Fix null defaults for Spring query parameters

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

@@ -1749,10 +1749,14 @@ private String getDefaultCollectionType(String arrayFormat, String defaultValues
 
     @Override
     public String toDefaultParameterValue(final Schema<?> schema) {
-        Object defaultValue = schema.get$ref() != null ? ModelUtils.getReferencedSchema(openAPI, schema).getDefault() : schema.getDefault();
+        Schema<?> referencedSchema = schema.get$ref() != null ? ModelUtils.getReferencedSchema(openAPI, schema) : schema;
+        Object defaultValue = schema.getDefault() != null ? schema.getDefault() : referencedSchema.getDefault();
         if (defaultValue == null) {
             return null;
         }
+        if (isNullNode(defaultValue)) {
+            return null;
+        }
         if (defaultValue instanceof Date) {
             Date date = (Date) schema.getDefault();
             LocalDate localDate = date.toInstant().atZone(ZoneId.systemDefault()).toLocalDate();

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

@@ -3029,6 +3029,58 @@ public void shouldIgnorePageableForSpringHttpInterface_issue24720() throws IOExc
                 .doesNotHaveParameter("pageable");
     }
 
+    @Test
+    public void nullSchemaDefaultIsNotRenderedAsSpringRequestParamDefault() throws IOException {
+        Map<String, Object> additionalProperties = new HashMap<>();
+        additionalProperties.put(INTERFACE_ONLY, true);
+        additionalProperties.put(SKIP_DEFAULT_INTERFACE, true);
+        additionalProperties.put(USE_SPRING_BOOT4, true);
+        additionalProperties.put("useJackson3", true);
+        additionalProperties.put(USE_JSPECIFY, true);
+        additionalProperties.put(USE_DEDUCTION_FOR_ONE_OF_INTERFACES, true);
+        additionalProperties.put(AbstractJavaCodegen.CONTAINER_DEFAULT_TO_NULL, true);
+        additionalProperties.put(GENERATE_JSON_INCLUDE_ANNOTATIONS, true);
+        additionalProperties.put(PERFORM_BEANVALIDATION, true);
+        additionalProperties.put(AbstractJavaCodegen.OPENAPI_NULLABLE, false);
+
+        Map<String, File> files = generateFromContract(
+                "src/test/resources/3_1/spring/optional-query-null-default.yaml", SPRING_HTTP_INTERFACE, additionalProperties);
+
+        JavaFileAssert api = JavaFileAssert.assertThat(files.get("PlacesApi.java"));
+        api.assertMethod("getPlaces")
+                .assertParameter("ids").hasType("List<String>")
+                .assertParameterAnnotations()
+                .containsWithNameAndDoesNotContainAttributes("RequestParam", List.of("defaultValue"));
+        api.assertMethod("getPlaces")
+                .assertParameter("search")
+                .assertParameterAnnotations()
+                .containsWithNameAndDoesNotContainAttributes("RequestParam", List.of("defaultValue"));
+        api.assertMethod("getPlaces")
+                .assertParameter("limit")
+                .assertParameterAnnotations()
+                .containsWithNameAndAttributes("RequestParam", ImmutableMap.of("defaultValue", "\"10\""));
+        api.assertMethod("getPlaces")
+                .assertParameter("requiredIds")
+                .assertParameterAnnotations()
+                .containsWithNameAndAttributes("RequestParam", ImmutableMap.of("required", "true"));
+        api.assertMethod("getPlaces")
+                .assertParameter("defaultIds")
+                .assertParameterAnnotations()
+                .containsWithNameAndAttributes("RequestParam", ImmutableMap.of("defaultValue", "\"a,b\""));
+        api.assertMethod("getPlaces")
+                .assertParameter("literalNull")
+                .assertParameterAnnotations()
+                .containsWithNameAndAttributes("RequestParam", ImmutableMap.of("defaultValue", "\"null\""));
+        api.assertMethod("getPlaces")
+                .assertParameter("referencedLiteralNull")
+                .assertParameterAnnotations()
+                .containsWithNameAndAttributes("RequestParam", ImmutableMap.of("defaultValue", "\"null\""));
+        api.assertMethod("getPlaces")
+                .assertParameter("referencedNullDefault")
+                .assertParameterAnnotations()
+                .containsWithNameAndDoesNotContainAttributes("RequestParam", List.of("defaultValue"));
+    }
+
     @Test
     public void explicitXSpringPaginatedFalseKeepsQueryParamsForSpringCloud_issue24720() throws IOException {
         // Regression #24720: x-spring-paginated: false on a spring-cloud operation must NOT be

modules/openapi-generator/src/test/resources/3_1/spring/optional-query-null-default.yaml

@@ -0,0 +1,68 @@
+openapi: 3.1.0
+info:
+  title: Optional query null default
+  version: 1.0.0
+paths:
+  /places:
+    get:
+      operationId: getPlaces
+      parameters:
+        - name: ids
+          in: query
+          required: false
+          schema:
+            type: array
+            items:
+              type: string
+            default: null
+        - name: search
+          in: query
+          required: false
+          schema:
+            type: string
+        - name: limit
+          in: query
+          required: false
+          schema:
+            type: integer
+            default: 10
+        - name: requiredIds
+          in: query
+          required: true
+          schema:
+            type: array
+            items:
+              type: string
+        - name: defaultIds
+          in: query
+          required: false
+          schema:
+            type: array
+            items:
+              type: string
+            default: [a, b]
+        - name: literalNull
+          in: query
+          required: false
+          schema:
+            type: string
+            default: "null"
+        - name: referencedLiteralNull
+          in: query
+          required: false
+          schema:
+            $ref: '#/components/schemas/stringWithNullDefault'
+            default: "null"
+        - name: referencedNullDefault
+          in: query
+          required: false
+          schema:
+            $ref: '#/components/schemas/stringWithNullDefault'
+      responses:
+        "200":
+          description: OK
+components:
+  schemas:
+    stringWithNullDefault:
+      type: string
+      default: null