Restrict Kotlin serialization when alternative is available

This commit applies similar changes already contributed to the
`HttpMessageConverter` stack for MVC applications.
Since there was no auto-configuration for Kotlinx JSON Serialization on
the reactive side, this commit adds a relevant `CodecCustomizer` that
will use an available `Json` bean and use it to configure a codec that:

* will only consider `@Serializable`-annotated types if another JSON
  library is available
* will use a broader support with Kotlinx Serialization otherwise

Fixes gh-48070

Author: bclozel

module/spring-boot-http-codec/build.gradle

@@ -32,6 +32,7 @@ dependencies {
 	optional(project(":core:spring-boot-test"))
 	optional(project(":module:spring-boot-jackson"))
 	optional(project(":module:spring-boot-jackson2"))
+	optional(project(":module:spring-boot-kotlinx-serialization-json"))
 	optional("org.springframework:spring-webflux")
 
 	testImplementation(project(":core:spring-boot-test"))

module/spring-boot-http-codec/src/main/java/org/springframework/boot/http/codec/autoconfigure/CodecsAutoConfiguration.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.http.codec.autoconfigure;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
+import kotlinx.serialization.json.Json;
 import org.jspecify.annotations.Nullable;
 import tools.jackson.databind.json.JsonMapper;
 
@@ -35,9 +36,13 @@
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.Ordered;
 import org.springframework.core.annotation.Order;
+import org.springframework.core.io.ResourceLoader;
 import org.springframework.http.codec.CodecConfigurer;
 import org.springframework.http.codec.json.JacksonJsonDecoder;
 import org.springframework.http.codec.json.JacksonJsonEncoder;
+import org.springframework.http.codec.json.KotlinSerializationJsonDecoder;
+import org.springframework.http.codec.json.KotlinSerializationJsonEncoder;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.unit.DataSize;
 import org.springframework.web.reactive.function.client.WebClient;
 
@@ -93,6 +98,29 @@ CodecCustomizer jackson2CodecCustomizer(ObjectMapper objectMapper) {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	@ConditionalOnClass(Json.class)
+	static class KotlinxSerializationJsonCodecConfiguration {
+
+		@Bean
+		@ConditionalOnBean(Json.class)
+		CodecCustomizer kotlinxJsonCodecCustomizer(Json json, ResourceLoader resourceLoader) {
+			ClassLoader classLoader = resourceLoader.getClassLoader();
+			boolean hasAnyJsonSupport = ClassUtils.isPresent("tools.jackson.databind.json.JsonMapper", classLoader)
+					|| ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", classLoader)
+					|| ClassUtils.isPresent("com.google.gson.Gson", classLoader);
+
+			return (configurer) -> {
+				CodecConfigurer.DefaultCodecs defaults = configurer.defaultCodecs();
+				defaults.kotlinSerializationJsonEncoder(hasAnyJsonSupport ? new KotlinSerializationJsonEncoder(json)
+						: new KotlinSerializationJsonEncoder(json, (type) -> true));
+				defaults.kotlinSerializationJsonDecoder(hasAnyJsonSupport ? new KotlinSerializationJsonDecoder(json)
+						: new KotlinSerializationJsonDecoder(json, (type) -> true));
+			};
+		}
+
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	@EnableConfigurationProperties(HttpCodecsProperties.class)
 	static class DefaultCodecsConfiguration {

module/spring-boot-http-codec/src/test/java/org/springframework/boot/http/codec/autoconfigure/CodecsAutoConfigurationTests.java

@@ -17,8 +17,10 @@
 package org.springframework.boot.http.codec.autoconfigure;
 
 import java.util.List;
+import java.util.Map;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
+import kotlinx.serialization.json.Json;
 import org.junit.jupiter.api.Test;
 import tools.jackson.databind.json.JsonMapper;
 
@@ -30,8 +32,13 @@
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.Ordered;
+import org.springframework.core.ResolvableType;
+import org.springframework.http.MediaType;
 import org.springframework.http.codec.CodecConfigurer;
 import org.springframework.http.codec.CodecConfigurer.DefaultCodecs;
+import org.springframework.http.codec.EncoderHttpMessageWriter;
+import org.springframework.http.codec.ServerCodecConfigurer;
+import org.springframework.http.codec.json.KotlinSerializationJsonEncoder;
 import org.springframework.http.codec.support.DefaultClientCodecConfigurer;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -132,6 +139,40 @@ void maxInMemorySizeEnforcedInDefaultCodecs() {
 					1048576));
 	}
 
+	@Test
+	void kotlinSerializationUsesLimitedPredicateWhenOtherJsonConverterIsAvailable() {
+		this.contextRunner.withUserConfiguration(KotlinxJsonConfiguration.class).run((context) -> {
+			KotlinSerializationJsonEncoder encoder = findEncoder(context, KotlinSerializationJsonEncoder.class);
+			assertThat(encoder.canEncode(ResolvableType.forClass(Map.class), MediaType.APPLICATION_JSON)).isFalse();
+		});
+	}
+
+	@Test
+	void kotlinSerializationUsesUnrestrictedPredicateWhenNoOtherJsonConverterIsAvailable() {
+		FilteredClassLoader classLoader = new FilteredClassLoader(JsonMapper.class.getPackage().getName(),
+				ObjectMapper.class.getPackage().getName());
+		this.contextRunner.withClassLoader(classLoader)
+			.withUserConfiguration(KotlinxJsonConfiguration.class)
+			.run((context) -> {
+				KotlinSerializationJsonEncoder encoder = findEncoder(context, KotlinSerializationJsonEncoder.class);
+				assertThat(encoder.canEncode(ResolvableType.forClass(Map.class), MediaType.APPLICATION_JSON)).isTrue();
+			});
+	}
+
+	@SuppressWarnings("unchecked")
+	private <T> T findEncoder(AssertableApplicationContext context, Class<T> encoderClass) {
+		ServerCodecConfigurer configurer = ServerCodecConfigurer.create();
+		context.getBeansOfType(CodecCustomizer.class).values().forEach((codec) -> codec.customize(configurer));
+		return (T) configurer.getWriters()
+			.stream()
+			.filter((writer) -> writer instanceof EncoderHttpMessageWriter<?>)
+			.map((writer) -> (EncoderHttpMessageWriter<?>) writer)
+			.map(EncoderHttpMessageWriter::getEncoder)
+			.filter((encoder) -> encoderClass.isAssignableFrom(encoder.getClass()))
+			.findFirst()
+			.orElseThrow();
+	}
+
 	private DefaultCodecs defaultCodecs(AssertableApplicationContext context) {
 		CodecCustomizer customizer = context.getBean(CodecCustomizer.class);
 		CodecConfigurer configurer = new DefaultClientCodecConfigurer();
@@ -159,6 +200,16 @@ ObjectMapper objectMapper() {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	static class KotlinxJsonConfiguration {
+
+		@Bean
+		Json kotlinxJson() {
+			return Json.Default;
+		}
+
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class CodecCustomizerConfiguration {