The problem with your test is that you are trying to use to MockitoJUnitRunner.class in a wrong way.

If you are mocking a Service using @InjectMocks you need to make sure you need to return the value Service.getProperty() by mocking the service call. If you are using SpringRunner.class then you shouldn't have @InjectMocks but should have @Autowired for the service. Following test works.

@RunWith(SpringRunner.class)
@SpringBootTest
public class ServiceTest {
    @Autowired
    Service service;

    @Autowired
    Environment environment;

    @Value("${prop}")
    private String expectedProperty;

    @Test
    public void testGetPropertyWithValueAnnotation() {
        assertEquals(expectedProperty, service.getProperty());
    }

    @Test
    public void testGetPropertyWithEnvironment() {
        assertEquals(environment.getProperty("prop"), service.getProperty());
    }
}

The problem with your test is that you are trying to use to MockitoJUnitRunner.class in a wrong way.

If you are mocking a Service using @InjectMocks you need to make sure you need to return the value Service.getProperty() by mocking the service call. If you are using SpringRunner.class then you shouldn't have @InjectMocks but should have @Autowired for the service. Following test works.

@RunWith(SpringRunner.class)
@SpringBootTest
public class ServiceTest {
    @Autowired
    Service service;

    @Autowired
    Environment environment;

    @Value("${prop}")
    private String expectedProperty;

    @Test
    public void testGetPropertyWithValueAnnotation() {
        assertEquals(expectedProperty, service.getProperty());
    }

    @Test
    public void testGetPropertyWithEnvironment() {
        assertEquals(environment.getProperty("prop"), service.getProperty());
    }
}