Support open polymorphism with Kotlin Serialization

This commit introduces open polymorphism support with Kotlin
Serialization in HTTP converters and codecs as a follow-up of gh-34410
which considers Kotlin Serialization as a Jackson/Gson/Jsonb equivalent
(it is not anymore configured before Jackson).

Closes gh-34433

spring-web/src/test/kotlin/org/springframework/http/codec/cbor/KotlinSerializationCborDecoderTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 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.
@@ -42,7 +42,6 @@ import org.springframework.http.MediaType
 @ExperimentalSerializationApi
 class KotlinSerializationCborDecoderTests : AbstractDecoderTests<KotlinSerializationCborDecoder>(KotlinSerializationCborDecoder()) {
 
-	@Suppress("UsePropertyAccessSyntax", "DEPRECATION")
 	@Test
 	override fun canDecode() {
 		assertThat(decoder.canDecode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_CBOR)).isTrue()
@@ -51,11 +50,13 @@ class KotlinSerializationCborDecoderTests : AbstractDecoderTests<KotlinSerializa
 		assertThat(decoder.canDecode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_XML)).isFalse()
 
 		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Int::class.java), MediaType.APPLICATION_CBOR)).isTrue()
-		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), MediaType.APPLICATION_CBOR)).isTrue()
+		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, OrderedImpl::class.java), MediaType.APPLICATION_CBOR)).isFalse()
 		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Pojo::class.java), MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_PDF)).isFalse()
-		assertThat(decoder.canDecode(ResolvableType.forClass(Ordered::class.java), MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(decoder.canDecode(ResolvableType.forClass(Ordered::class.java), MediaType.APPLICATION_CBOR)).isTrue()
+		assertThat(decoder.canDecode(ResolvableType.forClass(OrderedImpl::class.java), MediaType.APPLICATION_CBOR)).isFalse()
 	}
 
 	@Test
@@ -92,6 +93,14 @@ class KotlinSerializationCborDecoderTests : AbstractDecoderTests<KotlinSerializa
 		}
 	}
 
+
+	@Serializable
+	data class Pojo(val foo: String, val bar: String, val pojo: Pojo? = null)
+
+	class OrderedImpl : Ordered {
+		override fun getOrder(): Int {
+			return 0
+		}
+	}
+
 }
-@Serializable
-data class Pojo(val foo: String, val bar: String, val pojo: Pojo? = null)

spring-web/src/test/kotlin/org/springframework/http/codec/cbor/KotlinSerializationCborEncoderTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 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.
@@ -50,10 +50,9 @@ class KotlinSerializationCborEncoderTests : AbstractEncoderTests<KotlinSerializa
 		assertThat(encoder.canEncode(pojoType, null)).isTrue()
 
 		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Int::class.java), MediaType.APPLICATION_CBOR)).isTrue()
-		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Pojo::class.java), MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_CBOR)).isTrue()
-		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_PDF)).isFalse()
 	}
 
 	@Test
@@ -93,11 +92,18 @@ class KotlinSerializationCborEncoderTests : AbstractEncoderTests<KotlinSerializa
 		assertThat(encoder.canEncode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_XML)).isFalse()
 		val sseType = ResolvableType.forClass(ServerSentEvent::class.java)
 		assertThat(encoder.canEncode(sseType, MediaType.APPLICATION_CBOR)).isFalse()
-		assertThat(encoder.canEncode(ResolvableType.forClass(Ordered::class.java), MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, OrderedImpl::class.java), MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_PDF)).isFalse()
 	}
 
 
 	@Serializable
 	data class Pojo(val foo: String, val bar: String, val pojo: Pojo? = null)
 
