Align HttpMessageConverters builder with WebFlux codecs variants

As of #33894, we introduced a new `HttpMessageConverters` API.
While this achieved our goal of focusing converters classpath detection
in a single place and avoiding waste, a single `HttpMessageConverters`
instance for both client and server added more complexity for developers.

This commit aligns the API here with the WebFlux `CodecsConfigurer` to
opt for a client/server flavor as the first step in the builder.
While this make the sharing of converter instances between server and
client impossible, this allows for a simpler API and separates concerns.

Closes gh-35187

framework-docs/src/main/java/org/springframework/docs/web/webmvc/mvcconfig/mvcconfigmessageconverters/WebConfiguration.java

@@ -34,7 +34,7 @@ import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
 public class WebConfiguration implements WebMvcConfigurer {
 
 	@Override
-	public void configureMessageConverters(HttpMessageConverters.Builder builder) {
+	public void configureMessageConverters(HttpMessageConverters.ServerBuilder builder) {
 		JsonMapper jsonMapper = JsonMapper.builder()
 				.findAndAddModules()
 				.enable(SerializationFeature.INDENT_OUTPUT)

framework-docs/src/main/kotlin/org/springframework/docs/web/webmvc/mvcconfig/mvcconfigmessageconverters/WebConfiguration.kt

@@ -16,7 +16,7 @@ import java.text.SimpleDateFormat
 @Configuration
 class WebConfiguration : WebMvcConfigurer {
 
-	override fun configureMessageConverters(builder: HttpMessageConverters.Builder) {
+	override fun configureMessageConverters(builder: HttpMessageConverters.ServerBuilder) {
 		val jsonMapper = JsonMapper.builder()
 			.findAndAddModules()
 			.enable(SerializationFeature.INDENT_OUTPUT)

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

@@ -19,6 +19,7 @@ package org.springframework.http.converter;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.Iterator;
 import java.util.List;
 import java.util.function.Consumer;
 
@@ -55,113 +56,19 @@ import org.springframework.util.ClassUtils;
 @SuppressWarnings("removal")
 class DefaultHttpMessageConverters implements HttpMessageConverters {
 
-	private final List<HttpMessageConverter<?>> clientMessageConverters;
+	private final List<HttpMessageConverter<?>> messageConverters;
 
-	private final List<HttpMessageConverter<?>> serverMessageConverters;
-
-	DefaultHttpMessageConverters(List<HttpMessageConverter<?>> clientMessageConverters, List<HttpMessageConverter<?>> serverMessageConverters) {
-		this.clientMessageConverters = clientMessageConverters;
-		this.serverMessageConverters = serverMessageConverters;
-	}
-
-	@Override
-	public Iterable<HttpMessageConverter<?>> forClient() {
-		return this.clientMessageConverters;
+	DefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
+		this.messageConverters = messageConverters;
 	}
 
 	@Override
-	public Iterable<HttpMessageConverter<?>> forServer() {
-		return this.serverMessageConverters;
+	public Iterator<HttpMessageConverter<?>> iterator() {
+		return this.messageConverters.iterator();
 	}
 
-	static class DefaultBuilder implements HttpMessageConverters.Builder {
-
-		private final DefaultMessageConverterConfigurer commonMessageConverters;
-
-		private final DefaultClientMessageConverterConfigurer clientMessageConverterConfigurer;
-
-		private final DefaultServerMessageConverterConfigurer serverMessageConverterConfigurer;
-
-
-		DefaultBuilder(boolean registerDefaults) {
-			this(registerDefaults, DefaultHttpMessageConverters.class.getClassLoader());
-		}
-
-		DefaultBuilder(boolean registerDefaults, ClassLoader classLoader) {
-			this.commonMessageConverters = new DefaultMessageConverterConfigurer();
-			this.clientMessageConverterConfigurer = new DefaultClientMessageConverterConfigurer(this.commonMessageConverters);
-			this.serverMessageConverterConfigurer = new DefaultServerMessageConverterConfigurer(this.commonMessageConverters);
-			if (registerDefaults) {
-				this.commonMessageConverters.registerDefaults();
-				this.clientMessageConverterConfigurer.registerDefaults();
-				this.serverMessageConverterConfigurer.registerDefaults();
-			}
-		}
-
-		@Override
-		public Builder configureClient(Consumer<ClientMessageConverterConfigurer> consumer) {
-			consumer.accept(this.clientMessageConverterConfigurer);
-			return this;
-		}
-
-		@Override
-		public Builder configureServer(Consumer<ServerMessageConverterConfigurer> consumer) {
-			consumer.accept(this.serverMessageConverterConfigurer);
-			return this;
-		}
 
-		@Override
-		public Builder stringMessageConverter(HttpMessageConverter<?> stringMessageConverter) {
-			this.commonMessageConverters.setStringMessageConverter(stringMessageConverter);
-			return this;
-		}
-
-		@Override
-		public DefaultBuilder jsonMessageConverter(HttpMessageConverter<?> jsonMessageConverter) {
-			this.commonMessageConverters.setJsonMessageConverter(jsonMessageConverter);
-			return this;
-		}
-
-		@Override
-		public DefaultBuilder xmlMessageConverter(HttpMessageConverter<?> xmlMessageConverter) {
-			this.commonMessageConverters.setXmlMessageConverter(xmlMessageConverter);
-			return this;
-		}
-
-		@Override
-		public DefaultBuilder smileMessageConverter(HttpMessageConverter<?> smileMessageConverter) {
-			this.commonMessageConverters.setSmileMessageConverter(smileMessageConverter);
-			return this;
-		}
-
-		@Override
-		public Builder cborMessageConverter(HttpMessageConverter<?> cborMessageConverter) {
-			this.commonMessageConverters.setCborMessageConverter(cborMessageConverter);
-			return this;
-		}
-
-		@Override
-		public Builder yamlMessageConverter(HttpMessageConverter<?> yamlMessageConverter) {
-			this.commonMessageConverters.setYamlMessageConverter(yamlMessageConverter);
-			return this;
-		}
-
-		@Override
-		public DefaultBuilder additionalMessageConverter(HttpMessageConverter<?> customConverter) {
-			Assert.notNull(customConverter, "'customConverter' must not be null");
-			this.commonMessageConverters.customMessageConverters.add(customConverter);
-			return this;
-		}
-
-		@Override
-		public DefaultHttpMessageConverters build() {
-			return new DefaultHttpMessageConverters(this.clientMessageConverterConfigurer.getMessageConverters(),
-					this.serverMessageConverterConfigurer.getMessageConverters());
-		}
-
-	}
-
-	static class DefaultMessageConverterConfigurer {
+	abstract static class DefaultBuilder {
 
 		private static final boolean isJacksonPresent;
 
@@ -197,35 +104,37 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 
 		private static final boolean isRomePresent;
 
+		boolean registerDefaults;
 
-		private final @Nullable DefaultMessageConverterConfigurer inheritedMessageConverters;
-
-		private @Nullable ByteArrayHttpMessageConverter byteArrayMessageConverter;
+		@Nullable ByteArrayHttpMessageConverter byteArrayMessageConverter;
 
-		private @Nullable HttpMessageConverter<?> stringMessageConverter;
+		@Nullable HttpMessageConverter<?> stringMessageConverter;
 
 		List<HttpMessageConverter<?>> resourceMessageConverters = Collections.emptyList();
 
-		private @Nullable HttpMessageConverter<?> jsonMessageConverter;
+		@Nullable Consumer<HttpMessageConverter<?>> configurer;
 
-		private @Nullable HttpMessageConverter<?> xmlMessageConverter;
+		@Nullable HttpMessageConverter<?> jsonMessageConverter;
 
-		private @Nullable HttpMessageConverter<?> smileMessageConverter;
+		@Nullable HttpMessageConverter<?> xmlMessageConverter;
 
-		private @Nullable HttpMessageConverter<?> cborMessageConverter;
+		@Nullable HttpMessageConverter<?> smileMessageConverter;
 
-		private @Nullable HttpMessageConverter<?> yamlMessageConverter;
+		@Nullable HttpMessageConverter<?> cborMessageConverter;
 
-		private @Nullable HttpMessageConverter<?> protobufMessageConverter;
+		@Nullable HttpMessageConverter<?> yamlMessageConverter;
 
