Testing my AWS Lambda function with a fake S3Event

Question

I am implementing a Java 8 package for a AWS Lambda that gets triggered from S3, gets some file data and puts into another S3 bucket.

This is my current Handler:

public class Handler implements RequestHandler<S3Event, Void> {
    private static final String TARGET_BUCKET = "some-bucket";

    private AmazonS3Client s3Client = new AmazonS3Client(new DefaultAWSCredentialsProviderChain());
    private Runner runner = new Runner(s3Client, TARGET_BUCKET);

    @Override
    public Void handleRequest(S3Event s3Event, Context context) {
        runner.run(s3Event, context);

        return null;
    }
}

I have moved the business logic to my Runner class, so that I can properly test it (following the AWS Lambda best practices white paper).

However, I am struggling to see how I can pass a fake S3Event to test my run function.

My test currently is:

@Test
public void putsDataIntoS3() throws IOException {
    runner.run(new ObjectMapper().readValue(loadJsonFromFile("s3-event.json"), S3Event.class), context);

    assertTrue(true);
}

Where loadJsonFromFile gets the resource with the filename I am passing, converts it into an input stream and then into a String.

However, that results in an error:

com.fasterxml.jackson.databind.JsonMappingException: No suitable constructor found for type [simple type, class com.amazonaws.services.lambda.runtime.events.S3Event]: can not instantiate from JSON object (missing default constructor or creator, or perhaps need to add/enable type information?)

So my question is, how can I properly test my run function by passing fake S3Event JSON?

These are the aws-related dependencies I am using:

<dependency>
    <groupId>com.amazonaws</groupId>
    <artifactId>aws-lambda-java-core</artifactId>
    <version>1.1.0</version>
</dependency>
<dependency>
    <groupId>com.amazonaws</groupId>
    <artifactId>aws-lambda-java-events</artifactId>
    <version>2.2.2</version>
</dependency>
<dependency>
    <groupId>com.amazonaws</groupId>
    <artifactId>aws-java-sdk-s3</artifactId>
    <version>1.11.455</version>
</dependency>

EDIT:

I can use the S3Event.parseJson function like this: I've also seen the parseJson function, and I can use it like this:

@Test
public void putsFilteredDataIntoS3() throws IOException {
    runner.run(new S3Event(S3Event.parseJson(loadJsonFromFile("s3-event.json")).getRecords()), context);

    assertTrue(true);
}

But is doing all this a best practice?

Answer 1

You can use Mockito library and try to mock this S3Event.

Another option to create S3Event from JSON:

S3EventNotification notification = S3EventNotification.parseJson(loadJsonFromFile("s3-event.json"));
S3Event event = new S3Event(notification.getRecords());

EDIT: Third option is to update your aws-lambda-java-events to version 2.2.4, where they added default constructor for S3Event so you will be able to deserialize it like this:

objectMapper.readValue(loadJsonFromFile("s3-event.json"), S3Event.class)