+	class OrderedImpl : Ordered {
+		override fun getOrder(): Int {
+			return 0
+		}
+	}
+
 }

spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonDecoderTests.kt

@@ -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.
@@ -17,6 +17,8 @@
 package org.springframework.http.codec.json
 
 import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.modules.SerializersModule
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Test
 import org.springframework.core.MethodParameter
@@ -24,11 +26,11 @@ import org.springframework.core.Ordered
 import org.springframework.core.ResolvableType
 import org.springframework.core.codec.DecodingException
 import org.springframework.core.io.buffer.DataBuffer
-import org.springframework.core.io.buffer.DataBufferUtils
 import org.springframework.core.testfixture.codec.AbstractDecoderTests
 import org.springframework.http.MediaType
 import reactor.core.publisher.Flux
 import reactor.core.publisher.Mono
+import reactor.test.StepVerifier
 import reactor.test.StepVerifier.FirstStep
 import java.math.BigDecimal
 import java.nio.charset.Charset
@@ -58,11 +60,13 @@ class KotlinSerializationJsonDecoderTests : AbstractDecoderTests<KotlinSerializa
 				MediaType("application", "json", StandardCharsets.ISO_8859_1))).isTrue()
 
 		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Int::class.java), MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, OrderedImpl::class.java), MediaType.APPLICATION_JSON)).isFalse()
 		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Pojo::class.java), MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_PDF)).isFalse()
-		assertThat(decoder.canDecode(ResolvableType.forClass(Ordered::class.java), MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(decoder.canDecode(ResolvableType.forClass(Ordered::class.java), MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(decoder.canDecode(ResolvableType.forClass(OrderedImpl::class.java), MediaType.APPLICATION_JSON)).isFalse()
 		assertThat(decoder.canDecode(ResolvableType.NONE, MediaType.APPLICATION_JSON)).isFalse()
 		assertThat(decoder.canDecode(ResolvableType.forClass(BigDecimal::class.java), MediaType.APPLICATION_JSON)).isFalse()
 
@@ -84,6 +88,31 @@ class KotlinSerializationJsonDecoderTests : AbstractDecoderTests<KotlinSerializa
 		}, null, null)
 	}
 
+	@Test
+	@Suppress("UNCHECKED_CAST")
+	fun polymorphicDecode() {
+		val json = Json {
+			serializersModule = SerializersModule {
+				polymorphic(ISimpleSerializableBean::class, SimpleSerializableBean::class, SimpleSerializableBean.serializer())
+			}
+		}
+		val customDecoder = KotlinSerializationJsonDecoder(json)
+		val input = Flux.concat(
+			stringBuffer("{\"type\":\"org.springframework.http.codec.json.KotlinSerializationJsonDecoderTests.SimpleSerializableBean\",\"name\":\"foo\"}\n"),
+			stringBuffer("{\"type\":\"org.springframework.http.codec.json.KotlinSerializationJsonDecoderTests.SimpleSerializableBean\",\"name\":\"bar\"}\n"),
+			stringBuffer("{\"type\":\"org.springframework.http.codec.json.KotlinSerializationJsonDecoderTests.SimpleSerializableBean\",\"name\":\"baz\"}\n")
+		)
+
+		val resolvableType = ResolvableType.forClass(ISimpleSerializableBean::class.java)
+		val result: Flux<ISimpleSerializableBean> = customDecoder.decode(input, resolvableType, null, null) as Flux<ISimpleSerializableBean>
+		val step: FirstStep<ISimpleSerializableBean> = StepVerifier.create(result)
+
+		step.expectNext(SimpleSerializableBean("foo"))
+			.expectNext(SimpleSerializableBean("bar"))
+			.expectNext(SimpleSerializableBean("baz"))
+			.verifyComplete()
+	}
+
 	@Test
 	fun decodeWithUnexpectedFormat() {
 		val input = Flux.concat(
@@ -188,4 +217,17 @@ class KotlinSerializationJsonDecoderTests : AbstractDecoderTests<KotlinSerializa
 
 	fun handleMapWithNullable(map: Map<String, String?>) = map
 
+	interface ISimpleSerializableBean {
+		val name: String
+	}
+
+	@Serializable
+	data class SimpleSerializableBean(override val name: String): ISimpleSerializableBean
+
+	class OrderedImpl : Ordered {
+		override fun getOrder(): Int {
+			return 0
+		}
+	}
+
 }

spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt

@@ -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.
@@ -17,11 +17,14 @@
 package org.springframework.http.codec.json
 
 import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.modules.SerializersModule
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Test
 import org.springframework.core.MethodParameter
 import org.springframework.core.Ordered
 import org.springframework.core.ResolvableType
+import org.springframework.core.io.buffer.DataBuffer
 import org.springframework.core.testfixture.codec.AbstractEncoderTests
 import org.springframework.http.MediaType
 import org.springframework.http.codec.ServerSentEvent
@@ -53,11 +56,9 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 				MediaType("application", "json", StandardCharsets.US_ASCII))).isTrue()
 
