Fix multiple HTTP message converters of the same type being lost

module/spring-boot-http-converter/src/main/java/org/springframework/boot/http/converter/autoconfigure/DefaultClientHttpMessageConvertersCustomizer.java

@@ -17,6 +17,9 @@
 package org.springframework.boot.http.converter.autoconfigure;
 
 import java.util.Collection;
+import java.util.EnumSet;
+import java.util.function.BiConsumer;
+import java.util.function.Predicate;
 
 import org.jspecify.annotations.Nullable;
 
@@ -47,24 +50,32 @@ public void customize(ClientBuilder builder) {
 		}
 		else {
 			builder.registerDefaults();
-			this.converters.forEach((converter) -> {
-				if (converter instanceof StringHttpMessageConverter) {
-					builder.withStringConverter(converter);
-				}
-				else if (converter instanceof KotlinSerializationJsonHttpMessageConverter) {
-					builder.withKotlinSerializationJsonConverter(converter);
-				}
-				else if (supportsMediaType(converter, MediaType.APPLICATION_JSON)) {
-					builder.withJsonConverter(converter);
-				}
-				else if (supportsMediaType(converter, MediaType.APPLICATION_XML)) {
-					builder.withXmlConverter(converter);
+			EnumSet<ConverterType> registered = EnumSet.noneOf(ConverterType.class);
+			for (HttpMessageConverter<?> converter : this.converters) {
+				ConverterType type = findConverterType(converter);
+				if (type != null) {
+					if (!registered.contains(type)) {
+						type.registerWith(builder, converter);
+						registered.add(type);
+					}
+					else {
+						builder.addCustomConverter(converter);
+					}
 				}
 				else {
 					builder.addCustomConverter(converter);
 				}
-			});
+			}
+		}
+	}
+
+	private static @Nullable ConverterType findConverterType(HttpMessageConverter<?> converter) {
+		for (ConverterType type : ConverterType.values()) {
+			if (type.matches(converter)) {
+				return type;
+			}
 		}
+		return null;
 	}
 
 	private static boolean supportsMediaType(HttpMessageConverter<?> converter, MediaType mediaType) {
@@ -76,4 +87,36 @@ private static boolean supportsMediaType(HttpMessageConverter<?> converter, Medi
 		return false;
 	}
 
+	private enum ConverterType {
+
+		STRING(StringHttpMessageConverter.class::isInstance, ClientBuilder::withStringConverter),
+
+		KOTLIN_SERIALIZATION_JSON(KotlinSerializationJsonHttpMessageConverter.class::isInstance,
+				ClientBuilder::withKotlinSerializationJsonConverter),
+
+		JSON(converter -> supportsMediaType(converter, MediaType.APPLICATION_JSON),
+				ClientBuilder::withJsonConverter),
+
+		XML(converter -> supportsMediaType(converter, MediaType.APPLICATION_XML), ClientBuilder::withXmlConverter);
+
+		private final Predicate<HttpMessageConverter<?>> matcher;
+
+		private final BiConsumer<ClientBuilder, HttpMessageConverter<?>> registrar;
+
+		ConverterType(Predicate<HttpMessageConverter<?>> matcher,
+				BiConsumer<ClientBuilder, HttpMessageConverter<?>> registrar) {
+			this.matcher = matcher;
+			this.registrar = registrar;
+		}
+
+		boolean matches(HttpMessageConverter<?> converter) {
+			return this.matcher.test(converter);
+		}
+
+		void registerWith(ClientBuilder builder, HttpMessageConverter<?> converter) {
+			this.registrar.accept(builder, converter);
+		}
+
+	}
+
 }

module/spring-boot-http-converter/src/main/java/org/springframework/boot/http/converter/autoconfigure/DefaultServerHttpMessageConvertersCustomizer.java

@@ -17,6 +17,9 @@
 package org.springframework.boot.http.converter.autoconfigure;
 
 import java.util.Collection;
+import java.util.EnumSet;
+import java.util.function.BiConsumer;
+import java.util.function.Predicate;
 
 import org.jspecify.annotations.Nullable;
 
