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 spring-projectsgh-34410
which considers Kotlin Serialization as a Jackson/Gson/Jsonb equivalent
(it is not anymore configured before Jackson).

Closes spring-projectsgh-34433

Author: sdeleuze

spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationBinaryDecoder.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.
@@ -37,6 +37,10 @@
  * Abstract base class for {@link Decoder} implementations that defer to Kotlin
  * {@linkplain BinaryFormat binary serializers}.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * @author Sebastien Deleuze
  * @author Iain Henderson
  * @author Arjen Poutsma

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

@@ -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.
@@ -38,6 +38,10 @@
  * Abstract base class for {@link Encoder} implementations that defer to Kotlin
  * {@linkplain BinaryFormat binary serializers}.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * @author Sebastien Deleuze
  * @author Iain Henderson
  * @author Arjen Poutsma

spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringDecoder.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.
@@ -38,6 +38,10 @@
  * Abstract base class for {@link Decoder} implementations that defer to Kotlin
  * {@linkplain StringFormat string serializers}.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * @author Sebastien Deleuze
  * @author Iain Henderson
  * @author Arjen Poutsma

spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringEncoder.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.
@@ -43,6 +43,10 @@
  * Abstract base class for {@link Encoder} implementations that defer to Kotlin
  * {@linkplain StringFormat string serializers}.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * @author Sebastien Deleuze
  * @author Iain Henderson
  * @author Arjen Poutsma

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

@@ -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.
@@ -19,10 +19,8 @@
 import java.lang.reflect.Method;
 import java.lang.reflect.Type;
 import java.util.Arrays;
-import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
-import java.util.Set;
 
 import kotlin.reflect.KFunction;
 import kotlin.reflect.KType;
@@ -31,8 +29,6 @@
 import kotlinx.serialization.KSerializer;
 import kotlinx.serialization.SerialFormat;
 import kotlinx.serialization.SerializersKt;
-import kotlinx.serialization.descriptors.PolymorphicKind;
-import kotlinx.serialization.descriptors.SerialDescriptor;
 import org.jspecify.annotations.Nullable;
 
 import org.springframework.core.KotlinDetector;
@@ -46,6 +42,10 @@
  * Base class providing support methods for encoding and decoding with Kotlin
  * serialization.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * @author Sebastien Deleuze
  * @author Iain Henderson
  * @author Arjen Poutsma
@@ -143,9 +143,6 @@ private boolean supports(@Nullable MimeType mimeType) {
 						catch (IllegalArgumentException ignored) {
 						}
 						if (serializer != null) {
-							if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
-								return null;
-							}
 							this.kTypeSerializerCache.put(type, serializer);
 						}
 					}
@@ -162,27 +159,9 @@ private boolean supports(@Nullable MimeType mimeType) {
 			catch (IllegalArgumentException ignored) {
 			}
 			if (serializer != null) {
-				if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
-					return null;
-				}
 				this.typeSerializerCache.put(type, serializer);
 			}
 		}
 		return serializer;
 	}
-
-	private static boolean hasPolymorphism(SerialDescriptor descriptor, Set<String> alreadyProcessed) {
-		alreadyProcessed.add(descriptor.getSerialName());
-		if (descriptor.getKind().equals(PolymorphicKind.OPEN.INSTANCE)) {
-			return true;
-		}
-		for (int i = 0 ; i < descriptor.getElementsCount() ; i++) {
-			SerialDescriptor elementDescriptor = descriptor.getElementDescriptor(i);
-			if (!alreadyProcessed.contains(elementDescriptor.getSerialName()) && hasPolymorphism(elementDescriptor, alreadyProcessed)) {
-				return true;
-			}
-		}
-		return false;
-	}
-
 }

spring-web/src/main/java/org/springframework/http/codec/cbor/KotlinSerializationCborDecoder.java

@@ -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.
@@ -24,12 +24,12 @@
 /**
  * Decode a byte stream into CBOR and convert to Objects with
  * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
- *
- * <p>This decoder can be used to bind {@code @Serializable} Kotlin classes,
- * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphic serialization</a>
- * is not supported.
  * It supports {@code application/cbor}.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * <p>Decoding streams is not supported yet, see
  * <a href="https://github.com/Kotlin/kotlinx.serialization/issues/1073">kotlinx.serialization/issues/1073</a>
  * related issue.

spring-web/src/main/java/org/springframework/http/codec/cbor/KotlinSerializationCborEncoder.java

@@ -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.
@@ -24,12 +24,12 @@
 /**
  * Encode from an {@code Object} stream to a byte stream of CBOR objects using
  * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
- *
- * <p>This encoder can be used to bind {@code @Serializable} Kotlin classes,
- * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphic serialization</a>
- * is not supported.
  * It supports {@code application/cbor}.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * @author Iain Henderson
  * @since 6.0
  */

spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonDecoder.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.
@@ -24,13 +24,13 @@
 /**
  * Decode a byte stream into JSON and convert to Object's with
  * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
- *
- * <p>This decoder can be used to bind {@code @Serializable} Kotlin classes,
- * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphic serialization</a>
- * is not supported.
  * It supports {@code application/json} and {@code application/*+json} with
  * various character sets, {@code UTF-8} being the default.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * <p>Decoding streams is not supported yet, see
  * <a href="https://github.com/Kotlin/kotlinx.serialization/issues/1073">kotlinx.serialization/issues/1073</a>
  * related issue.

spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonEncoder.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.
@@ -35,13 +35,13 @@
 /**
  * Encode from an {@code Object} stream to a byte stream of JSON objects using
  * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
- *
- * <p>This encoder can be used to bind {@code @Serializable} Kotlin classes,
- * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphic serialization</a>
- * is not supported.
  * It supports {@code application/json}, {@code application/x-ndjson} and {@code application/*+json} with
  * various character sets, {@code UTF-8} being the default.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * @author Sebastien Deleuze
  * @author Iain Henderson
  * @since 5.3

spring-web/src/main/java/org/springframework/http/codec/protobuf/KotlinSerializationProtobufDecoder.java

@@ -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.
@@ -23,12 +23,12 @@
 /**
  * Decode a byte stream into a protocol Buffer and convert to Objects with
  * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
- *
- * <p>This decoder can be used to bind {@code @Serializable} Kotlin classes,
- * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphic serialization</a>
- * is not supported.
  * It supports {@code application/x-protobuf}, {@code application/octet-stream}, and {@code application/vnd.google.protobuf}.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * <p>Decoding streams is not supported yet, see
  * <a href="https://github.com/Kotlin/kotlinx.serialization/issues/1073">kotlinx.serialization/issues/1073</a>
  * related issue.

spring-web/src/main/java/org/springframework/http/codec/protobuf/KotlinSerializationProtobufEncoder.java

@@ -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.
@@ -23,12 +23,12 @@
 /**
  * Decode a byte stream into a Protocol Buffer and convert to Objects with
  * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
- *
- * <p>This decoder can be used to bind {@code @Serializable} Kotlin classes,
- * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphic serialization</a>
- * is not supported.
  * It supports {@code application/x-protobuf}, {@code application/octet-stream}, and {@code application/vnd.google.protobuf}.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * <p>Decoding streams is not supported yet, see
  * <a href="https://github.com/Kotlin/kotlinx.serialization/issues/1073">kotlinx.serialization/issues/1073</a>
  * related issue.

spring-web/src/main/java/org/springframework/http/converter/AbstractKotlinSerializationHttpMessageConverter.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.
@@ -19,9 +19,7 @@
 import java.io.IOException;
 import java.lang.reflect.Method;
 import java.lang.reflect.Type;
-import java.util.HashSet;
 import java.util.Map;
-import java.util.Set;
 
 import kotlin.reflect.KFunction;
 import kotlin.reflect.KType;
@@ -30,8 +28,6 @@
 import kotlinx.serialization.KSerializer;
 import kotlinx.serialization.SerialFormat;
 import kotlinx.serialization.SerializersKt;
-import kotlinx.serialization.descriptors.PolymorphicKind;
-import kotlinx.serialization.descriptors.SerialDescriptor;
 import org.jspecify.annotations.Nullable;
 
 import org.springframework.core.KotlinDetector;
@@ -48,6 +44,10 @@
  * Abstract base class for {@link HttpMessageConverter} implementations that
  * use Kotlin serialization.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * @author Andreas Ahlenstorf
  * @author Sebastien Deleuze
  * @author Juergen Hoeller
@@ -160,9 +160,6 @@ protected abstract void writeInternal(Object object, KSerializer<Object> seriali
 						catch (IllegalArgumentException ignored) {
 						}
 						if (serializer != null) {
-							if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
-								return null;
-							}
 							this.kTypeSerializerCache.put(type, serializer);
 						}
 					}
@@ -179,29 +176,12 @@ protected abstract void writeInternal(Object object, KSerializer<Object> seriali
 			catch (IllegalArgumentException ignored) {
 			}
 			if (serializer != null) {
-				if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
-					return null;
-				}
 				this.typeSerializerCache.put(type, serializer);
 			}
 		}
 		return serializer;
 	}
 
-	private boolean hasPolymorphism(SerialDescriptor descriptor, Set<String> alreadyProcessed) {
-		alreadyProcessed.add(descriptor.getSerialName());
-		if (descriptor.getKind().equals(PolymorphicKind.OPEN.INSTANCE)) {
-			return true;
-		}
-		for (int i = 0 ; i < descriptor.getElementsCount() ; i++) {
-			SerialDescriptor elementDescriptor = descriptor.getElementDescriptor(i);
-			if (!alreadyProcessed.contains(elementDescriptor.getSerialName()) && hasPolymorphism(elementDescriptor, alreadyProcessed)) {
-				return true;
-			}
-		}
-		return false;
-	}
-
 	@Override
 	protected boolean supportsRepeatableWrites(Object object) {
 		return true;

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

@@ -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.
@@ -31,6 +31,10 @@
  * Abstract base class for {@link HttpMessageConverter} implementations that
  * defer to Kotlin {@linkplain BinaryFormat binary serializers}.
  *
+ * <p>As of Spring Framework 7.0,
+ * <a href="https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#open-polymorphism">open polymorphism</a>
+ * is supported.
+ *
  * @author Andreas Ahlenstorf
  * @author Sebastien Deleuze
  * @author Juergen Hoeller