[KOTLIN-SPRING] - add support for 'openApiNullable' - jackson nullable library to handle non-required AND nullable: true properties (OpenAPITools#23804)

* feat(kotlin): add support for OpenAPI Jackson Nullable library to handle optional and nullable properties

* feat(kotlin): add support for OpenAPI Jackson Nullable library to handle optional and nullable properties

* add more tests for prefix/suffix

* improve springBootApplication.mustache structure

* regenerate samples

* fix test

* update documentation

* fix jackson-annotations import package. regenerate samples

* refactor import mappings for Jackson 3.x support and improve clarity

Author: Picazsoo

bin/configs/kotlin-spring-boot-sort-validation.yaml

@@ -14,5 +14,6 @@ additionalProperties:
   useSpringBoot3: "true"
   generateSortValidation: "true"
   generatePageableConstraintValidation: "true"
+  openApiNullable: "true"
   useTags: "true"
   requestMappingMode: api_interface

docs/generators/kotlin-spring.md

@@ -44,6 +44,7 @@ These options may be applied as additional-properties (cli) or configOptions (pl
 |library|library template (sub-template)|<dl><dt>**spring-boot**</dt><dd>Spring-boot Server application.</dd><dt>**spring-cloud**</dt><dd>Spring-Cloud-Feign client with Spring-Boot auto-configured settings.</dd><dt>**spring-declarative-http-interface**</dt><dd>Spring Declarative Interface client</dd></dl>|spring-boot|
 |modelMutable|Create mutable models| |false|
 |modelPackage|model package for generated code| |org.openapitools.model|
+|openApiNullable|Enable OpenAPI Jackson Nullable library (jackson-databind-nullable) for optional + nullable properties (required: false, nullable: true). When enabled, such properties use JsonNullable&lt;T&gt; = JsonNullable.undefined() so callers can distinguish between a missing key and an explicitly provided null. Requires jackson-databind-nullable &gt;= 0.2.10 when used with useJackson3.| |false|
 |packageName|Generated artifact package name.| |org.openapitools|
 |parcelizeModels|toggle &quot;@Parcelize&quot; for generated models| |null|
 |reactive|use coroutines for reactive behavior| |false|
@@ -64,7 +65,7 @@ These options may be applied as additional-properties (cli) or configOptions (pl
 |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|
+|useJackson3|Use Jackson 3 dependencies (tools.jackson package). Only available with `useSpringBoot4`. Defaults to true when `useSpringBoot4` is enabled.| |false|
 |useResponseEntity|Whether (when false) to return actual type (e.g. List&lt;Fruit&gt;) and handle non-happy path responses via exceptions flow or (when true) return entire ResponseEntity (e.g. ResponseEntity&lt;List&lt;Fruit&gt;&gt;). If disabled, method are annotated using a @ResponseStatus annotation, which has the status of the first response declared in the Api definition| |true|
 |useSealedResponseInterfaces|Generate sealed interfaces for endpoint responses that all possible response types implement. Allows controllers to return any valid response type in a type-safe manner (e.g., sealed interface CreateUserResponse implemented by User, ConflictResponse, ErrorResponse)| |false|
 |useSpringBoot3|Generate code and provide dependencies for use with Spring Boot &ge; 3 (use jakarta instead of javax in imports). Enabling this option will also enable `useJakartaEe`.| |false|

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

@@ -229,6 +229,9 @@ public class CodegenConstants {
     public static final String NULLABLE_REFERENCE_TYPES = "nullableReferenceTypes";
     public static final String NULLABLE_REFERENCE_TYPES_DESC = "Use nullable annotations in the project. Only supported on C# 8 / ASP.NET Core 3.1 or newer.";
 
+    public static final String OPENAPI_NULLABLE = "openApiNullable";
+    public static final String OPENAPI_NULLABLE_DESC = "Enable OpenAPI Jackson Nullable library (jackson-databind-nullable) for optional + nullable properties.";
+
     public static final String TEMPLATING_ENGINE = "templatingEngine";
     public static final String TEMPLATING_ENGINE_DESC = "The templating engine plugin to use: \"mustache\" (default) or \"handlebars\" (beta)";
 