@@ -47,24 +50,32 @@ public void customize(ServerBuilder builder) {
 		}
 		else {
 			builder.registerDefaults();
-			this.converters.forEach((converter) -> {
-				if (converter instanceof StringHttpMessageConverter) {
-					builder.withStringConverter(converter);
-				}
-				else if (converter instanceof KotlinSerializationJsonHttpMessageConverter) {
-					builder.withKotlinSerializationJsonConverter(converter);
-				}
-				else if (supportsMediaType(converter, MediaType.APPLICATION_JSON)) {
-					builder.withJsonConverter(converter);
-				}
-				else if (supportsMediaType(converter, MediaType.APPLICATION_XML)) {
-					builder.withXmlConverter(converter);
+			EnumSet<ConverterType> registered = EnumSet.noneOf(ConverterType.class);
+			for (HttpMessageConverter<?> converter : this.converters) {
+				ConverterType type = findConverterType(converter);
+				if (type != null) {
+					if (!registered.contains(type)) {
+						type.registerWith(builder, converter);
+						registered.add(type);
+					}
+					else {
+						builder.addCustomConverter(converter);
+					}
 				}
 				else {
 					builder.addCustomConverter(converter);
 				}
-			});
+			}
+		}
+	}
+
+	private static @Nullable ConverterType findConverterType(HttpMessageConverter<?> converter) {
+		for (ConverterType type : ConverterType.values()) {
+			if (type.matches(converter)) {
+				return type;
+			}
 		}
+		return null;
 	}
 
 	private static boolean supportsMediaType(HttpMessageConverter<?> converter, MediaType mediaType) {
@@ -76,4 +87,36 @@ private static boolean supportsMediaType(HttpMessageConverter<?> converter, Medi
 		return false;
 	}
 
+	private enum ConverterType {
+
+		STRING(StringHttpMessageConverter.class::isInstance, ServerBuilder::withStringConverter),
+
+		KOTLIN_SERIALIZATION_JSON(KotlinSerializationJsonHttpMessageConverter.class::isInstance,
+				ServerBuilder::withKotlinSerializationJsonConverter),
+
+		JSON(converter -> supportsMediaType(converter, MediaType.APPLICATION_JSON),
+				ServerBuilder::withJsonConverter),
+
+		XML(converter -> supportsMediaType(converter, MediaType.APPLICATION_XML), ServerBuilder::withXmlConverter);
+
+		private final Predicate<HttpMessageConverter<?>> matcher;
+
+		private final BiConsumer<ServerBuilder, HttpMessageConverter<?>> registrar;
+
+		ConverterType(Predicate<HttpMessageConverter<?>> matcher,
+				BiConsumer<ServerBuilder, HttpMessageConverter<?>> registrar) {
+			this.matcher = matcher;
+			this.registrar = registrar;
+		}
+
+		boolean matches(HttpMessageConverter<?> converter) {
+			return this.matcher.test(converter);
+		}
+
+		void registerWith(ServerBuilder builder, HttpMessageConverter<?> converter) {
+			this.registrar.accept(builder, converter);
+		}
+
+	}
+
 }

module/spring-boot-http-converter/src/test/java/org/springframework/boot/http/converter/autoconfigure/DefaultServerHttpMessageConvertersCustomizerTests.java

