[KOTLIN-SPRING] Feature - add support for 'useDeductionForOneOfInterfaces' (#23677)

* feat(kotlin-spring): add support for deduction in oneOf interfaces with Jackson annotations

* update samples

* Revert "update samples"

This reverts commit 71cdb09.

* Reapply "update samples"

This reverts commit dc0bac6.

* Revert "Reapply "update samples""

This reverts commit e45f0e6.

* Reapply "Reapply "update samples""

This reverts commit 050092c.

* white space change to trigger tests

* Revert "white space change to trigger tests"

This reverts commit ddde44c.

Author: Picazsoo

docs/generators/kotlin-spring.md

@@ -61,6 +61,7 @@ These options may be applied as additional-properties (cli) or configOptions (pl
 |substituteGenericPagedModel|Detect schemas that represent paginated responses (an object with a 'content' array property and a 'page' pagination-metadata property) and replace their generated references with PagedModel<T>. By default this uses a generated type in the config package (default 'org.openapitools.configuration'), but `importMappings.PagedModel` can override it to a custom/FQCN-mapped type. The detected page schemas and the pagination metadata schema are suppressed from code generation.| |false|
 |title|server title name or client service name| |OpenAPI Kotlin Spring|
 |useBeanValidation|Use BeanValidation API annotations to validate data types| |true|
+|useDeductionForOneOfInterfaces|Annotate discriminator-free oneOf interfaces with Jackson's @JsonTypeInfo(use = Id.DEDUCTION) and @JsonSubTypes so the concrete subtype is resolved from the JSON field set rather than a type-tag property. Has no effect when a discriminator is present (name-based resolution is used instead). Requires subtypes to have structurally distinct sets of properties.| |false|
 |useFeignClientUrl|Whether to generate Feign client with url parameter.| |true|
 |useFlowForArrayReturnType|Whether to use Flow for array/collection return types when reactive is enabled. If false, will use List instead.| |true|
 |useJackson3|Use Jackson 3 dependencies (tools.jackson package). Only available with `useSpringBoot4`. Defaults to true when `useSpringBoot4` is enabled. Incompatible with `openApiNullable`.| |false|

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

@@ -181,6 +181,8 @@ public String getDescription() {
     @Setter private boolean substituteGenericPagedModel = false;
     @Setter private boolean useSealedResponseInterfaces = false;
     @Setter private boolean companionObject = false;
+    @Getter @Setter
+    protected boolean useDeductionForOneOfInterfaces = false;
 
     @Getter @Setter
     protected boolean useSpringBoot3 = false;
@@ -311,6 +313,7 @@ public KotlinSpringServerCodegen() {
                 + "schema are suppressed from code generation.",
                 substituteGenericPagedModel);
         addSwitch(COMPANION_OBJECT, "Whether to generate companion objects in data classes, enabling companion extensions.", companionObject);
+        cliOptions.add(CliOption.newBoolean(CodegenConstants.USE_DEDUCTION_FOR_ONE_OF_INTERFACES, CodegenConstants.USE_DEDUCTION_FOR_ONE_OF_INTERFACES_DESC, useDeductionForOneOfInterfaces));
         supportedLibraries.put(SPRING_BOOT, "Spring-boot Server application.");
         supportedLibraries.put(SPRING_CLOUD_LIBRARY,
                 "Spring-Cloud-Feign client with Spring-Boot auto-configured settings.");
@@ -572,6 +575,8 @@ public void processOpts() {
             additionalProperties.put(COMPANION_OBJECT, companionObject);
         }
 
+        convertPropertyToBooleanAndWriteBack(CodegenConstants.USE_DEDUCTION_FOR_ONE_OF_INTERFACES, this::setUseDeductionForOneOfInterfaces);
+
         additionalProperties.put("springHttpStatus", new SpringHttpStatusLambda());
 
         // Set basePackage from invokerPackage

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

@@ -556,6 +556,7 @@ public void processOpts() {
         }
         convertPropertyToBooleanAndWriteBack(OPTIONAL_ACCEPT_NULLABLE, this::setOptionalAcceptNullable);
         convertPropertyToBooleanAndWriteBack(USE_SPRING_BUILT_IN_VALIDATION, this::setUseSpringBuiltInValidation);
+        convertPropertyToBooleanAndWriteBack(CodegenConstants.USE_DEDUCTION_FOR_ONE_OF_INTERFACES, this::setUseDeductionForOneOfInterfaces);
 
         additionalProperties.put("springHttpStatus", new SpringHttpStatusLambda());
 

modules/openapi-generator/src/main/resources/kotlin-spring/oneof_interface.mustache

@@ -4,7 +4,14 @@
 {{#discriminator}}
 {{>typeInfoAnnotation}}
 {{/discriminator}}
-{{#additionalModelTypeAnnotations}}
+{{^discriminator}}{{#useDeductionForOneOfInterfaces}}{{#jackson}}
+@JsonTypeInfo(use = JsonTypeInfo.Id.DEDUCTION)
+@JsonSubTypes(
+  {{#interfaceModels}}
+    JsonSubTypes.Type(value = {{classname}}::class){{^-last}},{{/-last}}
+  {{/interfaceModels}}
+)
+{{/jackson}}{{/useDeductionForOneOfInterfaces}}{{/discriminator}}{{#additionalModelTypeAnnotations}}
 {{{.}}}
 {{/additionalModelTypeAnnotations}}
 {{#vendorExtensions.x-class-extra-annotation}}

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

@@ -6050,6 +6050,40 @@ public void testOneOfRefEnumDiscriminatorResolvesType() throws IOException {
         );
     }
 
+    @Test(description = "oneOf without discriminator with useDeductionForOneOfInterfaces generates @JsonTypeInfo(DEDUCTION) annotation")
+    public void testOneOfDeductionWithoutDiscriminatorGeneratesDeductionAnnotation() throws IOException {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+
+        new DefaultGenerator().opts(new ClientOptInput()
+                        .openAPI(new OpenAPIParser().readLocation("src/test/resources/3_0/oneof_polymorphism_and_inheritance.yaml", null, new ParseOptions()).getOpenAPI())
+                        .config(new KotlinSpringServerCodegen() {{
+                            setOutputDir(output.getAbsolutePath());
+                            additionalProperties().put(CodegenConstants.USE_DEDUCTION_FOR_ONE_OF_INTERFACES, "true");
+                        }}))
+                .generate();
+
+        String outputPath = output.getAbsolutePath() + "/src/main/kotlin/org/openapitools/model";
+
+        // Animal has oneOf [Dog, Cat] with NO discriminator → deduction should be applied
+        assertFileContains(Paths.get(outputPath + "/Animal.kt"),
+                "sealed interface Animal",
+                "@JsonTypeInfo(use = JsonTypeInfo.Id.DEDUCTION)",
+                "@JsonSubTypes(",
+                "JsonSubTypes.Type(value = Dog::class)",
+                "JsonSubTypes.Type(value = Cat::class)"
+        );
+
+        // Fruit has oneOf [Apple, Banana] WITH a discriminator → must NOT use deduction
+        assertFileNotContains(Paths.get(outputPath + "/Fruit.kt"),
+                "JsonTypeInfo.Id.DEDUCTION"
+        );
+        assertFileContains(Paths.get(outputPath + "/Fruit.kt"),
+                "sealed interface Fruit",
+                "@JsonTypeInfo(use = JsonTypeInfo.Id.NAME"
+        );
+    }
+
     @Test
     public void testSealedResponseInterfacesWithDeclarativeHttpInterface() throws IOException {
         File output = Files.createTempDirectory("test").toFile().getCanonicalFile();