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?
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