Mockito: How to mock spring special DI that the injected object doesn't have no-arg constructor

Question

I'm using Mockito 3.4.6 in unit test, actually, i have integrated Mockito to my unit test and it works well. While, now i need to optimize some unit test, it's a special dependency injection that the injected object doesn't have no-arg constructor, I tried @Spy but it didn't work.

My Test: I tried 1. @Spy; 2. @Spy with setting instance using = getDtInsightApi(); 3. @Spy with @InjectMocks, all of tests are failed. As Mockito docs said, seems it can't work for this case.

@InjectMocks Mockito will try to inject mocks only either by constructor injection, setter injection, or property injection in order and as described below.

Also if only use @Spy, it will throw MockitoException:

org.mockito.exceptions.base.MockitoException: 
Failed to release mocks

This should not happen unless you are using a third-part mock maker

...
Caused by: org.mockito.exceptions.base.MockitoException: Unable to initialize @Spy annotated field 'api'.
Please ensure that the type 'DtInsightApi' has a no-arg constructor.
...
Caused by: org.mockito.exceptions.base.MockitoException: Please ensure that the type 'DtInsightApi' has a no-arg constructor.

See my pseudocode as below:

configure class:

@Configuration
public class SdkConfig {

    @Resource
    private EnvironmentContext environmentContext;

    @Bean(name = "api")
    public DtInsightApi getApi() {
     
        DtInsightApi.ApiBuilder builder = new DtInsightApi.ApiBuilder()
                    .setServerUrls("sdkUrls")
        return builder.buildApi();
    }
}

DtInsightApi class with no public no-arg constructor and get instance by its inner class

public class DtInsightApi {
    private String[] serverUrls;

    DtInsightApi(String[] serverUrls) {
        this.serverUrls = serverUrls;
    }
    
    // inner class
    public static class ApiBuilder {
        String[] serverUrls;

        public ApiBuilder() {
        }
        ...code...

        public DtInsightApi buildApi() {
           return new DtInsightApi(this.serverUrls);
        }
    }

    ...code...

}

unit test class:

public Test{
   
   @Autowired
   private PendingTestService service;

   @Spy
   private Api api = getDtInsightApi();

   @Mock
   private MockService mockService;

   @Before
    public void setUp() throws Exception {
        // open mock
        MockitoAnnotations.openMocks(this);
        // i use doReturn(...).when() for @Spy object
        Mockito.doReturn(mockService).when(api)
                   .getSlbApiClient(MockService.class);
        Mockito.when(mockService.addOrUpdate(any(MockDTO.class)))
                   .thenReturn(BaseObject.getApiResponseWithSuccess());
    }

    public DtInsightApi getDtInsightApi () {
        return new DtInsightApi.ApiBuilder()
                .setServerUrls(new String[]{"localhost:8080"})
                .buildApi();
    }

    @Test
    public void testUpdate() {
        service.update();
    }
}

PendingTestService:

@Service
public class PendingTestService{
   
   @Autowired
   DtInsightApi api;

   public void update() {
      // here mockService isn't the object i mocked
      MockService mockService = api.getSlbApiClient(MockService.class);
      mockService.update();
   }
}

Question: How to mock the DI object DtInsightApi which doesn't have no-arg constructor.

Answer 1

After checked Spring docs about unit test, I found a solution using @MockBean.

Spirng docs:https://docs.spring.io/spring-boot/docs/1.5.2.RELEASE/reference/html/boot-features-testing.html

According to Spring docs, you can use @MockBean to mock a bean inside your ApplicationContext, so i can use @MockBean to mock DtInsightApi.

It’s sometimes necessary to mock certain components within your application context when running tests. For example, you may have a facade over some remote service that’s unavailable during development. Mocking can also be useful when you want to simulate failures that might be hard to trigger in a real environment.

Spring Boot includes a @MockBean annotation that can be used to define a Mockito mock for a bean inside your ApplicationContext. You can use the annotation to add new beans, or replace a single existing bean definition. The annotation can be used directly on test classes, on fields within your test, or on @Configuration classes and fields. When used on a field, the instance of the created mock will also be injected. Mock beans are automatically reset after each test method.

My Solution: Use @MockBean and BDDMockito.given(...).willReturn(...), use @Qualifier("api") to specify the bean name because @MockBean injected by class type, if you have same class beans, you need to specify bean name.

My code in test class:

public class Test{
    @MockBean
    @Qualifier("api")
    private DtInsightApi api;

   @Mock
   private MockService mockService;

    @Before
    public void setUp() throws Exception {
        // open mock
        MockitoAnnotations.openMocks(this);
        BDDMockito.given(this.api.getSlbApiClient(MockService.class)).willReturn(mockService);
    }

    @Autowired
    private PendingTestService service;


    @Test
    public void testUpdate() {
        service.update();
    }
}

Debug the mockService you can see mockService instance is generated by Mockito, mock succeed.

You can also refer to Spring docs example: mock RemoteService in unit test.

import org.junit.*;
import org.junit.runner.*;
import org.springframework.beans.factory.annotation.*;
import org.springframework.boot.test.context.*;
import org.springframework.boot.test.mock.mockito.*;
import org.springframework.test.context.junit4.*;

import static org.assertj.core.api.Assertions.*;
import static org.mockito.BDDMockito.*;

@RunWith(SpringRunner.class)
@SpringBootTest
public class MyTests {

    @MockBean
    private RemoteService remoteService;

    @Autowired
    private Reverser reverser;

    @Test
    public void exampleTest() {
        // RemoteService has been injected into the reverser bean
        given(this.remoteService.someCall()).willReturn("mock");
        String reverse = reverser.reverseSomeCall();
        assertThat(reverse).isEqualTo("kcom");
    }

}