Mocking Libraries for Clojure Testing

15.5.2 Mocking Libraries

As experienced Java developers transitioning to Clojure, you are likely familiar with the concept of mocking in unit tests. Mocking allows you to isolate the unit of work by replacing dependencies with controlled substitutes. In Clojure, mocking can be achieved through libraries like clojure.test.mock or by using built-in techniques such as with-redefs. This section will guide you through these tools and techniques, helping you to write effective and reliable tests in Clojure.

Understanding Mocking in Clojure

Mocking in Clojure serves the same purpose as in Java: to simulate the behavior of complex, unpredictable, or external systems during testing. However, Clojure’s functional nature and emphasis on immutability introduce unique approaches to mocking.

Key Differences from Java

• Immutability: Clojure’s immutable data structures mean that state changes are not as prevalent as in Java, reducing the need for mocks that track state changes.
• First-Class Functions: Functions in Clojure can be passed around and redefined easily, allowing for more straightforward mocking techniques.
• Dynamic Binding: Clojure’s dynamic nature allows for temporary redefinition of functions, which can be leveraged for mocking.

Mocking Libraries in Clojure

Several libraries and techniques can be used for mocking in Clojure. Let’s explore some of the most popular options.

clojure.test.mock

clojure.test.mock is a library that provides a straightforward way to create mocks and stubs in Clojure. It is designed to integrate seamlessly with clojure.test, the standard testing framework in Clojure.

Installation

To use clojure.test.mock, add it to your project.clj or deps.edn file:

;; project.clj
:dependencies [[org.clojure/clojure "1.10.3"]
               [clojure.test.mock "0.1.0"]]

;; deps.edn
{:deps {org.clojure/clojure {:mvn/version "1.10.3"}
        clojure.test.mock {:mvn/version "0.1.0"}}}

Basic Usage

Here’s a simple example of how to use clojure.test.mock to mock a function:

(ns myapp.core-test
  (:require [clojure.test :refer :all]
            [clojure.test.mock :refer :all]
            [myapp.core :as core]))

(deftest test-my-function
  (with-mocks [core/dependency-fn (mock (returns 42))]
    (is (= 42 (core/my-function)))))

In this example, core/dependency-fn is mocked to always return 42. The with-mocks macro temporarily replaces the function during the test.

Advanced Features

clojure.test.mock also supports more advanced features such as:

• Argument Matching: Specify expected arguments for mocked functions.
• Call Verification: Verify that mocked functions are called with expected arguments.

(deftest test-advanced-mocking
  (with-mocks [core/dependency-fn (mock (returns 42) (args [1 2]))]
    (core/my-function 1 2)
    (verify-mock core/dependency-fn)))

Using with-redefs for Mocking

Clojure provides a built-in mechanism for temporarily redefining functions using with-redefs. This is particularly useful for mocking functions in a test context.

Basic Usage

with-redefs allows you to redefine a function within a specific scope:

(ns myapp.core-test
  (:require [clojure.test :refer :all]
            [myapp.core :as core]))

(deftest test-my-function
  (with-redefs [core/dependency-fn (fn [& _] 42)]
    (is (= 42 (core/my-function)))))

In this example, core/dependency-fn is redefined to return 42 during the execution of the test.

Advantages and Limitations

• Advantages: Simple to use and does not require additional dependencies.
• Limitations: Does not provide built-in support for argument matching or call verification.

Comparing Mocking in Java and Clojure

Let’s compare how mocking is typically done in Java using a library like Mockito and how it differs in Clojure.

Java Example with Mockito

import static org.mockito.Mockito.*;
import org.junit.Test;

public class MyServiceTest {
    @Test
    public void testMyMethod() {
        Dependency dependency = mock(Dependency.class);
        when(dependency.someMethod()).thenReturn(42);

        MyService service = new MyService(dependency);
        assertEquals(42, service.myMethod());
    }
}

Clojure Equivalent

(ns myapp.core-test
  (:require [clojure.test :refer :all]
            [myapp.core :as core]))

(deftest test-my-function
  (with-redefs [core/dependency-fn (fn [& _] 42)]
    (is (= 42 (core/my-function)))))

Comparison:

• Setup: Java requires a mocking library like Mockito, while Clojure can use built-in features like with-redefs.
• Syntax: Clojure’s syntax is more concise, leveraging its functional nature.
• Flexibility: Clojure’s dynamic nature allows for more flexible mocking without additional libraries.

Try It Yourself

Experiment with the following code snippets to deepen your understanding of mocking in Clojure:

1. Modify the with-redefs example to mock a function that takes multiple arguments and returns a computed value.
2. Use clojure.test.mock to verify that a mocked function is called with specific arguments.

Diagrams and Visuals

Below is a diagram illustrating the flow of data through a mocked function using with-redefs:

    graph TD;
	    A[Original Function] -->|Redefinition| B[Mocked Function];
	    B --> C[Test Execution];
	    C --> D[Assertions];

Diagram 1: Flow of data through a mocked function using with-redefs.

Exercises

1. Exercise 1: Write a test using clojure.test.mock to mock a function that interacts with an external API. Verify that the function is called with the correct parameters.
2. Exercise 2: Refactor a Java test that uses Mockito into a Clojure test using with-redefs.

Key Takeaways

• Mocking in Clojure: Leverage clojure.test.mock and with-redefs to mock dependencies in your tests.
• Functional Approach: Clojure’s functional nature simplifies mocking by allowing functions to be easily redefined.
• Comparison with Java: Clojure offers a more concise and flexible approach to mocking compared to Java.

By mastering these mocking techniques, you can write more effective and reliable tests in Clojure, ensuring your applications are robust and maintainable.

Further Reading

• Official Clojure Documentation
• ClojureDocs
• clojure.test.mock GitHub Repository

Quiz: Mastering Mocking Libraries in Clojure