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

@@ -102,7 +102,7 @@ public abstract class AbstractJavaCodegen extends DefaultCodegen implements Code
     public static final String ADDITIONAL_ONE_OF_TYPE_ANNOTATIONS = "additionalOneOfTypeAnnotations";
     public static final String ADDITIONAL_ENUM_TYPE_ANNOTATIONS = "additionalEnumTypeAnnotations";
     public static final String DISCRIMINATOR_CASE_SENSITIVE = "discriminatorCaseSensitive";
-    public static final String OPENAPI_NULLABLE = "openApiNullable";
+    public static final String OPENAPI_NULLABLE = CodegenConstants.OPENAPI_NULLABLE;
     public static final String JACKSON = "jackson";
     public static final String TEST_OUTPUT = "testOutput";
     public static final String IMPLICIT_HEADERS = "implicitHeaders";

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

@@ -181,6 +181,7 @@ public String getDescription() {
     @Setter private boolean substituteGenericPagedModel = false;
     @Setter private boolean useSealedResponseInterfaces = false;
     @Setter private boolean companionObject = false;
+    @Getter @Setter private boolean openApiNullable = false;
     @Getter @Setter
     protected boolean useDeductionForOneOfInterfaces = false;
 
@@ -290,7 +291,7 @@ public KotlinSpringServerCodegen() {
                 " (contexts) added to single project.", beanQualifiers);
         addSwitch(USE_SPRING_BOOT3, "Generate code and provide dependencies for use with Spring Boot ≥ 3 (use jakarta instead of javax in imports). Enabling this option will also enable `useJakartaEe`.", useSpringBoot3);
         addSwitch(USE_SPRING_BOOT4, "Generate code and provide dependencies for use with Spring Boot 4.x. Enabling this option will also enable `useJakartaEe`.", useSpringBoot4);
-        addSwitch(USE_JACKSON_3, "Use Jackson 3 dependencies (tools.jackson package). Only available with `useSpringBoot4`. Defaults to true when `useSpringBoot4` is enabled. Incompatible with `openApiNullable`.", useJackson3);
+        addSwitch(USE_JACKSON_3, "Use Jackson 3 dependencies (tools.jackson package). Only available with `useSpringBoot4`. Defaults to true when `useSpringBoot4` is enabled.", useJackson3);
         addSwitch(USE_FLOW_FOR_ARRAY_RETURN_TYPE, "Whether to use Flow for array/collection return types when reactive is enabled. If false, will use List instead.", useFlowForArrayReturnType);
         addSwitch(INCLUDE_HTTP_REQUEST_CONTEXT, "Whether to include HttpServletRequest (blocking) or ServerWebExchange (reactive) as additional parameter in generated methods.", includeHttpRequestContext);
         addSwitch(USE_RESPONSE_ENTITY,
@@ -314,6 +315,12 @@ public KotlinSpringServerCodegen() {
                 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));
+        addSwitch(CodegenConstants.OPENAPI_NULLABLE,
+                "Enable OpenAPI Jackson Nullable library (jackson-databind-nullable) for optional + nullable "
+                + "properties (required: false, nullable: true). When enabled, such properties use "
+                + "JsonNullable<T> = JsonNullable.undefined() so callers can distinguish between a missing key "
+                + "and an explicitly provided null. Requires jackson-databind-nullable >= 0.2.10 when used with useJackson3.",
+                openApiNullable);
         supportedLibraries.put(SPRING_BOOT, "Spring-boot Server application.");
         supportedLibraries.put(SPRING_CLOUD_LIBRARY,
                 "Spring-Cloud-Feign client with Spring-Boot auto-configured settings.");
@@ -543,10 +550,14 @@ public void processOpts() {
         // used later in recursive import in postProcessingModels
         importMapping.put("com.fasterxml.jackson.annotation.JsonProperty", "com.fasterxml.jackson.annotation.JsonCreator");
 
