15.5.1 Purpose of Mocks and Stubs

Understanding Mocks and Stubs in Clojure Testing

Testing is a crucial aspect of software development, ensuring that the code behaves as expected under various conditions. As we build applications with Clojure, it’s important to address how to handle tests involving side effects or interactions with external systems—which is where mocking and stubbing come into play.

What Are Mocks and Stubs?

• Mocks are objects created to simulate the behavior of real objects in controlled ways. Mocks can be programmed to verify interactions, such as whether certain methods were called, with what arguments, and how many times.

• Stubs are also simulated objects but are generally used to provide predetermined responses to specific calls, mainly to isolate the tests from unwanted side effects or dependencies.

Why Use Mocks and Stubs?

1. Isolation: By simulating dependencies like databases, web services, or any external systems, mocks and stubs keep unit tests isolated, focusing on the function being tested.

2. Control: They give developers control over the behavior of out-of-bound components, allowing tests to cover scenarios that might be unpredictable or difficult to reproduce in a live system.

3. Efficiency: Tests run faster as they aren’t reliant on the performance or reliability of external systems.

4. Safety: Avoid repeated and irreversible side effects like database writes during testing, preserving state and integrity.

When to Mock or Stub

• Testing External Dependencies: Use a mock when your function interacts with an external dependency, such as a microservice or a third-party API call, and you want to assert how your code interacts with that service.

• Control Over Test Environment: Use stubs to simulate responses from dependencies known in advance, like reading from a predefined file or database record.

• Complex Interactions: In scenarios where services involve complex setups or interactions, mocks and stubs enable assembling a predictable environment without cumbersome configuration.

Example: Mocking a Web Service

Let’s consider a scenario where a function retrieves user data from a user service. Instead of calling the real service, we use a mock to simulate both a successful call and a failure, ensuring our function handles all cases.

;; Java approach might resemble using a framework like Mockito
User mockUser = mock(UserService.class);
when(mockUser.getUserData(anyString())).thenReturn(new User("MockUser"));

;; Clojure equivalent using a mock function
(defn fetch-user-data [user-service user-id]
  (user-service user-id))

(defmock-special-mock
  (fetch-user-data (fn [id]
                     ;; Check behavior for mocked user data calling
                     {:name "MockUser"})))

Best Practices

• Specify Only What’s Necessary: When stubbing or mocking, specify only the characteristics necessary for the test.

• Test Realistic Scenarios: Ensure mocked behavior aligns closely with plausible scenarios to keep tests relevant.

• Maintain Readability: Use mocks and stubs to enhance test coverage, but avoid overusing them to prevent complex and unreadable tests.

Conclusion

Mocks and stubs are invaluable tools in testing Clojure applications, providing the ability to simulate external systems and control side-effects. They enhance the robustness of tests, promoting better code quality and confidence in your software. Embrace these techniques to pave the way for more reliable, maintainable Clojure applications.