-		private @Nullable HttpMessageConverter<?> atomMessageConverter;
+		@Nullable HttpMessageConverter<?> protobufMessageConverter;
 
-		private @Nullable HttpMessageConverter<?> rssMessageConverter;
+		@Nullable HttpMessageConverter<?> atomMessageConverter;
+
+		@Nullable HttpMessageConverter<?> rssMessageConverter;
+
+		final List<HttpMessageConverter<?>> customMessageConverters = new ArrayList<>();
 
-		private final List<HttpMessageConverter<?>> customMessageConverters = new ArrayList<>();
 
 		static {
-			ClassLoader classLoader = DefaultClientMessageConverterConfigurer.class.getClassLoader();
+			ClassLoader classLoader = DefaultBuilder.class.getClassLoader();
 			isJacksonPresent = ClassUtils.isPresent("tools.jackson.databind.ObjectMapper", classLoader);
 			isJackson2Present = ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", classLoader) &&
 						ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator", classLoader);
@@ -246,13 +155,6 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 			isRomePresent = ClassUtils.isPresent("com.rometools.rome.feed.WireFeed", classLoader);
 		}
 
-		DefaultMessageConverterConfigurer() {
-			this(null);
-		}
-
-		DefaultMessageConverterConfigurer(@Nullable DefaultMessageConverterConfigurer inheritedMessageConverters) {
-			this.inheritedMessageConverters = inheritedMessageConverters;
-		}
 
 		void setStringMessageConverter(HttpMessageConverter<?> stringMessageConverter) {
 			Assert.isTrue(stringMessageConverter.getSupportedMediaTypes().contains(MediaType.TEXT_PLAIN),
@@ -290,22 +192,23 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 			this.yamlMessageConverter = yamlMessageConverter;
 		}
 
+		void addCustomMessageConverter(HttpMessageConverter<?> customConverter) {
+			Assert.notNull(customConverter, "'customConverter' must not be null");
+			this.customMessageConverters.add(customConverter);
+		}
+
+		void addMessageConverterConfigurer(Consumer<HttpMessageConverter<?>> configurer) {
+			this.configurer = (this.configurer != null) ? configurer.andThen(this.configurer) : configurer;
+		}
+
 		List<HttpMessageConverter<?>> getBaseConverters() {
 			List<HttpMessageConverter<?>> converters = new ArrayList<>();
 			if (this.byteArrayMessageConverter != null) {
 				converters.add(this.byteArrayMessageConverter);
 			}
-			else if (this.inheritedMessageConverters != null &&
-					this.inheritedMessageConverters.byteArrayMessageConverter != null) {
-				converters.add(this.inheritedMessageConverters.byteArrayMessageConverter);
-			}
 			if (this.stringMessageConverter != null) {
 				converters.add(this.stringMessageConverter);
 			}
-			else if (this.inheritedMessageConverters != null &&
-					this.inheritedMessageConverters.stringMessageConverter != null) {
-				converters.add(this.inheritedMessageConverters.stringMessageConverter);
-			}
 			return converters;
 		}
 
@@ -314,305 +217,277 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 			if (this.jsonMessageConverter != null) {
 				converters.add(this.jsonMessageConverter);
 			}
-			else if (this.inheritedMessageConverters != null &&
-					this.inheritedMessageConverters.jsonMessageConverter != null) {
-				converters.add(this.inheritedMessageConverters.jsonMessageConverter);
-			}
 			if (this.smileMessageConverter != null) {
 				converters.add(this.smileMessageConverter);
 			}
-			else if (this.inheritedMessageConverters != null &&
-					this.inheritedMessageConverters.smileMessageConverter != null) {
-				converters.add(this.inheritedMessageConverters.smileMessageConverter);
-			}
 			if (this.cborMessageConverter!= null) {
 				converters.add(this.cborMessageConverter);
 			}
-			else if (this.inheritedMessageConverters != null &&
-					this.inheritedMessageConverters.cborMessageConverter != null) {
-				converters.add(this.inheritedMessageConverters.cborMessageConverter);
-			}
 			if (this.yamlMessageConverter!= null) {
 				converters.add(this.yamlMessageConverter);
 			}
-			else if (this.inheritedMessageConverters != null &&
-					this.inheritedMessageConverters.yamlMessageConverter != null) {
-				converters.add(this.inheritedMessageConverters.yamlMessageConverter);
-			}
 			if (this.xmlMessageConverter!= null) {
 				converters.add(this.xmlMessageConverter);
 			}
-			else if (this.inheritedMessageConverters != null &&
-					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.customMessageConverters);
-			if (this.inheritedMessageConverters != null) {
-				result.addAll(this.inheritedMessageConverters.customMessageConverters);
-			}
-			return result;
+			return this.customMessageConverters;
 		}
 
-		void registerDefaults() {
+		void detectMessageConverters() {
 			this.byteArrayMessageConverter = new ByteArrayHttpMessageConverter();
 			this.stringMessageConverter = new StringHttpMessageConverter();
 
-			if (isJacksonPresent) {
-				this.jsonMessageConverter = new JacksonJsonHttpMessageConverter();
-			}
-			else if (isJackson2Present) {
-				this.jsonMessageConverter = new MappingJackson2HttpMessageConverter();
-			}
-			else if (isGsonPresent) {
-				this.jsonMessageConverter = new GsonHttpMessageConverter();
-			}
-			else if (isJsonbPresent) {
-				this.jsonMessageConverter = new JsonbHttpMessageConverter();
-			}
-			else if (isKotlinSerializationJsonPresent) {
-				this.jsonMessageConverter = new KotlinSerializationJsonHttpMessageConverter();
-			}
-
-			if (isJacksonXmlPresent) {
-				this.xmlMessageConverter = new JacksonXmlHttpMessageConverter();
-			}
-			else if (isJackson2XmlPresent) {
-				this.xmlMessageConverter = new MappingJackson2XmlHttpMessageConverter();
-			}
-			else if (isJaxb2Present) {
-				this.xmlMessageConverter = new Jaxb2RootElementHttpMessageConverter();
-			}
-
-			if (isJacksonSmilePresent) {
-				this.smileMessageConverter = new JacksonSmileHttpMessageConverter();
-			}
-			else if (isJackson2SmilePresent) {
-				this.smileMessageConverter = new MappingJackson2SmileHttpMessageConverter();
-			}
-			if (isJacksonCborPresent) {
-				this.cborMessageConverter = new JacksonCborHttpMessageConverter();
-			}
-			else if (isJackson2CborPresent) {
-				this.cborMessageConverter = new MappingJackson2CborHttpMessageConverter();
-			}
-			else if (isKotlinSerializationCborPresent) {
-				this.cborMessageConverter = new KotlinSerializationCborHttpMessageConverter();
-			}
-
-			if (isJacksonYamlPresent) {
-				this.yamlMessageConverter = new JacksonYamlHttpMessageConverter();
-			}
-			else if (isJackson2YamlPresent) {
-				this.yamlMessageConverter = new MappingJackson2YamlHttpMessageConverter();
-			}
-
-			if (isKotlinSerializationProtobufPresent) {
-				this.protobufMessageConverter = new KotlinSerializationProtobufHttpMessageConverter();
+			if (this.jsonMessageConverter == null) {
+				if (isJacksonPresent) {
+					this.jsonMessageConverter = new JacksonJsonHttpMessageConverter();
+				}
+				else if (isJackson2Present) {
+					this.jsonMessageConverter = new MappingJackson2HttpMessageConverter();
+				}
+				else if (isGsonPresent) {
+					this.jsonMessageConverter = new GsonHttpMessageConverter();
+				}
+				else if (isJsonbPresent) {
+					this.jsonMessageConverter = new JsonbHttpMessageConverter();
+				}
+				else if (isKotlinSerializationJsonPresent) {
+					this.jsonMessageConverter = new KotlinSerializationJsonHttpMessageConverter();
+				}
+			}
+
+			if (this.xmlMessageConverter == null) {
+				if (isJacksonXmlPresent) {
+					this.xmlMessageConverter = new JacksonXmlHttpMessageConverter();
+				}
+				else if (isJackson2XmlPresent) {
+					this.xmlMessageConverter = new MappingJackson2XmlHttpMessageConverter();
+				}
+				else if (isJaxb2Present) {
+					this.xmlMessageConverter = new Jaxb2RootElementHttpMessageConverter();
+				}
+			}
+
+			if (this.smileMessageConverter == null) {
+				if (isJacksonSmilePresent) {
+					this.smileMessageConverter = new JacksonSmileHttpMessageConverter();
+				}
+				else if (isJackson2SmilePresent) {
+					this.smileMessageConverter = new MappingJackson2SmileHttpMessageConverter();
+				}
+			}
+
+			if (this.cborMessageConverter == null) {
+				if (isJacksonCborPresent) {
+					this.cborMessageConverter = new JacksonCborHttpMessageConverter();
+				}
+				else if (isJackson2CborPresent) {
+					this.cborMessageConverter = new MappingJackson2CborHttpMessageConverter();
+				}
+				else if (isKotlinSerializationCborPresent) {
+					this.cborMessageConverter = new KotlinSerializationCborHttpMessageConverter();
+				}
+			}
+
+			if (this.yamlMessageConverter == null) {
+				if (isJacksonYamlPresent) {
+					this.yamlMessageConverter = new JacksonYamlHttpMessageConverter();
+				}
+				else if (isJackson2YamlPresent) {
+					this.yamlMessageConverter = new MappingJackson2YamlHttpMessageConverter();
+				}
+			}
+
+			if (this.protobufMessageConverter == null) {
+				if (isKotlinSerializationProtobufPresent) {
+					this.protobufMessageConverter = new KotlinSerializationProtobufHttpMessageConverter();
+				}
 			}
 