-        if (isUseJackson3()) {
-            // Override databind imports for Jackson 3
-            importMapping.put("JsonDeserialize", "tools.jackson.databind.annotation.JsonDeserialize");
-        }
+        // Jackson 3.x intentionally kept jackson-annotations at 2.x (com.fasterxml.jackson.annotation).
+        // Only jackson-databind moved to tools.jackson.databind in Jackson 3.x.
+        importMapping.put("JsonSetter", "com.fasterxml.jackson.annotation.JsonSetter");
+        importMapping.put("Nulls", "com.fasterxml.jackson.annotation.Nulls");
+        // jackson-databind-nullable >= 0.2.10 supports both Jackson 2 and 3.
+        importMapping.put("JsonNullable", "org.openapitools.jackson.nullable.JsonNullable");
+        // JsonDeserialize lives in jackson-databind which moved packages in Jackson 3.x.
+        importMapping.put("JsonDeserialize", (isUseJackson3() ? JACKSON3_PACKAGE : JACKSON2_PACKAGE) + ".databind.annotation.JsonDeserialize");
 
         // Spring-specific import mappings for x-spring-paginated support
         importMapping.put("ParameterObject", "org.springdoc.api.annotations.ParameterObject");
@@ -767,10 +778,10 @@ public void processOpts() {
             throw new IllegalArgumentException("useJackson3 is only available with Spring Boot >= 4");
         }
 
-        if (isUseJackson3() && additionalProperties.containsKey("openApiNullable")
-                && Boolean.parseBoolean(additionalProperties.get("openApiNullable").toString())) {
-            throw new IllegalArgumentException("openApiNullable cannot be set with useJackson3");
+        if (additionalProperties.containsKey(CodegenConstants.OPENAPI_NULLABLE)) {
+            this.setOpenApiNullable(convertPropertyToBoolean(CodegenConstants.OPENAPI_NULLABLE));
         }
+        writePropertyBack(CodegenConstants.OPENAPI_NULLABLE, openApiNullable);
 
         if (isUseSpringBoot3() || isUseSpringBoot4()) {
             if (AnnotationLibrary.SWAGGER1.equals(getAnnotationLibrary())) {
@@ -1321,6 +1332,21 @@ public void postProcessModelProperty(CodegenModel model, CodegenProperty propert
             property.example = null;
         }
 
+        // Scenario 3: optional + non-nullable → block explicit JSON nulls via @JsonSetter(nulls = Nulls.FAIL).
+        // Missing keys still succeed (default = null is used), but explicit {"field": null} fails deserialization.
+        if (!Boolean.TRUE.equals(property.required) && !Boolean.TRUE.equals(property.isNullable)) {
+            property.vendorExtensions.put("x-has-json-setter-nulls-fail", true);
+            model.imports.add("JsonSetter");
+            model.imports.add("Nulls");
+        }
+
+        // Scenario 4: optional + nullable with openApiNullable → use JsonNullable<T> = JsonNullable.undefined()
+        // so callers can distinguish between a missing key and an explicitly provided null.
+        if (openApiNullable && !Boolean.TRUE.equals(property.required) && Boolean.TRUE.equals(property.isNullable)) {
+            property.vendorExtensions.put("x-is-jackson-optional-nullable", true);
+            model.imports.add("JsonNullable");
+        }
+
         //Add imports for Jackson
         if (!Boolean.TRUE.equals(model.isEnum)) {
             model.imports.add("JsonProperty");
@@ -1475,6 +1501,23 @@ public ModelsMap postProcessModelsEnum(ModelsMap objs) {
         //Add imports for Jackson
         List<Map<String, String>> imports = objs.getImports();
 