@@ -0,0 +1,131 @@
+/*
+ * Copyright 2012-present 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.boot.http.converter.autoconfigure;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.http.MediaType;
+import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.HttpMessageConverters.ServerBuilder;
+import org.springframework.http.converter.json.GsonHttpMessageConverter;
+import org.springframework.http.converter.json.JacksonJsonHttpMessageConverter;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.BDDMockito.then;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+
+/**
+ * Tests for {@link DefaultServerHttpMessageConvertersCustomizer}.
+ *
+ * @author Tommy Karlsson
+ */
+@SuppressWarnings("deprecation")
+class DefaultServerHttpMessageConvertersCustomizerTests {
+
+	@Test
+	void customizeWithTwoJsonConvertersConfiguresFirstAsJsonConverterAndSecondAsCustomConverter() {
+		// Create two JSON converters
+		JacksonJsonHttpMessageConverter jsonConverter1 = new JacksonJsonHttpMessageConverter();
+		GsonHttpMessageConverter jsonConverter2 = new GsonHttpMessageConverter();
+
+		// Create the customizer with both converters
+		List<HttpMessageConverter<?>> converters = List.of(jsonConverter1, jsonConverter2);
+		DefaultServerHttpMessageConvertersCustomizer customizer = new DefaultServerHttpMessageConvertersCustomizer(null,
+				converters);
+
+		// Mock the ServerBuilder
+		ServerBuilder builder = mock(ServerBuilder.class);
+
+		// Execute the customize method
+		customizer.customize(builder);
+
+		// Verify that registerDefaults was called
+		then(builder).should().registerDefaults();
+
+		// Verify that the first JSON converter is configured as the JSON converter
+		then(builder).should(times(1)).withJsonConverter(jsonConverter1);
+
+		// Verify that the second JSON converter is configured as a custom converter
+		// (since there should only be one JSON converter registered via withJsonConverter)
+		then(builder).should(times(1)).addCustomConverter(jsonConverter2);
+
+		// Verify that withJsonConverter was only called once (for the first converter)
+		then(builder).should(times(1)).withJsonConverter(any());
+	}
+
+	@Test
+	void customizeWithJsonConverterVerifiesJsonConverterConfiguration() {
+		// Create a single JSON converter
+		JacksonJsonHttpMessageConverter jsonConverter = new JacksonJsonHttpMessageConverter();
+
+		// Verify it supports JSON
+		boolean supportsJson = jsonConverter.getSupportedMediaTypes()
+			.stream()
+			.anyMatch(mediaType -> mediaType.equalsTypeAndSubtype(MediaType.APPLICATION_JSON));
+		assert supportsJson : "Converter should support APPLICATION_JSON";
+
+		// Create the customizer
+		List<HttpMessageConverter<?>> converters = List.of(jsonConverter);
+		DefaultServerHttpMessageConvertersCustomizer customizer = new DefaultServerHttpMessageConvertersCustomizer(null,
+				converters);
+
+		// Mock the ServerBuilder
+		ServerBuilder builder = mock(ServerBuilder.class);
+
+		// Execute the customize method
+		customizer.customize(builder);
+
+		// Verify that registerDefaults was called
+		then(builder).should().registerDefaults();
+
+		// Verify that withJsonConverter was called with the JSON converter
+		then(builder).should().withJsonConverter(jsonConverter);
+
+		// Verify that addCustomConverter was not called for the JSON converter
+		then(builder).should(times(0)).addCustomConverter(any());
+	}
+
+	@Test
+	void customizeWithNonJsonConverterConfiguresAsCustomConverter() {
+		// Create a custom converter that doesn't support JSON
+		HttpMessageConverter<?> customConverter = mock(HttpMessageConverter.class);
+
+		// Create the customizer
+		List<HttpMessageConverter<?>> converters = List.of(customConverter);
+		DefaultServerHttpMessageConvertersCustomizer customizer = new DefaultServerHttpMessageConvertersCustomizer(null,
+				converters);
+
+		// Mock the ServerBuilder
+		ServerBuilder builder = mock(ServerBuilder.class);
+
+		// Execute the customize method
+		customizer.customize(builder);
+
+		// Verify that registerDefaults was called
+		then(builder).should().registerDefaults();
+
+		// Verify that addCustomConverter was called with the custom converter
+		then(builder).should().addCustomConverter(customConverter);
+
+		// Verify that withJsonConverter was not called
+		then(builder).should(times(0)).withJsonConverter(any());
+	}
+
+}