fix(java): serialize primitive arrays in Gson oneOf

modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/oneof_model.mustache

@@ -81,9 +81,16 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
                     if (value.getActualInstance() instanceof {{#isArray}}List<?>{{/isArray}}{{#isMap}}Map<?, ?>{{/isMap}}{{^isMap}}{{^isArray}}{{{dataType}}}{{/isArray}}{{/isMap}}) {
                     {{#isPrimitiveType}}
                     {{^isMap}}
+                    {{#isArray}}
+                        JsonArray array = adapter{{#sanitizeDataType}}{{{dataType}}}{{/sanitizeDataType}}.toJsonTree(({{{dataType}}})value.getActualInstance()).getAsJsonArray();
+                        elementAdapter.write(out, array);
+                        return;
+                    {{/isArray}}
+                    {{^isArray}}
                         JsonPrimitive primitive = adapter{{#sanitizeDataType}}{{{dataType}}}{{/sanitizeDataType}}.toJsonTree(({{{dataType}}})value.getActualInstance()).getAsJsonPrimitive();
                         elementAdapter.write(out, primitive);
                         return;
+                    {{/isArray}}
                     {{/isMap}}
                     {{#isMap}}
                         JsonObject object = adapter{{#sanitizeDataType}}{{{dataType}}}{{/sanitizeDataType}}.toJsonTree(({{{dataType}}})value.getActualInstance()).getAsJsonObject();

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

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2026 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.openapitools.codegen.java;
+
+import com.github.javaparser.StaticJavaParser;
+import com.github.javaparser.ast.body.ClassOrInterfaceDeclaration;
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.TypeAdapterFactory;
+import org.openapitools.codegen.DefaultGenerator;
+import org.openapitools.codegen.config.CodegenConfigurator;
+import org.testng.annotations.Test;
+
+import javax.tools.ToolProvider;
+import java.io.ByteArrayOutputStream;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.List;
+
+import static org.openapitools.codegen.TestUtils.newTempFolder;
+import static org.testng.Assert.*;
+
+public class JavaOkHttpGsonOneOfPrimitiveArrayTest {
+    @Test
+    public void testPrimitiveArrayRoundTrip() throws Exception {
+        Path output = newTempFolder();
+        CodegenConfigurator config = new CodegenConfigurator().setGeneratorName("java").setLibrary("okhttp-gson")
+                .setInputSpec("src/test/resources/3_0/java/oneof-primitive-array.yaml").setOutputDir(output.toString())
+                .addGlobalProperty("models", "").addGlobalProperty("modelDocs", "false")
+                .addGlobalProperty("modelTests", "false");
+        new DefaultGenerator().opts(config.toClientOptInput()).generate();
+        ClassOrInterfaceDeclaration union = StaticJavaParser.parse(output.resolve("src/main/java/org/openapitools/client/model/Union.java"))
+                .getClassByName("Union").orElseThrow();
+        ClassOrInterfaceDeclaration factory = union.getMembers().stream()
+                .filter(m -> m instanceof ClassOrInterfaceDeclaration).map(m -> (ClassOrInterfaceDeclaration) m)
+                .filter(c -> c.getNameAsString().equals("CustomTypeAdapterFactory")).findFirst().orElseThrow();
+
+        // Exercise the generated factory with a minimal model stub and the real Gson runtime.
+        String fixture = "import com.google.gson.*;\nimport com.google.gson.reflect.*;\n"
+                + "import com.google.gson.stream.*;\nimport java.io.*;\nimport java.lang.reflect.Type;\n"
+                + "import java.util.*;\nimport java.util.logging.*;\n"
+                + "public class UnionFixture { public static class Union {\n"
+                + "private static final Logger log = Logger.getLogger(Union.class.getName());\n"
+                + "private Object value; public Object getActualInstance() { return value; }\n"
+                + "public void setActualInstance(Object value) { this.value = value; }\n"
+                + factory + "\n"
+                + "public static class Variant { public String kind; "
+                + "public static void validateJsonElement(JsonElement e) throws IOException { "
+                + "if (!e.isJsonObject() || !e.getAsJsonObject().has(\"kind\")) throw new IOException(); } }\n"
+                + "} }\n";
+        Path source = output.resolve("UnionFixture.java");
+        Files.writeString(source, fixture);
+        String classpath = Paths.get(Gson.class.getProtectionDomain().getCodeSource().getLocation().toURI()).toString();
+        ByteArrayOutputStream diagnostics = new ByteArrayOutputStream();
+        assertNotNull(ToolProvider.getSystemJavaCompiler());
+        assertEquals(ToolProvider.getSystemJavaCompiler().run(null, diagnostics, diagnostics,
+                "--release", "8", "-classpath", classpath, "-d", output.toString(), source.toString()),
+                0, diagnostics.toString(StandardCharsets.UTF_8));
+        try (URLClassLoader loader = new URLClassLoader(new URL[]{output.toUri().toURL()}, getClass().getClassLoader())) {
+            Class<?> type = loader.loadClass("UnionFixture$Union");
+            TypeAdapterFactory instance = (TypeAdapterFactory) loader.loadClass("UnionFixture$Union$CustomTypeAdapterFactory")
+                    .getConstructor().newInstance();
+            Gson gson = new GsonBuilder().registerTypeAdapterFactory(instance).create();
+            String json = "[\"alpha\",\"beta\"]";
+            Object result = gson.fromJson(json, type);
+            assertTrue(type.getMethod("getActualInstance").invoke(result) instanceof List);
+            assertEquals(gson.toJson(result, type), json);
+            json = "{\"kind\":\"known\"}";
+            result = gson.fromJson(json, type);
+            assertEquals(type.getMethod("getActualInstance").invoke(result).getClass().getSimpleName(), "Variant");
+            assertEquals(gson.toJson(result, type), json);
+        }
+    }
+}

