NullPointerException 而 mocking s3Service + junit

Question

I am using amazon s3 for the document storage. I need to write test cases for the service class which is using s3.

How to mock the s3 object?

I have tried the below, but getting the NPE.

sample test case:

@ExtendWith(MockitoExtension.class)
class MyServiceTest {

    @InjectMocks
    private MyService myService;

    @InjectMocks
    private S3Service s3Service;
    @Mock
    private S3Configuration.S3Storage s3Storage;
    @Mock
    private AmazonS3 amazonS3;

    @BeforeEach
    public void setup() {
        ReflectionTestUtils.setField(myService, "s3Service", s3Service);
        ReflectionTestUtils.setField(s3Service, "s3Storage", s3Storage);
    }

 @Test
    void getDetailTest() {

        given(s3Storage.getClient()).willReturn(amazonS3);
        given(s3Service.getBucket()).willReturn("BUCKET1");
        given(s3Service.readFromS3(s3Service.getBucket(),"myfile1.txt")).willReturn("hello from s3 file"); //Null pointer exception
}
}

Below is the s3 service class sample where it is throwing NPE.

public String readFromS3(String bucketName, String key) {
        var s3object = s3Storage.getClient().getObject(new GetObjectRequest(bucketName, key));
//s3Object is getting null when run the test case.
    //more logic

}

How to mock s3Service.readFromS3() from MyService class?

Answer 1

You don't need to mock each object in your test, it's quite fine to construct your service using mocks...

@ExtendWith(MockitoExtension.class)
class MyServiceTest {

    private MyService myService;

    @Mock
    private AmazonS3 amazonS3;

    @BeforeEach
    public void setup() {
        this.myService = new MyService(amazonS3);
    }

    @Test
    void getDetailTest() {
        given(this.amazonS3.getXXX()).willReturn(new Yyyyy());
        assertEqual("ok", this.myService.doSmth());
    }
}