[kotlin] Add companionObject option to generate companion objects in data classes

Add a new boolean CLI option `companionObject` (default: false) that generates
an empty `companion object { }` on all data class models, enabling users to add
companion extensions from outside the generated code.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: thejeff77

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

@@ -106,6 +106,8 @@ public class KotlinClientCodegen extends AbstractKotlinCodegen {
 
     public static final String GENERATE_ONEOF_ANYOF_WRAPPERS = "generateOneOfAnyOfWrappers";
 
+    public static final String COMPANION_OBJECT = "companionObject";
+
     protected static final String VENDOR_EXTENSION_BASE_NAME_LITERAL = "x-base-name-literal";
 
 
@@ -123,6 +125,7 @@ public class KotlinClientCodegen extends AbstractKotlinCodegen {
     @Setter protected boolean mapFileBinaryToByteArray = false;
     @Setter protected boolean generateOneOfAnyOfWrappers = true;
     @Getter @Setter protected boolean failOnUnknownProperties = false;
+    @Setter protected boolean companionObject = false;
 
     protected String authFolder;
 
@@ -288,6 +291,8 @@ public KotlinClientCodegen() {
 
         cliOptions.add(CliOption.newBoolean(GENERATE_ONEOF_ANYOF_WRAPPERS, "Generate oneOf, anyOf schemas as wrappers. Only `jvm-retrofit2`(library) with `gson` or `kotlinx_serialization`(serializationLibrary) support this option."));
 
+        cliOptions.add(CliOption.newBoolean(COMPANION_OBJECT, "Whether to generate companion objects in data classes, enabling companion extensions.", false));
+
         CliOption serializationLibraryOpt = new CliOption(CodegenConstants.SERIALIZATION_LIBRARY, SERIALIZATION_LIBRARY_DESC);
         cliOptions.add(serializationLibraryOpt.defaultValue(serializationLibrary.name()));
 
@@ -322,6 +327,10 @@ public boolean getGenerateOneOfAnyOfWrappers() {
         return generateOneOfAnyOfWrappers;
     }
 
+    public boolean getCompanionObject() {
+        return companionObject;
+    }
+
     public void setGenerateRoomModels(Boolean generateRoomModels) {
         this.generateRoomModels = generateRoomModels;
     }
@@ -484,6 +493,12 @@ public void processOpts() {
             setFailOnUnknownProperties(false);
         }
 
+        if (additionalProperties.containsKey(COMPANION_OBJECT)) {
+            setCompanionObject(convertPropertyToBooleanAndWriteBack(COMPANION_OBJECT));
+        } else {
+            additionalProperties.put(COMPANION_OBJECT, companionObject);
+        }
+
         commonSupportingFiles();
 
         switch (getLibrary()) {
@@ -936,7 +951,7 @@ public ModelsMap postProcessModels(ModelsMap objs) {
 
         for (ModelMap mo : objects.getModels()) {
             CodegenModel cm = mo.getModel();
-            if (getGenerateRoomModels() || getGenerateOneOfAnyOfWrappers()) {
+            if (getGenerateRoomModels() || getGenerateOneOfAnyOfWrappers() || getCompanionObject()) {
                 cm.vendorExtensions.put("x-has-data-class-body", true);
             }
 

modules/openapi-generator/src/main/resources/kotlin-client/data_class.mustache

@@ -123,7 +123,8 @@ import {{packageName}}.infrastructure.ITransformForStorage
         private const val serialVersionUID: Long = 123
     }
 {{/serializableModel}}
-{{#discriminator}}{{#vars}}{{#required}}
+{{^serializableModel}}{{^generateRoomModels}}{{^generateOneOfAnyOfWrappers}}{{#companionObject}}    {{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}companion object { }
+{{/companionObject}}{{/generateOneOfAnyOfWrappers}}{{/generateRoomModels}}{{/serializableModel}}{{#discriminator}}{{#vars}}{{#required}}
 {{>interface_req_var}}{{/required}}{{^required}}
 {{>interface_opt_var}}{{/required}}{{/vars}}{{/discriminator}}
 {{#hasEnums}}

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

@@ -43,6 +43,7 @@
 import java.util.Map;
 
 import static org.openapitools.codegen.CodegenConstants.*;
+import static org.openapitools.codegen.languages.KotlinClientCodegen.COMPANION_OBJECT;
 import static org.openapitools.codegen.languages.KotlinClientCodegen.GENERATE_ONEOF_ANYOF_WRAPPERS;
 
 @SuppressWarnings("static-method")
@@ -878,6 +879,50 @@ public void emptyModelKotlinxSerializationTest() throws IOException {
         TestUtils.assertFileNotContains(modelKt, "data class EmptyModel");
     }
 
+    @Test
+    public void testCompanionObjectAdditionalProperty() {
+        final KotlinClientCodegen codegen = new KotlinClientCodegen();
+
+        // Default case, nothing provided
+        codegen.processOpts();
+
+        ConfigAssert configAssert = new ConfigAssert(codegen.additionalProperties());
+        // Default to false
+        configAssert.assertValue(COMPANION_OBJECT, codegen::getCompanionObject, Boolean.FALSE);
+
+        // Provide true
+        codegen.additionalProperties().put(COMPANION_OBJECT, true);
+        codegen.processOpts();
+
+        // Should be true
+        configAssert.assertValue(COMPANION_OBJECT, codegen::getCompanionObject, Boolean.TRUE);
+
+        // Provide false
+        codegen.additionalProperties().put(COMPANION_OBJECT, false);
+        codegen.processOpts();
+
+        // Should be false
+        configAssert.assertValue(COMPANION_OBJECT, codegen::getCompanionObject, Boolean.FALSE);
+    }
+
+    @Test
+    public void testCompanionObjectGeneratesCompanionInModel() throws IOException {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("kotlin")
+                .addAdditionalProperty(COMPANION_OBJECT, true)
+                .setInputSpec("src/test/resources/3_0/petstore.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        DefaultGenerator generator = new DefaultGenerator();
+        generator.opts(configurator.toClientOptInput()).generate();
+
+        Path petModel = Paths.get(output.getAbsolutePath() + "/src/main/kotlin/org/openapitools/client/models/Pet.kt");
+        TestUtils.assertFileContains(petModel, "companion object { }");
+    }
+
     private static class ModelNameTest {
         private final String expectedName;
         private final String expectedClassName;