Unit test using junit and mockito for getObject method in AmazonS3

Question

I have method which uses s3.getObject to get S3Object and writes the contents of the object into a temporary file

@Override
    public Optional<String> getObject(String s3BucketName, String s3Path) {
        try {
            S3Object s3Object = s3Client.getObject(new GetObjectRequest(s3BucketName, s3Path));
            try (S3ObjectInputStream s3ObjectInputStream = s3Object.getObjectContent()){
                File tmp = File.createTempFile("/tmp/" + UUID.randomUUID().toString(), ".json");
                IOUtils.copy(s3ObjectInputStream, new FileOutputStream(tmp));
                return Optional.of(tmp.getAbsolutePath());
            } catch (Exception e) {
                System.err.println(e.getMessage());
            }
        } catch (AmazonServiceException e) {
            String msg = String.format("Service error while getting object=%s in bucket=%s",
                    s3Path, s3BucketName);
            throw new RuntimeException(msg, e);
        } catch (SdkClientException e) {
            String msg = String.format("Client error while getting object=%s in bucket=%s",
                    s3Path, s3BucketName);
            throw new RuntimeException(msg + e.getMessage());
        }
        return Optional.empty();
    }

I'm not sure I understand how to write the unit test for this method. Here is what I have tried

@Test
    public void getObjectTest() throws UnsupportedEncodingException {
        S3Object s3Object = Mockito.mock(S3Object.class);
        s3Object.setObjectContent(new StringInputStream(TEST_STRING));
        Mockito.when(mockS3Client.getObject(new GetObjectRequest(TEST_S3BUCKET, TEST_S3OBJECT))).thenReturn(s3Object);
        s3Accessor.getObject(TEST_S3BUCKET, TEST_S3OBJECT);
        verify(mockS3Client).getObject(new GetObjectRequest(TEST_S3BUCKET, TEST_S3OBJECT));
    }

I'm new to unit tests and I'm not sure what I can assert here since I'm getting only an absolute path of the file from the method. Can someone advice me on this?

Answer 1

There are a few things I would suggest changing in the test Do not mock the S3Object rather build the object from a local file, setting the value to the mocked object isn't correct.

You should add tests for exceptions as well.

Check out the below code

import com.amazonaws.AmazonServiceException;
import com.amazonaws.services.s3.AmazonS3Client;
import com.amazonaws.services.s3.model.GetObjectRequest;
import com.amazonaws.services.s3.model.S3Object;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.junit.MockitoJUnitRunner;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.UnsupportedEncodingException;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertThrows;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

@RunWith(MockitoJUnitRunner.class)
public class ExampleServiceTest {

    @Mock
    private AmazonS3Client mockS3Client;

    @Test
    public void getObjectTest() throws UnsupportedEncodingException, FileNotFoundException {
        String testBucketName = "test_bucket", s3Path = "/files/A.json";
        S3Object s3Object = buildS3Object();
        when(mockS3Client.getObject(new GetObjectRequest(testBucketName, s3Path))).thenReturn(s3Object);
        ExampleService exampleService = new ExampleService(mockS3Client);

        exampleService.getObject(testBucketName, s3Path);

        verify(mockS3Client).getObject(new GetObjectRequest(testBucketName, s3Path));
    }


    @Test
    public void ShouldThrowRuntimeExceptionServiceErrorMessageWhenAmazonServiceException() throws FileNotFoundException {
        String testBucketName = "test_bucket", s3Path = "/files/A.json";

        when(mockS3Client.getObject(new GetObjectRequest(testBucketName, s3Path))).thenThrow(AmazonServiceException.class);
        ExampleService exampleService = new ExampleService(mockS3Client);

        Exception exception = assertThrows(RuntimeException.class, () -> {
            exampleService.getObject(testBucketName, s3Path);
        });

        assertEquals("Service error while getting object=/files/A.json in bucket=test_bucket", exception.getMessage());
    }

    private S3Object buildS3Object() throws FileNotFoundException {
        S3Object s3Object = new S3Object();
        s3Object.setObjectContent(new FileInputStream("your_path/src/test/resources/A.json"));
        return s3Object;
    }
}