How can I unit test the getObject method of the AWS S3 SDK using Java?

Question

I'm working with Java and I'm using the AWS SDK for interact with S3. I've the following method and I want to unit test it

private final S3Client s3Client;
...
...
public byte[] download(String key) throws IOException {
    GetObjectRequest getObjectRequest = GetObjectRequest.builder()
            .bucket("myBucket")
            .key(key)
            .build();
    return s3Client.getObject(getObjectRequest).readAllBytes();
}

For this purpose I'm using JUnit 5 and Mockito. The problem is that I don't know how to mock the result of

s3Client.getObject(getObjectRequest) 

because the return type

ResponseInputStream<GetObjectResponse> 

is a final class.

Any idea or suggestions? Thank you

Answer 1

In case anyone is still looking for a different solution this is how I did it:
This is the code that needs to be mocked:

InputStream objectStream =
    this.s3Client.getObject(
        GetObjectRequest.builder().bucket(bucket).key(key).build(),
        ResponseTransformer.toInputStream());

This is how to mock it:

S3Client s3Client = Mockito.mock(S3Client.class);
String bucket = "bucket";
String key = "key";
InputStream objectStream = getFakeInputStream();
when(s3Client.getObject(
        Mockito.any(GetObjectRequest.class),
        ArgumentMatchers
            .<ResponseTransformer<GetObjectResponse, ResponseInputStream<GetObjectResponse>>>
                any()))
    .then(
        invocation -> {
          GetObjectRequest getObjectRequest = invocation.getArgument(0);
          assertEquals(bucket, getObjectRequest.bucket());
          assertEquals(key, getObjectRequest.key());

          return new ResponseInputStream<>(
              GetObjectResponse.builder().build(), AbortableInputStream.create(objectStream));
        });

Answer 2

The problem is solved. In a maven project you can add a file named "org.mockito.plugins.MockMaker" in the folder "src/test/resources/mockito-extensions".

Inside the file, add "mock-maker-inline" without quotes.

From now Mockito will be able to mock final classes also.