 		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Int::class.java), MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Pojo::class.java), MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_PDF)).isFalse()
-		assertThat(encoder.canEncode(ResolvableType.NONE, MediaType.APPLICATION_JSON)).isFalse()
 
 		assertThat(encoder.canEncode(pojoType, MediaType.APPLICATION_NDJSON)).isTrue()
 	}
@@ -116,6 +117,28 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 		}
 	}
 
+	@Test
+	fun encodePolymorphicStream() {
+		val json = Json {
+			serializersModule = SerializersModule {
+				polymorphic(ISimpleSerializableBean::class, SimpleSerializableBean::class, SimpleSerializableBean.serializer())
+			}
+		}
+		val customEncoder = KotlinSerializationJsonEncoder(json)
+		val input = Flux<ISimpleSerializableBean>.just(
+			SimpleSerializableBean("foo"),
+			SimpleSerializableBean("bar"),
+			SimpleSerializableBean("baz")
+		)
+		val result = customEncoder.encode(input, this.bufferFactory, ResolvableType.forClass(ISimpleSerializableBean::class.java), MediaType.APPLICATION_NDJSON, null)
+		val step = StepVerifier.create<DataBuffer>(result)
+		step
+			.consumeNextWith { expectString("{\"type\":\"org.springframework.http.codec.json.KotlinSerializationJsonEncoderTests.SimpleSerializableBean\",\"name\":\"foo\"}\n").accept(it) }
+			.consumeNextWith { expectString("{\"type\":\"org.springframework.http.codec.json.KotlinSerializationJsonEncoderTests.SimpleSerializableBean\",\"name\":\"bar\"}\n").accept(it) }
+			.consumeNextWith { expectString("{\"type\":\"org.springframework.http.codec.json.KotlinSerializationJsonEncoderTests.SimpleSerializableBean\",\"name\":\"baz\"}\n").accept(it) }
+			.verifyComplete()
+	}
+
 	@Test
 	fun encodeMono() {
 		val input = Mono.just(Pojo("foo", "bar"))
@@ -141,8 +164,10 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 		assertThat(encoder.canEncode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_XML)).isFalse()
 		val sseType = ResolvableType.forClass(ServerSentEvent::class.java)
 		assertThat(encoder.canEncode(sseType, MediaType.APPLICATION_JSON)).isFalse()
-		assertThat(encoder.canEncode(ResolvableType.forClass(Ordered::class.java), MediaType.APPLICATION_JSON)).isFalse()
 		assertThat(encoder.canEncode(ResolvableType.forClass(BigDecimal::class.java), null)).isFalse()
+		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, OrderedImpl::class.java), MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), MediaType.APPLICATION_PDF)).isFalse()
+		assertThat(encoder.canEncode(ResolvableType.NONE, MediaType.APPLICATION_JSON)).isFalse()
 	}
 
 	@Test
@@ -165,4 +190,17 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 	val value: Int
 		get() = 42
 
+	interface ISimpleSerializableBean {
+		val name: String
+	}
+
+	@Serializable
+	data class SimpleSerializableBean(override val name: String): ISimpleSerializableBean
+
+	class OrderedImpl : Ordered {
+		override fun getOrder(): Int {
+			return 0
+		}
+	}
+
 }

spring-web/src/test/kotlin/org/springframework/http/codec/protobuf/KotlinSerializationProtobufDecoderTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 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.
@@ -48,10 +48,12 @@ class KotlinSerializationProtobufDecoderTests : AbstractDecoderTests<KotlinSeria
 			assertThat(decoder.canDecode(ResolvableType.forClass(Pojo::class.java),mimeType)).isTrue()
 
