[#380] fix: add dedicated JsonMetaDataSerializer

kotlin/src/main/kotlin/org/axonframework/extensions/kotlin/serialization/AxonSerializers.kt

@@ -110,7 +110,7 @@ val AxonSerializersModule = SerializersModule {
         subclass(MultipleInstancesResponseTypeSerializer)
         subclass(ArrayResponseTypeSerializer)
     }
-    contextual(MetaData::class) { MetaDataSerializer }
+    contextual(MetaData::class) { ComposedMetaDataSerializer }
 }
 
 /**

kotlin/src/main/kotlin/org/axonframework/extensions/kotlin/serialization/MetaDataSerializer.kt

@@ -5,6 +5,7 @@ import kotlinx.serialization.SerializationException
 import kotlinx.serialization.builtins.MapSerializer
 import kotlinx.serialization.builtins.serializer
 import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encodeToString
 import kotlinx.serialization.encoding.Decoder
 import kotlinx.serialization.encoding.Encoder
 import kotlinx.serialization.json.*
@@ -13,11 +14,44 @@ import java.time.Instant
 import java.util.UUID
 
 /**
- * A Kotlinx [KSerializer] for Axon Framework's [MetaData] type, supporting serialization across any format.
+ * A composite Kotlinx [KSerializer] for Axon Framework's [MetaData] type that selects the
+ * appropriate serializer based on the encoder/decoder type.
+ *
+ * This serializer delegates to:
+ * - [JsonMetaDataSerializer] when used with [JsonEncoder]/[JsonDecoder]
+ * - [StringMetaDataSerializer] for all other encoder/decoder types
+ *
+ * This allows efficient JSON serialization without unnecessary string encoding, while
+ * maintaining compatibility with all other serialization formats through string-based
+ * serialization.
+ *
+ * @author Mateusz Nowak
+ * @since 4.11.2
+ */
+object ComposedMetaDataSerializer : KSerializer<MetaData> {
+    override val descriptor: SerialDescriptor = StringMetaDataSerializer.descriptor
+
+    override fun serialize(encoder: Encoder, value: MetaData) {
+        when (encoder) {
+            is JsonEncoder -> JsonMetaDataSerializer.serialize(encoder, value)
+            else -> StringMetaDataSerializer.serialize(encoder, value)
+        }
+    }
+
+    override fun deserialize(decoder: Decoder): MetaData {
+        return when (decoder) {
+            is JsonDecoder -> JsonMetaDataSerializer.deserialize(decoder)
+            else -> StringMetaDataSerializer.deserialize(decoder)
+        }
+    }
+}
+
+/**
+ * A Kotlinx [KSerializer] for Axon Framework's [MetaData] type, suitable for serialization across any format.
  *
  * This serializer converts a [MetaData] instance to a JSON-encoded [String] using a recursive conversion
- * of all entries into [JsonElement]s. This JSON string is then serialized using [String.serializer],
- * ensuring compatibility with any [kotlinx.serialization.encoding.Encoder]—including formats such as JSON, CBOR, ProtoBuf, or Avro.
+ * of all entries into [JsonElement]s. This JSON string is then serialized using [String.serializer()],
+ * ensuring compatibility with any serialization format.
  *
  * ### Supported value types
  * Each entry in the MetaData map must conform to one of the following:
@@ -31,13 +65,12 @@ import java.util.UUID
  * - Custom types that do not fall into the above categories will throw a [SerializationException]
  * - Deserialized non-primitive types (like [UUID], [Instant]) are restored as [String], not their original types
  *
- * This serializer guarantees structural integrity of nested metadata (e.g. map within list within map), while remaining format-agnostic.
+ * This serializer guarantees structural integrity of nested metadata while remaining format-agnostic.
  *
  * @author Mateusz Nowak
  * @since 4.11.1
  */
-object MetaDataSerializer : KSerializer<MetaData> {
-
+object StringMetaDataSerializer : KSerializer<MetaData> {
     private val json = Json { encodeDefaults = true; ignoreUnknownKeys = true }
 