+        // Set 4-state nullable/required vendor extensions on optionalVars instances.
+        // optionalVars are cloned independently from vars by removeAllDuplicatedProperty(),
+        // so they require a separate pass here — postProcessModelProperty only covers vars.
+        for (ModelMap mo : objs.getModels()) {
+            CodegenModel cm = mo.getModel();
+            for (CodegenProperty var : cm.optionalVars) {
+                // Scenario 3: optional + non-nullable → block explicit JSON nulls via @JsonSetter(nulls = Nulls.FAIL)
+                if (!var.required && !var.isNullable) {
+                    var.vendorExtensions.put("x-has-json-setter-nulls-fail", true);
+                }
+                // Scenario 4: optional + nullable with openApiNullable → use JsonNullable<T>
+                if (openApiNullable && !var.required && var.isNullable) {
+                    var.vendorExtensions.put("x-is-jackson-optional-nullable", true);
+                }
+            }
+        }
+
         objs.getModels().stream()
                 .map(ModelMap::getModel)
                 .filter(cm -> Boolean.TRUE.equals(cm.isEnum) && cm.allowableValues != null)

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

@@ -2,5 +2,6 @@
     @Schema({{#example}}example = "{{#lambdaRemoveLineBreak}}{{#lambdaEscapeInNormalString}}{{{.}}}{{/lambdaEscapeInNormalString}}{{/lambdaRemoveLineBreak}}", {{/example}}{{#isReadOnly}}readOnly = {{{isReadOnly}}}, {{/isReadOnly}}description = "{{{description}}}"){{/swagger2AnnotationLibrary}}{{#swagger1AnnotationLibrary}}
     @ApiModelProperty({{#example}}example = "{{#lambdaRemoveLineBreak}}{{#lambdaEscapeInNormalString}}{{{.}}}{{/lambdaEscapeInNormalString}}{{/lambdaRemoveLineBreak}}", {{/example}}{{#isReadOnly}}readOnly = {{{isReadOnly}}}, {{/isReadOnly}}value = "{{{description}}}"){{/swagger1AnnotationLibrary}}{{#deprecated}}
     @Deprecated(message = ""){{/deprecated}}{{#vendorExtensions.x-field-extra-annotation}}
-    {{{.}}}{{/vendorExtensions.x-field-extra-annotation}}
-    @get:JsonProperty("{{{baseName}}}"){{#isInherited}} override{{/isInherited}} {{>modelMutable}} {{{name}}}: {{#isEnum}}{{#isArray}}{{baseType}}<{{/isArray}}{{classname}}.{{{nameInPascalCase}}}{{#isArray}}>{{/isArray}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}? = {{^defaultValue}}null{{/defaultValue}}{{#defaultValue}}{{^isNumber}}{{{defaultValue}}}{{/isNumber}}{{#isNumber}}{{{dataType}}}("{{{defaultValue}}}"){{/isNumber}}{{/defaultValue}}
+    {{{.}}}{{/vendorExtensions.x-field-extra-annotation}}{{#vendorExtensions.x-has-json-setter-nulls-fail}}
+    @field:JsonSetter(nulls = Nulls.FAIL){{/vendorExtensions.x-has-json-setter-nulls-fail}}
+    @get:JsonProperty("{{{baseName}}}"){{#isInherited}} override{{/isInherited}} {{>modelMutable}} {{{name}}}: {{#vendorExtensions.x-is-jackson-optional-nullable}}JsonNullable<{{#isEnum}}{{#isArray}}{{baseType}}<{{/isArray}}{{classname}}.{{{nameInPascalCase}}}{{#isArray}}>{{/isArray}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}>{{/vendorExtensions.x-is-jackson-optional-nullable}}{{^vendorExtensions.x-is-jackson-optional-nullable}}{{#isEnum}}{{#isArray}}{{baseType}}<{{/isArray}}{{classname}}.{{{nameInPascalCase}}}{{#isArray}}>{{/isArray}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}?{{/vendorExtensions.x-is-jackson-optional-nullable}} = {{#vendorExtensions.x-is-jackson-optional-nullable}}JsonNullable.undefined(){{/vendorExtensions.x-is-jackson-optional-nullable}}{{^vendorExtensions.x-is-jackson-optional-nullable}}{{^defaultValue}}null{{/defaultValue}}{{#defaultValue}}{{^isNumber}}{{{defaultValue}}}{{/isNumber}}{{#isNumber}}{{{dataType}}}("{{{defaultValue}}}"){{/isNumber}}{{/defaultValue}}{{/vendorExtensions.x-is-jackson-optional-nullable}}

modules/openapi-generator/src/main/resources/kotlin-spring/libraries/spring-boot/buildGradle-sb3-Kts.mustache

