[kotlin][jvm-spring-restclient] honour nullableReturnType in body access

When `nullableReturnType=true` is set, the generated API function
signature correctly returns a nullable type (e.g. `String?`), but the
implementation force-unwraps the response body with `!!`. This throws
a NullPointerException at runtime whenever the server returns an empty
body, even though the signature promises that null is a valid return
value.

This change:

* Drops the `!!` in `api.mustache` when `nullableReturnType` is enabled.
* Propagates nullability into the generic type parameter of the
  `WithHttpInfo` variant for consistency.
* Relaxes the type parameter in `ApiClient.kt.mustache` from `T: Any`
  to `T: Any?` so that `ResponseEntity<T?>` compiles.
* Adds a new sample `kotlin-jvm-spring-3-restclient-nullable-return`
  that exercises `nullableReturnType=true`, including a regression
  test that reproduces the original NPE when the fix is reverted.

Author: takkiraz

.github/workflows/samples-kotlin-client.yaml

@@ -66,6 +66,7 @@ jobs:
           - samples/client/echo_api/kotlin-jvm-spring-3-webclient
           - samples/client/petstore/kotlin-jvm-spring-3-restclient
           - samples/client/echo_api/kotlin-jvm-spring-3-restclient
+          - samples/client/others/kotlin-jvm-spring-3-restclient-nullable-return
           - samples/client/petstore/kotlin-name-parameter-mappings
           - samples/client/others/kotlin-jvm-okhttp-parameter-tests
           - samples/client/others/kotlin-jvm-okhttp-path-comments

bin/configs/kotlin-jvm-spring-3-restclient-nullable-return.yaml

@@ -0,0 +1,11 @@
+generatorName: kotlin
+outputDir: samples/client/others/kotlin-jvm-spring-3-restclient-nullable-return
+library: jvm-spring-restclient
+inputSpec: modules/openapi-generator/src/test/resources/3_0/kotlin/nullable-return-type.yaml
+templateDir: modules/openapi-generator/src/main/resources/kotlin-client
+additionalProperties:
+  artifactId: kotlin-jvm-spring-3-restclient-nullable-return
+  enumUnknownDefaultCase: true
+  serializationLibrary: jackson
+  useSpringBoot3: true
+  nullableReturnType: true

bin/utils/test_file_list.yaml

@@ -66,3 +66,5 @@
   sha256: 82a6be39c1ed3dada96dfa1833a6709834cb3f9f9d50a19cbd9d49699e46df4f
 - filename: "samples/client/petstore/kotlin-jvm-spring-3-restclient/src/test/kotlin/org/openapitools/integration/UserApiTest.kt"
   sha256: bc64fb94857a3598e1332f1278307c3078ea9ec4b4aa75690e6eda86e9729a8d
+- filename: "samples/client/others/kotlin-jvm-spring-3-restclient-nullable-return/src/test/kotlin/org/openapitools/integration/NullableReturnTypeTest.kt"
+  sha256: 41ad4769f4d1bbf616056865deca44ed6dc414ca20d43e38fd9d40a1a81e9c19

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-spring-restclient/api.mustache

@@ -69,7 +69,7 @@ import {{packageName}}.infrastructure.*
     {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}fun {{operationId}}({{#allParams}}{{{paramName}}}: {{#isEnum}}{{#isContainer}}kotlin.collections.List<{{enumName}}{{operationIdCamelCase}}>{{/isContainer}}{{^isContainer}}{{enumName}}{{operationIdCamelCase}}{{/isContainer}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}{{^required}}?{{#defaultValue}} = {{>param_default_value}}{{/defaultValue}}{{^defaultValue}} = null{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}): {{#returnType}}{{{returnType}}}{{#nullableReturnType}}?{{/nullableReturnType}}{{/returnType}}{{^returnType}}Unit{{/returnType}} {
         {{#returnType}}val result = {{/returnType}}{{operationId}}WithHttpInfo({{#allParams}}{{{paramName}}} = {{{paramName}}}{{^-last}}, {{/-last}}{{/allParams}})
         {{#returnType}}
-        return result.body!!
+        return result.body{{^nullableReturnType}}!!{{/nullableReturnType}}
         {{/returnType}}
     }
 
