Serivce "entity" inside the controller getiing null object when test with testng & mockito and sends the response to the Bad Request i.e testfails

ControllerClass, entity object inside the controller getting null return from service.save when run with testng

@RestController
@RequestMapping("/api")
public class PostController{

 @PostMapping("/post")
public ResponseEntity<PostResponse> savePostInfo(@RequestBody PostRequest req) throws Exception {


    PostEntity entity = postService.savePost(req.getData()); 
   //above line returning null when testng test perform
   
    PostResponse response = new PostResponse();
    if (entity != null) {
     
        response.setId(req.getData().getPostId());
        response.setData(req.getData());
        response.setStatus(Status.SUCCESS);
        response.setMessage("Post Information Saved");
        response.setIsAuthenticated(Boolean.TRUE);
        return new ResponseEntity<>(response, HttpStatus.OK);

     } else {
         
        response.setStatus(Status.FAILURE);
        response.setMessage("Data not Saved");
        return new ResponseEntity<>(response, HttpStatus.BAD_REQUEST);

     }
   }
}

TestClass

@RunWith(SpringRunner.class)  
@ContextConfiguration(classes=PostController.class)
@WebAppConfiguration
@WebMvcTest
public class PostControllerTest extends AbstractTestNGSpringContextTests{

private static final Logger logger = LoggerFactory.getLogger(PostControllerTest.class);

@Autowired
private WebApplicationContext wac;

@Autowired
private PostController postController;

  @Autowired
  ObjectMapper mapper;


  @MockBean
  private PostService postServiceUT = mock(PostService.class);
     
  private MockMvc mockMvc;

before method

  @BeforeMethod
  public void init() {
  mockMvc = MockMvcBuilders.webAppContextSetup(wac).build();
  } 

test method, mockMvc.perform getting the status.BAD_REQUEST due to PostEntity entity = postService.savePost(req.getData()) returning null in testng test, Can someone help me how to write a test for above controller method.

 @Test
 public void savePostInfoTest() throws Exception {
    
       PostEntity entity =new PostEntity();

       entity.setAccountId(11);
       entity.setCampaignId(12);
       entity.setContent("one");
       entity.setDateTime("now");
       entity.setExternalAccountId(13);
       entity.setExternalLink("link1");
       entity.setPostId(14);
       entity.setStatusId(15);
       entity.setTypeId(16);
       
       PostRequest req = new PostRequest();

       req.setData(entity);
      
       PostResponse response = new PostResponse();
         
       response.setId(entity.getPostId());
       response.setData(entity);
       response.setStatus(Status.SUCCESS);
       response.setMessage("Post Information Saved");
       response.setIsAuthenticated(Boolean.TRUE);
            
        logger.info("-------inside logger info  before Mockito.when");
                    
          
            
        Mockito.when(postServiceUT.savePost(Mockito.any(PostEntity.class))).thenReturn(entity);
        
          logger.info("-------inside logger info  before mvc.perform");
          logger.info("-------------data ="+req.getData());
          
     
          
       mockMvc.perform(post("/api/post").contentType(MediaType.APPLICATION_JSON_VALUE)
                .accept(MediaType.APPLICATION_JSON).characterEncoding("UTF-8")
                    .content(mapper.writeValueAsString(req)))
                    .andDo(print())
                    .andExpect(status().isOk())
                    .andExpect(jsonPath("$.message", is("Post Information Saved")))
                    .andExpect(jsonPath("$.status", is("SUCCESS"))) 
                    .andExpect(jsonPath("$.isAuthenticated", is(true)));
        
}}