Don't Be Mocked by your Mocks - Best Practices using Mocks

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Don't be Mocked by your Mocks
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Victor Rentea
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Java PerformanceReactive
Design Patterns Refactoring Unit Testing
any
lang
uage

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Decouple a Dependency in Tests
Isolated
versus external systems
Fast
DB, APIs
Test Less Code
Test a layer, by faking the layer below:
WHY?!
Cheating?
layers

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him.slap();

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Mock
Verify a method call
verify(repo).save(…);
Stub
Respond to method calls
when(mock.method())
.thenReturn(…);
Dummy
Avoids Null Pointers
Mock w/o behavior
system.setAuthorizer(new NoopAuthorizer());
https://blog.cleancoder.com/uncle-bob/2014/05/14/TheLittleMocker.html
Fake
Alternative
implementationTest Double

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Fake
Alternative
implementation
Test Implementation
(eg. storing in a hash map)
when(userRepo.findById(1L))
.thenReturn(u);
// prodCall()
userRepo.save(u); //id=1
// prodCall()
Vs
C# Repository pattern + LINQ queries.
Given you trust LINQ, can be tested with just an in-memory list.
TIP: Consider
if copy-pasting
stubbing
when(...)
.thenAnswer(<callback>);
.. or instead of
dynamic stubs
verify(orderRepo).save(...);
list= orderRepo.findAll();
assert list contains new order

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Mock
Verify a method call
verify(repo).save(…);
Stub
Respond to method calls
when(mock.method())
.thenReturn(…);
Dummy
Avoids Null Pointers
= Mock w/o behavior
system.setAuthorizer(new NoopAuthorizer());
Fake
Alternative
implementationTest Double
you stub or mock a method

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Mocks aren't Stubs
https://martinfowler.com/articles/mocksArentStubs.html
Stub Queries, Expect Actions
orderRepo.save(Order)userRepo.findById(id):User
Verify what tested code does
based on user's state
when(repo.findById(…)).thenReturn(user);
out = testedCode();
verify(repo).findById(…);
Since Mockito 2.x stubbed methods are automatically verified by default
2
Command-Query Separation Principle
Need Both?

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No side effects
No INSERTs, POSTs, queues, files, fields,…
𝑒𝑔. 𝑀𝑎𝑡ℎ𝑒𝑚𝑎𝑡𝑖𝑐𝑎𝑙 𝐹𝑢𝑛𝑐𝑡𝑖𝑜𝑛𝑠: 𝑓 𝑥, 𝑦 = 𝑥2
+ 𝑦
(logging doesn't count)
Referential Transparent
Same arguments ➔ same result
No current time, random, GET, SELECT…
Pure Functions

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Side-effects
1) On external systems:
2) On in-memory objects
➔ assert their state after
What can a Unit Test check?
OutputInput Params Feature
(method)
Data from
dependencies
Input+Deps → Output
a) Real: SELECT from test to check the INSERT
b) Mocked: verify repo.save() was called
Input → Output
Pass More IN
Philosophic Slide
aka Interaction Testing
Return Change
Pure Function: 𝑓 𝑥 = 𝑥2
The Simplest Tests
a) Real: SELECT
b) Stub: when.thenReturn.

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Extract Method
mock
verify(orderRepo).save(…);
stub
when(userRepo.findById()).thenReturn(…);
No mocks
Stubs & Mocks
ArgumentCaptor
SimplerTests
Test This:
x 8 tests

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= Pure Function
public
Cons: parameter count++
Cons: complex return

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The Circle of Purity
* idea inspired by Venkat Subramaniam

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infrastructure
domainfunctional core
3
The Universal Architecture
Side-effects &
Dependencies
Code Dependency
aka Onion
aka Hexagonal
aka Ports-and-Adapters

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Design the Most Complex Parts of Logic
as Pure Functions
My Point?

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Pure Functions
Easy to Understand Easy to Test

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How to test this one ?
NPE
Should other tests also execute createOrder() ?

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NPE
Test OverlapHeavy Tests
YES
Complex
Logic+
NO
(Partial Mock)
YES
Few?
(eg. NPE)
many

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Partial Mock?!
(eg @Spy in Mockito)

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Partial Mock
Mocked Methods
createOrder
Tested Methods
placeOrder
Same Object

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Missed Design Hint
NPE
Tests
Test OverlapHeavy Tests
YES NO
(Partial Mock)
Complex
Logic
OrderFactory
NO
(Extract to New Class)
Separation By Layers of Abstraction
Complex
Logic
@Mock
many
4

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What's so bad about them after all ?
But I ❤️ Mocks!
I use them everywhere!
Fragile Tests Inconsistent Tests Incorrect Tests

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The Downfall of Mocks

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You have to do a difficult change.
First, you make that change easy (this might be difficult).
Then, you do the easy change.
-- Kent Beck
Changing Existing Code
Preparatory Refactoring
The Need for Refactoring
Production Code

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We write tests to refactor safely

