fix(jaxrs): correct add/remove methods for JsonNullable<List<T>> fields

modules/openapi-generator/src/main/resources/JavaJaxRS/spec/pojo.mustache

@@ -173,36 +173,84 @@ public class {{classname}} {{#parent}}extends {{{.}}}{{/parent}} {{#vendorExtens
 
   {{#isArray}}
   public {{classname}} add{{nameInPascalCase}}Item({{{items.datatypeWithEnum}}} {{name}}Item) {
+    {{#vendorExtensions.x-is-jackson-optional-nullable}}
+    if (this.{{name}} == null || !this.{{name}}.isPresent() || this.{{name}}.get() == null) {
+      this.{{name}} = JsonNullable.<{{{datatypeWithEnum}}}>of({{{defaultValue}}}{{^defaultValue}}new {{#uniqueItems}}LinkedHashSet{{/uniqueItems}}{{^uniqueItems}}ArrayList{{/uniqueItems}}<>(){{/defaultValue}});
+    }
+    try {
+      this.{{name}}.get().add({{name}}Item);
+    } catch (java.util.NoSuchElementException e) {
+      // this can never happen, as we make sure above that the value is present
+    }
+    return this;
+    {{/vendorExtensions.x-is-jackson-optional-nullable}}
+    {{^vendorExtensions.x-is-jackson-optional-nullable}}
     if (this.{{name}} == null) {
       this.{{name}} = {{{defaultValue}}}{{^defaultValue}}new {{#uniqueItems}}LinkedHashSet{{/uniqueItems}}{{^uniqueItems}}ArrayList{{/uniqueItems}}<>(){{/defaultValue}};
     }
 
     this.{{name}}.add({{name}}Item);
     return this;
+    {{/vendorExtensions.x-is-jackson-optional-nullable}}
   }
 
   public {{classname}} remove{{nameInPascalCase}}Item({{{items.datatypeWithEnum}}} {{name}}Item) {
+    {{#vendorExtensions.x-is-jackson-optional-nullable}}
+    if ({{name}}Item != null && this.{{name}} != null && this.{{name}}.isPresent()) {
+      try {
+        this.{{name}}.get().remove({{name}}Item);
+      } catch (java.util.NoSuchElementException e) {
+        // this can never happen, as we make sure above that the value is present
+      }
+    }
+    {{/vendorExtensions.x-is-jackson-optional-nullable}}
+    {{^vendorExtensions.x-is-jackson-optional-nullable}}
     if ({{name}}Item != null && this.{{name}} != null) {
       this.{{name}}.remove({{name}}Item);
     }
+    {{/vendorExtensions.x-is-jackson-optional-nullable}}
 
     return this;
   }
   {{/isArray}}
   {{#isMap}}
   public {{classname}} put{{nameInPascalCase}}Item(String key, {{{items.datatypeWithEnum}}} {{name}}Item) {
+    {{#vendorExtensions.x-is-jackson-optional-nullable}}
+    if (this.{{name}} == null || !this.{{name}}.isPresent() || this.{{name}}.get() == null) {
+      this.{{name}} = JsonNullable.<{{{datatypeWithEnum}}}>of({{{defaultValue}}}{{^defaultValue}}new HashMap<>(){{/defaultValue}});
+    }
+    try {
+      this.{{name}}.get().put(key, {{name}}Item);
+    } catch (java.util.NoSuchElementException e) {
+      // this can never happen, as we make sure above that the value is present
+    }
+    return this;
+    {{/vendorExtensions.x-is-jackson-optional-nullable}}
+    {{^vendorExtensions.x-is-jackson-optional-nullable}}
     if (this.{{name}} == null) {
       this.{{name}} = {{{defaultValue}}}{{^defaultValue}}new HashMap<>(){{/defaultValue}};
     }
 
     this.{{name}}.put(key, {{name}}Item);
     return this;
+    {{/vendorExtensions.x-is-jackson-optional-nullable}}
   }
 
