Refine Kotlin serialization converters/codecs conditions

This commit is a follow-up of 34410 to refine the activation conditions
of Kotlin serialization converters/codecs.

Closes gh-34438

spring-web/src/main/java/org/springframework/http/codec/support/BaseDefaultCodecs.java

@@ -411,10 +411,6 @@ class BaseDefaultCodecs implements CodecConfigurer.DefaultCodecs, CodecConfigure
 			addCodec(this.typedReaders, new DecoderHttpMessageReader<>(this.protobufDecoder != null ?
 					(ProtobufDecoder) this.protobufDecoder : new ProtobufDecoder()));
 		}
-		else if (kotlinSerializationProtobufPresent) {
-			addCodec(this.typedReaders, new DecoderHttpMessageReader<>(this.kotlinSerializationProtobufDecoder != null ?
-					(KotlinSerializationProtobufDecoder) this.kotlinSerializationProtobufDecoder : new KotlinSerializationProtobufDecoder()));
-		}
 		addCodec(this.typedReaders, new FormHttpMessageReader());
 		if (this.multipartReader != null) {
 			addCodec(this.typedReaders, this.multipartReader);

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -87,9 +87,6 @@ public class AllEncompassingFormHttpMessageConverter extends FormHttpMessageConv
 			addPartConverter(new Jaxb2RootElementHttpMessageConverter());
 		}
 
-		if (kotlinSerializationJsonPresent) {
-			addPartConverter(new KotlinSerializationJsonHttpMessageConverter());
-		}
 		if (jackson2Present) {
 			addPartConverter(new MappingJackson2HttpMessageConverter());
 		}
@@ -99,6 +96,9 @@ public class AllEncompassingFormHttpMessageConverter extends FormHttpMessageConv
 		else if (jsonbPresent) {
 			addPartConverter(new JsonbHttpMessageConverter());
 		}
+		else if (kotlinSerializationJsonPresent) {
+			addPartConverter(new KotlinSerializationJsonHttpMessageConverter());
+		}
 
 		if (jackson2XmlPresent) {
 			addPartConverter(new MappingJackson2XmlHttpMessageConverter());
@@ -111,15 +111,14 @@ public class AllEncompassingFormHttpMessageConverter extends FormHttpMessageConv
 		if (jackson2CborPresent) {
 			addPartConverter(new MappingJackson2CborHttpMessageConverter());
 		}
+		else if (kotlinSerializationCborPresent) {
+			addPartConverter(new KotlinSerializationCborHttpMessageConverter());
+		}
 
 		if (jackson2YamlPresent) {
 			addPartConverter(new MappingJackson2YamlHttpMessageConverter());
 		}
 
-		if (kotlinSerializationCborPresent) {
-			addPartConverter(new KotlinSerializationCborHttpMessageConverter());
-		}
-
 		if (kotlinSerializationProtobufPresent) {
 			addPartConverter(new KotlinSerializationProtobufHttpMessageConverter());
 		}

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

@@ -419,9 +419,6 @@ final class DefaultRestClientBuilder implements RestClient.Builder {
 			this.messageConverters.add(new ResourceHttpMessageConverter(false));
 			this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
 
-			if (kotlinSerializationJsonPresent) {
-				this.messageConverters.add(new KotlinSerializationJsonHttpMessageConverter());
-			}
 			if (jackson2Present) {
 				this.messageConverters.add(new MappingJackson2HttpMessageConverter());
 			}
@@ -431,6 +428,9 @@ final class DefaultRestClientBuilder implements RestClient.Builder {
 			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/main/java/org/springframework/web/client/RestTemplate.java

@@ -204,9 +204,6 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 			this.messageConverters.add(new KotlinSerializationProtobufHttpMessageConverter());
 		}
 
-		if (kotlinSerializationJsonPresent) {
-			this.messageConverters.add(new KotlinSerializationJsonHttpMessageConverter());
-		}
 		if (jackson2Present) {
 			this.messageConverters.add(new MappingJackson2HttpMessageConverter());
 		}
@@ -216,6 +213,9 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 		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

@@ -108,14 +108,16 @@ class RestTemplateTests {
 		template.setErrorHandler(errorHandler);
 	}
 
-	@Test // gh-29008
+	@Test
-	void defaultMessageConvertersWithKotlinSerialization() {
+	void defaultMessageConvertersWithoutKotlinSerialization() {
 		RestTemplate restTemplate = new RestTemplate();
 		List<HttpMessageConverter<?>> httpMessageConverters = restTemplate.getMessageConverters();
 		assertThat(httpMessageConverters).extracting("class").containsOnlyOnce(
-			KotlinSerializationJsonHttpMessageConverter.class,
 			MappingJackson2HttpMessageConverter.class
 		);
+		assertThat(httpMessageConverters).extracting("class").doesNotContain(
+				KotlinSerializationJsonHttpMessageConverter.class
+		);
 	}
 
 	@Test

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

@@ -891,9 +891,6 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 			messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
 		}
 
-		if (kotlinSerializationCborPresent) {
-			messageConverters.add(new KotlinSerializationCborHttpMessageConverter());
-		}
 		if (kotlinSerializationProtobufPresent) {
 			messageConverters.add(new KotlinSerializationProtobufHttpMessageConverter());
 		}
@@ -929,6 +926,9 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 			}
 			messageConverters.add(new MappingJackson2CborHttpMessageConverter(builder.build()));
 		}
+		else if (kotlinSerializationCborPresent) {
+			messageConverters.add(new KotlinSerializationCborHttpMessageConverter());
+		}
 		if (jackson2YamlPresent) {
 			Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.yaml();
 			if (this.applicationContext != null) {

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

@@ -172,7 +172,7 @@ class WebMvcConfigurationSupportTests {
 		ApplicationContext context = initContext(WebConfig.class);
 		RequestMappingHandlerAdapter adapter = context.getBean(RequestMappingHandlerAdapter.class);
 		List<HttpMessageConverter<?>> converters = adapter.getMessageConverters();
-		assertThat(converters).hasSizeGreaterThanOrEqualTo(14);
+		assertThat(converters).hasSizeGreaterThanOrEqualTo(13);
 		converters.stream()
 				.filter(AbstractJackson2HttpMessageConverter.class::isInstance)
 				.forEach(converter -> {