使用 static 方法初始化的私有字段的单元测试方法

Question

In my class TwitchService I'm creating a private field twitchClient using TwitchClientBuilder . How can I approach testing method sendChatMessage() which is using twitchClient ?

public class TwitchService {

    public TwitchService(){
        twitchClient = TwitchClientBuilder.builder().build();
    }

    public void sendChatMessage(String content) throws Exception {
        if(content.isEmpty()){
            throw new Exception("Cannot send empty message!");
        }

        twitchClient.getChat().sendMessage(content);
    }
}

So far, I have tried mocking TwitchClientBuilder.builder() and twitchCliendBuilder.build() like this:

@RunWith(MockitoJUnitRunner.class)
public class TwitchServiceTest {

    private TwitchService twitchService;

    @Mock
    private TwitchClientBuilder twitchClientBuilder;


    @Test
    public void sendChatMessage() throws Exception {

        try (MockedStatic<TwitchClientBuilder> twitchClientBuilderMockedStatic = Mockito.mockStatic(TwitchClientBuilder.class)){
            twitchClientBuilderMockedStatic.when(() -> TwitchClientBuilder.builder()).thenReturn(mock(TwitchClientBuilder.class));

            when(twitchClientBuilder.build()).thenReturn(mock(TwitchClient.class));
            twitchService = new TwitchService("x", "y");
            twitchService.sendChatMessage("test message");
        }

    }
}

But I get NullPointerException on twitchClient in sendChatMessage() Any ideas how to properly mock fields like this?

Answer 1

Use constructor dependency injection for TwitchClient , instead of hard-coding it by calling builder.

Simply re-write your service, like so:

public class TwitchService {

    private final TwitchClient twitchClient

    public TwitchService(final TwitchClient twitchClient) {
        this.twitchClient = twitchClient;
    }

...
}

TwitchService is now loosely-coupled with the implementation of TwitchClient , which gives you ability to mock the client in the unit tests and use real-implementation (unit tested by itself) in real-code.

So in unit-test, we can mock TwitchClient :

@RunWith(MockitoJUnitRunner.class)
public class TwitchServiceTest {

    @Mock
    private TwitchClient mockedTwitchClient;

    // Tested instance
    private TwitchService twitchService;

    @Before
    public void before() {
        twitchService = new TwitchService(mockedTwitchClient);
    }

    @Test
    public void ourTest() {
        when(mockedTwitchClient.xyz()).thenReturn(...);
        ...
    }

and in the real code, we "inject" real TwitchClient built by TwitchClientBuilder :

...
twitchService = new TwitchService(TwitchClientBuilder.builder().build());
...