 			assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Int::class.java), mimeType)).isTrue()
-			assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), mimeType)).isFalse()
+			assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), mimeType)).isTrue()
+			assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, OrderedImpl::class.java), mimeType)).isFalse()
 			assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(List::class.java, Pojo::class.java), mimeType)).isTrue()
 			assertThat(decoder.canDecode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), mimeType)).isTrue()
-			assertThat(decoder.canDecode(ResolvableType.forClass(Ordered::class.java), mimeType)).isFalse()
+			assertThat(decoder.canDecode(ResolvableType.forClass(Ordered::class.java), mimeType)).isTrue()
+			assertThat(decoder.canDecode(ResolvableType.forClass(OrderedImpl::class.java), mimeType)).isFalse()
 		}
 		assertThat(decoder.canDecode(ResolvableType.forClass(Pojo::class.java), null)).isTrue()
 		assertThat(decoder.canDecode(ResolvableType.forClass(String::class.java), null)).isFalse()
@@ -111,6 +113,14 @@ class KotlinSerializationProtobufDecoderTests : AbstractDecoderTests<KotlinSeria
 		}
 	}
 
+
+	@Serializable
+	data class Pojo(val foo: String, val bar: String, val pojo: Pojo? = null)
+
+	class OrderedImpl : Ordered {
+		override fun getOrder(): Int {
+			return 0
+		}
+	}
+
 }
-@Serializable
-data class Pojo(val foo: String, val bar: String, val pojo: Pojo? = null)

spring-web/src/test/kotlin/org/springframework/http/codec/protobuf/KotlinSerializationProtobufEncoderTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 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.
@@ -39,7 +39,6 @@ import reactor.test.StepVerifier.FirstStep
  * @author Sebastien Deleuze
  * @author Iain Henderson
  */
-@Suppress("UsePropertyAccessSyntax")
 @ExperimentalSerializationApi
 class KotlinSerializationProtobufEncoderTests : AbstractEncoderTests<KotlinSerializationProtobufEncoder>(KotlinSerializationProtobufEncoder()) {
 
@@ -59,7 +58,7 @@ class KotlinSerializationProtobufEncoderTests : AbstractEncoderTests<KotlinSeria
 			assertThat(encoder.canEncode(pojoType, mediaType)).isTrue()
 
 			assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Int::class.java), mimeType)).isTrue()
-			assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), mimeType)).isFalse()
+			assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Ordered::class.java), mimeType)).isTrue()
 			assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(List::class.java, Pojo::class.java), mimeType)).isTrue()
 			assertThat(encoder.canEncode(ResolvableType.forClassWithGenerics(ArrayList::class.java, Int::class.java), mimeType)).isTrue()
 		}
@@ -104,7 +103,7 @@ class KotlinSerializationProtobufEncoderTests : AbstractEncoderTests<KotlinSeria
 		val sseType = ResolvableType.forClass(ServerSentEvent::class.java)
 		for (mediaType in mediaTypes) {
 			assertThat(encoder.canEncode(sseType, mediaType)).isFalse()
-			assertThat(encoder.canEncode(ResolvableType.forClass(Ordered::class.java), mediaType)).isFalse()
+			assertThat(encoder.canEncode(ResolvableType.forClass(OrderedImpl::class.java), mediaType)).isFalse()
 		}
 	}
 
@@ -112,4 +111,10 @@ class KotlinSerializationProtobufEncoderTests : AbstractEncoderTests<KotlinSeria
 	@Serializable
 	data class Pojo(val foo: String, val bar: String, val pojo: Pojo? = null)
 
+	class OrderedImpl : Ordered {
+		override fun getOrder(): Int {
+			return 0
+		}
+	}
+
 }

spring-web/src/test/kotlin/org/springframework/http/converter/cbor/KotlinSerializationCborHttpMessageConverterTests.kt