 			if (isRomePresent) {
-				this.atomMessageConverter = new AtomFeedHttpMessageConverter();
-				this.rssMessageConverter = new RssChannelHttpMessageConverter();
+				if (this.atomMessageConverter == null) {
+					this.atomMessageConverter = new AtomFeedHttpMessageConverter();
+				}
+				if (this.rssMessageConverter == null) {
+					this.rssMessageConverter = new RssChannelHttpMessageConverter();
+				}
 			}
 		}
 
 	}
 
-	static class DefaultClientMessageConverterConfigurer extends DefaultMessageConverterConfigurer implements ClientMessageConverterConfigurer {
-
-		private @Nullable Consumer<HttpMessageConverter<?>> configurer;
-
-		private final DefaultMessageConverterConfigurer clientMessageConverters;
-
-
-		public DefaultClientMessageConverterConfigurer(DefaultMessageConverterConfigurer parentMessageConverters) {
-			this.clientMessageConverters = new DefaultMessageConverterConfigurer(parentMessageConverters);
-		}
+	static class DefaultClientBuilder extends DefaultBuilder implements ClientBuilder {
 
 		@Override
-		public ClientMessageConverterConfigurer stringMessageConverter(HttpMessageConverter<?> stringMessageConverter) {
-			this.clientMessageConverters.setStringMessageConverter(stringMessageConverter);
+		public DefaultClientBuilder registerDefaults() {
+			this.registerDefaults = true;
 			return this;
 		}
 
 		@Override
-		public ClientMessageConverterConfigurer jsonMessageConverter(HttpMessageConverter<?> jsonMessageConverter) {
-			this.clientMessageConverters.setJsonMessageConverter(jsonMessageConverter);
+		public ClientBuilder stringMessageConverter(HttpMessageConverter<?> stringMessageConverter) {
+			setStringMessageConverter(stringMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ClientMessageConverterConfigurer xmlMessageConverter(HttpMessageConverter<?> xmlMessageConverter) {
-			this.clientMessageConverters.setXmlMessageConverter(xmlMessageConverter);
+		public ClientBuilder jsonMessageConverter(HttpMessageConverter<?> jsonMessageConverter) {
+			setJsonMessageConverter(jsonMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ClientMessageConverterConfigurer smileMessageConverter(HttpMessageConverter<?> smileMessageConverter) {
-			this.clientMessageConverters.setSmileMessageConverter(smileMessageConverter);
+		public ClientBuilder xmlMessageConverter(HttpMessageConverter<?> xmlMessageConverter) {
+			setXmlMessageConverter(xmlMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ClientMessageConverterConfigurer cborMessageConverter(HttpMessageConverter<?> cborMessageConverter) {
-			this.clientMessageConverters.setCborMessageConverter(cborMessageConverter);
+		public ClientBuilder smileMessageConverter(HttpMessageConverter<?> smileMessageConverter) {
+			setSmileMessageConverter(smileMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ClientMessageConverterConfigurer yamlMessageConverter(HttpMessageConverter<?> yamlMessageConverter) {
-			this.clientMessageConverters.setYamlMessageConverter(yamlMessageConverter);
+		public ClientBuilder cborMessageConverter(HttpMessageConverter<?> cborMessageConverter) {
+			setCborMessageConverter(cborMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ClientMessageConverterConfigurer additionalMessageConverter(HttpMessageConverter<?> customConverter) {
-			Assert.notNull(customConverter, "'customConverter' must not be null");
-			this.clientMessageConverters.customMessageConverters.add(customConverter);
+		public ClientBuilder yamlMessageConverter(HttpMessageConverter<?> yamlMessageConverter) {
+			setYamlMessageConverter(yamlMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ClientMessageConverterConfigurer configureClientMessageConverters(Consumer<HttpMessageConverter<?>> configurer) {
-			this.configurer = (this.configurer != null) ? configurer.andThen(this.configurer) : configurer;
+		public ClientBuilder customMessageConverter(HttpMessageConverter<?> customConverter) {
+			addCustomMessageConverter(customConverter);
 			return this;
 		}
 
 		@Override
-		void registerDefaults() {
-			this.resourceMessageConverters = Collections.singletonList(new ResourceHttpMessageConverter(false));
+		public ClientBuilder configureMessageConverters(Consumer<HttpMessageConverter<?>> configurer) {
+			addMessageConverterConfigurer(configurer);
+			return this;
 		}
 
-		List<HttpMessageConverter<?>> getMessageConverters() {
+		@Override
+		public HttpMessageConverters build() {
+			if (this.registerDefaults) {
+				this.resourceMessageConverters = Collections.singletonList(new ResourceHttpMessageConverter(false));
+				detectMessageConverters();
+			}
 			List<HttpMessageConverter<?>> allConverters = new ArrayList<>();
 			List<HttpMessageConverter<?>> partConverters = new ArrayList<>();
+			partConverters.addAll(this.getCustomConverters());
+			partConverters.addAll(this.getCoreConverters());
 
-			partConverters.addAll(this.clientMessageConverters.getCustomConverters());
-			partConverters.addAll(this.clientMessageConverters.getCoreConverters());
-
-			allConverters.addAll(this.clientMessageConverters.getCustomConverters());
-			allConverters.addAll(this.clientMessageConverters.getBaseConverters());
+			allConverters.addAll(this.getCustomConverters());
+			allConverters.addAll(this.getBaseConverters());
 			allConverters.addAll(this.resourceMessageConverters);
 			if (!partConverters.isEmpty()) {
 				allConverters.add(new AllEncompassingFormHttpMessageConverter(partConverters));
 			}
-			allConverters.addAll(this.clientMessageConverters.getCoreConverters());
-
+			allConverters.addAll(this.getCoreConverters());
 			if (this.configurer != null) {
 				allConverters.forEach(this.configurer);
 			}
-			return allConverters;
+			return new DefaultHttpMessageConverters(allConverters);
 		}
 	}
 
-	static class DefaultServerMessageConverterConfigurer extends DefaultMessageConverterConfigurer implements ServerMessageConverterConfigurer {
-
-		private @Nullable Consumer<HttpMessageConverter<?>> configurer;
-
-		private final DefaultMessageConverterConfigurer serverMessageConverters;
+	static class DefaultServerBuilder extends DefaultBuilder implements ServerBuilder {
 
 
-		DefaultServerMessageConverterConfigurer(DefaultMessageConverterConfigurer commonMessageConverters) {
-			this.serverMessageConverters = new DefaultMessageConverterConfigurer(commonMessageConverters);
-		}
-
 		@Override
-		public ServerMessageConverterConfigurer stringMessageConverter(HttpMessageConverter<?> stringMessageConverter) {
-			this.serverMessageConverters.setStringMessageConverter(stringMessageConverter);
+		public ServerBuilder registerDefaults() {
+			this.registerDefaults = true;
 			return this;
 		}
 
