From 4ee0a8ee3321df8055dce16eb645aca8bf6f1ee5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Deleuze?=
 <sdeleuze@users.noreply.github.com>
Date: Mon, 24 Nov 2025 17:43:21 +0100
Subject: [PATCH] Refine Kotlin Serialization codec type checks

ServerSentEvent and String checks, removed from
KotlinSerializationSupport in Spring Framework 7.0, are reintroduced by
this commit at the right level (KotlinSerializationSupport for
ServerSentEvent and KotlinSerializationString(Decoder|Encoder) for
String).

Closes gh-35885
---
 .../http/codec/KotlinSerializationStringDecoder.java            | 2 +-
 .../http/codec/KotlinSerializationStringEncoder.java            | 2 +-
 .../springframework/http/codec/KotlinSerializationSupport.java  | 2 +-
 .../http/codec/json/KotlinSerializationJsonDecoderTests.kt      | 2 +-
 .../http/codec/json/KotlinSerializationJsonEncoderTests.kt      | 2 +-
 5 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringDecoder.java b/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringDecoder.java
index 684df592298..2529a81997a 100644
--- a/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringDecoder.java
+++ b/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringDecoder.java
@@ -102,7 +102,7 @@ public abstract class KotlinSerializationStringDecoder<T extends StringFormat> e
 
 	@Override
 	public boolean canDecode(ResolvableType elementType, @Nullable MimeType mimeType) {
-		return canSerialize(elementType, mimeType);
+		return canSerialize(elementType, mimeType) && !CharSequence.class.isAssignableFrom(elementType.toClass());
 	}
 
 	@Override
diff --git a/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringEncoder.java b/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringEncoder.java
index 18d7330b453..c9046bee552 100644
--- a/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringEncoder.java
+++ b/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringEncoder.java
@@ -101,7 +101,7 @@ public abstract class KotlinSerializationStringEncoder<T extends StringFormat> e
 
 	@Override
 	public boolean canEncode(ResolvableType elementType, @Nullable MimeType mimeType) {
-		return canSerialize(elementType, mimeType);
+		return canSerialize(elementType, mimeType) && !String.class.isAssignableFrom(elementType.toClass());
 	}
 
 	@Override
diff --git a/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationSupport.java b/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationSupport.java
index 1d144926c0a..b69dae1253a 100644
--- a/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationSupport.java
+++ b/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationSupport.java
@@ -117,7 +117,7 @@ public abstract class KotlinSerializationSupport<T extends SerialFormat> {
 		if (!this.typePredicate.test(type) || ResolvableType.NONE.equals(type)) {
 			return false;
 		}
-		return serializer(type) != null && supports(mimeType);
+		return serializer(type) != null && supports(mimeType) && !ServerSentEvent.class.isAssignableFrom(type.toClass());
 	}
 
 	private boolean supports(@Nullable MimeType mimeType) {
diff --git a/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonDecoderTests.kt b/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonDecoderTests.kt
index 10689531892..28c8f87ec5b 100644
--- a/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonDecoderTests.kt
+++ b/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonDecoderTests.kt
@@ -80,7 +80,7 @@ class KotlinSerializationJsonDecoderTests : AbstractDecoderTests<KotlinSerializa
 		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(Pojo::class.java), jsonSubtype)).isTrue()
 		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(Pojo::class.java), null)).isTrue()
-		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(String::class.java), null)).isTrue()
+		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(String::class.java), null)).isFalse()
 		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_XML)).isFalse()
 		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(Pojo::class.java),
 			MediaType("application", "json", StandardCharsets.UTF_8))).isTrue()
diff --git a/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt b/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt
index 9da09e3bdac..59bf4c71131 100644
--- a/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt
+++ b/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt
@@ -91,7 +91,7 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 
 		assertThat(encoderForAllTypes.canEncode(pojoType, MediaType.APPLICATION_NDJSON)).isTrue()
 
-		assertThat(encoderForAllTypes.canEncode(ResolvableType.forClass(String::class.java), null)).isTrue()
+		assertThat(encoderForAllTypes.canEncode(ResolvableType.forClass(String::class.java), null)).isFalse()
 		assertThat(encoderForAllTypes.canEncode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_XML)).isFalse()
 		val sseType = ResolvableType.forClass(ServerSentEvent::class.java)
 		assertThat(encoderForAllTypes.canEncode(sseType, MediaType.APPLICATION_JSON)).isFalse()