@@ -43,7 +43,6 @@ import org.springframework.web.testfixture.http.MockHttpOutputMessage
  * @author Sebastien Deleuze
  * @author Iain Henderson
  */
-@Suppress("UsePropertyAccessSyntax")
 @ExperimentalSerializationApi
 class KotlinSerializationCborHttpMessageConverterTests {
 
@@ -66,19 +65,21 @@ class KotlinSerializationCborHttpMessageConverterTests {
 		assertThat(converter.canRead(String::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canRead(NotSerializableBean::class.java, MediaType.APPLICATION_CBOR)).isFalse()
 
-		assertThat(converter.canRead(Map::class.java, MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(converter.canRead(Map::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canRead(resolvableTypeOf<Map<String, SerializableBean>>(), MediaType.APPLICATION_CBOR)).isTrue()
-		assertThat(converter.canRead(List::class.java, MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(converter.canRead(List::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canRead(resolvableTypeOf<List<SerializableBean>>(), MediaType.APPLICATION_CBOR)).isTrue()
-		assertThat(converter.canRead(Set::class.java, MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(converter.canRead(Set::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canRead(resolvableTypeOf<Set<SerializableBean>>(), MediaType.APPLICATION_CBOR)).isTrue()
 
 		assertThat(converter.canRead(resolvableTypeOf<List<Int>>(), MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canRead(resolvableTypeOf<ArrayList<Int>>(), MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canRead(resolvableTypeOf<List<Int>>(), MediaType.APPLICATION_JSON)).isFalse()
 
-		assertThat(converter.canRead(resolvableTypeOf<Ordered>(), MediaType.APPLICATION_CBOR)).isFalse()
-		assertThat(converter.canRead(resolvableTypeOf<List<Ordered>>(), MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(converter.canRead(resolvableTypeOf<Ordered>(), MediaType.APPLICATION_CBOR)).isTrue()
+		assertThat(converter.canRead(resolvableTypeOf<List<Ordered>>(), MediaType.APPLICATION_CBOR)).isTrue()
+		assertThat(converter.canRead(resolvableTypeOf<OrderedImpl>(), MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(converter.canRead(resolvableTypeOf<List<OrderedImpl>>(), MediaType.APPLICATION_CBOR)).isFalse()
 	}
 
 	@Test
@@ -88,18 +89,19 @@ class KotlinSerializationCborHttpMessageConverterTests {
 		assertThat(converter.canWrite(String::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canWrite(NotSerializableBean::class.java, MediaType.APPLICATION_CBOR)).isFalse()
 
-		assertThat(converter.canWrite(Map::class.java, MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(converter.canWrite(Map::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canWrite(resolvableTypeOf<Map<String, SerializableBean>>(), Map::class.java, MediaType.APPLICATION_CBOR)).isTrue()
-		assertThat(converter.canWrite(List::class.java, MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(converter.canWrite(List::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canWrite(resolvableTypeOf<List<SerializableBean>>(), List::class.java, MediaType.APPLICATION_CBOR)).isTrue()
-		assertThat(converter.canWrite(Set::class.java, MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(converter.canWrite(Set::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canWrite(resolvableTypeOf<Set<SerializableBean>>(), Set::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 
 		assertThat(converter.canWrite(resolvableTypeOf<List<Int>>(), List::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canWrite(resolvableTypeOf<ArrayList<Int>>(), List::class.java, MediaType.APPLICATION_CBOR)).isTrue()
 		assertThat(converter.canWrite(resolvableTypeOf<List<Int>>(), List::class.java, MediaType.APPLICATION_JSON)).isFalse()
 
-		assertThat(converter.canWrite(resolvableTypeOf<Ordered>(), Ordered::class.java, MediaType.APPLICATION_CBOR)).isFalse()
+		assertThat(converter.canWrite(resolvableTypeOf<Ordered>(), Ordered::class.java, MediaType.APPLICATION_CBOR)).isTrue()
+		assertThat(converter.canWrite(resolvableTypeOf<OrderedImpl>(), Ordered::class.java, MediaType.APPLICATION_CBOR)).isFalse()
 	}
 
