Configure custom converters ahead in HttpMessageConverters

This commit aligns `HttpMessageConverters` with the WebFlux codecs
configuration by adding custom converters first, before the
auto-detected and well-known ones.

This is a general approach in Spring Framework, where custom
implementations are more likely to handle specific uses cases and should
be involved before considering the more general implementations.

Fixes gh-35177

spring-web/src/main/java/org/springframework/http/converter/DefaultHttpMessageConverters.java

@@ -149,7 +149,7 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 		@Override
 		public DefaultBuilder additionalMessageConverter(HttpMessageConverter<?> customConverter) {
 			Assert.notNull(customConverter, "'customConverter' must not be null");
-			this.commonMessageConverters.additionalMessageConverters.add(customConverter);
+			this.commonMessageConverters.customMessageConverters.add(customConverter);
 			return this;
 		}
 
@@ -216,7 +216,13 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 
 		private @Nullable HttpMessageConverter<?> yamlMessageConverter;
 
-		private final List<HttpMessageConverter<?>> additionalMessageConverters = new ArrayList<>();
+		private @Nullable HttpMessageConverter<?> protobufMessageConverter;
+
+		private @Nullable HttpMessageConverter<?> atomMessageConverter;
+
+		private @Nullable HttpMessageConverter<?> rssMessageConverter;
+
+		private final List<HttpMessageConverter<?>> customMessageConverters = new ArrayList<>();
 
 		static {
 			ClassLoader classLoader = DefaultClientMessageConverterConfigurer.class.getClassLoader();
@@ -340,13 +346,34 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 					this.inheritedMessageConverters.xmlMessageConverter != null) {
 				converters.add(this.inheritedMessageConverters.xmlMessageConverter);
 			}
+			if (this.protobufMessageConverter != null) {
+				converters.add(this.protobufMessageConverter);
+			}
+			else if (this.inheritedMessageConverters != null &&
+					this.inheritedMessageConverters.protobufMessageConverter != null) {
+				converters.add(this.inheritedMessageConverters.protobufMessageConverter);
+			}
+			if (this.atomMessageConverter != null) {
+				converters.add(this.atomMessageConverter);
+			}
+			else if (this.inheritedMessageConverters != null &&
+					this.inheritedMessageConverters.atomMessageConverter != null) {
+				converters.add(this.inheritedMessageConverters.atomMessageConverter);
+			}
+			if (this.rssMessageConverter != null) {
+				converters.add(this.rssMessageConverter);
+			}
+			else if (this.inheritedMessageConverters != null &&
+					this.inheritedMessageConverters.rssMessageConverter != null) {
+				converters.add(this.inheritedMessageConverters.rssMessageConverter);
+			}
 			return converters;
 		}
 
 		List<HttpMessageConverter<?>> getCustomConverters() {
-			List<HttpMessageConverter<?>> result = new ArrayList<>(this.additionalMessageConverters);
+			List<HttpMessageConverter<?>> result = new ArrayList<>(this.customMessageConverters);
 			if (this.inheritedMessageConverters != null) {
-				result.addAll(this.inheritedMessageConverters.additionalMessageConverters);
+				result.addAll(this.inheritedMessageConverters.customMessageConverters);
 			}
 			return result;
 		}
@@ -405,12 +432,12 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 			}
 
 			if (isKotlinSerializationProtobufPresent) {
-				this.additionalMessageConverters.add(new KotlinSerializationProtobufHttpMessageConverter());
+				this.protobufMessageConverter = new KotlinSerializationProtobufHttpMessageConverter();
 			}
 
 			if (isRomePresent) {
-				this.additionalMessageConverters.add(new AtomFeedHttpMessageConverter());
+				this.atomMessageConverter = new AtomFeedHttpMessageConverter();
-				this.additionalMessageConverters.add(new RssChannelHttpMessageConverter());
+				this.rssMessageConverter = new RssChannelHttpMessageConverter();
 			}
 		}
 
