Allow array items in kotlin to be nullable (#19080)

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

@@ -347,6 +347,12 @@ public abstract class AbstractKotlinCodegen extends DefaultCodegen implements Co
         return toModelName(type);
     }
 
+    private String getItemsTypeDeclaration(Schema items) {
+        String itemsTypeDeclaration = getTypeDeclaration(items);
+        String nullable = items.getNullable() != null && items.getNullable() && !itemsTypeDeclaration.endsWith("?") ? "?" : "";
+        return itemsTypeDeclaration + nullable;
+    }
+
     /**
      * Output the type declaration of the property
      *
@@ -359,7 +365,7 @@ public abstract class AbstractKotlinCodegen extends DefaultCodegen implements Co
         Schema<?> target = ModelUtils.isGenerateAliasAsModel() ? p : schema;
         if (ModelUtils.isArraySchema(target)) {
             Schema<?> items = ModelUtils.getSchemaItems( schema);
-            return getSchemaType(target) + "<" + getTypeDeclaration(items) + ">";
+            return getSchemaType(target) + "<" + getItemsTypeDeclaration(items) + ">";
         } else if (ModelUtils.isMapSchema(target)) {
             // Note: ModelUtils.isMapSchema(p) returns true when p is a composed schema that also defines
             // additionalproperties: true

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

@@ -362,6 +362,37 @@ public class KotlinSpringServerCodegenTest {
             "ApiUtil");
     }
 
+    @Test
+    public void arrayItemsCanBeNullable() throws IOException {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+        String outputPath = output.getAbsolutePath().replace('\\', '/');
+
+        OpenAPI openAPI = new OpenAPIParser()
+                .readLocation("src/test/resources/3_0/array-nullable-items.yaml", null, new ParseOptions()).getOpenAPI();
+
+        KotlinSpringServerCodegen codegen = new KotlinSpringServerCodegen();
+        codegen.setOutputDir(output.getAbsolutePath());
+        codegen.additionalProperties().put(CXFServerFeatures.LOAD_TEST_DATA_FROM_FILE, "true");
+
+        ClientOptInput input = new ClientOptInput();
+        input.openAPI(openAPI);
+        input.config(codegen);
+
+        DefaultGenerator generator = new DefaultGenerator();
+
+        generator.setGeneratorPropertyDefault(CodegenConstants.MODELS, "true");
+        generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_TESTS, "false");
+        generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_DOCS, "false");
+        generator.setGeneratorPropertyDefault(CodegenConstants.APIS, "true");
+        generator.setGeneratorPropertyDefault(CodegenConstants.SUPPORTING_FILES, "false");
+
+        generator.opts(input).generate();
+
+        assertFileContains(Paths.get(outputPath + "/src/main/kotlin/org/openapitools/model/ArrayWithNullableItemsModel.kt"), "List<kotlin.String?>");
+    }
+
+
     @Test
     public void doNotGenerateRequestParamForObjectQueryParam() throws IOException {
         File output = Files.createTempDirectory("test").toFile().getCanonicalFile();

modules/openapi-generator/src/test/resources/3_0/array-nullable-items.yaml

@@ -0,0 +1,26 @@
+openapi: 3.0.0
+info:
+  title: 'Array nullable items'
+  version: latest
+paths:
+  '/':
+    get:
+      operationId: operation
+      responses:
+        '200':
+          description: Success
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ArrayWithNullableItemsModel'
+components:
+  schemas:
+    ArrayWithNullableItemsModel:
+      required:
+        - foo
+      properties:
+        foo:
+          type: array
+          items:
+            type: string
+            nullable: true