fix: normalize OAS 3.1 schemas with type:[object,"null"] to set nullable:true correctly

modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java

@@ -980,6 +980,18 @@ public Schema normalizeSchema(Schema schema, Set<Schema> visitedSchemas) {
 
             return schema;
         } else if (ModelUtils.hasProperties(schema)) {
+            // OAS 3.1: if the type array includes "null", extract it and set nullable:true
+            // on the parent schema before normalizing its child properties.
+            // We intentionally do NOT call the full processNormalize31Spec here because
+            // that method can replace a JsonSchema with properties (but no explicit type)
+            // with an empty schema, discarding all properties.
+            if (getRule(NORMALIZE_31SPEC) && schema.getTypes() != null && schema.getTypes().contains("null")) {
+                schema.setNullable(true);
+                schema.getTypes().remove("null");
+                if (schema.getTypes().size() == 1) {
+                    schema.setType(String.valueOf(schema.getTypes().iterator().next()));
+                }
+            }
             normalizeProperties(schema, visitedSchemas);
         } else if (schema.getAdditionalProperties() instanceof Schema) { // map
             normalizeMapSchema(schema);

modules/openapi-generator/src/test/java/org/openapitools/codegen/OpenAPINormalizerTest.java

@@ -1896,4 +1896,62 @@ public void testLooseNullDefinitions() {
         ModelUtils.looseNullDefinitions = false;
     }
 
+    /**
+     * Verify that a schema defined as type:[object,"null"] WITH properties (OAS 3.1 style)
+     * is correctly normalized so that nullable:true is set on the schema itself.
+     * Regression test for https://github.com/OpenAPITools/openapi-generator/issues/24139
+     */
+    @Test
+    public void testIssue24139NullableObjectWithPropertiesGetsNullableTrue() {
+        OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_1/issue_24139.yaml");
+
+        // Before normalization: NestedNullable has types=[object,null], nullable is not yet set
+        Schema<?> nestedNullableBefore = openAPI.getComponents().getSchemas().get("NestedNullable");
+        assertNotNull(nestedNullableBefore);
+        assertNotNull(nestedNullableBefore.getProperties());
+
+        Map<String, String> options = new HashMap<>();
+        options.put("NORMALIZE_31SPEC", "true");
+        OpenAPINormalizer openAPINormalizer = new OpenAPINormalizer(openAPI, options);
+        openAPINormalizer.normalize();
+
+        // After normalization: nullable must be true and type must be "object"
+        Schema<?> nestedNullableAfter = openAPI.getComponents().getSchemas().get("NestedNullable");
+        assertNotNull(nestedNullableAfter);
+        assertEquals(nestedNullableAfter.getNullable(), Boolean.TRUE,
+                "NestedNullable with type:[object,\"null\"] should have nullable:true after normalization");
+        assertEquals(nestedNullableAfter.getType(), "object");
+        assertNotNull(nestedNullableAfter.getProperties(),
+                "NestedNullable properties must be preserved after normalization");
+    }
+
+    /**
+     * Regression test: an OAS 3.1 schema with properties but NO explicit type declaration
+     * must keep its properties after NORMALIZE_31SPEC normalization.
+     * Regression for a potential regression introduced by the fix for issue 24139.
+     */
+    @Test
+    public void testIssue24139ImpliedObjectSchemaKeepsProperties() {
+        OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_1/issue_24139.yaml");
+
+        Schema<?> impliedBefore = openAPI.getComponents().getSchemas().get("ImpliedObject");
+        assertNotNull(impliedBefore);
+        assertNotNull(impliedBefore.getProperties());
+
+        Map<String, String> options = new HashMap<>();
+        options.put("NORMALIZE_31SPEC", "true");
+        OpenAPINormalizer openAPINormalizer = new OpenAPINormalizer(openAPI, options);
+        openAPINormalizer.normalize();
+
+        Schema<?> impliedAfter = openAPI.getComponents().getSchemas().get("ImpliedObject");
+        assertNotNull(impliedAfter);
+        assertNotNull(impliedAfter.getProperties(),
+                "ImpliedObject (no explicit type, just properties) must keep its properties after normalization");
+        assertNotNull(impliedAfter.getProperties().get("name"),
+                "ImpliedObject.name property must be preserved after normalization");
+        assertNull(impliedAfter.getNullable(),
+                "ImpliedObject must not be marked nullable (no null type was declared)");
+    }
+
 }
+

