {
+			this.metadataExtractors.add(consumer);
+			return this;
+		}
+
 		@Override
 		public RSocketStrategies build() {
 			RouteMatcher matcher = (this.routeMatcher != null ? this.routeMatcher : initRouteMatcher());
@@ -213,6 +222,10 @@ final class DefaultRSocketStrategies implements RSocketStrategies {
 			MetadataExtractor extractor = (this.metadataExtractor != null ?
 					this.metadataExtractor : new DefaultMetadataExtractor(this.decoders));
 
+			if (extractor instanceof MetadataExtractorRegistry) {
+				this.metadataExtractors.forEach(consumer -> consumer.accept((MetadataExtractorRegistry) extractor));
+			}
+
 			return new DefaultRSocketStrategies(
 					this.encoders, this.decoders, matcher, registry, factory, extractor);
 		}

spring-messaging/src/main/java/org/springframework/messaging/rsocket/MetadataExtractor.java

@@ -31,6 +31,7 @@ import org.springframework.util.MimeType;
  *
  * @author Rossen Stoyanchev
  * @since 5.2
+ * @see MetadataExtractorRegistry
  */
 public interface MetadataExtractor {
 

spring-messaging/src/main/java/org/springframework/messaging/rsocket/MetadataExtractorRegistry.java

@@ -0,0 +1,90 @@
+/*
+ * Copyright 2002-2019 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
+ *
+ *      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.messaging.rsocket;
+
+import java.util.Map;
+import java.util.function.BiConsumer;
+
+import org.springframework.core.ParameterizedTypeReference;
+import org.springframework.lang.Nullable;
+import org.springframework.util.MimeType;
+
+/**
+ * Stores registrations of extractors for metadata entries.
+ * Each metadata entry is decoded based on its {@code MimeType} and a name
+ * is assigned to the decoded value.
+ *
+ * @author Rossen Stoyanchev
+ * @author Brian Clozel
+ * @since 5.2
+ * @see MetadataExtractor
+ */
+public interface MetadataExtractorRegistry {
+
+	/**
+	 * Decode metadata entries with the given {@link MimeType} to the specified
+	 * target class, and store the decoded value in the output map under the
+	 * given name.
+	 * @param mimeType the mime type of metadata entries to extract
+	 * @param targetType the target value type to decode to
+	 * @param name assign a name for the decoded value; if not provided, then
+	 * the mime type is used as the key
+	 */
+	default void metadataToExtract(MimeType mimeType, Class<?> targetType, @Nullable String name) {
+		String key = name != null ? name : mimeType.toString();
+		metadataToExtract(mimeType, targetType, (value, map) -> map.put(key, value));
+	}
+
+	/**
+	 * Variant of {@link #metadataToExtract(MimeType, Class, String)} that accepts
+	 * {@link ParameterizedTypeReference} instead of {@link Class} for
+	 * specifying a target type with generic parameters.
+	 * @param mimeType the mime type of metadata entries to extract
+	 * @param targetType the target value type to decode to
+	 */
+	default void metadataToExtract(
+			MimeType mimeType, ParameterizedTypeReference<?> targetType, @Nullable String name) {
+
+		String key = name != null ? name : mimeType.toString();
+		metadataToExtract(mimeType, targetType, (value, map) -> map.put(key, value));
+	}
+
+	/**
+	 * Variant of {@link #metadataToExtract(MimeType, Class, String)} that allows
+	 * custom logic to be used to map the decoded value to any number of values
+	 * in the output map.
+	 * @param mimeType the mime type of metadata entries to extract
+	 * @param targetType the target value type to decode to
+	 * @param mapper custom logic to add the decoded value to the output map
+	 * @param <T> the target value type
+	 */
+	<T> void metadataToExtract(
+			MimeType mimeType, Class<T> targetType, BiConsumer<T, Map<String, Object>> mapper);
+
+	/**
+	 * Variant of {@link #metadataToExtract(MimeType, Class, BiConsumer)} that
+	 * accepts {@link ParameterizedTypeReference} instead of {@link Class} for
+	 * specifying a target type with generic parameters.
+	 * @param mimeType the mime type of metadata entries to extract
+	 * @param type the target value type to decode to
+	 * @param mapper custom logic to add the decoded value to the output map
+	 * @param <T> the target value type
+	 */
+	<T> void metadataToExtract(
+			MimeType mimeType, ParameterizedTypeReference<T> type, BiConsumer<T, Map<String, Object>> mapper);
+
+}

spring-messaging/src/main/java/org/springframework/messaging/rsocket/RSocketStrategies.java

@@ -219,6 +219,12 @@ public interface RSocketStrategies {
 		 */
 		Builder metadataExtractor(@Nullable MetadataExtractor metadataExtractor);
 
+		/**
+		 * Apply the consumer to the {@link MetadataExtractorRegistry} in order
+		 * to register extra metadata entry extractors.
+		 */
+		Builder metadataExtractors(Consumer<MetadataExtractorRegistry> extractorRegistry);
+
 		/**
 		 * Build the {@code RSocketStrategies} instance.
 		 */

spring-messaging/src/test/java/org/springframework/messaging/rsocket/DefaultRSocketStrategiesTests.java

@@ -15,6 +15,8 @@
  */
 package org.springframework.messaging.rsocket;
 
+import java.util.function.Consumer;
+
 import org.junit.jupiter.api.Test;
 
 import org.springframework.core.ReactiveAdapterRegistry;
@@ -30,16 +32,20 @@ import org.springframework.util.AntPathMatcher;
 import org.springframework.util.SimpleRouteMatcher;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
 
 /**
  * Unit tests for {@link RSocketStrategies}.
  * @author Rossen Stoyanchev
  * @since 5.2
  */
-public class DefaultRSocketStrategiesTests {
+class DefaultRSocketStrategiesTests {
 
 	@Test
-	public void defaultSettings() {
+	void defaultSettings() {
 		RSocketStrategies strategies = RSocketStrategies.create();
 
 		assertThat(strategies.encoders()).hasSize(4).hasOnlyElementsOfTypes(
@@ -62,8 +68,7 @@ public class DefaultRSocketStrategiesTests {
 	}
 
 	@Test
-	public void explicitValues() {
-
+	void explicitValues() {
 		SimpleRouteMatcher matcher = new SimpleRouteMatcher(new AntPathMatcher());
 		DefaultMetadataExtractor extractor = new DefaultMetadataExtractor();
 		ReactiveAdapterRegistry registry = new ReactiveAdapterRegistry();
@@ -90,7 +95,7 @@ public class DefaultRSocketStrategiesTests {
 	}
 
 	@Test
-	public void copyConstructor() {
+	void copyConstructor() {
 		RSocketStrategies strategies1 = RSocketStrategies.create();
 		RSocketStrategies strategies2 = strategies1.mutate().build();
 
@@ -101,4 +106,12 @@ public class DefaultRSocketStrategiesTests {
 		assertThat(strategies1.reactiveAdapterRegistry()).isSameAs(strategies2.reactiveAdapterRegistry());
 	}
 
+	@Test
+	@SuppressWarnings("unchecked")
+	void applyMetadataExtractors() {
+		Consumer<MetadataExtractorRegistry> consumer = (Consumer<MetadataExtractorRegistry>) mock(Consumer.class);
+		RSocketStrategies strategies = RSocketStrategies.builder().metadataExtractors(consumer).build();
+		verify(consumer, times(1)).accept(any());
+	}
+
 }