@@ -79,7 +79,7 @@ import {{packageName}}.infrastructure.*
     {{/isDeprecated}}
     {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}fun {{operationId}}WithHttpInfo({{#allParams}}{{{paramName}}}: {{#isEnum}}{{#isContainer}}kotlin.collections.List<{{enumName}}{{operationIdCamelCase}}>{{/isContainer}}{{^isContainer}}{{enumName}}{{operationIdCamelCase}}{{/isContainer}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}{{^required}}?{{#defaultValue}} = {{>param_default_value}}{{/defaultValue}}{{^defaultValue}} = null{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}): ResponseEntity<{{#returnType}}{{{returnType}}}{{#nullableReturnType}}?{{/nullableReturnType}}{{/returnType}}{{^returnType}}Unit{{/returnType}}> {
         val localVariableConfig = {{operationId}}RequestConfig({{#allParams}}{{{paramName}}} = {{{paramName}}}{{^-last}}, {{/-last}}{{/allParams}})
-        return request<{{#hasBodyParam}}{{#bodyParams}}{{{dataType}}}{{/bodyParams}}{{/hasBodyParam}}{{^hasBodyParam}}{{^hasFormParams}}Unit{{/hasFormParams}}{{#hasFormParams}}Map<String, PartConfig<*>>{{/hasFormParams}}{{/hasBodyParam}}, {{{returnType}}}{{^returnType}}Unit{{/returnType}}>(
+        return request<{{#hasBodyParam}}{{#bodyParams}}{{{dataType}}}{{/bodyParams}}{{/hasBodyParam}}{{^hasBodyParam}}{{^hasFormParams}}Unit{{/hasFormParams}}{{#hasFormParams}}Map<String, PartConfig<*>>{{/hasFormParams}}{{/hasBodyParam}}, {{#returnType}}{{{returnType}}}{{#nullableReturnType}}?{{/nullableReturnType}}{{/returnType}}{{^returnType}}Unit{{/returnType}}>(
             localVariableConfig
         )
     }

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-spring-restclient/infrastructure/ApiClient.kt.mustache

@@ -10,13 +10,13 @@ import org.springframework.util.LinkedMultiValueMap
 
 {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}open class ApiClient(protected val client: RestClient) {
 
-    protected inline fun <reified I : Any, reified T: Any> request(requestConfig: RequestConfig<I>): ResponseEntity<T> {
+    protected inline fun <reified I : Any?, reified T: Any?> request(requestConfig: RequestConfig<I>): ResponseEntity<T> {
         return prepare(defaults(requestConfig))
             .retrieve()
             .toEntity(object : ParameterizedTypeReference<T>() {})
     }
 
-    protected fun <I : Any> prepare(requestConfig: RequestConfig<I>) =
+    protected fun <I : Any?> prepare(requestConfig: RequestConfig<I>) =
         client.method(requestConfig)
             .uri(requestConfig)
             .headers(requestConfig)
@@ -45,7 +45,7 @@ import org.springframework.util.LinkedMultiValueMap
     private fun <I> RestClient.RequestBodySpec.headers(requestConfig: RequestConfig<I>) =
         apply { requestConfig.headers.forEach { (name, value) -> header(name, value) } }
 
-    private fun <I : Any> RestClient.RequestBodySpec.nullableBody(requestConfig: RequestConfig<I>): RestClient.RequestBodySpec {
+    private fun <I : Any?> RestClient.RequestBodySpec.nullableBody(requestConfig: RequestConfig<I>): RestClient.RequestBodySpec {
         when {
             requestConfig.headers[HttpHeaders.CONTENT_TYPE] == MediaType.MULTIPART_FORM_DATA_VALUE -> {
                 val parts = LinkedMultiValueMap<String, Any>()
@@ -59,7 +59,7 @@ import org.springframework.util.LinkedMultiValueMap
             }
 
             else -> {
-                return apply { if (requestConfig.body != null) body(requestConfig.body) }
+                return apply { if (requestConfig.body != null) body(requestConfig.body as Any) }
             }
         }
     }

modules/openapi-generator/src/test/resources/3_0/kotlin/nullable-return-type.yaml