 		@Override
-		public ServerMessageConverterConfigurer jsonMessageConverter(HttpMessageConverter<?> jsonMessageConverter) {
-			this.serverMessageConverters.setJsonMessageConverter(jsonMessageConverter);
+		public ServerBuilder stringMessageConverter(HttpMessageConverter<?> stringMessageConverter) {
+			setStringMessageConverter(stringMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ServerMessageConverterConfigurer xmlMessageConverter(HttpMessageConverter<?> xmlMessageConverter) {
-			this.serverMessageConverters.setXmlMessageConverter(xmlMessageConverter);
+		public ServerBuilder jsonMessageConverter(HttpMessageConverter<?> jsonMessageConverter) {
+			setJsonMessageConverter(jsonMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ServerMessageConverterConfigurer smileMessageConverter(HttpMessageConverter<?> smileMessageConverter) {
-			this.serverMessageConverters.setSmileMessageConverter(smileMessageConverter);
+		public ServerBuilder xmlMessageConverter(HttpMessageConverter<?> xmlMessageConverter) {
+			setXmlMessageConverter(xmlMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ServerMessageConverterConfigurer cborMessageConverter(HttpMessageConverter<?> cborMessageConverter) {
-			this.serverMessageConverters.setCborMessageConverter(cborMessageConverter);
+		public ServerBuilder smileMessageConverter(HttpMessageConverter<?> smileMessageConverter) {
+			setSmileMessageConverter(smileMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ServerMessageConverterConfigurer yamlMessageConverter(HttpMessageConverter<?> yamlMessageConverter) {
-			this.serverMessageConverters.setYamlMessageConverter(yamlMessageConverter);
+		public ServerBuilder cborMessageConverter(HttpMessageConverter<?> cborMessageConverter) {
+			setCborMessageConverter(cborMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ServerMessageConverterConfigurer additionalMessageConverter(HttpMessageConverter<?> customConverter) {
-			Assert.notNull(customConverter, "'customConverter' must not be null");
-			this.serverMessageConverters.customMessageConverters.add(customConverter);
+		public ServerBuilder yamlMessageConverter(HttpMessageConverter<?> yamlMessageConverter) {
+			setYamlMessageConverter(yamlMessageConverter);
 			return this;
 		}
 
 		@Override
-		public ServerMessageConverterConfigurer configureServerMessageConverters(Consumer<HttpMessageConverter<?>> configurer) {
-			this.configurer = (this.configurer != null) ? configurer.andThen(this.configurer) : configurer;
+		public ServerBuilder customMessageConverter(HttpMessageConverter<?> customConverter) {
+			addCustomMessageConverter(customConverter);
 			return this;
 		}
 
 		@Override
-		void registerDefaults() {
-			this.resourceMessageConverters = Arrays.asList(new ResourceHttpMessageConverter(), new ResourceRegionHttpMessageConverter());
+		public ServerBuilder configureMessageConverters(Consumer<HttpMessageConverter<?>> configurer) {
+			addMessageConverterConfigurer(configurer);
+			return this;
 		}
 
-		List<HttpMessageConverter<?>> getMessageConverters() {
+		@Override
+		public HttpMessageConverters build() {
+			if (this.registerDefaults) {
+				this.resourceMessageConverters = Arrays.asList(new ResourceHttpMessageConverter(), new ResourceRegionHttpMessageConverter());
+				detectMessageConverters();
+			}
 			List<HttpMessageConverter<?>> allConverters = new ArrayList<>();
 			List<HttpMessageConverter<?>> partConverters = new ArrayList<>();
 
-			partConverters.addAll(this.serverMessageConverters.getCustomConverters());
-			partConverters.addAll(this.serverMessageConverters.getCoreConverters());
+			partConverters.addAll(this.getCustomConverters());
+			partConverters.addAll(this.getCoreConverters());
 
-			allConverters.addAll(this.serverMessageConverters.getCustomConverters());
-			allConverters.addAll(this.serverMessageConverters.getBaseConverters());
+			allConverters.addAll(this.getCustomConverters());
+			allConverters.addAll(this.getBaseConverters());
 			allConverters.addAll(this.resourceMessageConverters);
 			if (!partConverters.isEmpty()) {
 				allConverters.add(new AllEncompassingFormHttpMessageConverter(partConverters));
 			}
-			allConverters.addAll(this.serverMessageConverters.getCoreConverters());
+			allConverters.addAll(this.getCoreConverters());
 			if (this.configurer != null) {
 				allConverters.forEach(this.configurer);
 			}
-			return allConverters;
+			return new DefaultHttpMessageConverters(allConverters);
 		}
 	}
 

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

@@ -20,79 +20,69 @@ import java.util.function.Consumer;
 
 /**
  * Utility for building and configuring an immutable collection of {@link HttpMessageConverter}
- * instances for client and server usage. You can {@link #create() create}
- * a new empty instance or ask to {@link #withDefaults() register default converters},
- * if available in your classpath.
- *
- * <p>This class offers a flexible arrangement for {@link HttpMessageConverters.Builder configuring message converters shared between}
- * client and server, or {@link HttpMessageConverters.Builder#configureClient(Consumer) configuring client-specific}
- * and {@link HttpMessageConverters.Builder#configureServer(Consumer) server-specific} converters.
- *
- * <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
- *     <li>a Multipart converter, using all detected and custom converters for part conversion
- *     <li>A JSON converter
- *     <li>A Smile converter
- *     <li>A CBOR converter
- *     <li>A YAML converter
- *     <li>An XML converter
- *     <li>An ProtoBuf converter
- *     <li>ATOM and RSS converters
- * </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}
- *     <li>{@link ResourceRegionHttpMessageConverter}
- *     <li>A JSON converter
- *     <li>A Smile converter
- *     <li>A CBOR converter
- *     <li>A YAML converter
- *     <li>An XML converter
- *     <li>An ProtoBuf converter
- *     <li>ATOM and RSS converters
- *     <li>a Multipart converter, using all detected and custom converters for part conversion
- * </ol>
+ * instances for {@link #forClient() client} or {@link #forServer() server} usage. You can
+ * ask to {@link Builder#registerDefaults() register default converters with classpath detection},
+ * add custom converters and post-process configured converters.
  *
  * @author Brian Clozel
  * @since 7.0
  */
-public interface HttpMessageConverters {
+public interface HttpMessageConverters extends Iterable<HttpMessageConverter<?>> {
 
-	/**
-	 * Return the list of configured message converters, tailored for HTTP client usage.
-	 */
-	Iterable<HttpMessageConverter<?>> forClient();
-
-	/**
-	 * Return the list of configured message converters, tailored for HTTP server usage.
-	 */
-	Iterable<HttpMessageConverter<?>> forServer();
 
 	/**
-	 * Create a builder instance, without any message converter pre-configured.
+	 * Create a builder instance, tailored for HTTP client usage.
+	 * <p>The following HTTP message converters can be detected and registered if available, in order:
+	 * <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
+	 *     <li>a Multipart converter, using all detected and custom converters for part conversion
+	 *     <li>A JSON converter
+	 *     <li>A Smile converter
+	 *     <li>A CBOR converter
+	 *     <li>A YAML converter
+	 *     <li>An XML converter
+	 *     <li>A ProtoBuf converter
+	 *     <li>ATOM and RSS converters
+	 * </ol>
 	 */
-	static Builder create() {
-		return new DefaultHttpMessageConverters.DefaultBuilder(false);
+	static ClientBuilder forClient() {
+		return new DefaultHttpMessageConverters.DefaultClientBuilder();
 	}
 
 	/**
-	 * Create a builder instance with default message converters pre-configured.
+	 * Create a builder instance, tailored for HTTP server usage.
+	 * <p>The following HTTP message converters can be detected and registered if available, in order:
+	 * <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}
+	 *     <li>{@link ResourceRegionHttpMessageConverter}
+	 *     <li>A JSON converter
+	 *     <li>A Smile converter
+	 *     <li>A CBOR converter
+	 *     <li>A YAML converter
+	 *     <li>An XML converter
+	 *     <li>A ProtoBuf converter
+	 *     <li>ATOM and RSS converters
+	 *     <li>a Multipart converter, using all detected and custom converters for part conversion
+	 * </ol>
 	 */
-	static Builder withDefaults() {
-		return new DefaultHttpMessageConverters.DefaultBuilder(true);
+	static ServerBuilder forServer() {
+		return new DefaultHttpMessageConverters.DefaultServerBuilder();
 	}
 