 	@Test
@@ -228,4 +230,10 @@ class KotlinSerializationCborHttpMessageConverterTests {
 		return ResolvableType.forType((superType as ParameterizedType).actualTypeArguments.first()!!)
 	}
 
+	class OrderedImpl : Ordered {
+		override fun getOrder(): Int {
+			return 0
+		}
+	}
+
 }

spring-web/src/test/kotlin/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverterTests.kt

@@ -17,6 +17,8 @@
 package org.springframework.http.converter.json
 
 import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.modules.SerializersModule
 import org.assertj.core.api.Assertions.assertThat
 import org.assertj.core.api.Assertions.assertThatExceptionOfType
 import org.junit.jupiter.api.Test
@@ -53,19 +55,21 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		assertThat(converter.canRead(String::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canRead(NotSerializableBean::class.java, MediaType.APPLICATION_JSON)).isFalse()
 
-		assertThat(converter.canRead(Map::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canRead(Map::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canRead(resolvableTypeOf<Map<String, SerializableBean>>(), MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canRead(List::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canRead(List::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canRead(resolvableTypeOf<List<SerializableBean>>(), MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canRead(Set::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canRead(Set::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canRead(resolvableTypeOf<Set<SerializableBean>>(), MediaType.APPLICATION_JSON)).isTrue()
 
 		assertThat(converter.canRead(resolvableTypeOf<List<Int>>(), MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canRead(resolvableTypeOf<ArrayList<Int>>(), MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canRead(resolvableTypeOf<List<Int>>(), MediaType.APPLICATION_PDF)).isFalse()
 
-		assertThat(converter.canRead(resolvableTypeOf<Ordered>(), MediaType.APPLICATION_JSON)).isFalse()
-		assertThat(converter.canRead(resolvableTypeOf<List<Ordered>>(), MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canRead(resolvableTypeOf<Ordered>(), MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canRead(resolvableTypeOf<List<Ordered>>(), MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canRead(resolvableTypeOf<OrderedImpl>(), MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canRead(resolvableTypeOf<List<OrderedImpl>>(), MediaType.APPLICATION_JSON)).isFalse()
 
 		assertThat(converter.canRead(ResolvableType.forType(ResolvableType.NONE.type), MediaType.APPLICATION_JSON)).isFalse()
 
@@ -79,18 +83,19 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		assertThat(converter.canWrite(String::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canWrite(NotSerializableBean::class.java, MediaType.APPLICATION_JSON)).isFalse()
 
-		assertThat(converter.canWrite(Map::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canWrite(Map::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canWrite(resolvableTypeOf<Map<String, SerializableBean>>(), Map::class.java, MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canWrite(List::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canWrite(List::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canWrite(resolvableTypeOf<List<SerializableBean>>(), List::class.java, MediaType.APPLICATION_JSON)).isTrue()
-		assertThat(converter.canWrite(Set::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canWrite(Set::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canWrite(resolvableTypeOf<Set<SerializableBean>>(), Set::class.java, MediaType.APPLICATION_JSON)).isTrue()
 
 		assertThat(converter.canWrite(resolvableTypeOf<List<Int>>(), List::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canWrite(resolvableTypeOf<ArrayList<Int>>(), List::class.java, MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(converter.canWrite(resolvableTypeOf<List<Int>>(), List::class.java, MediaType.APPLICATION_PDF)).isFalse()
 
-		assertThat(converter.canWrite(resolvableTypeOf<Ordered>(), Ordered::class.java, MediaType.APPLICATION_JSON)).isFalse()
+		assertThat(converter.canWrite(resolvableTypeOf<Ordered>(), Ordered::class.java, MediaType.APPLICATION_JSON)).isTrue()
+		assertThat(converter.canWrite(resolvableTypeOf<OrderedImpl>(), OrderedImpl::class.java, MediaType.APPLICATION_JSON)).isFalse()
 
 		assertThat(converter.canWrite(ResolvableType.NONE, SerializableBean::class.java, MediaType.APPLICATION_JSON)).isFalse()
 
@@ -246,6 +251,26 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		assertThat(result).containsExactlyEntriesOf(mapOf("value" to null))
 	}
 
