Refine multipart converter registration in HttpMessageConverters

As of #33894, `HttpMessageConverters` auto-detects converters and use
custom-provided ones to configure a collection of converters for the
client or the server.
Right now the multipart converter is only configured if core converters
(JSON, XML...) are configured/detected. We do not reuse the base
converters (resource, string, byte array) for the multipart converter
as it applies different encoding defaults (ISO for the main ones, UTF-8
for multipart).

This commit refines the configuration to not only include the multipart
converter when core converters are present, but also if any other
converter was configured.

Closes gh-35203

Author: bclozel

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

@@ -17,8 +17,6 @@
 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;
@@ -110,7 +108,9 @@ abstract static class DefaultBuilder {
 
 		@Nullable HttpMessageConverter<?> stringMessageConverter;
 
-		List<HttpMessageConverter<?>> resourceMessageConverters = Collections.emptyList();
+		@Nullable HttpMessageConverter<?> resourceMessageConverter;
+
+		@Nullable HttpMessageConverter<?> resourceRegionMessageConverter;
 
 		@Nullable Consumer<HttpMessageConverter<?>> configurer;
 
@@ -386,7 +386,7 @@ public ClientBuilder configureMessageConverters(Consumer<HttpMessageConverter<?>
 		@Override
 		public HttpMessageConverters build() {
 			if (this.registerDefaults) {
-				this.resourceMessageConverters = Collections.singletonList(new ResourceHttpMessageConverter(false));
+				this.resourceMessageConverter = new ResourceHttpMessageConverter(false);
 				detectMessageConverters();
 			}
 			List<HttpMessageConverter<?>> allConverters = new ArrayList<>();
@@ -396,8 +396,10 @@ public HttpMessageConverters build() {
 
 			allConverters.addAll(this.getCustomConverters());
 			allConverters.addAll(this.getBaseConverters());
-			allConverters.addAll(this.resourceMessageConverters);
-			if (!partConverters.isEmpty()) {
+			if (this.resourceMessageConverter != null) {
+				allConverters.add(this.resourceMessageConverter);
+			}
+			if (!partConverters.isEmpty() || !allConverters.isEmpty()) {
 				allConverters.add(new AllEncompassingFormHttpMessageConverter(partConverters));
 			}
 			allConverters.addAll(this.getCoreConverters());
@@ -468,7 +470,8 @@ public ServerBuilder configureMessageConverters(Consumer<HttpMessageConverter<?>
 		@Override
 		public HttpMessageConverters build() {
 			if (this.registerDefaults) {
-				this.resourceMessageConverters = Arrays.asList(new ResourceHttpMessageConverter(), new ResourceRegionHttpMessageConverter());
+				this.resourceMessageConverter = new ResourceHttpMessageConverter();
+				this.resourceRegionMessageConverter = new ResourceRegionHttpMessageConverter();
 				detectMessageConverters();
 			}
 			List<HttpMessageConverter<?>> allConverters = new ArrayList<>();
@@ -479,8 +482,13 @@ public HttpMessageConverters build() {
 
 			allConverters.addAll(this.getCustomConverters());
 			allConverters.addAll(this.getBaseConverters());
-			allConverters.addAll(this.resourceMessageConverters);
-			if (!partConverters.isEmpty()) {
+			if (this.resourceMessageConverter != null) {
+				allConverters.add(this.resourceMessageConverter);
+			}
+			if (this.resourceRegionMessageConverter != null) {
+				allConverters.add(this.resourceRegionMessageConverter);
+			}
+			if (!partConverters.isEmpty() || !allConverters.isEmpty()) {
 				allConverters.add(new AllEncompassingFormHttpMessageConverter(partConverters));
 			}
 			allConverters.addAll(this.getCoreConverters());

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

@@ -160,6 +160,13 @@ void registerCustomConverterInMultipartConverter() {
 			assertThat(multipartConverter.getPartConverters()).hasAtLeastOneElementOfType(CustomHttpMessageConverter.class);
 		}
 
+		@Test
+		void registerMultipartConverterWhenOtherConvertersPresent() {
+			var converters = HttpMessageConverters.forClient()
+					.stringMessageConverter(new StringHttpMessageConverter()).build();
+			assertThat(converters).hasExactlyElementsOfTypes(StringHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
+		}
+
 		@Test
 		void shouldUseSpecificConverter() {
 			var jacksonConverter = new JacksonJsonHttpMessageConverter();
@@ -248,7 +255,14 @@ void registerCustomConverterInMultipartConverter() {
 		}
 
 		@Test
-		void shouldUseServerSpecificConverter() {
+		void registerMultipartConverterWhenOtherConvertersPresent() {
+			var converters = HttpMessageConverters.forServer()
+					.stringMessageConverter(new StringHttpMessageConverter()).build();
+			assertThat(converters).hasExactlyElementsOfTypes(StringHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
+		}
+
+		@Test
+		void shouldUseSpecificConverter() {
 			var jacksonConverter = new JacksonJsonHttpMessageConverter();
 			var converters = HttpMessageConverters.forServer().registerDefaults()
 					.jsonMessageConverter(jacksonConverter).build();

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

@@ -33,6 +33,7 @@
 import org.springframework.http.client.support.BasicAuthenticationInterceptor;
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.http.converter.StringHttpMessageConverter;
+import org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter;
 import org.springframework.web.util.DefaultUriBuilderFactory;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -150,7 +151,7 @@ void configureMessageConverters() {
 
 		assertThat(fieldValue("messageConverters", defaultBuilder))
 				.asInstanceOf(InstanceOfAssertFactories.LIST)
-				.containsExactly(stringConverter);
+				.hasExactlyElementsOfTypes(StringHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
 	}
 
 	@Test