Avoiding Object Instantiation in Test Files: A Guide to Testing with Scala and Redis

Learn how to prevent unwanted object instantiation in your Scala tests, particularly when dealing with Redis in Jenkins. Discover effective strategies to streamline your testing process.
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This video is based on the question https://stackoverflow.com/q/71658636/ asked by the user 'AlleXyS' ( https://stackoverflow.com/u/9275568/ ) and on the answer https://stackoverflow.com/a/71658961/ provided by the user 'Tim' ( https://stackoverflow.com/u/7662670/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Avoiding Object Instantiation in Test Files: A Guide to Testing with Scala and Redis

When designing software in Scala, especially when using libraries like Akka and Redis, we sometimes encounter the challenge of managing test environments. A frequent issue developers face is unnecessary object instantiation during test runs. Specifically, if your tests inadvertently instantiate a singleton object that interfaces with a live Redis server, it can lead to failures and unwanted dependencies. In this guide, we'll explore the problem and provide a clean solution to avoid this common pitfall.

The Challenge: Unnecessary Instantiation

In our case study, we have an object called RedisPubSubService that sets up a connection to a Redis server. During testing—particularly when running in CI/CD environments like Jenkins—this object gets instantiated even if it’s not required for the tests being executed. This leads to failed tests due to timeout errors when the non-existent Redis server is accessed.

The main challenge here is that the RedisPubSubService object initializes resources that are outside the control of our tests, making the test suite unreliable and fragile.

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When we instantiate our application in the test file, all the val fields in RedisPubSubService get initialized, which can cause unwanted delays and failures:

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Since StartUpService is a singleton, it’s not as simple to disable the instantiation of other objects.

Solution: Modularize Your Code

Based on previous experiences and community input, a clean solution would involve creating a separate module for testing. Here’s how to implement this solution step by step:

Step 1: Create a Test Module

By defining a new TestModule, you can selectively bind only the components you want during your tests. In this case, we’ll bind TraitRepository and omit StartUpService, effectively isolating the dependencies that cause issues.

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Step 2: Override the Existing Module

In your test setup, you can now instruct the GuiceApplicationBuilder to disable the original module and use TestModule instead. This way, you avoid initializing unnecessary resources:

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By applying this design pattern, you prevent instantiating the RedisPubSubService class during your tests, helping you avoid connection issues and timeout exceptions.

Alternative Approaches

Use Lazy Initialization: You can also mark relevant vals as lazy, ensuring they're only initialized when invoked. This can further help reduce unwanted instantiation issues.

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Mocking Services: For unit testing, another approach is to use mocks. Create a mock version of your services, ensuring that your tests run cleanly without external dependencies. A mock service could be defined in a separate object like RedisPubSubMock, which mimics the real service but only initializes during tests.

Conclusion

By introducing test-specific modules and making strategic changes like lazy initialization or mocking, you can create a robust testing environment that remains resilient to external dependencies. This approach will streamline your Continuous Integration workflows and keep your tests green—ensuring you can develop with confidence.

For developers using Scala with Redis or similar technologies, these practices can be invaluable in maintaining the quality and reliability of your software.

If you found this guide helpful, feel free to share it with your fellow developers encountering similar challenges in test setups!

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