Using Mockito to help test retry code...

There are times when you want to make sure that your code can retry an operation that might occaisionally fail due to environment issues.  For example, while using AWS SDK to get an object from S3 you might sometimes get an AmazonClientException - perhaps because of an endpoint being temporarily unreachable. You can simulate this situation using Mockito for the AmazonS3Client.

Mockito provides a fluent style way of how your mock objects respond.  Here is an example:

    AmazonS3Client mockClient = mock(AmazonS3Client.class);
    S3Object mockObject = mock(S3Object.class);

    when(mockClient.getObject(anyString(), anyString()))
        .thenThrow(new AmazonClientException("Something bad happened."))
        .thenReturn(mockObject);

This will cause the code to throw an AmazonClientException on the first call to getObject(), but then return the mocked S3Object on the second call to getObject().

This means that you can have happy path tests where the call eventually succeeds, and a test that will show how your code handles all retries failing, by just setting up your mock with the desired configuration of "thenThrow" and "thenReturn" entries.