modules/openapi-generator/src/test/java/org/openapitools/codegen/kotlin/spring/KotlinSpringServerCodegenTest.java

@@ -6813,4 +6813,35 @@ public void paramJsonPropertyAnnotationWithDigitStartingPropertyName() throws IO
                 "@param:JsonProperty(\"2nd_field\")\n    @get:JsonProperty(\"2nd_field\") val `2ndField`"
         );
     }
+
+    /**
+     * Regression test for https://github.com/OpenAPITools/openapi-generator/issues/24139
+     * A property that $ref's an OAS 3.1 schema with type:[object,"null"] is nullable and must
+     * NOT receive @field:JsonSetter(nulls = Nulls.FAIL).
+     */
+    @Test(description = "issue 24139: nullable $ref (type:[object,null]) must not get @JsonSetter(nulls = Nulls.FAIL)")
+    public void testIssue24139NullableRefNoJsonSetterNullsFail() throws IOException {
+        Map<String, Object> additionalProperties = new HashMap<>();
+        additionalProperties.put("useBeanValidation", true);
+        additionalProperties.put("openApiNullable", "true");
+
+        Map<String, File> files = generateFromContract(
+                "src/test/resources/3_1/issue_24139.yaml",
+                additionalProperties
+        );
+
+        File itemFile = files.get("Item.kt");
+        assertThat(itemFile).isNotNull();
+
+        // nestedNullable: $ref to NestedNullable (type:[object,"null"]) — nullable, no @JsonSetter(nulls = Nulls.FAIL)
+        assertFileNotContains(itemFile.toPath(), "nestedNullable: NestedNullable");
+        // The field must NOT have @JsonSetter(nulls = Nulls.FAIL) because the referenced schema is nullable
+        String content = org.apache.commons.io.FileUtils.readFileToString(itemFile, StandardCharsets.UTF_8);
+        // Extract the nestedNullable field block and verify annotation absence
+        Assert.assertFalse(
+                content.contains("@field:JsonSetter(nulls = Nulls.FAIL)\n    @param:JsonProperty(\"nestedNullable\")") ||
+                content.contains("@field:JsonSetter(nulls = Nulls.FAIL)\n    @get:JsonProperty(\"nestedNullable\")"),
+                "nestedNullable ($ref to nullable schema) must not have @JsonSetter(nulls = Nulls.FAIL)"
+        );
+    }
 }

modules/openapi-generator/src/test/resources/3_1/issue_24139.yaml

@@ -0,0 +1,37 @@
+openapi: 3.1.1
+info:
+  title: Test issue 24139
+  version: 1.0.0
+
+components:
+  schemas:
+    NestedNullable:
+      type: [ object, "null" ]
+      properties:
+        value:
+          type: [ number, "null" ]
+    Nested:
+      type: object
+      properties:
+        value:
+          type: [ number, "null" ]
+    # OAS 3.1 allows object schemas with properties but no explicit type declaration.
+    # This must not lose its properties after normalization.
+    ImpliedObject:
+      properties:
+        name:
+          type: string
+
+    Item:
+      type: object
+      properties:
+        nestedNullable:
+          $ref: '#/components/schemas/NestedNullable'
+
+        nestedNullable2:
+          anyOf:
+            - $ref: '#/components/schemas/Nested'
+            - type: 'null'
+
+        nested:
+          $ref: '#/components/schemas/Nested'