Closes gh-36043

Signed-off-by: Hayden Rear <hayden.rear@gmail.com>

Author: haydenrear

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

@@ -217,9 +217,9 @@ private void checkConverterSupports(HttpMessageConverter<?> converter, MediaType
 			throw new IllegalArgumentException("converter should support '" + mediaType + "'");
 		}
 
-		void addCustomMessageConverter(HttpMessageConverter<?> customConverter) {
+		void addCustomMessageConverter(int index, HttpMessageConverter<?> customConverter) {
 			Assert.notNull(customConverter, "'customConverter' must not be null");
-			this.customConverters.add(customConverter);
+			this.customConverters.add(index, customConverter);
 		}
 
 		void addMessageConverterConfigurer(Consumer<HttpMessageConverter<?>> configurer) {
@@ -432,7 +432,13 @@ public ClientBuilder withYamlConverter(HttpMessageConverter<?> yamlConverter) {
 
 		@Override
 		public ClientBuilder addCustomConverter(HttpMessageConverter<?> customConverter) {
-			addCustomMessageConverter(customConverter);
+			addCustomConverter(0, customConverter);
+			return this;
+		}
+
+		@Override
+		public ClientBuilder addCustomConverter(int index, HttpMessageConverter<?> customConverter) {
+			addCustomMessageConverter(index, customConverter);
 			return this;
 		}
 
@@ -529,7 +535,13 @@ public ServerBuilder withYamlConverter(HttpMessageConverter<?> yamlConverter) {
 
 		@Override
 		public ServerBuilder addCustomConverter(HttpMessageConverter<?> customConverter) {
-			addCustomMessageConverter(customConverter);
+			addCustomMessageConverter(0, customConverter);
+			return this;
+		}
+
+		@Override
+		public ServerBuilder addCustomConverter(int index, HttpMessageConverter<?> customConverter) {
+			addCustomMessageConverter(0, customConverter);
 			return this;
 		}
 

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

@@ -166,6 +166,13 @@ interface Builder<T extends Builder<T>> {
 		 */
 		T addCustomConverter(HttpMessageConverter<?> customConverter);
 
+
+		/**
+		 * Add a custom {@code HttpMessageConverter} to the list of converters, at the specified index, pushing all existing message converters forward one.
+		 * @param customConverter the converter instance to add
+		 */
+		T addCustomConverter(int index, HttpMessageConverter<?> customConverter);
+
 		/**
 		 * Add a consumer for configuring the selected message converters.
 		 * @param configurer the configurer to use

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

@@ -147,6 +147,18 @@ void registerCustomMessageConverter() {
 			assertThat(converters).hasExactlyElementsOfTypes(CustomHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
 		}
 
+		@Test
+		void registerCustomMessageConverterAtIndex() {
+			HttpMessageConverters.ClientBuilder clientBuilder = HttpMessageConverters.forClient()
+					.addCustomConverter(0, new CustomHttpMessageConverter());
+			var converters = clientBuilder.build();
+			assertThat(converters).hasExactlyElementsOfTypes(CustomHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
+
+			clientBuilder.addCustomConverter(0, new CustomHttpMessageConverter());
+			converters = clientBuilder.build();
+			assertThat(converters).hasExactlyElementsOfTypes(CustomHttpMessageConverter.class, CustomHttpMessageConverter.class, AllEncompassingFormHttpMessageConverter.class);
+		}
+
 		@Test
 		void registerCustomMessageConverterAheadOfDefaults() {
 			var converters = HttpMessageConverters.forClient().registerDefaults()