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Which is
easier to write?
MockEverything
LessMocks
(w/o functional change requests)
Which is stabler?
A
B
C D
Bdirect dependency
(mock) Dfinal real system
(SELECT)
(mock)
Ctest A+B
together
TEST
At which API to verify?Easier to Write Reliable & Stable
Mocks "Freeze" APIs
eg. Repo
Closer to Spec
B1
B2
Are these Unit Tests?
(all are fresh classes)

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Unit Testing
What’s a Unit?
It's a unit of behavior
= the smallest part of a feature that you can test in isolation
a method ?
a class ?
a full use-case?
- Kent Beck, inventor of TDD, XP, Unit Testing
It's perfectly fine for unit tests to talk to databases and filesystems! – talk
... as long as your tests are isolated ➔ in-memory DB; Docker

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Don't Mock between newly-created classes
vs Stable Interfaces
✓Standard API
(lib, framework, "platform")
✓Multiple implems
✓Old API
Actually, there's an entire TDD style relying
on mocking not-yet-implemented classes

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Mocks "Freeze" APIs
Only Mock Stable Interfaces

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A
B
TEST
Kept in Sync
Inconsistencies
when(b.f(7)).thenReturn(1)
assertEquals(1, b.f(7));
Impossible Test Cases
assertEquals(-1, b.f(null));
b.f(x * 2);Production code:
assertEquals(2, b.f(7));
You test them
separately

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Testing on the Toilet

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Test requirements, not implementation details!
James Coplien: https://rbcs-us.com/documents/Why-Most-Unit-Testing-is-Waste.pdf

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Fragile Tests
(when design changes)
Inconsistencies
(when requirements change)
Incorrect Tests
(when testing in isolation)
Extensive Mocking

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Test more classes together!
(versus mocking every dependency of the tested class)

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Large Input Data
Many Tests
A
B
C D
final real system
(SELECT)
TEST
eg. Repo
Testing
Framework
(testing DSL)
GET
Blackbox
Test
verify via
another API call
Build Deeper Tests
Reliable & Stable
Closer to Spec
POST
As deep as you can to keep them
maintainable and reliable

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A
B
C D
final real system
(SELECT)
TEST
eg. Repo
Testing
Framework
(testing DSL)
GET
Blackbox
Test
verify via
another API call
Build Deeper Tests
Reliable & Stable
Closer to Spec
POST
As deep as you can to keep them
understandable, reliable & fast
Large Input Data
Many Tests

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It may be cheaper to build
a testing framework
On the medium-long term
test it end-to-end
Like Uncle Bob did (2018)
than mocking the little parts
and

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is impossible for some use-cases
(exponential number of execution paths)
Decompose Complex Flows in Components
tested independently
But to test it end-to-end
Mocks are Inevitable

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Mock Roles, Not Objects without a clear contract
http://jmock.org/oopsla2004.pdf
Challenge: 🥇
Try to extract an interface from the class you want to mock!
Redesign until that interface makes sense
6

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A) Test More
STOP
and refactor testsA or prodB,C
How many mocks per @Test
Using MORE?
A
B
C
D
E F
(Including @Before and test super-classes)
High Coupling
C) Redesign Responsibilities8
= dependencies count

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Hidden Dependencies

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Hidden Dependencies
Static Library Calls
LocalDate.now();
new
new Date();
Encapsulate in mockable
instances passed in as
dependencies
private TimeProvider time;
System.currentTimeMillis();

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Best if simple and pure
If you need to Mock it
It should not be an Util!
no polymorphism (proxies, strategy...), global state, no lifecycle management, uncontrolled usage

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Mocks returning Mocks
Mock data objects (Entities, Value Objects, ...)
Too many mocks
Partial Mocks
Mock Statics
Verify stubbed methods
Check how many times() a call happened
Verify no extra call happen or never()
Capture and assert every argument
Populate Test Instances
Mocking Worst Practices

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Minimalistic Testing
Test Where The Risk is!

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Then, What to mock?
Legacy Code
As an interim stage
Libraries or External Systems
Mock your adapters
Slow or Unreliable Resources
If impossible to run them in-memory / Dockerized
Well Defined Roles
Try to extract a well defined interface
WireMock?

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Don't Mock Stuff
just because it's a bit harder to test the entire flow

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DEVELOPERS!!
What are you?

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Pure Functions

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226
¡¡ PLEASE !!
Ask me questions!

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Thank You!
VictorRentea.ro
Blog⭐, Company Training, Masterclasses, Best Talks,...
@VictorRenteavictorrentea@gmail.com
¡¡ PLEASE !!
Ask me questions!
Training Topics:
▪ Clean Code + Refactoring
▪ Design Patterns
▪ Unit Testing + TDD
▪ Advanced FP with Java
▪ Spring
▪ Hibernate/JPA
▪ Reactive Programming
▪ Java Performance
▪ Pragmatic DDD

Don't Be Mocked by your Mocks - Best Practices using Mocks

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