Mockito - returning null

Question

I have the following implementation of class A, using spring boot. A is an abstraction over restTemplate to make GET/POST/PUT RestAPI calls. Tests are written using Mockito.

Class A {

@Bean
RestTemplate restTemplate;

public class A(RestTemplate restTemplate){
    this.restTemplate = restTemplate;
}

public ResponseEntity perform(String endPoint, String requestBody, String auth, HttpMethod httpMethod){
    ...code to create parameters to pass to the exchange method
    ResponseEntity responseEntity = restTemplate.exchange(url, httpMethod, requestEntity, String.class)
    return responseEntity;
 }

}

Unit Test With Mockito:

class ATest{
    A a;

    @Spy
    RestTemplate restTemplate;

    @Before
    public void setUp() {
        a = Mockito.spy(new A(restTemplate));
    }
    
      @Test
    public void testHttpUtil(){
        ResponseEntity responseEntity = new ResponseEntity(HttpStatus.OK);
        HttpHeaders headers = new HttpHeaders();
        headers.set("Authorization" , "testAuth");
        HttpEntity<String> requestEntity = new HttpEntity<String>("testPayload", headers);
        Mockito.doReturn(responseEntity).when(restTemplate).exchange(Mockito.any(),Mockito.eq(HttpMethod.POST), Mockito.any(HttpEntity.class), Mockito.eq(String.class));
        ResponseEntity responseEntity1 = a.perform("https://example.com/v1/testapi", "testpayload", "testauthinfo", MediaType.APPLICATION_JSON_VALUE, HttpMethod.POST );
        Assert.assertNotNull(responseEntity1);
        Mockito.verify(restTemplate, Mockito.atMost(1)).exchange(Mockito.any(),Mockito.eq(HttpMethod.POST), Mockito.any(HttpEntity.class), Mockito.eq(String.class));

    }
}

My thought behind this implementation is to mock the restTemplate's exchange method, and return a response, upon calling A's perform method. Right now null is being returned upon A's perform method. Looks like I am doing something wrong. Can someone pls assist me on this?

To circumvent this, I mocked the RestTemplate and used

Mockito.verify(restTemplate, Mockito.atMost(1)).exchange(Mockito.any(),Mockito.eq(HttpMethod.POST), Mockito.any(HttpEntity.class), Mockito.eq(String.class)); to find out how many times restTemplate.exchange method has been called. Tests are passing now. But still want to know what's wrong with the implementation posted in the question

Answer 1

Update: On instantiating the restTemplate, the test passed. Did the following change to the test code.

@Spy
RestTemplate restTemplate = new RestTemplate();