@@ -0,0 +1,34 @@
+openapi: 3.0.3
+info:
+  title: Nullable Return Type Test
+  version: 1.0.0
+paths:
+  /nullable-string:
+    post:
+      operationId: returnNullableString
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/PingRequest'
+      responses:
+        "200":
+          description: Successful response, body may be empty
+          content:
+            text/html:
+              schema:
+                type: string
+  /always-empty:
+    get:
+      operationId: returnNothing
+      responses:
+        "204":
+          description: No content
+components:
+  schemas:
+    PingRequest:
+      type: object
+      properties:
+        msg:
+          type: string

samples/client/echo_api/kotlin-jvm-spring-3-restclient/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,13 +10,13 @@ import org.springframework.util.LinkedMultiValueMap
 
 open class ApiClient(protected val client: RestClient) {
 
-    protected inline fun <reified I : Any, reified T: Any> request(requestConfig: RequestConfig<I>): ResponseEntity<T> {
+    protected inline fun <reified I : Any?, reified T: Any?> request(requestConfig: RequestConfig<I>): ResponseEntity<T> {
         return prepare(defaults(requestConfig))
             .retrieve()
             .toEntity(object : ParameterizedTypeReference<T>() {})
     }
 
-    protected fun <I : Any> prepare(requestConfig: RequestConfig<I>) =
+    protected fun <I : Any?> prepare(requestConfig: RequestConfig<I>) =
         client.method(requestConfig)
             .uri(requestConfig)
             .headers(requestConfig)
@@ -45,7 +45,7 @@ open class ApiClient(protected val client: RestClient) {
     private fun <I> RestClient.RequestBodySpec.headers(requestConfig: RequestConfig<I>) =
         apply { requestConfig.headers.forEach { (name, value) -> header(name, value) } }
 
-    private fun <I : Any> RestClient.RequestBodySpec.nullableBody(requestConfig: RequestConfig<I>): RestClient.RequestBodySpec {
+    private fun <I : Any?> RestClient.RequestBodySpec.nullableBody(requestConfig: RequestConfig<I>): RestClient.RequestBodySpec {
         when {
             requestConfig.headers[HttpHeaders.CONTENT_TYPE] == MediaType.MULTIPART_FORM_DATA_VALUE -> {
                 val parts = LinkedMultiValueMap<String, Any>()
@@ -59,7 +59,7 @@ open class ApiClient(protected val client: RestClient) {
             }
 
             else -> {
-                return apply { if (requestConfig.body != null) body(requestConfig.body) }
+                return apply { if (requestConfig.body != null) body(requestConfig.body as Any) }
             }
         }
     }

samples/client/others/kotlin-jvm-spring-3-restclient-nullable-return/.openapi-generator-ignore

@@ -0,0 +1,23 @@
+# OpenAPI Generator Ignore
+# Generated by openapi-generator https://github.com/openapitools/openapi-generator
+
+# Use this file to prevent files from being overwritten by the generator.
+# The patterns follow closely to .gitignore or .dockerignore.
+
+# As an example, the C# client generator defines ApiClient.cs.
+# You can make changes and tell OpenAPI Generator to ignore just this file by uncommenting the following line:
+#ApiClient.cs
+
+# You can match any string of characters against a directory, file or extension with a single asterisk (*):
+#foo/*/qux
+# The above matches foo/bar/qux and foo/baz/qux, but not foo/bar/baz/qux
+
+# You can recursively match patterns against a directory, file or extension with a double asterisk (**):
+#foo/**/qux
+# This matches foo/bar/qux, foo/baz/qux, and foo/bar/baz/qux
+
+# You can also negate patterns with an exclamation (!).
+# For example, you can ignore all files in a docs folder with the file extension .md:
+#docs/*.md
+# Then explicitly reverse the ignore rule for a single file:
+#!docs/README.md

samples/client/others/kotlin-jvm-spring-3-restclient-nullable-return/.openapi-generator/FILES

@@ -0,0 +1,17 @@
+README.md
+build.gradle
+docs/DefaultApi.md
+docs/PingRequest.md
+gradle/wrapper/gradle-wrapper.jar
+gradle/wrapper/gradle-wrapper.properties
+gradlew
+gradlew.bat
+settings.gradle
+src/main/kotlin/org/openapitools/client/apis/DefaultApi.kt
+src/main/kotlin/org/openapitools/client/infrastructure/ApiAbstractions.kt
+src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt
+src/main/kotlin/org/openapitools/client/infrastructure/PartConfig.kt
+src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt
+src/main/kotlin/org/openapitools/client/infrastructure/RequestMethod.kt
+src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt
+src/main/kotlin/org/openapitools/client/models/PingRequest.kt

samples/client/others/kotlin-jvm-spring-3-restclient-nullable-return/.openapi-generator/VERSION

@@ -0,0 +1 @@
+7.22.0-SNAPSHOT