Test case issue with Java

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Hi I am having trouble with writing test cases when using java+springboot+temporal stack.
the test class & the workflowimpl class are attached below. if instead of UserModel Object i return a string value it works fine but if i return the userModel object or object of any kind it fails with null value.
It might be a dumb question or a silly mistake on my part but i have tried alot of approaches from my end. if someone can help me that would be great. the full project is here https://github.com/Ranjan-A/temporal-poc/tree/main/temporal-trial-2/temporal

public class UserWorkflowImplTest {
    private TestWorkflowEnvironment testEnv;
    private Worker worker;
    private WorkflowClient client;

    @Before
    public void setUp(){
        testEnv = TestWorkflowEnvironment.newInstance();
        worker = testEnv.newWorker("123");
        worker.registerWorkflowImplementationTypes(UserWorkflowImpl.class);
        client = testEnv.getWorkflowClient();
    }
    @After
    public void tearDown(){
        testEnv.close();
    }
    @Test
    public void testMockedflow(){
        UserActivity userActivity = 
        Mockito.mock(UserActivity.class,Mockito.withSettings().withoutAnnotations());
        UserModel userModel = new UserModel();
        userModel.setId("Ashish");
        Mockito.when(userActivity.userGreeting("Ashish")).thenReturn(userModel);
        Mockito.when(userActivity.conveyGreeting(userModel)).thenReturn("Hello Ashish");
        worker.registerActivitiesImplementations(userActivity);
        testEnv.start();
        UserWorkflow workflow = client.newWorkflowStub(
                UserWorkflow.class, WorkflowOptions.newBuilder().setTaskQueue("123").build()
        );
        String greeting = workflow.userGreeting("Ashish");
        assertEquals("Hello Ashish",greeting);
    }
}
 
@WorkflowImpl(taskQueues = "123")
public class UserWorkflowImpl implements UserWorkflow{
    private ActivityOptions options = 
    ActivityOptions.newBuilder().setStartToCloseTimeout(Duration.ofSeconds(30))
            .build();
    private UserActivity activity = Workflow.newActivityStub(UserActivity.class,options);
    @Override
    public String userGreeting(String name) {
        String greeting="";
       //String greeting = activity.greetingActivity(name);
        UserModel userModel = activity.userGreeting(name);
        System.out.println(userModel.toString());
        if(userModel.getId()!=null){
            greeting=activity.conveyGreeting(userModel);
        }
       return greeting;
    }
}