   public {{classname}} remove{{nameInPascalCase}}Item(String key) {
+    {{#vendorExtensions.x-is-jackson-optional-nullable}}
+    if (this.{{name}} != null && this.{{name}}.isPresent()) {
+      try {
+        this.{{name}}.get().remove(key);
+      } catch (java.util.NoSuchElementException e) {
+        // this can never happen, as we make sure above that the value is present
+      }
+    }
+    {{/vendorExtensions.x-is-jackson-optional-nullable}}
+    {{^vendorExtensions.x-is-jackson-optional-nullable}}
     if (this.{{name}} != null) {
       this.{{name}}.remove(key);
     }
+    {{/vendorExtensions.x-is-jackson-optional-nullable}}
 
     return this;
   }
@@ -283,4 +331,4 @@ public class {{classname}} {{#parent}}extends {{{.}}}{{/parent}} {{#vendorExtens
     }
     {{/vars}}
   }{{/additionalProperties}}{{/generateBuilders}}
-}
+}

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/jaxrs/JavaJAXRSSpecServerCodegenTest.java

@@ -1236,5 +1236,43 @@ public void disableGenerateJsonCreator() throws Exception {
 
         assertFileNotContains(files.get("RequiredProperties.java").toPath(), "@JsonCreator");
     }
+
+    @Test
+    public void testGenerateJsonNullableListFieldsHelperMethodReferences_issue23251() throws Exception {
+        Map<String, Object> properties = new HashMap<>();
+        properties.put(OPENAPI_NULLABLE, "true");
+
+        File output = Files.createTempDirectory("test").toFile();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+            .setGeneratorName("jaxrs-spec")
+            .setAdditionalProperties(properties)
+            .setInputSpec("src/test/resources/bugs/issue_23251.yaml")
+            .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        final ClientOptInput clientOptInput = configurator.toClientOptInput();
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(clientOptInput).generate();
+
+        validateJavaSourceFiles(files);
+
+        TestUtils.ensureContainsFile(files, output, "src/gen/java/org/openapitools/model/BugResponse.java");
+
+        // Assert that the generated model contains JsonNullable fields
+        assertFileContains(output.toPath().resolve("src/gen/java/org/openapitools/model/BugResponse.java"),
+            "private JsonNullable<String> nullableField = JsonNullable.<String>undefined();",
+            "private JsonNullable<List<String>> nullableList = JsonNullable.<List<String>>undefined();",
+            "private JsonNullable<List<@Valid NestedResponse>> nullableObjectList = JsonNullable.<List<@Valid NestedResponse>>undefined();"
+        );
+
+        // Assert that the generated model contains correct add and remove helper methods reference for JsonNullable fields
+        assertFileContains(output.toPath().resolve("src/gen/java/org/openapitools/model/BugResponse.java"),
+            "this.nullableList.get().add(nullableListItem);",
+                "this.nullableList.get().remove(nullableListItem);",
+            "this.nullableObjectList.get().add(nullableObjectListItem);",
+            "this.nullableObjectList.get().remove(nullableObjectListItem);");
+
+        output.deleteOnExit();
+    }
 
 }

modules/openapi-generator/src/test/resources/bugs/issue_23251.yaml

@@ -0,0 +1,44 @@
+openapi: 3.0.2
+info:
+  title: Reproduce
+  version: 1.0.0
+paths:
+  /test:
+    get:
+      summary: test
+      operationId: test
+      responses:
+        200:
+          description: Test
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/BugResponse'
+components:
+  schemas:
+    BugResponse:
+      type: object
+      properties:
+        nullableField:
+          type: string
+          nullable: true
+          x-is-jackson-optional-nullable: true
+        nullableList:
+          type: array
+          items:
+            type: string
+          nullable: true
+          x-is-jackson-optional-nullable: true
+        nullableObjectList:
+          type: array
+          items:
+            $ref: '#/components/schemas/NestedResponse'
+          nullable: true
+          x-is-jackson-optional-nullable: true
+    NestedResponse:
+      type: object
+      properties:
+        nestedField:
+          type: string
+          nullable: true
+          x-is-jackson-optional-nullable: true