+	@Test
+	@Suppress("UNCHECKED_CAST")
+	fun readWithPolymorphism() {
+		val json = Json {
+			serializersModule = SerializersModule {
+				polymorphic(ISimpleSerializableBean::class, SimpleSerializableBean::class, SimpleSerializableBean.serializer())
+			}
+		}
+		val customConverter = KotlinSerializationJsonHttpMessageConverter(json)
+		val body = """[{"type":"org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverterTests.SimpleSerializableBean","name":"foo"},""" +
+				"""{"type":"org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverterTests.SimpleSerializableBean","name":"bar"}]"""
+
+		val inputMessage = MockHttpInputMessage(body.toByteArray(StandardCharsets.UTF_8))
+		inputMessage.headers.contentType = MediaType.APPLICATION_JSON
+		val resolvableType = ResolvableType.forClassWithGenerics(List::class.java, ISimpleSerializableBean::class.java)
+		val result = customConverter.read(resolvableType, inputMessage, null) as List<ISimpleSerializableBean>
+
+		assertThat(result).containsExactly(SimpleSerializableBean("foo"), SimpleSerializableBean("bar"))
+	}
+
 	@Test
 	fun writeObject() {
 		val outputMessage = MockHttpOutputMessage()
@@ -398,6 +423,27 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		assertThat(result).isEqualTo("42")
 	}
 
+	@Test
+	fun writeWithPolymorphism() {
+		val json = Json {
+			serializersModule = SerializersModule {
+				polymorphic(ISimpleSerializableBean::class, SimpleSerializableBean::class, SimpleSerializableBean.serializer())
+			}
+		}
+		val customConverter = KotlinSerializationJsonHttpMessageConverter(json)
+
+		val outputMessage = MockHttpOutputMessage()
+		val value = listOf(SimpleSerializableBean("foo"), SimpleSerializableBean("bar"))
+		customConverter.write(value, ResolvableType.forClassWithGenerics(List::class.java, ISimpleSerializableBean::class.java), null, outputMessage, null)
+
+		val result = outputMessage.getBodyAsString(StandardCharsets.UTF_8)
+
+		assertThat(outputMessage.headers.containsHeaderValue("Content-Type", "application/json")).isTrue()
+		assertThat(result).isEqualTo(
+			"""[{"type":"org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverterTests.SimpleSerializableBean","name":"foo"},""" +
+					"""{"type":"org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverterTests.SimpleSerializableBean","name":"bar"}]""")
+	}
+
 
 	@Serializable
 	@Suppress("ArrayInDataClass")
@@ -426,4 +472,17 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 	val value: Int
 		get() = 42
 
+	interface ISimpleSerializableBean {
+		val name: String
+	}
+
+	@Serializable
+	data class SimpleSerializableBean(override val name: String): ISimpleSerializableBean
+
+	class OrderedImpl : Ordered {
+		override fun getOrder(): Int {
+			return 0
+		}
+	}
+
 }

spring-web/src/test/kotlin/org/springframework/http/converter/protobuf/KotlinSerializationProtobufHttpMessageConverterTests.kt

@@ -41,7 +41,6 @@ import kotlin.reflect.typeOf
  * @author Sebastien Deleuze
  * @author Iain Henderson
  */
-@Suppress("UsePropertyAccessSyntax")
 @ExperimentalSerializationApi
 class KotlinSerializationProtobufHttpMessageConverterTests {
 
@@ -68,18 +67,20 @@ class KotlinSerializationProtobufHttpMessageConverterTests {
 			assertThat(converter.canRead(String::class.java, mimeType)).isTrue()
 			assertThat(converter.canRead(NotSerializableBean::class.java, mimeType)).isFalse()
 