+	interface Builder<T extends Builder<T>> {
 
-	interface MessageConverterConfigurer<T extends MessageConverterConfigurer<T>> {
+		/**
+		 * Register default converters using classpath detection.
+		 * Manual registrations like {@link #jsonMessageConverter(HttpMessageConverter)} will
+		 * override auto-detected ones.
+		 */
+		T registerDefaults();
 
 		/**
 		 * Override the default String {@code HttpMessageConverter}
@@ -146,27 +136,13 @@ public interface HttpMessageConverters {
 		 * Add a custom {@code HttpMessageConverter} to the list of converters.
 		 * @param customConverter the converter instance to add
 		 */
-		T additionalMessageConverter(HttpMessageConverter<?> customConverter);
-
-	}
-
-	/**
-	 * Builder for an {@link HttpMessageConverters}.
-	 * This builder manages the configuration of common and client/server-specific message converters.
-	 */
-	interface Builder extends MessageConverterConfigurer<Builder> {
-
-		/**
-		 * Configure client-specific message converters.
-		 * If no opinion is provided here, message converters defined in this builder will be used.
-		 */
-		Builder configureClient(Consumer<ClientMessageConverterConfigurer> consumer);
+		T customMessageConverter(HttpMessageConverter<?> customConverter);
 
 		/**
-		 * Configure server-specific message converters.
-		 * If no opinion is provided here, message converters defined in this builder will be used.
+		 * Add a consumer for configuring the selected message converters.
+		 * @param configurer the configurer to use
 		 */
-		Builder configureServer(Consumer<ServerMessageConverterConfigurer> consumer);
+		T configureMessageConverters(Consumer<HttpMessageConverter<?>> configurer);
 
 		/**
 		 * Build and return the {@link HttpMessageConverters} instance configured by this builder.
@@ -174,25 +150,17 @@ public interface HttpMessageConverters {
 		HttpMessageConverters build();
 	}
 
-	interface ClientMessageConverterConfigurer extends MessageConverterConfigurer<ClientMessageConverterConfigurer> {
-
-		/**
-		 * Register a consumer to apply to configured converter instances.
-		 * This can be used to configure rather than replace one or more specific converters.
-		 * @param configurer the consumer to apply
-		 */
-		ClientMessageConverterConfigurer configureClientMessageConverters(Consumer<HttpMessageConverter<?>> configurer);
+	/**
+	 * Client builder for an {@link HttpMessageConverters} instance.
+	 */
+	interface ClientBuilder extends Builder<ClientBuilder> {
 
 	}
 
-	interface ServerMessageConverterConfigurer extends MessageConverterConfigurer<ServerMessageConverterConfigurer> {
-
-		/**
-		 * Register a consumer to apply to configured converter instances.
-		 * This can be used to configure rather than replace one or more specific converters.
-		 * @param configurer the consumer to apply
-		 */
-		ServerMessageConverterConfigurer configureServerMessageConverters(Consumer<HttpMessageConverter<?>> configurer);
+	/**
+	 * Server builder for an {@link HttpMessageConverters} instance.
+	 */
+	interface ServerBuilder extends Builder<ServerBuilder> {
 
 	}
 

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

@@ -39,7 +39,7 @@ public class AllEncompassingFormHttpMessageConverter extends FormHttpMessageConv
 	 */
 	@SuppressWarnings("removal")
 	public AllEncompassingFormHttpMessageConverter() {
-		HttpMessageConverters.withDefaults().build().forClient().forEach(this::addPartConverter);
+		HttpMessageConverters.forClient().registerDefaults().build().forEach(this::addPartConverter);
 	}
 
 	/**

spring-web/src/main/java/org/springframework/web/client/DefaultRestClientBuilder.java

@@ -361,6 +361,7 @@ final class DefaultRestClientBuilder implements RestClient.Builder {
 	}
 
 	@Override
+	@SuppressWarnings("removal")
 	public RestClient.Builder messageConverters(Consumer<List<HttpMessageConverter<?>>> configurer) {
 		configurer.accept(initMessageConverters());
 		validateConverters(this.messageConverters);
@@ -376,6 +377,13 @@ final class DefaultRestClientBuilder implements RestClient.Builder {
 		return this;
 	}
 
+	@Override
+	public RestClient.Builder configureMessageConverters(Consumer<HttpMessageConverters.ClientBuilder> configurer) {
+		HttpMessageConverters.ClientBuilder clientBuilder = HttpMessageConverters.forClient();
+		configurer.accept(clientBuilder);
+		return messageConverters(clientBuilder.build());
+	}
+
 	@Override
 	public RestClient.Builder observationRegistry(ObservationRegistry observationRegistry) {
 		Assert.notNull(observationRegistry, "observationRegistry must not be null");
@@ -399,7 +407,7 @@ final class DefaultRestClientBuilder implements RestClient.Builder {
 	private List<HttpMessageConverter<?>> initMessageConverters() {
 		if (this.messageConverters == null) {
 			this.messageConverters = new ArrayList<>();
-			HttpMessageConverters.withDefaults().build().forClient().forEach(this.messageConverters::add);
+			HttpMessageConverters.forClient().registerDefaults().build().forEach(this.messageConverters::add);
 		}
 		return this.messageConverters;
 	}

spring-web/src/main/java/org/springframework/web/client/RestClient.java

@@ -47,6 +47,7 @@ import org.springframework.http.client.ClientHttpRequestInterceptor;
 import org.springframework.http.client.ClientHttpResponse;
 import org.springframework.http.client.observation.ClientRequestObservationConvention;
 import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.HttpMessageConverters;
 import org.springframework.lang.CheckReturnValue;
 import org.springframework.util.MultiValueMap;
 import org.springframework.web.util.DefaultUriBuilderFactory;
@@ -452,7 +453,9 @@ public interface RestClient {
 		 * {@link HttpMessageConverter} pre-initialized
 		 * @return this builder
 		 * @see #messageConverters(Iterable)
+		 * @deprecated since 7.0 in favor of {@link #configureMessageConverters(Consumer)}
 		 */
+		@Deprecated(since = "7.0", forRemoval = true)
 		Builder messageConverters(Consumer<List<HttpMessageConverter<?>>> configurer);
 
 		/**
@@ -460,10 +463,18 @@ public interface RestClient {
 		 * @param messageConverters the list of {@link HttpMessageConverter} to use
 		 * @return this builder
 		 * @since 6.2
-		 * @see #messageConverters(Consumer)
+		 * @see #configureMessageConverters(Consumer)
 		 */
 		Builder messageConverters(Iterable<HttpMessageConverter<?>> messageConverters);
 
+		/**
+		 * Configure the message converters for the {@code RestClient} to use.
+		 * @param configurer the configurer to apply on an empty {@link HttpMessageConverters.ClientBuilder}.
+		 * @return this builder
+		 * @since 7.0
+		 */
+		Builder configureMessageConverters(Consumer<HttpMessageConverters.ClientBuilder> configurer);
+
 		/**
 		 * Configure the {@link io.micrometer.observation.ObservationRegistry} to use
 		 * for recording HTTP client observations.

spring-web/src/main/java/org/springframework/web/client/RestTemplate.java

@@ -124,9 +124,7 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 	 * Default {@link HttpMessageConverter HttpMessageConverters} are initialized.
 	 */
 	public RestTemplate() {
-		HttpMessageConverters.withDefaults().build().forClient().forEach(this.messageConverters::add);
-		updateErrorHandlerConverters();
-		this.uriTemplateHandler = initUriTemplateHandler();
+		this(HttpMessageConverters.forClient().registerDefaults().build());
 	}
 
 	/**

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

@@ -17,11 +17,6 @@
 package org.springframework.http.converter;
 
 import java.io.IOException;
-import java.net.URL;
-import java.net.URLClassLoader;
-import java.util.Collection;
-import java.util.HashSet;
-import java.util.Set;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
 
