Problem

• Complexity

• False sense of security.

• Parallel/Paired objects (real and mocks) can lead to maintainability issues.

• Maintainability

Solution

• Mock just non-business entities.

• Remove the mock if its interface has too much behavior.

Sample Code

Wrong

class PaymentTest {
    @Test
    fun `process payment should return true on successful payment`() {
        val paymentDetails = mapOf(
            "amount" to "100",
            "currency" to "USD",
            "cardNumber" to "1234567890123456",
            "expiryDate" to "12/20",
            "cvv" to "123"
        )

        // We should not mock a business object
        val payment = mockk<Payment>()
        every { payment.process(any(), any()) } returns true

        // This is an external and coupled system.
        // We have no control on it so tests might be fragile
        val authorizeNet = AuthorizeNetAIM(Payment.API_ID, Payment.TRANSACTION_KEY)

        val result = payment.process(authorizeNet, paymentDetails)
        assertTrue(result)
    }
}

Right

class PaymentTest {
    @Test
    fun `process payment should return true on successful payment`() {
        val paymentDetails = mapOf(
            "amount" to "100",
            "currency" to "USD",
            "cardNumber" to "1234567890123456",
            "expiryDate" to "12/20",
            "cvv" to "123"
        )

        val payment = Payment()

        // External system is mocked
        val response = Response(approved = true, transactionId = "1234567890")
        val authorizeNet = mockk<AuthorizeNetAIM>()
        every { authorizeNet.authorizeAndCapture() } returns response

        val result = payment.process(authorizeNet, paymentDetails)
        assertTrue(result)
    }
}

Exceptions

• Mocking accidental problems (serialization, databases, APIs) is a very good habit to avoid coupling.

Conclusion

Mocks, like other test doubles, are valuable tools. Using them judiciously is an art.

Imagine a play in which each actor, instead of rehearsing with other actors, had to interact with 25 scriptwriters. The actors would never rehearse together. How would the result of the play be?

Credits

• Code Smell 30 - Mocking Business by Maximiliano Contieri