Revisit Kotlin Serialization integration

This commit removes the "kotlin-serialization" option from the
`spring.http.converters.preferred-json-mapper` and configures the kotlin
serialization http message converter ahead of the preferred JSON
converter.

This effectively makes Kotlin Serialization a converter that is
considered first for JSON support, and then Jackson/Jsonb/Gson is
considered as fallback.

Closes gh-47178

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

@@ -42,7 +42,7 @@ class GsonHttpMessageConvertersConfiguration {
 
 	@Configuration(proxyBeanMethods = false)
 	@ConditionalOnBean(Gson.class)
-	@Conditional(PreferGsonOrOtherJsonLibraryUnavailableCondition.class)
+	@Conditional(PreferGsonOrJacksonAndJsonbUnavailableCondition.class)
 	static class GsonHttpMessageConverterConfiguration {
 
 		@Bean
@@ -55,9 +55,9 @@ class GsonHttpMessageConvertersConfiguration {
 
 	}
 
-	private static class PreferGsonOrOtherJsonLibraryUnavailableCondition extends AnyNestedCondition {
+	private static class PreferGsonOrJacksonAndJsonbUnavailableCondition extends AnyNestedCondition {
 
-		PreferGsonOrOtherJsonLibraryUnavailableCondition() {
+		PreferGsonOrJacksonAndJsonbUnavailableCondition() {
 			super(ConfigurationPhase.REGISTER_BEAN);
 		}
 
@@ -67,16 +67,16 @@ class GsonHttpMessageConvertersConfiguration {
 
 		}
 
-		@Conditional(OtherJsonLibrariesUnavailableCondition.class)
+		@Conditional(JacksonAndJsonbUnavailableCondition.class)
 		static class JacksonJsonbUnavailable {
 
 		}
 
 	}
 
-	private static class OtherJsonLibrariesUnavailableCondition extends NoneNestedConditions {
+	private static class JacksonAndJsonbUnavailableCondition extends NoneNestedConditions {
 
-		OtherJsonLibrariesUnavailableCondition() {
+		JacksonAndJsonbUnavailableCondition() {
 			super(ConfigurationPhase.REGISTER_BEAN);
 		}
 
@@ -91,12 +91,6 @@ class GsonHttpMessageConvertersConfiguration {
 
 		}
 
-		@ConditionalOnProperty(name = HttpMessageConvertersAutoConfiguration.PREFERRED_MAPPER_PROPERTY,
-				havingValue = "kotlin-serialization")
-		static class KotlinSerializationPreferred {
-
-		}
-
 	}
 
 }

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

@@ -23,7 +23,6 @@ import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
-import org.springframework.boot.autoconfigure.condition.NoneNestedConditions;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.Configuration;
@@ -42,7 +41,7 @@ class JsonbHttpMessageConvertersConfiguration {
 
 	@Configuration(proxyBeanMethods = false)
 	@ConditionalOnBean(Jsonb.class)
-	@Conditional(PreferJsonbOrOtherJsonLibrariesMissingCondition.class)
+	@Conditional(PreferJsonbOrMissingJacksonAndGsonCondition.class)
 	static class JsonbHttpMessageConverterConfiguration {
 
 		@Bean
@@ -55,9 +54,9 @@ class JsonbHttpMessageConvertersConfiguration {
 
 	}
 
-	private static class PreferJsonbOrOtherJsonLibrariesMissingCondition extends AnyNestedCondition {
+	private static class PreferJsonbOrMissingJacksonAndGsonCondition extends AnyNestedCondition {
 
-		PreferJsonbOrOtherJsonLibrariesMissingCondition() {
+		PreferJsonbOrMissingJacksonAndGsonCondition() {
 			super(ConfigurationPhase.REGISTER_BEAN);
 		}
 