-			assertThat(converter.canRead(Map::class.java, mimeType)).isFalse()
+			assertThat(converter.canRead(Map::class.java, mimeType)).isTrue()
 			assertThat(converter.canRead(resolvableTypeOf<Map<String, SerializableBean>>(), mimeType)).isTrue()
-			assertThat(converter.canRead(List::class.java, mimeType)).isFalse()
+			assertThat(converter.canRead(List::class.java, mimeType)).isTrue()
 			assertThat(converter.canRead(resolvableTypeOf<List<SerializableBean>>(), mimeType)).isTrue()
-			assertThat(converter.canRead(Set::class.java, mimeType)).isFalse()
+			assertThat(converter.canRead(Set::class.java, mimeType)).isTrue()
 			assertThat(converter.canRead(resolvableTypeOf<Set<SerializableBean>>(), mimeType)).isTrue()
 
 			assertThat(converter.canRead(resolvableTypeOf<List<Int>>(), mimeType)).isTrue()
 			assertThat(converter.canRead(resolvableTypeOf<ArrayList<Int>>(),mimeType)).isTrue()
 
-			assertThat(converter.canRead(resolvableTypeOf<Ordered>(), mimeType)).isFalse()
-			assertThat(converter.canRead(resolvableTypeOf<List<Ordered>>(), mimeType)).isFalse()
+			assertThat(converter.canRead(resolvableTypeOf<Ordered>(), mimeType)).isTrue()
+			assertThat(converter.canRead(resolvableTypeOf<List<Ordered>>(), mimeType)).isTrue()
+			assertThat(converter.canRead(resolvableTypeOf<OrderedImpl>(), mimeType)).isFalse()
+			assertThat(converter.canRead(resolvableTypeOf<List<OrderedImpl>>(), mimeType)).isFalse()
 		}
 		assertThat(converter.canRead(SerializableBean::class.java, MediaType.APPLICATION_JSON)).isFalse()
 		assertThat(converter.canRead(resolvableTypeOf<List<Int>>(), MediaType.APPLICATION_JSON)).isFalse()
@@ -92,17 +93,18 @@ class KotlinSerializationProtobufHttpMessageConverterTests {
 			assertThat(converter.canWrite(String::class.java, mimeType)).isTrue()
 			assertThat(converter.canWrite(NotSerializableBean::class.java, mimeType)).isFalse()
 
-			assertThat(converter.canWrite(Map::class.java, mimeType)).isFalse()
+			assertThat(converter.canWrite(Map::class.java, mimeType)).isTrue()
 			assertThat(converter.canWrite(resolvableTypeOf<Map<String, SerializableBean>>(), Map::class.java, mimeType)).isTrue()
-			assertThat(converter.canWrite(List::class.java, mimeType)).isFalse()
+			assertThat(converter.canWrite(List::class.java, mimeType)).isTrue()
 			assertThat(converter.canWrite(resolvableTypeOf<List<SerializableBean>>(), List::class.java, mimeType)).isTrue()
-			assertThat(converter.canWrite(Set::class.java, mimeType)).isFalse()
+			assertThat(converter.canWrite(Set::class.java, mimeType)).isTrue()
 			assertThat(converter.canWrite(resolvableTypeOf<Set<SerializableBean>>(), Set::class.java, mimeType)).isTrue()
 
 			assertThat(converter.canWrite(resolvableTypeOf<List<Int>>(), List::class.java, mimeType)).isTrue()
 			assertThat(converter.canWrite(resolvableTypeOf<ArrayList<Int>>(), List::class.java, mimeType)).isTrue()
 
-			assertThat(converter.canWrite(resolvableTypeOf<Ordered>(), Ordered::class.java, mimeType)).isFalse()
+			assertThat(converter.canWrite(resolvableTypeOf<Ordered>(), Ordered::class.java, mimeType)).isTrue()
+			assertThat(converter.canWrite(resolvableTypeOf<Ordered>(), OrderedImpl::class.java, mimeType)).isTrue()
 		}
 
 		assertThat(converter.canWrite(SerializableBean::class.java, MediaType.APPLICATION_JSON)).isFalse()
@@ -243,4 +245,10 @@ class KotlinSerializationProtobufHttpMessageConverterTests {
 		return ResolvableType.forType((superType as ParameterizedType).actualTypeArguments.first()!!)
 	}
 
+	class OrderedImpl : Ordered {
+		override fun getOrder(): Int {
+			return 0
+		}
+	}
+
 }