Unit testing - 9 design hints

Your tests are trying
to tell you something
by Victor Rentea at
9 design hints you were missing
Read the article:
https://victorrentea.ro/blog/design-insights-from-unit-testing/
Get the code:
https://github.com/victorrentea/unit-testing.git Branch: 9hints_talk
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a training by
&
(referring to an old Romanian saying)
- NO COMMENT -

a training by
▪Better Signatures ⌨= code
▪Immutable Objects ⌨
▪CQS Principle ⌨
▪Functional Core / Imperative Shell ⌨
▪Separation by Layers of Abstraction ⌨
▪Decouple Unrelated Complexity ⌨
▪Role-based Design
▪Onion Architecture
▪Breakdown Data Objects ⌨
Design Improvements hinted by Unit Tests

a training by
Are you agile?
"Emergent Architecture"
Ever heard of the

a training by
Are you agile?
Tests give you hints
on how to break
and design complex logic
"Emergent Architecture"
that never emerges 
Under-design is the default today?

a training by
Do you write Tests
BEFORE the implementation?
Writing #responsible Unit Tests will 🚀 YOUR design skills
If you would, you'd get
✔ more real coverage => courage
✔ more early design feedback
The point of this talk
→ Mocks

a training by
Mocks make our tests...
Repeatable
isolated from external systems
Fast ✔
DB, external APIs
Reasonably Complex ✔
When facing high cyclomatic complexity ➔
Alternatives:
in-memory DB (H2)
Testcontainers
WireMock

a training by
Logic
to test
Cyclomatic Complexity
= number of independent paths through code
Test
Test
Test
Test
Test
Test
Test
CC=5 CC=6
f(a) g(b)
calls
CC=30
Too Many
To cover all branches
Too Heavy
in setup and input data (a and b)
End-to-end tests grow...

a training by
⛔
If there's not much complexity,
avoid mocks
A B
Instead,
test clusters of objects
Internal refactoring
won't break tests 👍

a training by
(The rest of the talk assumes we have serious complexity)
Unit Testing intensifies design
around complex logic

a training by

a training by
Precise Signatures
Methods taking less data as arguments are easier to understand; ✔
and less coupled
method(heavyObject)
HeavyObject having dozens of fields
method(part1, part2)
only takes necessary parts of HeavyObject
Tests will get simpler
new HeavyObject(..)

a training by
Immutable Objects ✔
eg. Mutating a parameter often causes awkward bugs.
method(obj) {
obj.setX(..);
}
method(..) {
return x;
}
when(b.method(..)).thenReturn("stub");
doAnswer(invocation -> .. obj.setX("stub"))
.when(b).method(any());
Mocking such a method is painful:

a training by
do side-effects
return void sendEmail(Email):void
Command-Query Separation
setActive(true):void
return results
search(criteria):List
computePrice(flight):int
Bertrand Meyer, in 1994:
Pure Functions

a training by
No side effects
(doesn't change anything)
No INSERTs, POSTs, queues, files, NO Fields changed,…
𝑒𝑔: 𝑀𝑎𝑡ℎ𝑒𝑚𝑎𝑡𝑖𝑐𝑎𝑙 𝐹𝑢𝑛𝑐𝑡𝑖𝑜𝑛𝑠: 𝑓 𝑥, 𝑦 = 𝑥2
+ 𝑦
+
Referential Transparent
(same inputs produce the same output)
No current time, random, GET, SELECT…
Pure Functions

a training by
Command-Query Separation Principle
A method should be either
a query (pure function, returning data), or
a command (side-effecting, returning void)
If you need to both stub (when...) and verify the same method,
you are breaking CQS:
Exception: external calls should happen ONCE
eg. REST/WSDL calls that are critical, cost money, or take time
when(b.query(..)).thenReturn(X);
prod.call();
verify(b).command(Y);
when(b.god(..)).thenReturn(X);
prod.call();
verify(b).god(Y);
Apply inside core logic
https://martinfowler.com/articles/mocksArentStubs.html