     override val descriptor: SerialDescriptor = String.serializer().descriptor
@@ -46,14 +79,93 @@ object MetaDataSerializer : KSerializer<MetaData> {
         val map: Map<String, JsonElement> = value.entries.associate { (key, rawValue) ->
             key to toJsonElement(rawValue)
         }
-        val jsonString = json.encodeToString(MapSerializer(String.serializer(), JsonElement.serializer()), map)
-        encoder.encodeSerializableValue(String.serializer(), jsonString)
+        val jsonString = json.encodeToString(JsonObject(map))
+        encoder.encodeString(jsonString)
+    }
+
+    override fun deserialize(decoder: Decoder): MetaData {
+        val jsonString = decoder.decodeString()
+        val jsonObject = json.parseToJsonElement(jsonString).jsonObject
+        val reconstructed = jsonObject.mapValues { (_, jsonElement) ->
+            fromJsonElement(jsonElement)
+        }
+        return MetaData(reconstructed)
+    }
+
+    private fun toJsonElement(value: Any?): JsonElement = when (value) {
+        null -> JsonNull
+        is String -> JsonPrimitive(value)
+        is Boolean -> JsonPrimitive(value)
+        is Int -> JsonPrimitive(value)
+        is Long -> JsonPrimitive(value)
+        is Float -> JsonPrimitive(value)
+        is Double -> JsonPrimitive(value)
+        is UUID -> JsonPrimitive(value.toString())
+        is Instant -> JsonPrimitive(value.toString())
+        is Map<*, *> -> JsonObject(value.entries.associate { (k, v) ->
+            k.toString() to toJsonElement(v)
+        })
+        is Collection<*> -> JsonArray(value.map { toJsonElement(it) })
+        is Array<*> -> JsonArray(value.map { toJsonElement(it) })
+        else -> throw SerializationException("Unsupported type: ${value::class}")
+    }
+
+    private fun fromJsonElement(element: JsonElement): Any? = when (element) {
+        is JsonNull -> null
+        is JsonPrimitive -> {
+            if (element.isString) {
+                element.content
+            } else {
+                element.booleanOrNull ?: element.intOrNull ?: element.longOrNull ?:
+                element.floatOrNull ?: element.doubleOrNull ?: element.content
+            }
+        }
+        is JsonObject -> element.mapValues { fromJsonElement(it.value) }
+        is JsonArray -> element.map { fromJsonElement(it) }
+    }
+}
+
+/**
+ * A Kotlinx [KSerializer] for Axon Framework's [MetaData] type, optimized for JSON serialization.
+ *
+ * This serializer converts a [MetaData] instance directly to a JSON object structure,
+ * avoiding the string-encoding that [StringMetaDataSerializer] uses. This ensures JSON values
+ * are properly encoded without quote escaping.
+ *
+ * ### Supported value types
+ * Each entry in the MetaData map must conform to one of the following:
+ * - Primitives: [String], [Int], [Long], [Float], [Double], [Boolean]
+ * - Complex types: [UUID], [Instant]
+ * - Collections: [Collection], [List], [Set]
+ * - Arrays: [Array]
+ * - Nested Maps: [Map] with keys convertible to [String]
+ *
+ * ### Limitations
+ * - Custom types that do not fall into the above categories will throw a [SerializationException]
+ * - Deserialized non-primitive types (like [UUID], [Instant]) are restored as [String], not their original types
+ *
+ * This serializer is specifically optimized for JSON serialization formats.
+ *
+ * @author Mateusz Nowak
+ * @since 4.11.2
+ */
+object JsonMetaDataSerializer : KSerializer<MetaData> {
+    private val mapSerializer = MapSerializer(String.serializer(), JsonElement.serializer())
+
+    override val descriptor: SerialDescriptor = mapSerializer.descriptor
+
+    override fun serialize(encoder: Encoder, value: MetaData) {
+        val jsonMap = value.entries.associate { (key, rawValue) ->
+            key to toJsonElement(rawValue)
+        }
+        encoder.encodeSerializableValue(mapSerializer, jsonMap)
     }
 