@@ -466,7 +493,7 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 		@Override
 		public ClientMessageConverterConfigurer additionalMessageConverter(HttpMessageConverter<?> customConverter) {
 			Assert.notNull(customConverter, "'customConverter' must not be null");
-			this.clientMessageConverters.additionalMessageConverters.add(customConverter);
+			this.clientMessageConverters.customMessageConverters.add(customConverter);
 			return this;
 		}
 
@@ -485,16 +512,16 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 			List<HttpMessageConverter<?>> allConverters = new ArrayList<>();
 			List<HttpMessageConverter<?>> partConverters = new ArrayList<>();
 
-			partConverters.addAll(this.clientMessageConverters.getCoreConverters());
 			partConverters.addAll(this.clientMessageConverters.getCustomConverters());
+			partConverters.addAll(this.clientMessageConverters.getCoreConverters());
 
+			allConverters.addAll(this.clientMessageConverters.getCustomConverters());
 			allConverters.addAll(this.clientMessageConverters.getBaseConverters());
 			allConverters.addAll(this.resourceMessageConverters);
 			if (!partConverters.isEmpty()) {
 				allConverters.add(new AllEncompassingFormHttpMessageConverter(partConverters));
 			}
 			allConverters.addAll(this.clientMessageConverters.getCoreConverters());
-			allConverters.addAll(this.clientMessageConverters.getCustomConverters());
 
 			if (this.configurer != null) {
 				allConverters.forEach(this.configurer);
@@ -553,7 +580,7 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 		@Override
 		public ServerMessageConverterConfigurer additionalMessageConverter(HttpMessageConverter<?> customConverter) {
 			Assert.notNull(customConverter, "'customConverter' must not be null");
-			this.serverMessageConverters.additionalMessageConverters.add(customConverter);
+			this.serverMessageConverters.customMessageConverters.add(customConverter);
 			return this;
 		}
 
@@ -572,16 +599,16 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 			List<HttpMessageConverter<?>> allConverters = new ArrayList<>();
 			List<HttpMessageConverter<?>> partConverters = new ArrayList<>();
 
-			partConverters.addAll(this.serverMessageConverters.getCoreConverters());
 			partConverters.addAll(this.serverMessageConverters.getCustomConverters());
+			partConverters.addAll(this.serverMessageConverters.getCoreConverters());
 
+			allConverters.addAll(this.serverMessageConverters.getCustomConverters());
 			allConverters.addAll(this.serverMessageConverters.getBaseConverters());
 			allConverters.addAll(this.resourceMessageConverters);
 			if (!partConverters.isEmpty()) {
 				allConverters.add(new AllEncompassingFormHttpMessageConverter(partConverters));
 			}
 			allConverters.addAll(this.serverMessageConverters.getCoreConverters());
-			allConverters.addAll(this.serverMessageConverters.getCustomConverters());
 			if (this.configurer != null) {
 				allConverters.forEach(this.configurer);
 			}

spring-web/src/main/java/org/springframework/http/converter/HttpMessageConverters.java

@@ -31,6 +31,7 @@ import java.util.function.Consumer;
  * <p>The following HTTP message converters will be detected and registered if available, in order.
  * For {@link #forClient() client side converters}:
  * <ol>
+ *     <li>All custom message converters configured with the builder
  *     <li>{@link ByteArrayHttpMessageConverter}
  *     <li>{@link StringHttpMessageConverter} with the {@link java.nio.charset.StandardCharsets#ISO_8859_1} charset
  *     <li>{@link ResourceHttpMessageConverter}, with resource streaming support disabled
@@ -42,11 +43,11 @@ import java.util.function.Consumer;
  *     <li>An XML converter
  *     <li>An ProtoBuf converter
  *     <li>ATOM and RSS converters
- *     <li>All custom message converters configured with the builder
  * </ol>
  *
  * For {@link #forClient() client side converters}:
  * <ol>
+ *     <li>All custom message converters configured with the builder
  *     <li>{@link ByteArrayHttpMessageConverter}
  *     <li>{@link StringHttpMessageConverter} with the {@link java.nio.charset.StandardCharsets#ISO_8859_1} charset
  *     <li>{@link ResourceHttpMessageConverter}
@@ -58,7 +59,6 @@ import java.util.function.Consumer;
  *     <li>An XML converter
  *     <li>An ProtoBuf converter
  *     <li>ATOM and RSS converters
- *     <li>All custom message converters configured with the builder
  *     <li>a Multipart converter, using all detected and custom converters for part conversion
  * </ol>
  *

