fix: Kotlin nullability interpretation of Any? type

springdoc-openapi-starter-common/src/main/java/org/springdoc/core/customizers/KotlinNullablePropertyCustomizer.kt

@@ -128,8 +128,15 @@ class KotlinNullablePropertyCustomizer(
 	 * Marks a non-$ref property as nullable.
 	 * - OAS 3.0: `nullable: true`
 	 * - OAS 3.1: adds `"null"` to the `types` set
+	 *
+	 * A schema representing `Any?` carries no `type`/`types` constraint, i.e. it is an
+	 * "any" schema which already permits any value including `null`. Such a schema is left
+	 * untouched (kept as an empty schema) rather than being narrowed to `type: "null"`
+	 * (3.1) or decorated with a redundant `nullable: true` (3.0).
 	 */
 	private fun markNullable(property: Schema<*>, specVersion: SpecVersion) {
+		if (isAnySchema(property)) return
+
 		if (specVersion == SpecVersion.V31) {
 			val currentTypes = property.types ?: property.type?.let { setOf(it) } ?: emptySet()
 			if ("null" !in currentTypes) {
@@ -140,6 +147,16 @@ class KotlinNullablePropertyCustomizer(
 		}
 	}
 
+	/**
+	 * Returns true when the schema imposes no type constraint (i.e. represents `Any`),
+	 * in which case it is treated as an empty schema that already allows any value,
+	 * including `null`.
+	 */
+	private fun isAnySchema(property: Schema<*>): Boolean =
+		property.`$ref` == null &&
+			property.type == null &&
+			property.types.isNullOrEmpty()
+
 	/**
 	 * Wraps a $ref property in a nullable composite schema. A fresh wrapper schema is returned
 	 * (the original property object is left untouched) with any sibling attributes such as

springdoc-openapi-tests/springdoc-openapi-kotlin-webmvc-tests/src/test/kotlin/test/org/springdoc/api/v30/app18/NullableController.kt

@@ -8,6 +8,7 @@ data class NullableFieldsResponse(
 	val requiredField: String,
 	val nullableString: String? = null,
 	val nullableInt: Int? = null,
+	val nullableAny: Any? = null,
 	@field:Schema(description = "The nested object")
 	val nullableNested: NestedObject? = null,
 )

springdoc-openapi-tests/springdoc-openapi-kotlin-webmvc-tests/src/test/kotlin/test/org/springdoc/api/v30/app19/NullableController.kt

@@ -7,6 +7,7 @@ data class NullableFieldsResponse(
     val requiredField: String,
     val nullableString: String? = null,
     val nullableInt: Int? = null,
+    val nullableAny: Any? = null,
     val nullableNested: NestedObject? = null,
 )
 

springdoc-openapi-tests/springdoc-openapi-kotlin-webmvc-tests/src/test/kotlin/test/org/springdoc/api/v31/app23/NullableController.kt

@@ -8,6 +8,7 @@ data class NullableFieldsResponse(
 	val requiredField: String,
 	val nullableString: String? = null,
 	val nullableInt: Int? = null,
+	val nullableAny: Any? = null,
 	@field:Schema(description = "The nested object")
 	val nullableNested: NestedObject? = null,
 )

springdoc-openapi-tests/springdoc-openapi-kotlin-webmvc-tests/src/test/kotlin/test/org/springdoc/api/v31/app24/NullableController.kt

@@ -7,6 +7,7 @@ data class NullableFieldsResponse(
     val requiredField: String,
     val nullableString: String? = null,
     val nullableInt: Int? = null,
+    val nullableAny: Any? = null,
     val nullableNested: NestedObject? = null,
 )
 

springdoc-openapi-tests/springdoc-openapi-kotlin-webmvc-tests/src/test/resources/results/3.0.1/app18.json

@@ -68,6 +68,10 @@
             "format": "int32",
             "nullable": true
           },
+          "nullableAny" : {
+            "type" : "object",
+            "nullable" : true
+          },
           "nullableNested": {
             "nullable": true,
             "allOf": [

springdoc-openapi-tests/springdoc-openapi-kotlin-webmvc-tests/src/test/resources/results/3.0.1/app19.json

@@ -64,6 +64,9 @@
             "type": "integer",
             "format": "int32"
           },
+          "nullableAny" : {
+            "type" : "object"
+          },
           "nullableNested": {
             "$ref": "#/components/schemas/NestedObject"
           }

springdoc-openapi-tests/springdoc-openapi-kotlin-webmvc-tests/src/test/resources/results/3.1.0/app23.json

@@ -70,6 +70,7 @@
             ],
             "format": "int32"
           },
+          "nullableAny": { },
           "nullableNested": {
             "description": "The nested object",
             "oneOf": [

springdoc-openapi-tests/springdoc-openapi-kotlin-webmvc-tests/src/test/resources/results/3.1.0/app24.json

@@ -61,6 +61,7 @@
             "type": "integer",
             "format": "int32"
           },
+          "nullableAny": { },
           "nullableNested": {
             "$ref": "#/components/schemas/NestedObject"
           }