Fix Kotlin Serialization converter registration logic

This commit fixes Kotlin Serialization converter
registration logic in RestTemplate,
AbstractMessageBrokerConfiguration and
AllEncompassingFormHttpMessageConverter classes
to be similar to the one in
WebMvcConfigurationSupport.

Closes gh-29008

Author: meloning

spring-messaging/src/main/java/org/springframework/messaging/simp/config/AbstractMessageBrokerConfiguration.java

@@ -461,6 +461,9 @@ public CompositeMessageConverter brokerMessageConverter() {
 		if (registerDefaults) {
 			converters.add(new StringMessageConverter());
 			converters.add(new ByteArrayMessageConverter());
+			if (kotlinSerializationJsonPresent) {
+				converters.add(new KotlinSerializationJsonMessageConverter());
+			}
 			if (jackson2Present) {
 				converters.add(createJacksonConverter());
 			}
@@ -470,9 +473,6 @@ else if (gsonPresent) {
 			else if (jsonbPresent) {
 				converters.add(new JsonbMessageConverter());
 			}
-			else if (kotlinSerializationJsonPresent) {
-				converters.add(new KotlinSerializationJsonMessageConverter());
-			}
 		}
 		return new CompositeMessageConverter(converters);
 	}

spring-messaging/src/test/java/org/springframework/messaging/simp/config/MessageBrokerConfigurationTests.java

@@ -39,6 +39,7 @@
 import org.springframework.messaging.converter.CompositeMessageConverter;
 import org.springframework.messaging.converter.ContentTypeResolver;
 import org.springframework.messaging.converter.DefaultContentTypeResolver;
+import org.springframework.messaging.converter.KotlinSerializationJsonMessageConverter;
 import org.springframework.messaging.converter.MappingJackson2MessageConverter;
 import org.springframework.messaging.converter.MessageConverter;
 import org.springframework.messaging.converter.StringMessageConverter;
@@ -282,12 +283,13 @@ public void configureMessageConvertersDefault() {
 		CompositeMessageConverter compositeConverter = config.brokerMessageConverter();
 
 		List<MessageConverter> converters = compositeConverter.getConverters();
-		assertThat(converters).hasSize(3);
+		assertThat(converters).hasSize(4);
 		assertThat(converters.get(0)).isInstanceOf(StringMessageConverter.class);
 		assertThat(converters.get(1)).isInstanceOf(ByteArrayMessageConverter.class);
-		assertThat(converters.get(2)).isInstanceOf(MappingJackson2MessageConverter.class);
+		assertThat(converters.get(2)).isInstanceOf(KotlinSerializationJsonMessageConverter.class);
+		assertThat(converters.get(3)).isInstanceOf(MappingJackson2MessageConverter.class);
 
-		ContentTypeResolver resolver = ((MappingJackson2MessageConverter) converters.get(2)).getContentTypeResolver();
+		ContentTypeResolver resolver = ((MappingJackson2MessageConverter) converters.get(3)).getContentTypeResolver();
 		assertThat(((DefaultContentTypeResolver) resolver).getDefaultMimeType()).isEqualTo(MimeTypeUtils.APPLICATION_JSON);
 	}
 
@@ -340,11 +342,12 @@ protected boolean configureMessageConverters(List<MessageConverter> messageConve
 		};
 		CompositeMessageConverter compositeConverter = config.brokerMessageConverter();
 
-		assertThat(compositeConverter.getConverters()).hasSize(4);
+		assertThat(compositeConverter.getConverters()).hasSize(5);
 		Iterator<MessageConverter> iterator = compositeConverter.getConverters().iterator();
 		assertThat(iterator.next()).isEqualTo(testConverter);
 		assertThat(iterator.next()).isInstanceOf(StringMessageConverter.class);
 		assertThat(iterator.next()).isInstanceOf(ByteArrayMessageConverter.class);
+		assertThat(iterator.next()).isInstanceOf(KotlinSerializationJsonMessageConverter.class);
 		assertThat(iterator.next()).isInstanceOf(MappingJackson2MessageConverter.class);
 	}
 

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

@@ -87,6 +87,9 @@ public AllEncompassingFormHttpMessageConverter() {
 			}
 		}
 
+		if (kotlinSerializationJsonPresent) {
+			addPartConverter(new KotlinSerializationJsonHttpMessageConverter());
+		}
 		if (jackson2Present) {
 			addPartConverter(new MappingJackson2HttpMessageConverter());
 		}
@@ -96,9 +99,6 @@ else if (gsonPresent) {
 		else if (jsonbPresent) {
 			addPartConverter(new JsonbHttpMessageConverter());
 		}
-		else if (kotlinSerializationJsonPresent) {
-			addPartConverter(new KotlinSerializationJsonHttpMessageConverter());
-		}
 
 		if (jackson2XmlPresent && !shouldIgnoreXml) {
 			addPartConverter(new MappingJackson2XmlHttpMessageConverter());

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

@@ -174,6 +174,9 @@ else if (jaxb2Present) {
 			}
 		}
 
+		if (kotlinSerializationJsonPresent) {
+			this.messageConverters.add(new KotlinSerializationJsonHttpMessageConverter());
+		}
 		if (jackson2Present) {
 			this.messageConverters.add(new MappingJackson2HttpMessageConverter());
 		}
@@ -183,9 +186,6 @@ else if (gsonPresent) {
 		else if (jsonbPresent) {
 			this.messageConverters.add(new JsonbHttpMessageConverter());
 		}
-		else if (kotlinSerializationJsonPresent) {
-			this.messageConverters.add(new KotlinSerializationJsonHttpMessageConverter());
-		}
 
 		if (jackson2SmilePresent) {
 			this.messageConverters.add(new MappingJackson2SmileHttpMessageConverter());

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

@@ -51,6 +51,8 @@
 import org.springframework.http.client.SimpleClientHttpRequestFactory;
 import org.springframework.http.converter.GenericHttpMessageConverter;
 import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverter;
+import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
 import org.springframework.util.StreamUtils;
 import org.springframework.web.util.DefaultUriBuilderFactory;
 
@@ -103,6 +105,16 @@ void setup() {
 		template.setErrorHandler(errorHandler);
 	}
 
+	@Test // gh-29008
+	void defaultMessageConvertersWithKotlinSerialization() {
+		RestTemplate restTemplate = new RestTemplate();
+		List<HttpMessageConverter<?>> httpMessageConverters = restTemplate.getMessageConverters();
+		assertThat(httpMessageConverters).extracting("class").containsOnlyOnce(
+			KotlinSerializationJsonHttpMessageConverter.class,
+			MappingJackson2HttpMessageConverter.class
+		);
+	}
+
 	@Test
 	void constructorPreconditions() {
 		assertThatIllegalArgumentException()