spring-web/src/test/java/org/springframework/http/converter/DefaultHttpMessageConvertersTests.java

@@ -150,7 +150,21 @@ class DefaultHttpMessageConvertersTests {
 		void registerCustomMessageConverter() {
 			var converters = HttpMessageConverters.create()
 					.additionalMessageConverter(new CustomHttpMessageConverter()).build();
-			assertThat(converters.forClient()).hasExactlyElementsOfTypes(AllEncompassingFormHttpMessageConverter.class, CustomHttpMessageConverter.class);
+			assertThat(converters.forClient()).hasExactlyElementsOfTypes(CustomHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
+		}
+
+		@Test
+		void registerCustomMessageConverterAheadOfDefaults() {
+			var converters = HttpMessageConverters.withDefaults()
+					.additionalMessageConverter(new CustomHttpMessageConverter()).build();
+			assertThat(converters.forClient()).hasExactlyElementsOfTypes(
+					CustomHttpMessageConverter.class, ByteArrayHttpMessageConverter.class,
+					StringHttpMessageConverter.class, ResourceHttpMessageConverter.class,
+					AllEncompassingFormHttpMessageConverter.class,
+					JacksonJsonHttpMessageConverter.class, JacksonSmileHttpMessageConverter.class,
+					JacksonCborHttpMessageConverter.class, JacksonYamlHttpMessageConverter.class,
+					JacksonXmlHttpMessageConverter.class, KotlinSerializationProtobufHttpMessageConverter.class,
+					AtomFeedHttpMessageConverter.class, RssChannelHttpMessageConverter.class);
 		}
 
 		@Test
@@ -224,7 +238,22 @@ class DefaultHttpMessageConvertersTests {
 		void registerCustomMessageConverter() {
 			var converters = HttpMessageConverters.create()
 					.additionalMessageConverter(new CustomHttpMessageConverter()).build();
-			assertThat(converters.forServer()).hasExactlyElementsOfTypes(AllEncompassingFormHttpMessageConverter.class, CustomHttpMessageConverter.class);
+			assertThat(converters.forServer()).hasExactlyElementsOfTypes(CustomHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
+		}
+
+		@Test
+		void registerCustomMessageConverterAheadOfDefaults() {
+			var converters = HttpMessageConverters.withDefaults()
+					.additionalMessageConverter(new CustomHttpMessageConverter()).build();
+			assertThat(converters.forServer()).hasExactlyElementsOfTypes(
+					CustomHttpMessageConverter.class,
+					ByteArrayHttpMessageConverter.class, StringHttpMessageConverter.class,
+					ResourceHttpMessageConverter.class, ResourceRegionHttpMessageConverter.class,
+					AllEncompassingFormHttpMessageConverter.class,
+					JacksonJsonHttpMessageConverter.class, JacksonSmileHttpMessageConverter.class,
+					JacksonCborHttpMessageConverter.class, JacksonYamlHttpMessageConverter.class,
+					JacksonXmlHttpMessageConverter.class, KotlinSerializationProtobufHttpMessageConverter.class,
+					AtomFeedHttpMessageConverter.class, RssChannelHttpMessageConverter.class);
 		}
 
 		@Test