@@ -30,7 +25,6 @@ import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.MethodSource;
 
-import org.springframework.core.SmartClassLoader;
 import org.springframework.http.HttpInputMessage;
 import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.converter.cbor.JacksonCborHttpMessageConverter;
@@ -60,59 +54,50 @@ class DefaultHttpMessageConvertersTests {
 
 	static Stream<Iterable<HttpMessageConverter<?>>> emptyMessageConverters() {
 		return Stream.of(
-				HttpMessageConverters.create().build().forClient(),
-				HttpMessageConverters.create().build().forServer()
+				HttpMessageConverters.forClient().build(),
+				HttpMessageConverters.forServer().build()
 		);
 	}
 
-	@Test
-	void clientAndServerConvertersAreShared() {
-		var converters = HttpMessageConverters.withDefaults().build();
-		Set<HttpMessageConverter<?>> allConverters = new HashSet<>();
-		converters.forClient().forEach(allConverters::add);
-		converters.forServer().forEach(allConverters::add);
-		assertThat(allConverters).hasSize(15);
-	}
-
 	@Test
 	void failsWhenStringConverterDoesNotSupportMediaType() {
 		assertThatIllegalArgumentException()
-				.isThrownBy(() -> HttpMessageConverters.create().stringMessageConverter(new CustomHttpMessageConverter()).build())
+				.isThrownBy(() -> HttpMessageConverters.forClient().stringMessageConverter(new CustomHttpMessageConverter()).build())
 				.withMessage("stringMessageConverter should support 'text/plain'");
 	}
 
 	@Test
 	void failsWhenJsonConverterDoesNotSupportMediaType() {
 		assertThatIllegalArgumentException()
-				.isThrownBy(() -> HttpMessageConverters.create().jsonMessageConverter(new CustomHttpMessageConverter()).build())
+				.isThrownBy(() -> HttpMessageConverters.forClient().jsonMessageConverter(new CustomHttpMessageConverter()).build())
 				.withMessage("jsonMessageConverter should support 'application/json'");
 	}
 
 	@Test
 	void failsWhenXmlConverterDoesNotSupportMediaType() {
 		assertThatIllegalArgumentException()
-				.isThrownBy(() -> HttpMessageConverters.create().xmlMessageConverter(new CustomHttpMessageConverter()).build())
+				.isThrownBy(() -> HttpMessageConverters.forClient().xmlMessageConverter(new CustomHttpMessageConverter()).build())
 				.withMessage("xmlMessageConverter should support 'text/xml'");
 	}
 
 	@Test
 	void failsWhenSmileConverterDoesNotSupportMediaType() {
 		assertThatIllegalArgumentException()
-				.isThrownBy(() -> HttpMessageConverters.create().smileMessageConverter(new CustomHttpMessageConverter()).build())
+				.isThrownBy(() -> HttpMessageConverters.forClient().smileMessageConverter(new CustomHttpMessageConverter()).build())
 				.withMessage("smileMessageConverter should support 'application/x-jackson-smile'");
 	}
 
 	@Test
 	void failsWhenCborConverterDoesNotSupportMediaType() {
 		assertThatIllegalArgumentException()
-				.isThrownBy(() -> HttpMessageConverters.create().cborMessageConverter(new CustomHttpMessageConverter()).build())
+				.isThrownBy(() -> HttpMessageConverters.forClient().cborMessageConverter(new CustomHttpMessageConverter()).build())
 				.withMessage("cborMessageConverter should support 'application/cbor'");
 	}
 
 	@Test
 	void failsWhenYamlConverterDoesNotSupportMediaType() {
 		assertThatIllegalArgumentException()
-				.isThrownBy(() -> HttpMessageConverters.create().yamlMessageConverter(new CustomHttpMessageConverter()).build())
+				.isThrownBy(() -> HttpMessageConverters.forClient().yamlMessageConverter(new CustomHttpMessageConverter()).build())
 				.withMessage("yamlMessageConverter should support 'application/yaml'");
 	}
 
@@ -122,8 +107,8 @@ class DefaultHttpMessageConvertersTests {
 
 		@Test
 		void defaultConverters() {
-			var converters = HttpMessageConverters.withDefaults().build();
-			assertThat(converters.forClient()).hasExactlyElementsOfTypes(ByteArrayHttpMessageConverter.class,
+			var converters = HttpMessageConverters.forClient().registerDefaults().build();
+			assertThat(converters).hasExactlyElementsOfTypes(ByteArrayHttpMessageConverter.class,
 					StringHttpMessageConverter.class, ResourceHttpMessageConverter.class,
 					AllEncompassingFormHttpMessageConverter.class,
 					JacksonJsonHttpMessageConverter.class, JacksonSmileHttpMessageConverter.class,
@@ -134,8 +119,8 @@ class DefaultHttpMessageConvertersTests {
 
 		@Test
 		void multipartConverterContainsOtherConverters() {
-			var converters = HttpMessageConverters.withDefaults().build();
-			AllEncompassingFormHttpMessageConverter multipartConverter = findMessageConverter(AllEncompassingFormHttpMessageConverter.class, converters.forClient());
+			var converters = HttpMessageConverters.forClient().registerDefaults().build();
+			var multipartConverter = findMessageConverter(AllEncompassingFormHttpMessageConverter.class, converters);
 
 			assertThat(multipartConverter.getPartConverters()).hasExactlyElementsOfTypes(
 					ByteArrayHttpMessageConverter.class, StringHttpMessageConverter.class,
@@ -148,16 +133,16 @@ class DefaultHttpMessageConvertersTests {
 
 		@Test
 		void registerCustomMessageConverter() {
-			var converters = HttpMessageConverters.create()
-					.additionalMessageConverter(new CustomHttpMessageConverter()).build();
-			assertThat(converters.forClient()).hasExactlyElementsOfTypes(CustomHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
+			var converters = HttpMessageConverters.forClient()
+					.customMessageConverter(new CustomHttpMessageConverter()).build();
+			assertThat(converters).hasExactlyElementsOfTypes(CustomHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
 		}
 
 		@Test
 		void registerCustomMessageConverterAheadOfDefaults() {
-			var converters = HttpMessageConverters.withDefaults()
-					.additionalMessageConverter(new CustomHttpMessageConverter()).build();
-			assertThat(converters.forClient()).hasExactlyElementsOfTypes(
+			var converters = HttpMessageConverters.forClient().registerDefaults()
+					.customMessageConverter(new CustomHttpMessageConverter()).build();
+			assertThat(converters).hasExactlyElementsOfTypes(
 					CustomHttpMessageConverter.class, ByteArrayHttpMessageConverter.class,
 					StringHttpMessageConverter.class, ResourceHttpMessageConverter.class,
 					AllEncompassingFormHttpMessageConverter.class,
@@ -169,33 +154,31 @@ class DefaultHttpMessageConvertersTests {
 
 		@Test
 		void registerCustomConverterInMultipartConverter() {
-			var converters = HttpMessageConverters.withDefaults()
-					.additionalMessageConverter(new CustomHttpMessageConverter()).build();
-			AllEncompassingFormHttpMessageConverter multipartConverter = findMessageConverter(AllEncompassingFormHttpMessageConverter.class, converters.forClient());
+			var converters = HttpMessageConverters.forClient().registerDefaults()
+					.customMessageConverter(new CustomHttpMessageConverter()).build();
+			var multipartConverter = findMessageConverter(AllEncompassingFormHttpMessageConverter.class, converters);
 			assertThat(multipartConverter.getPartConverters()).hasAtLeastOneElementOfType(CustomHttpMessageConverter.class);
 		}
 
 		@Test
-		void shouldUseServerSpecificConverter() {
-			JacksonJsonHttpMessageConverter jacksonConverter = new JacksonJsonHttpMessageConverter();
-			var converters = HttpMessageConverters.withDefaults()
-					.configureClient(configurer -> configurer.jsonMessageConverter(jacksonConverter)).build();
+		void shouldUseSpecificConverter() {
+			var jacksonConverter = new JacksonJsonHttpMessageConverter();
+			var converters = HttpMessageConverters.forClient().registerDefaults()
+					.jsonMessageConverter(jacksonConverter).build();
 