a training by
Functional Core / Imperative Shell Segregation
The heavier some logic is, the less dependencies it should have.
Keep complex logic in pure functions. ✔
Complex logic requires many tests.
Shrink these tests by limiting mocks.
=> loose coupling
when.thenReturn(..);
when.thenReturn(..);
when.thenReturn(x);
when.thenReturn(y);
prod.complex();
verify(..).f(z);
verify(..).g();
verify(..).h();
x7=😖
when.thenReturn(..); // ideally 0 mocks
z = prod.complex(x, y);
assert..(z...);

a training by
Dependencies
Heavy complexity
Side-effects
FUNCTIONAL CORE
PURE
FUNCTION

a training by
Design the Most Complex Parts of Logic
as Pure Functions 💘
Easier to Understand Easier to Test

a training by
Separation by Layers of Abstraction
You have two complex functions in the same class, f() calling g()
Move g() in a separate class ✔
You test g() separately.
When testing f(), you don't want it to call g(), as g was tested
Using partial mocks (@Spy) leads to horrid tests. Solution:
class Big {
f() {
//complex
g();
}
g() {
//complex
}
}
class HighLevel {
LowLevel low;
f() {
//complex
low.g();
}
}
class LowLevel {
g() {
//complex
}
}

a training by
▪Better Signatures
▪Immutable Objects
▪CQS Principle
▪Functional Core / Imperative Shell
▪Separation by Layers of Abstraction
▪Decouple Unrelated Complexity
▪Role-based Design
▪Onion Architecture
▪Breakdown Data Objects
Design Improvements hinted by Unit Tests

a training by
Fixture Creep
2 complex functions in the same class, use different sets of dependencies.
Break unrelated complexity for clarity and loose coupling. ✔
// prod code
class Wide {
A a;
B b;
complex1() {
//uses a
}
complex2() {
//uses b
}
}
class WideTest {
@Mock A a;
@Mock B b;
@InjectMocks Wide wide;
@BeforeEach
init() {//shared fixture
when(a..).then
when(b..).then
}
// 5 tests for complex1
// 4 tests for complex2
}
class Complex1Test {
@Mock A a;
@Mock B b;
}
class Complex2Test {
@Mock B b;
@Mock C c;
}
Testing both in the same Test class leads to a
bloated fixture ("Before").
Hard to trace later what's used by one test.
Break the test class.
Also: Hierarchical Tests.

a training by
Mock Roles, not Objects
Bad Habit: You finish the implementation, then you write tests.
You [blindly] @Mock all dependencies.
Two years later, your tests are fragile and slow you down.
Before mocking a dependency, clarify its role. ✔
And test earlier.
http://jmock.org/oopsla2004.pdf

a training by
Problem: You allow external APIs, DTOs, and ugly frameworks
to enter freely in your core logic.
Testing your core logic now requires creating foreign objects,
or mocking an awkward library.
Onion Architecture
Pull core logic in an Agnostic Domain ✔
So most tests talk in terms of your internal Domain Model ✔
https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html

application
Value Object
Entity
id
Domain
Service
Domain
Service
(agnostic)
domain
DTO
External
API
DTO
UI,
Client
External
Systems
Façade
Controller
IRepo
Clean Pragmatic Architecture
at Devoxx Ukraine 2021
Curious for more?
↓
IAdapter
Adapter
⛔
⛔

a training by
Creepy Object Mother
You use large data structures in your core logic.
If the structures enforce their internal consistency,
(it's not an anemic model only having getters and setters)
then creating test data instances becomes cumbersome,
(unrelated fields to set)
so you share a global "TestData" class that builds correct instances.
TestData gets bloated and couples your tests together.
Break TestData🪓 in several classes
Break data structures🪓 in separate Bounded Contexts?
ie packages>modular monolith>microservices🤞
https://martinfowler.com/bliki/ObjectMother.html (2006)

a training by

a training by
▪More Calls from Tests > Better Signatures, Better Skills
▪Immutable Objects 🤘 (rock)
▪when.then xor verify > Command-Query Separation
▪More Tests ➔ ↓Mocks > Functional Core / Imperative Shell
▪Partial Mocks > Separation by Layers of Abstraction
▪Bloated Fixture > Break Test > Decouple Unrelated Complexity
▪Mock Roles not Objects > Role-based Design
▪Keep most tests on Domain > Onion Architecture
▪Creepy Object Mother > Break Data Objects
> 2 Bounded Contexts?
Recap

a training by

Unit testing - 9 design hints

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