Fix issue 22209: [JAVA-gen] Fix oneOf inheritance: prevent generation of abstract wrapper and ensure real POJO model is produced

- Add useOneOfPojo option to generate real POJO classes for oneOf schemas instead of wrapper models
- Override updateModelForComposedSchema() to restore original vars when option is enabled
- Process the option in processOpts() method with default value false for backward compatibility

fixes #22209

Author: Chhida

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

@@ -114,6 +114,7 @@ public abstract class AbstractJavaCodegen extends DefaultCodegen implements Code
 
     public static final String CAMEL_CASE_DOLLAR_SIGN = "camelCaseDollarSign";
     public static final String USE_ONE_OF_INTERFACES = "useOneOfInterfaces";
+    public static final String USE_ONE_OF_POJO = "useOneOfPojo";
     public static final String LOMBOK = "lombok";
     public static final String DEFAULT_TEST_FOLDER = "${project.build.directory}/generated-test-sources/openapi";
     public static final String GENERATE_CONSTRUCTOR_WITH_ALL_ARGS = "generateConstructorWithAllArgs";
@@ -343,6 +344,7 @@ public AbstractJavaCodegen() {
         cliOptions.add(CliOption.newBoolean(CodegenConstants.HIDE_GENERATION_TIMESTAMP, CodegenConstants.HIDE_GENERATION_TIMESTAMP_DESC, this.isHideGenerationTimestamp()));
         cliOptions.add(CliOption.newBoolean(WITH_XML, "whether to include support for application/xml content type and include XML annotations in the model (works with libraries that provide support for JSON and XML)"));
         cliOptions.add(CliOption.newBoolean(USE_ONE_OF_INTERFACES, "whether to use a java interface to describe a set of oneOf options, where each option is a class that implements the interface"));
+        cliOptions.add(CliOption.newBoolean(USE_ONE_OF_POJO, "Generate real POJO classes for oneOf schemas instead of wrapper models").defaultValue("false"));
 
         CliOption dateLibrary = new CliOption(DATE_LIBRARY, "Option. Date library to use").defaultValue(this.getDateLibrary());
         Map<String, String> dateOptions = new HashMap<>();
@@ -402,6 +404,31 @@ public AbstractJavaCodegen() {
         cliOptions.add(CliOption.newString(CodegenConstants.DEFAULT_TO_EMPTY_CONTAINER, CodegenConstants.DEFAULT_TO_EMPTY_CONTAINER_DESC));
     }
 
+    /**
+     * Do not merge properties for ComposedSchema avec schema.getOneOf() not empty
+     *
+     * {@inheritDoc}
+     * @see org.openapitools.codegen.DefaultCodegen#updateModelForComposedSchema(org.openapitools.codegen.CodegenModel, io.swagger.v3.oas.models.media.Schema, java.util.Map)
+     */
+    @SuppressWarnings("rawtypes")
+    @Override
+    protected void updateModelForComposedSchema(CodegenModel m, Schema schema, Map<String, Schema> allDefinitions)
+    {
+        final List<CodegenProperty> varsOrigin = new ArrayList<>(m.vars);
+        final List<CodegenProperty> optionalVarsOrigin = new ArrayList<>(m.optionalVars);
+
+        super.updateModelForComposedSchema(m, schema, allDefinitions);
+
+        if(schema.getOneOf() != null && !schema.getOneOf().isEmpty() ) {
+            LOGGER.info("Do not merge properties for ComposedSchema avec schema.getOneOf() not empty {} -> {} ", m.getClassname(), schema.getOneOf());
+            if (Boolean.TRUE.equals(additionalProperties.get(USE_ONE_OF_POJO))) {
+                m.vars = varsOrigin;
+                m.optionalVars = optionalVarsOrigin;
+                m.oneOf = new TreeSet<>();
+            }
+        }
+    }
+
     @Override
     public void processOpts() {
         useCodegenAsMustacheParentContext();
@@ -447,6 +474,17 @@ public void processOpts() {
             additionalProperties.put(ANNOTATION_LIBRARY, AnnotationLibrary.NONE);
         }
 
+        // -----------------------------------------------------------------
+        // `useOneOfPojo` – backward compatibility switch.
+        // set to true to get the POJO
+        // -----------------------------------------------------------------
+        if (additionalProperties.containsKey(USE_ONE_OF_POJO)) {
+            Boolean flag = Boolean.valueOf(additionalProperties.get(USE_ONE_OF_POJO).toString());
+            additionalProperties.put(USE_ONE_OF_POJO, flag);
+        } else {
+            additionalProperties.put(USE_ONE_OF_POJO, Boolean.FALSE);
+        }
+
         convertPropertyToBooleanAndWriteBack(GENERATE_CONSTRUCTOR_WITH_ALL_ARGS, this::setGenerateConstructorWithAllArgs);
         convertPropertyToBooleanAndWriteBack(GENERATE_BUILDERS, this::setGenerateBuilders);
         convertPropertyToBooleanAndWriteBack(DISABLE_DISCRIMINATOR_JSON_IGNORE_PROPERTIES, this::setDisableDiscriminatorJsonIgnoreProperties);