-			JacksonJsonHttpMessageConverter customConverter = findMessageConverter(JacksonJsonHttpMessageConverter.class, converters.forClient());
+			var customConverter = findMessageConverter(JacksonJsonHttpMessageConverter.class, converters);
 			assertThat(customConverter).isEqualTo(jacksonConverter);
 		}
 
 		@Test
 		void shouldConfigureConverter() {
-			CustomHttpMessageConverter customConverter = new CustomHttpMessageConverter();
-			HttpMessageConverters.withDefaults()
-					.additionalMessageConverter(customConverter)
-					.configureClient(configurer -> {
-						configurer.configureClientMessageConverters(converter -> {
-							if (converter instanceof CustomHttpMessageConverter custom) {
-								custom.processed = true;
-							}
-						});
+			var customConverter = new CustomHttpMessageConverter();
+			HttpMessageConverters.forClient()
+					.customMessageConverter(customConverter)
+					.configureMessageConverters(converter -> {
+						if (converter instanceof CustomHttpMessageConverter custom) {
+							custom.processed = true;
+						}
 					}).build();
 
 			assertThat(customConverter.processed).isTrue();
@@ -209,8 +192,8 @@ class DefaultHttpMessageConvertersTests {
 
 		@Test
 		void defaultConverters() {
-			var converters = HttpMessageConverters.withDefaults().build();
-			assertThat(converters.forServer()).hasExactlyElementsOfTypes(
+			var converters = HttpMessageConverters.forServer().registerDefaults().build();
+			assertThat(converters).hasExactlyElementsOfTypes(
 					ByteArrayHttpMessageConverter.class, StringHttpMessageConverter.class,
 					ResourceHttpMessageConverter.class, ResourceRegionHttpMessageConverter.class,
 					AllEncompassingFormHttpMessageConverter.class,
@@ -222,8 +205,8 @@ class DefaultHttpMessageConvertersTests {
 
 		@Test
 		void multipartConverterContainsOtherConverters() {
-			var converters = HttpMessageConverters.withDefaults().build();
-			AllEncompassingFormHttpMessageConverter multipartConverter = findMessageConverter(AllEncompassingFormHttpMessageConverter.class, converters.forServer());
+			var converters = HttpMessageConverters.forServer().registerDefaults().build();
+			var multipartConverter = findMessageConverter(AllEncompassingFormHttpMessageConverter.class, converters);
 
 			assertThat(multipartConverter.getPartConverters()).hasExactlyElementsOfTypes(
 					ByteArrayHttpMessageConverter.class, StringHttpMessageConverter.class,
@@ -236,16 +219,16 @@ class DefaultHttpMessageConvertersTests {
 
 		@Test
 		void registerCustomMessageConverter() {
-			var converters = HttpMessageConverters.create()
-					.additionalMessageConverter(new CustomHttpMessageConverter()).build();
-			assertThat(converters.forServer()).hasExactlyElementsOfTypes(CustomHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
+			var converters = HttpMessageConverters.forServer()
+					.customMessageConverter(new CustomHttpMessageConverter()).build();
+			assertThat(converters).hasExactlyElementsOfTypes(CustomHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
 		}
 
 		@Test
 		void registerCustomMessageConverterAheadOfDefaults() {
-			var converters = HttpMessageConverters.withDefaults()
-					.additionalMessageConverter(new CustomHttpMessageConverter()).build();
-			assertThat(converters.forServer()).hasExactlyElementsOfTypes(
+			var converters = HttpMessageConverters.forServer().registerDefaults()
+					.customMessageConverter(new CustomHttpMessageConverter()).build();
+			assertThat(converters).hasExactlyElementsOfTypes(
 					CustomHttpMessageConverter.class,
 					ByteArrayHttpMessageConverter.class, StringHttpMessageConverter.class,
 					ResourceHttpMessageConverter.class, ResourceRegionHttpMessageConverter.class,
@@ -258,33 +241,31 @@ class DefaultHttpMessageConvertersTests {
 
 		@Test
 		void registerCustomConverterInMultipartConverter() {
-			var converters = HttpMessageConverters.withDefaults()
-					.additionalMessageConverter(new CustomHttpMessageConverter()).build();
-			AllEncompassingFormHttpMessageConverter multipartConverter = findMessageConverter(AllEncompassingFormHttpMessageConverter.class, converters.forServer());
+			var converters = HttpMessageConverters.forServer().registerDefaults()
+					.customMessageConverter(new CustomHttpMessageConverter()).build();
+			var multipartConverter = findMessageConverter(AllEncompassingFormHttpMessageConverter.class, converters);
 			assertThat(multipartConverter.getPartConverters()).hasAtLeastOneElementOfType(CustomHttpMessageConverter.class);
 		}
 
 		@Test
 		void shouldUseServerSpecificConverter() {
-			JacksonJsonHttpMessageConverter jacksonConverter = new JacksonJsonHttpMessageConverter();
-			var converters = HttpMessageConverters.withDefaults()
-					.configureServer(configurer -> configurer.jsonMessageConverter(jacksonConverter)).build();
+			var jacksonConverter = new JacksonJsonHttpMessageConverter();
+			var converters = HttpMessageConverters.forServer().registerDefaults()
+					.jsonMessageConverter(jacksonConverter).build();
 
-			JacksonJsonHttpMessageConverter customConverter = findMessageConverter(JacksonJsonHttpMessageConverter.class, converters.forServer());
+			var customConverter = findMessageConverter(JacksonJsonHttpMessageConverter.class, converters);
 			assertThat(customConverter).isEqualTo(jacksonConverter);
 		}
 
 		@Test
 		void shouldConfigureConverter() {
-			CustomHttpMessageConverter customConverter = new CustomHttpMessageConverter();
-			HttpMessageConverters.withDefaults()
-					.additionalMessageConverter(customConverter)
-					.configureServer(configurer -> {
-						configurer.configureServerMessageConverters(converter -> {
-							if (converter instanceof CustomHttpMessageConverter custom) {
-								custom.processed = true;
-							}
-						});
+			var customConverter = new CustomHttpMessageConverter();
+			HttpMessageConverters.forServer().registerDefaults()
+					.customMessageConverter(customConverter)
+					.configureMessageConverters(converter -> {
+						if (converter instanceof CustomHttpMessageConverter custom) {
+							custom.processed = true;
+						}
 					}).build();
 
 			assertThat(customConverter.processed).isTrue();
@@ -299,32 +280,6 @@ class DefaultHttpMessageConvertersTests {
 				.findFirst().orElseThrow();
 	}
 
-
-	static class FilteredClassLoader extends URLClassLoader implements SmartClassLoader {
-
-		private final Collection<Class<?>> hiddenClasses;
-
-		public FilteredClassLoader(Class<?>... hiddenClasses) {
-			this(java.util.Arrays.asList(hiddenClasses));
-		}
-
-		FilteredClassLoader(Collection<Class<?>> hiddenClasses) {
-			super(new URL[0], FilteredClassLoader.class.getClassLoader());
-			this.hiddenClasses = hiddenClasses;
-		}
-
-		@Override
-		protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
-			for (Class<?> hiddenClass : this.hiddenClasses) {
-				if (hiddenClass.getName().equals(name)) {
-					throw new ClassNotFoundException();
-				}
-			}
-			return super.loadClass(name, resolve);
-		}
-
-	}
-
 	static class CustomHttpMessageConverter extends AbstractHttpMessageConverter<Object> {
 
 		boolean processed = false;

spring-web/src/test/java/org/springframework/web/client/RestClientBuilderTests.java

@@ -41,6 +41,7 @@ import static org.assertj.core.api.Assertions.fail;
 
 
 /**
+ * Tests for {@link DefaultRestClientBuilder}.
  * @author Arjen Poutsma
  * @author Sebastien Deleuze
  * @author Nicklas Wiegandt
@@ -139,6 +140,19 @@ public class RestClientBuilderTests {
 		assertThatIllegalArgumentException().isThrownBy(() -> builder.messageConverters(converters));
 	}
 