@@ -67,32 +66,8 @@ class JsonbHttpMessageConvertersConfiguration {
 
 		}
 
-		@Conditional(OtherJsonLibrariesMissingMissingCondition.class)
-		static class OtherJsonLibrariesMissingMissing {
-
-		}
-
-	}
-
-	private static class OtherJsonLibrariesMissingMissingCondition extends NoneNestedConditions {
-
-		OtherJsonLibrariesMissingMissingCondition() {
-			super(ConfigurationPhase.REGISTER_BEAN);
-		}
-
-		@ConditionalOnBean(JacksonJsonHttpMessageConverter.class)
-		static class JacksonAvailable {
-
-		}
-
-		@ConditionalOnBean(GsonHttpMessageConverter.class)
-		static class GsonAvailable {
-
-		}
-
-		@ConditionalOnProperty(name = HttpMessageConvertersAutoConfiguration.PREFERRED_MAPPER_PROPERTY,
-				havingValue = "kotlin-serialization")
-		static class KotlinPreferred {
+		@ConditionalOnMissingBean({ JacksonJsonHttpMessageConverter.class, GsonHttpMessageConverter.class })
+		static class JacksonAndGsonMissing {
 
 		}
 

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

@@ -16,61 +16,33 @@
 
 package org.springframework.boot.http.converter.autoconfigure;
 
+import kotlinx.serialization.Serializable;
 import kotlinx.serialization.json.Json;
 
-import org.springframework.boot.autoconfigure.condition.AnyNestedCondition;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
-import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.context.annotation.Bean;
-import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.Configuration;
-import org.springframework.http.converter.json.GsonHttpMessageConverter;
-import org.springframework.http.converter.json.JacksonJsonHttpMessageConverter;
-import org.springframework.http.converter.json.JsonbHttpMessageConverter;
+import org.springframework.core.annotation.Order;
 import org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverter;
 
 /**
  * Configuration for HTTP message converters that use Kotlin Serialization.
  *
+ * @author Brian Clozel
  * @author Dmitry Sulman
  */
 @Configuration(proxyBeanMethods = false)
-@ConditionalOnClass(Json.class)
+@ConditionalOnClass({ Serializable.class, Json.class })
+@ConditionalOnBean(Json.class)
 class KotlinSerializationHttpMessageConvertersConfiguration {
 
-	@Configuration(proxyBeanMethods = false)
-	@ConditionalOnBean(Json.class)
-	@Conditional(PreferKotlinSerializationOrOtherJsonLibrariesUnavailableCondition.class)
-	static class KotlinSerializationHttpMessageConverterConfiguration {
-
-		@Bean
-		@ConditionalOnMissingBean
-		KotlinSerializationJsonHttpMessageConverter kotlinSerializationJsonHttpMessageConverter(Json json) {
-			return new KotlinSerializationJsonHttpMessageConverter(json);
-		}
-
-	}
-
-	private static class PreferKotlinSerializationOrOtherJsonLibrariesUnavailableCondition extends AnyNestedCondition {
-
-		PreferKotlinSerializationOrOtherJsonLibrariesUnavailableCondition() {
-			super(ConfigurationPhase.REGISTER_BEAN);
-		}
-
-		@ConditionalOnProperty(name = HttpMessageConvertersAutoConfiguration.PREFERRED_MAPPER_PROPERTY,
-				havingValue = "kotlin-serialization")
-		static class KotlinSerializationPreferred {
-
-		}
-
-		@ConditionalOnMissingBean({ JacksonJsonHttpMessageConverter.class, JsonbHttpMessageConverter.class,
-				GsonHttpMessageConverter.class })
-		static class OtherJsonLibrariesUnavailable {
-
-		}
-
+	@Bean
+	@ConditionalOnMissingBean
+	@Order(-10) // configured ahead of JSON mappers
+	KotlinSerializationJsonHttpMessageConverter kotlinSerializationJsonHttpMessageConverter(Json json) {
+		return new KotlinSerializationJsonHttpMessageConverter(json);
 	}
 
 }

module/spring-boot-http-converter/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -19,9 +19,6 @@
         },
         {
           "value": "jsonb"
-        },
-        {
-          "value": "kotlin-serialization"
         }
       ],
       "providers": [

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

@@ -17,6 +17,7 @@
 package org.springframework.boot.http.converter.autoconfigure;
 
 import java.nio.charset.StandardCharsets;
+import java.util.List;
 
 import com.google.gson.Gson;
 import jakarta.json.bind.Jsonb;
@@ -131,7 +132,6 @@ class HttpMessageConvertersAutoConfigurationTests {
 			assertConverterBeanRegisteredWithHttpMessageConverters(context, GsonHttpMessageConverter.class);
 			assertThat(context).doesNotHaveBean(JsonbHttpMessageConverter.class);
 			assertThat(context).doesNotHaveBean(JacksonJsonHttpMessageConverter.class);
-			assertThat(context).doesNotHaveBean(KotlinSerializationJsonHttpMessageConverter.class);
 		});
 	}
 
