How to mock an s3ObjectStream - testing

Question

I have code to test as follows:
#BeforeEach
static void setup() throws IOException {
Context context = new MockLambdaContext();
mockAmazonS3Client();
}
#Test
#DisplayName("Testing handleRequest returns complete when middle base64 piece")
public void handleRequestTestMiddlePiece() throws IOException {
ApplicationHandler applicationHandler = new ApplicationHandler();
mockAmazonS3Client();
input.replace("tag","middle");
assertEquals("complete", applicationHandler.handleRequest(input, context));
}
public static void mockAmazonS3Client() throws IOException {
AmazonS3Client mockAmazonS3Client = mock(AmazonS3Client.class);
S3Object s3Object = mock(S3Object.class);
S3ObjectInputStream s3ObjectInputStream = mock(S3ObjectInputStream.class);
InputStream testInputStream = new ByteArrayInputStream("Test".getBytes());
when(mockAmazonS3Client.getObject(any(String.class), any(String.class))).thenAnswer(invocationOnMock -> {
return s3Object;
});
when(s3ObjectInputStream.read(any(byte[].class))).thenAnswer(invocation -> {
return testInputStream.read(invocation.getArgument(0));
});
when(s3Object.getObjectContent()).thenReturn(s3ObjectInputStream);
new MockUp<AmazonS3Client>() {
#Mock
public S3Object getObject(String bucketName, String key) throws SdkClientException, AmazonServiceException {
return s3Object;
}
};
new MockUp<AmazonS3Client>() {
#Mock
PutObjectResult putObject(PutObjectRequest var1) throws SdkClientException, AmazonServiceException {
return null;
}
};
new MockUp<AmazonS3Client>() {
#Mock
public S3ObjectInputStream getObjectContent() {
return s3ObjectInputStream;
}
};
}
In the mockAmazonS3Client I have mocked the S3Object and the S3ObjectStream that getObject produces using mockito. I have managed to use a 'when' to have getObjectContent call an S3ObjectStream. However, when I use another 'when' to mock what happens if the S3ObjectSteam is read it throws an exception where 0 bytes are returned:
java.io.IOException: Underlying input stream returned zero bytes
It could be the second 'when' that is failing for some reason, though I'm not sure. Any thoughts will be gratefully received. Thanks.