+	@Test
+	void configureMessageConverters() {
+		StringHttpMessageConverter stringConverter = new StringHttpMessageConverter();
+		RestClient.Builder builder = RestClient.builder();
+		builder.configureMessageConverters(clientBuilder -> clientBuilder.stringMessageConverter(stringConverter));
+		assertThat(builder).isInstanceOf(DefaultRestClientBuilder.class);
+		DefaultRestClientBuilder defaultBuilder = (DefaultRestClientBuilder) builder;
+
+		assertThat(fieldValue("messageConverters", defaultBuilder))
+				.asInstanceOf(InstanceOfAssertFactories.LIST)
+				.containsExactly(stringConverter);
+	}
+
 	@Test
 	void defaultCookieAddsCookieToDefaultCookiesMap() {
 		RestClient.Builder builder = RestClient.builder();

spring-web/src/test/java/org/springframework/web/client/RestClientObservationTests.java

@@ -44,7 +44,7 @@ import org.springframework.http.client.ClientHttpResponse;
 import org.springframework.http.client.observation.ClientRequestObservationContext;
 import org.springframework.http.client.observation.ClientRequestObservationConvention;
 import org.springframework.http.client.observation.DefaultClientRequestObservationConvention;
-import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.StringHttpMessageConverter;
 import org.springframework.util.StreamUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -70,8 +70,6 @@ class RestClientObservationTests {
 
 	private final ClientHttpResponse response = mock();
 
-	private final HttpMessageConverter<String> converter = mock();
-
 	private RestClient client;
 
 
@@ -84,7 +82,7 @@ class RestClientObservationTests {
 	RestClient.Builder createBuilder() {
 		return RestClient.builder()
 				.baseUrl("https://example.com/base")
-				.messageConverters(converters -> converters.add(0, this.converter))
+				.configureMessageConverters(converters -> converters.customMessageConverter(new StringHttpMessageConverter()))
 				.requestFactory(this.requestFactory)
 				.observationRegistry(this.observationRegistry);
 	}
@@ -203,7 +201,7 @@ class RestClientObservationTests {
 		assertThatExceptionOfType(RestClientException.class).isThrownBy(() ->
 				client.get().uri(url).retrieve().body(User.class));
 
-		assertThatHttpObservation().hasLowCardinalityKeyValue("exception", "RestClientException");
+		assertThatHttpObservation().hasLowCardinalityKeyValue("exception", "UnknownContentTypeException");
 	}
 
 	@Test

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/DelegatingWebMvcConfiguration.java

@@ -122,7 +122,7 @@ public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
 	}
 
 	@Override
-	protected void configureMessageConverters(HttpMessageConverters.Builder builder) {
+	protected void configureMessageConverters(HttpMessageConverters.ServerBuilder builder) {
 		this.configurers.configureMessageConverters(builder);
 	}
 

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurationSupport.java

@@ -863,9 +863,9 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 	 * @since 7.0
 	 */
 	protected HttpMessageConverters createMessageConverters() {
-		HttpMessageConverters.Builder builder = HttpMessageConverters.withDefaults();
-		configureMessageConverters(builder);
-		return builder.build();
+		HttpMessageConverters.ServerBuilder serverBuilder = HttpMessageConverters.forServer().registerDefaults();
+		configureMessageConverters(serverBuilder);
+		return serverBuilder.build();
 	}
 
 	/**
@@ -874,7 +874,7 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 	 * @param builder the {@code HttpMessageConverters} builder to configure
 	 * @since 7.0
 	 */
-	protected void configureMessageConverters(HttpMessageConverters.Builder builder) {
+	protected void configureMessageConverters(HttpMessageConverters.ServerBuilder builder) {
 	}
 
 	/**
@@ -885,7 +885,7 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 	 * otherwise be registered by default. Also see {@link #addDefaultHttpMessageConverters}
 	 * for adding default message converters.
 	 * @param converters a list to add message converters to (initially an empty list)
-	 * @deprecated since 7.0 in favor of {@link #configureMessageConverters(HttpMessageConverters.Builder)}
+	 * @deprecated since 7.0 in favor of {@link #configureMessageConverters(HttpMessageConverters.ServerBuilder)}
 	 */
 	@Deprecated(since = "7.0", forRemoval = true)
 	protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
@@ -897,7 +897,7 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 	 * to be registered and then insert a custom converter through this method.
 	 * @param converters the list of configured converters to extend
 	 * @since 4.1.3
-	 * @deprecated since 7.0 in favor of {@link #configureMessageConverters(HttpMessageConverters.Builder)}
+	 * @deprecated since 7.0 in favor of {@link #configureMessageConverters(HttpMessageConverters.ServerBuilder)}
 	 */
 	@Deprecated(since = "7.0", forRemoval = true)
 	protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
@@ -912,7 +912,7 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 	@Deprecated(since = "7.0", forRemoval = true)
 	protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
 		HttpMessageConverters converters = createMessageConverters();
-		converters.forServer().forEach(messageConverters::add);
+		converters.forEach(messageConverters::add);
 	}
 
 	/**

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurer.java

@@ -177,7 +177,7 @@ public interface WebMvcConfigurer {
 	 * @param builder the builder to configure
 	 * @since 7.0
 	 */
-	default void configureMessageConverters(HttpMessageConverters.Builder builder) {
+	default void configureMessageConverters(HttpMessageConverters.ServerBuilder builder) {
 	}
 
 	/**
@@ -195,7 +195,7 @@ public interface WebMvcConfigurer {
 	 * {@link #extendMessageConverters(java.util.List)} to modify the configured
 	 * list of message converters.
 	 * @param converters initially an empty list of converters
-	 * @deprecated since 7.0 in favor of configuring converters on {@link #configureMessageConverters(HttpMessageConverters.Builder)}
+	 * @deprecated since 7.0 in favor of configuring converters on {@link #configureMessageConverters(HttpMessageConverters.ServerBuilder)}
 	 */
 	@Deprecated(since = "7.0", forRemoval = true)
 	default void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
@@ -210,7 +210,7 @@ public interface WebMvcConfigurer {
 	 * the converters configured earlier will be preferred.
 	 * @param converters the list of configured converters to be extended
 	 * @since 4.1.3
-	 * @deprecated since 7.0 in favor of configuring converters on {@link #configureMessageConverters(HttpMessageConverters.Builder)}
+	 * @deprecated since 7.0 in favor of configuring converters on {@link #configureMessageConverters(HttpMessageConverters.ServerBuilder)}
 	 */
 	@Deprecated(since = "7.0", forRemoval = true)
 	default void extendMessageConverters(List<HttpMessageConverter<?>> converters) {

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurerComposite.java

@@ -142,7 +142,7 @@ class WebMvcConfigurerComposite implements WebMvcConfigurer {
 	}
 
 	@Override
-	public void configureMessageConverters(HttpMessageConverters.Builder builder) {
+	public void configureMessageConverters(HttpMessageConverters.ServerBuilder builder) {
 		for (WebMvcConfigurer delegate : this.delegates) {
 			delegate.configureMessageConverters(builder);
 		}

spring-webmvc/src/test/java/org/springframework/web/servlet/config/annotation/DelegatingWebMvcConfigurationTests.java

@@ -126,8 +126,8 @@ public class DelegatingWebMvcConfigurationTests {
 		WebMvcConfigurer configurer = new WebMvcConfigurer() {
 
 			@Override
-			public void configureMessageConverters(HttpMessageConverters.Builder builder) {
-				builder.additionalMessageConverter(customConverter);
+			public void configureMessageConverters(HttpMessageConverters.ServerBuilder builder) {
+				builder.customMessageConverter(customConverter);
 			}
 		};
 		webMvcConfig.setConfigurers(Collections.singletonList(configurer));

spring-webmvc/src/test/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurationSupportExtensionTests.java

@@ -356,11 +356,11 @@ class WebMvcConfigurationSupportExtensionTests {
 
 		@Override
 		protected HttpMessageConverters createMessageConverters() {
-			return HttpMessageConverters.create().jsonMessageConverter(new JacksonJsonHttpMessageConverter()).build();
+			return HttpMessageConverters.forServer().jsonMessageConverter(new JacksonJsonHttpMessageConverter()).build();
 		}
 
 		@Override
-		public void configureMessageConverters(HttpMessageConverters.Builder builder) {
+		public void configureMessageConverters(HttpMessageConverters.ServerBuilder builder) {
 		}
 
 		@Override