     override fun deserialize(decoder: Decoder): MetaData {
-        val jsonString = decoder.decodeSerializableValue(String.serializer())
-        val map = json.decodeFromString(MapSerializer(String.serializer(), JsonElement.serializer()), jsonString)
-        val reconstructed = map.mapValues { (_, jsonElement) -> fromJsonElement(jsonElement) }
+        val jsonMap = decoder.decodeSerializableValue(mapSerializer)
+        val reconstructed = jsonMap.mapValues { (_, jsonElement) ->
+            fromJsonElement(jsonElement)
+        }
         return MetaData(reconstructed)
     }
 
@@ -67,7 +179,9 @@ object MetaDataSerializer : KSerializer<MetaData> {
         is Double -> JsonPrimitive(value)
         is UUID -> JsonPrimitive(value.toString())
         is Instant -> JsonPrimitive(value.toString())
-        is Map<*, *> -> JsonObject(value.entries.associate { (k, v) -> k.toString() to toJsonElement(v) })
+        is Map<*, *> -> JsonObject(value.entries.associate { (k, v) ->
+            k.toString() to toJsonElement(v)
+        })
         is Collection<*> -> JsonArray(value.map { toJsonElement(it) })
         is Array<*> -> JsonArray(value.map { toJsonElement(it) })
         else -> throw SerializationException("Unsupported type: ${value::class}")
@@ -79,12 +193,8 @@ object MetaDataSerializer : KSerializer<MetaData> {
             if (element.isString) {
                 element.content
             } else {
-                element.booleanOrNull
-                    ?: element.intOrNull
-                    ?: element.longOrNull
-                    ?: element.floatOrNull
-                    ?: element.doubleOrNull
-                    ?: element.content
+                element.booleanOrNull ?: element.intOrNull ?: element.longOrNull ?:
+                element.floatOrNull ?: element.doubleOrNull ?: element.content
             }
         }
         is JsonObject -> element.mapValues { fromJsonElement(it.value) }

kotlin/src/test/kotlin/org/axonframework/extensions/kotlin/serializer/MetaDataSerializerTest.kt

@@ -218,6 +218,28 @@ class MetaDataSerializerTest {
         assertEquals(deserializedValue, complexStructure)
     }
 
+    @Test
+    fun `should not escape quotes in complex nested structures in MetaData`() {
+        val complexStructure = mapOf(
+            "string" to "value",
+            "number" to 42,
+            "boolean" to true,
+            "null" to null,
+            "list" to listOf(1, 2, 3),
+            "nestedMap" to mapOf(
+                "a" to "valueA",
+                "b" to listOf("x", "y", "z"),
+                "c" to mapOf("nested" to "deepValue")
+            )
+        )
+
+        val metaData = MetaData.with("complexValue", complexStructure)
+
+        val serialized = jsonSerializer.serialize(metaData, String::class.java)
+        val json = """{"complexValue":{"string":"value","number":42,"boolean":true,"null":null,"list":[1,2,3],"nestedMap":{"a":"valueA","b":["x","y","z"],"c":{"nested":"deepValue"}}}}"""
+        assertEquals(json, serialized.data);
+    }
+
     @Test
     fun `do not handle custom objects`() {
         data class Person(val name: String, val age: Int)
@@ -235,7 +257,7 @@ class MetaDataSerializerTest {
     fun `should throw exception when deserializing malformed JSON`() {
         val serializedType = SimpleSerializedType(MetaData::class.java.name, null)
 
-        val syntaxErrorJson = """{"key": value}"""  // missing quotes around value
+        val syntaxErrorJson = """{"key": value"""  // missing closing bracket around value
         val syntaxErrorObject = SimpleSerializedObject(syntaxErrorJson, String::class.java, serializedType)
 
         assertThrows<SerializationException> {