[JAVA] Fix jackson (de)serializer annotation breaking libs using gson…

… when `bigDecimalAsString=true` (OpenAPITools#18835)

* Implement regression test for bigDecimalAsString annotations

* Ensure @JsonSerialize and @JsonDeserialize is only generated when Jackson enabled

Fixes OpenAPITools#6496

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

@@ -1779,7 +1779,7 @@ private void addAdditionalImports(CodegenModel model, CodegenComposedSchemas com
 
     @Override
     public void postProcessModelProperty(CodegenModel model, CodegenProperty property) {
-        if (serializeBigDecimalAsString) {
+        if (serializeBigDecimalAsString && additionalProperties.containsKey(JACKSON)) {
             if ("decimal".equals(property.baseType) || "bigdecimal".equalsIgnoreCase(property.baseType)) {
                 // we serialize BigDecimal as `string` to avoid precision loss
                 property.vendorExtensions.put("x-extra-annotation", "@JsonSerialize(using = ToStringSerializer.class)");
@@ -1804,7 +1804,7 @@ public void postProcessModelProperty(CodegenModel model, CodegenProperty propert
             model.imports.add("Arrays");
         } else if ("set".equals(property.containerType)) {
             model.imports.add("LinkedHashSet");
-            if (!openApiNullable || !property.isNullable) { // cannot be wrapped to nullable
+            if ((!openApiNullable || !property.isNullable) && additionalProperties.containsKey(JACKSON)) { // cannot be wrapped to nullable
                 model.imports.add("JsonDeserialize");
                 property.vendorExtensions.put("x-setter-extra-annotation", "@JsonDeserialize(as = LinkedHashSet.class)");
             }

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

@@ -58,6 +58,7 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.entry;
+import static org.assertj.core.api.InstanceOfAssertFactories.FILE;
 import static org.openapitools.codegen.CodegenConstants.SERIALIZATION_LIBRARY;
 import static org.openapitools.codegen.TestUtils.validateJavaSourceFiles;
 import static org.openapitools.codegen.languages.JavaClientCodegen.*;
@@ -2610,6 +2611,62 @@ private void testHandleURIEnum(String library, String[] expectedInnerEnumLines,
         );
     }
 
+    /**
+     * Regression test for <a href="https://github.com/OpenAPITools/openapi-generator/issues/6496">#6496</a>
+     */
+    @Test void doesNotGenerateJacksonToStringSerializerAnnotation_whenLibraryIsGson_andSerializeBigDecimalAsStringIsTrue() {
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+            .setGeneratorName("java")
+            .setLibrary(JavaClientCodegen.OKHTTP_GSON)
+            .addAdditionalProperty(CodegenConstants.SERIALIZATION_LIBRARY, SERIALIZATION_LIBRARY_GSON)
+            .addAdditionalProperty(CodegenConstants.SERIALIZE_BIG_DECIMAL_AS_STRING, true)
+            .addGlobalProperty(CodegenConstants.MODELS, "FormatTest")
+            .addGlobalProperty(CodegenConstants.MODEL_DOCS, "false")
+            .addGlobalProperty(CodegenConstants.MODEL_TESTS, "false")
+            .setInputSpec("src/test/resources/2_0/java/issue-6496.yaml")
+            .setOutputDir(newTempFolder().toString().replace("\\", "/"));
+
+        List<File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
+
+        assertThat(files).hasSize(1).first(FILE).content()
+            .doesNotContain(
+                "@JsonDeserialize(as = LinkedHashSet.class)",
+                "@JsonSerialize(using = ToStringSerializer.class)",
+                "com.fasterxml.jackson.databind.ser.std.ToStringSerializer",
+                "com.fasterxml.jackson.databind.annotation.JsonDeserialize",
+                "com.fasterxml.jackson.databind.annotation.JsonSerialize"
+            );
+    }
+
+    /**
+     * Test that fix for <a href="https://github.com/OpenAPITools/openapi-generator/issues/6496">#6496</a> has
+     * no unwanted side effects on the existing feature (Jackson + bigDecimalAsString)
+     */
+    @Test void generatesJacksonToStringSerializerAnnotation_whenLibraryIsJackson_andSerializeBigDecimalAsStringIsTrue() {
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+            .setGeneratorName("java")
+            .setLibrary(JavaClientCodegen.NATIVE)
+            .addAdditionalProperty(CodegenConstants.SERIALIZATION_LIBRARY, SERIALIZATION_LIBRARY_JACKSON)
+            .addAdditionalProperty(CodegenConstants.SERIALIZE_BIG_DECIMAL_AS_STRING, true)
+            .addAdditionalProperty(OPENAPI_NULLABLE, false)
+            .addGlobalProperty(CodegenConstants.MODELS, "FormatTest")
+            .addGlobalProperty(CodegenConstants.MODEL_DOCS, "false")
+            .addGlobalProperty(CodegenConstants.MODEL_TESTS, "false")
+            .setInputSpec("src/test/resources/2_0/java/issue-6496.yaml")
+            .setOutputDir(newTempFolder().toString().replace("\\", "/"));
+
+        List<File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
+
+        assertThat(files).hasSize(1).first(FILE).content()
+            .contains(
+                "@JsonDeserialize(as = LinkedHashSet.class)",
+                "@JsonSerialize(using = ToStringSerializer.class)",
+                "com.fasterxml.jackson.databind.ser.std.ToStringSerializer",
+                "com.fasterxml.jackson.databind.annotation.JsonDeserialize",
+                "com.fasterxml.jackson.databind.annotation.JsonSerialize"
+            );
+    }
+
     static private Path newTempFolder() {
         try {
             var tempDir = Files.createTempDirectory("test");

modules/openapi-generator/src/test/resources/2_0/java/issue-6496.yaml

@@ -0,0 +1,88 @@
+swagger: '2.0'
+info:
+  description: "This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\"
+  version: 1.0.0
+  title: OpenAPI Petstore
+  license:
+    name: Apache-2.0
+    url: 'https://www.apache.org/licenses/LICENSE-2.0.html'
+basePath: /v2
+schemes:
+  - http
+paths:
+  /:
+    get:
+      operationId: addPet
+      responses:
+        '200':
+          description: successful operation
+definitions:
+  FormatTest:
+    type: object
+    required:
+      - number
+      - byte
+      - date
+      - password
+    properties:
+      integer:
+        type: integer
+        maximum: 100
+        minimum: 10
+      int32:
+        type: integer
+        format: int32
+        maximum: 200
+        minimum: 20
+      int64:
+        type: integer
+        format: int64
+      number:
+        maximum: 543.2
+        minimum: 32.1
+        type: number
+      float:
+        type: number
+        format: float
+        maximum: 987.6
+        minimum: 54.3
+      double:
+        type: number
+        format: double
+        maximum: 123.4
+        minimum: 67.8
+      string:
+        type: string
+        pattern: /[a-z]/i
+      byte:
+        type: string
+        format: byte
+      binary:
+        type: string
+        format: binary
+      date:
+        type: string
+        format: date
+      dateTime:
+        type: string
+        format: date-time
+      uuid:
+        type: string
+        format: uuid
+        example: 72f98069-206d-4f12-9f12-3d1e525a8e84
+      password:
+        type: string
+        format: password
+        maxLength: 64
+        minLength: 10
+      BigDecimal:
+        type: string
+        format: number
+      enumContainer:
+        type: array
+        uniqueItems: true
+        items:
+          enum:
+            - placed
+            - approved
+            - delivered