@@ -163,7 +163,6 @@ class HttpMessageConvertersAutoConfigurationTests {
 			assertConverterBeanRegisteredWithHttpMessageConverters(context, JsonbHttpMessageConverter.class);
 			assertThat(context).doesNotHaveBean(GsonHttpMessageConverter.class);
 			assertThat(context).doesNotHaveBean(JacksonJsonHttpMessageConverter.class);
-			assertThat(context).doesNotHaveBean(KotlinSerializationJsonHttpMessageConverter.class);
 		});
 	}
 
@@ -184,17 +183,15 @@ class HttpMessageConvertersAutoConfigurationTests {
 	}
 
 	@Test
-	void kotlinSerializationCanBePreferred() {
-		allOptionsRunner().withPropertyValues("spring.http.converters.preferred-json-mapper:kotlin-serialization")
-			.run((context) -> {
-				assertConverterBeanExists(context, KotlinSerializationJsonHttpMessageConverter.class,
-						"kotlinSerializationJsonHttpMessageConverter");
-				assertConverterBeanRegisteredWithHttpMessageConverters(context,
-						KotlinSerializationJsonHttpMessageConverter.class);
-				assertThat(context).doesNotHaveBean(GsonHttpMessageConverter.class);
-				assertThat(context).doesNotHaveBean(JsonbHttpMessageConverter.class);
-				assertThat(context).doesNotHaveBean(JacksonJsonHttpMessageConverter.class);
-			});
+	void kotlinSerializationOrderedAheadOfJsonConverter() {
+		allOptionsRunner().run((context) -> {
+			assertConverterBeanExists(context, KotlinSerializationJsonHttpMessageConverter.class,
+					"kotlinSerializationJsonHttpMessageConverter");
+			assertConverterBeanRegisteredWithHttpMessageConverters(context,
+					KotlinSerializationJsonHttpMessageConverter.class);
+			assertConvertersBeanRegisteredWithHttpMessageConverters(context,
+					List.of(KotlinSerializationJsonHttpMessageConverter.class, JacksonJsonHttpMessageConverter.class));
+		});
 	}
 
 	@Test
@@ -240,7 +237,6 @@ class HttpMessageConvertersAutoConfigurationTests {
 			assertConverterBeanRegisteredWithHttpMessageConverters(context, JacksonJsonHttpMessageConverter.class);
 			assertThat(context).doesNotHaveBean(GsonHttpMessageConverter.class);
 			assertThat(context).doesNotHaveBean(JsonbHttpMessageConverter.class);
-			assertThat(context).doesNotHaveBean(KotlinSerializationJsonHttpMessageConverter.class);
 		});
 	}
 
@@ -251,7 +247,6 @@ class HttpMessageConvertersAutoConfigurationTests {
 				assertConverterBeanExists(context, GsonHttpMessageConverter.class, "gsonHttpMessageConverter");
 				assertConverterBeanRegisteredWithHttpMessageConverters(context, GsonHttpMessageConverter.class);
 				assertThat(context).doesNotHaveBean(JsonbHttpMessageConverter.class);
-				assertThat(context).doesNotHaveBean(KotlinSerializationJsonHttpMessageConverter.class);
 			});
 	}
 
@@ -327,6 +322,15 @@ class HttpMessageConvertersAutoConfigurationTests {
 		assertThat(converters.getConverters()).contains(converter);
 	}
 
+	private void assertConvertersBeanRegisteredWithHttpMessageConverters(AssertableApplicationContext context,
+			List<Class<? extends HttpMessageConverter<?>>> types) {
+
+		List<? extends HttpMessageConverter<?>> converterInstances = types.stream().map(context::getBean).toList();
+
+		HttpMessageConverters converters = context.getBean(HttpMessageConverters.class);
+		assertThat(converters.getConverters()).containsSubsequence(converterInstances);
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class JacksonJsonMapperConfig {