@@ -52,6 +52,8 @@ dependencies {
 {{#useBeanValidation}}
     implementation("org.springframework.boot:spring-boot-starter-validation")
     implementation("jakarta.validation:jakarta.validation-api"){{/useBeanValidation}}
+{{#openApiNullable}}
+    implementation("org.openapitools:jackson-databind-nullable:0.2.10"){{/openApiNullable}}
     implementation("jakarta.annotation:jakarta.annotation-api:2.1.0")
 
     testImplementation("org.jetbrains.kotlin:kotlin-test-junit5")

modules/openapi-generator/src/main/resources/kotlin-spring/libraries/spring-boot/buildGradle-sb4-Kts.mustache

@@ -61,6 +61,8 @@ dependencies {
 {{#useBeanValidation}}
     implementation("org.springframework.boot:spring-boot-starter-validation")
     implementation("jakarta.validation:jakarta.validation-api"){{/useBeanValidation}}
+{{#openApiNullable}}
+    implementation("org.openapitools:jackson-databind-nullable:0.2.10"){{/openApiNullable}}
     implementation("jakarta.annotation:jakarta.annotation-api:3.0.0")
 
     testImplementation("org.jetbrains.kotlin:kotlin-test-junit5")

modules/openapi-generator/src/main/resources/kotlin-spring/libraries/spring-boot/buildGradleKts.mustache

@@ -59,6 +59,8 @@ dependencies {
 {{#useBeanValidation}}
     implementation("org.springframework.boot:spring-boot-starter-validation")
     implementation("javax.validation:validation-api"){{/useBeanValidation}}
+{{#openApiNullable}}
+    implementation("org.openapitools:jackson-databind-nullable:0.2.8"){{/openApiNullable}}
     implementation("javax.annotation:javax.annotation-api:1.3.2")
     testImplementation("org.jetbrains.kotlin:kotlin-test-junit5")
     testImplementation("org.springframework.boot:spring-boot-starter-test") {

modules/openapi-generator/src/main/resources/kotlin-spring/libraries/spring-boot/pom-sb3.mustache

@@ -13,9 +13,9 @@
         <swagger-annotations.version>2.2.15</swagger-annotations.version>{{/swagger2AnnotationLibrary}}{{/springDocDocumentationProvider}}
         <findbugs-jsr305.version>3.0.2</findbugs-jsr305.version>
         <jakarta-annotation.version>2.1.0</jakarta-annotation.version>
-        <kotlin-test-junit5.version>1.7.10</kotlin-test-junit5.version>
+        <kotlin-test-junit5.version>1.9.25</kotlin-test-junit5.version>
 
-        <kotlin.version>1.7.10</kotlin.version>
+        <kotlin.version>1.9.25</kotlin.version>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     </properties>
     <parent>
@@ -197,12 +197,23 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-validation</artifactId>
         </dependency>{{/useBeanValidation}}
+{{#openApiNullable}}
+        <dependency>
+            <groupId>org.openapitools</groupId>
+            <artifactId>jackson-databind-nullable</artifactId>
+            <version>0.2.10</version>
+        </dependency>{{/openApiNullable}}
         <dependency>
             <groupId>jakarta.annotation</groupId>
             <artifactId>jakarta.annotation-api</artifactId>
             <version>${jakarta-annotation.version}</version>
             <scope>provided</scope>
         </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-test</artifactId>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>org.jetbrains.kotlin</groupId>
             <artifactId>kotlin-test-junit5</artifactId>