How to mock Amazon S3 in an integration test

If you're ok with depending on a running docker container, and want something well-supported, you could consider using localstack

Before running your tests, start S3 like so:

docker run --name localstack -d -p 5000:5000 -e SERVICES=s3:5000 localstack/localstack

And then stop it when tests complete like so:

docker stop localstack

You'll need to configure your S3 client to point to localhost:5000 for tests. In Java, this can be done like so:

        AmazonS3ClientBuilder.standard()
            .withEndpointConfiguration(new AwsClientBuilder.EndpointConfiguration(
                 "http://localhost:5000", 
                 "us-west-2"))
            .build();

There is also an Findify s3mock tool written exactly for this purpose. It mocks the essential parts of AWS S3 API on top of local filesystem:

import io.findify.s3mock.S3Mock

S3Mock api = S3Mock.create(8001, "/tmp/s3");
api.start();

AmazonS3Client client = new AmazonS3Client(new AnonymousAWSCredentials());
// use local API mock, not the AWS one
client.setEndpoint("http://127.0.0.1:8001");
client.createBucket("testbucket");
client.putObject("testbucket", "file/name", "contents");

It's also easily embeddable and configuration-less.

Another option is S3 ninja - emulates the S3 API for development and testing purposes.