modules/openapi-generator/src/test/resources/3_0/java/oneof-primitive-array.yaml

@@ -0,0 +1,19 @@
+openapi: 3.0.3
+info:
+  title: oneOf primitive array
+  version: '1'
+paths: {}
+components:
+  schemas:
+    Union:
+      oneOf:
+        - $ref: '#/components/schemas/Variant'
+        - type: array
+          items:
+            type: string
+    Variant:
+      type: object
+      required: [kind]
+      properties:
+        kind:
+          type: string

samples/client/petstore/java/okhttp-gson-3.1/src/main/java/org/openapitools/client/model/RefToRefParameterAnyofRefToAnyofParameter.java

@@ -84,8 +84,8 @@ public void write(JsonWriter out, RefToRefParameterAnyofRefToAnyofParameter valu
                     }
                     // check if the actual instance is of the type `List<String>`
                     if (value.getActualInstance() instanceof List<?>) {
-                        JsonPrimitive primitive = adapterListString.toJsonTree((List<String>)value.getActualInstance()).getAsJsonPrimitive();
-                        elementAdapter.write(out, primitive);
+                        JsonArray array = adapterListString.toJsonTree((List<String>)value.getActualInstance()).getAsJsonArray();
+                        elementAdapter.write(out, array);
                         return;
                     }
                     throw new IOException("Failed to serialize as the type doesn't match oneOf schemas: List<String>, String");

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/ArrayOneOf.java

@@ -84,8 +84,8 @@ public void write(JsonWriter out, ArrayOneOf value) throws IOException {
                     }
                     // check if the actual instance is of the type `List<String>`
                     if (value.getActualInstance() instanceof List<?>) {
-                        JsonPrimitive primitive = adapterListString.toJsonTree((List<String>)value.getActualInstance()).getAsJsonPrimitive();
-                        elementAdapter.write(out, primitive);
+                        JsonArray array = adapterListString.toJsonTree((List<String>)value.getActualInstance()).getAsJsonArray();
+                        elementAdapter.write(out, array);
                         return;
                     }
                     throw new IOException("Failed to serialize as the type doesn't match oneOf schemas: Integer, List<String>");

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/FakeOneOfWIthSameErasureGet200Response.java

@@ -80,8 +80,8 @@ public void write(JsonWriter out, FakeOneOfWIthSameErasureGet200Response value)
 
                     // check if the actual instance is of the type `List<String>`
                     if (value.getActualInstance() instanceof List<?>) {
-                        JsonPrimitive primitive = adapterListString.toJsonTree((List<String>)value.getActualInstance()).getAsJsonPrimitive();
-                        elementAdapter.write(out, primitive);
+                        JsonArray array = adapterListString.toJsonTree((List<String>)value.getActualInstance()).getAsJsonArray();
+                        elementAdapter.write(out, array);
                         return;
                     }
                     throw new IOException("Failed to serialize as the type doesn't match oneOf schemas: List<Integer>, List<String>");