Imagine a world without mocks

Imagine a world
without mocks
@KenScambler
Scala Developer at

Me
14 years
5 years
5 years
when possible
when bored
when forced

http://techblog.realestate.com.au/to-kill-a-mockingtest/

Q: What’s so bad about
mocks & stubs?
A: The problem they solve is
“how to test poorly designed
code”

UserRepo
UserService
User getUser(UserId)
DB
AuthService
boolean authUser(UserId)
Collaborators galore!
Record selectUser(SQL)

UserRepo
UserService
DB
AuthService
boolean authUser(UserId)
How to separate UserService?

http://martinfowler.com/articles/mocksArentStubs.html
Stubs provide canned answers
to calls made during the test,
usually not responding at all to
anything outside what's
programmed in for the test.

UserService
If you ask me
“authUser(1234)”,
I’ll say “true”
Stub
UserRepo
DB

UserService
Stub
UserRepo
UserService’s
input is now
deterministic!
If you ask me
DB

http://martinfowler.com/articles/mocksArentStubs.html
Mocks are objects pre-
programmed with
expectations which form a
specification of the calls
they are expected to
receive.

UserService
I expect
“selectUser()”
to be called once
Mock
If you ask me

UserService
I expect
“selectUser()”
to be called once
Mock
UserService’s
output is now
deterministic!
If you ask me

UserService
Mock
Deterministic
output
Deterministic
input
= Fairly sane test

1.
Coupled to brittle
implementation
details.

UserService
Mock
What if an equivalent
method is called
instead? If you ask me
authLocalUser(1234)
Stub

UserService
Ah shit.
authLocalUser(1234)

UserService
Honestly, no
one’s asked me
that before.
authLocalUser(1234)

UserService
I’m just a sock.
authLocalUser(1234)

2.
Somewhat misses the
point of the test

UserService
authLocalUser(1234)
I expect
“selectUser()”
to be called once
Still no idea.

UserService
IT DIDN’T GET
CALLED!!! NOTHING
HAPPENED!!!!!

UserService
IT DIDN’T GET
CALLED!!! NOTHING
HAPPENED!!!!!
Real problem:
The stub configuration was
out of date.

public interface Config {
// Database stuff
String getDatabaseHost();
int getDatabasePort();
int getMaxThreads();
int getConnectionTimeout();
// Potato settings
String getDefaultPotatoVariety();
int getMaxPotatoes();
double getPotatoShininess();
// Sacrificial settings
int getBloodSacrificeGoatCount();
int getBloodSacrificeChickenCount();
int getBloodSacrificeSheepCount();
}

public class PotatoService {
public PotatoService(Config config) {
this.potatoVariety = config.getPotatoVariety();
this.maxPotatoes = config.getMaxPotatoes();
}
public Salad makePotatoSalad() {...}
}

public class PotatoServiceTest {
Config config = mock(Config.class)
@Before
public void before() {
when(config.getDefaultPotatoVariety())
.thenReturn(“pontiac”);
when(config.getMaxPotatoes())
.thenReturn(33);
}
public testMakeSalad() {
PotatoService service = new PotatoService();
Assert.equalTo(service.makeSalad(), ...);
}
}

Config config = mock(Config.class)
@Before
when(config.getDefaultPotatoVariety())
.thenReturn(“pontiac”);
when(config.getMaxPotatoes())
.thenReturn(33);
}
PotatoService service = new PotatoService();
}
}
Stub

Looks ok. But what is the
stub trying to tell us?

No-ones ever
going to
need all those
things at once.
public interface Config {
// Database stuff
// Potato settings
// Sacrificial settings
}

That’s better!
public interface DatabaseConfig {
}
public interface PotatoConfig {
}
public interface SacrificialConfig {
}

public PotatoService(PotatoConfig config) {
this.potatoVariety = config.getPotatoVariety();
this.maxPotatoes = config.getMaxPotatoes();
}
}
Don’t you just need the two
fields? Does it matter where
they come from?

public PotatoService(String variety, int max) {
this.potatoVariety = variety;
this.maxPotatoes = max;
}
}
The application wiring can
be someone else’s business.

PotatoService service =
new PotatoService(“pontiac”, 33);
}
}

- More modular
- More reusable
- Simpler
- Less code
- Stubs are gone

Case study #2
Unnecessary mutable
state

public interface Wallet {
int removeCoins(int amount);
int getAmount();
}
public interface VendingMachine {
void insertCoins(int amount);
Can collectCan();
int getStoredCash();
}
public interface Customer {
void buyDrink();
}

public class CustomerTest {
Wallet wallet = mock(Wallet.class);
VendingMachine machine = mock(VendingMachine.class);
@Before
when(wallet.removeCoins(3)).thenReturn(3);
when(vendingMachine.collectCan())
.thenReturn(new CokeCan());
}
public testBuyDrink() {
Customer c = new Customer();
c.buyDrink();
verify(wallet).removeCoins(3);
verify(vendingMachine).insertCoins(3);
verify(vendingMachine).collectCan();
}
}

Wallet wallet = mock(Wallet.class);
VendingMachine machine = mock(VendingMachine.class);
@Before
when(wallet.removeCoins(3)).thenReturn(3);
when(vendingMachine.collectCan())
.thenReturn(new CokeCan());
}
Customer c = new Customer();
c.buyDrink();
verify(wallet).removeCoins(3);
verify(vendingMachine).insertCoins(3);
verify(vendingMachine).collectCan();
}
}
Stub
Mock

The class under test is
separated now!
But what are the mocks
telling us?

int getAmount();
}
Can collectCan();
}
void buyDrink();
}
Surely we care about
the resulting state,
not the in-betweeny
verbs.

If the state is just
immutable
values, we don’t
have to force
isolation
int getAmount();
}
Can collectCan();
}
void buyDrink();
}

int getAmount();
Wallet removeCoins(int amount);
}
Optional<Can> getCanInTray();
List<Can> getCansInMachine();
VendingMachine insertCoins(int amount);
VendingMachine collectCan();
}
Wallet getWallet();
List<Can> getCansHeld();
Pair<VendingMachine, Customer>
buyDrink(VendingMachine vm);
}

int getAmount();
}
}
Wallet getWallet();
}
Immutable
state

int getAmount();
}
}
Wallet getWallet();
}
“Actions” just
return new
copies

Customer c = new Customer(new Wallet(23));
VendingMachine vm = new VendingMachine(10,30);
Pair<VendingMachine, Customer> result = c.buyDrink(vm);
Customer c2 = result.second();
VendingMachine vm2 = result.first();
Assert.equals(20, c2.getWallet().getAmount());
Assert.equals(9, vm2.getCansInMachine().size());
Assert.equals(33, vm2.getStoredCash());
}
}

- Less moving parts
- More reusable
- Simpler
- Easier
- Mocks & Stubs are
gone

“But then it’s an
integration test!”

1. Immutable data
structures are just
values.

2. We have no
business peeking at
a method’s tools;
only its results,
effects

3. Pure functions
are already
deterministic

Case study #3
Essential effects

public interface EmailSender {
void sendEmail(String addr, Email email);
}
public class SpecialOffers {
private final EmailSender sender;
void sendSpecialOffers(Customer c) {
if (!c.isUnsubscribed()) {
String content = "Hi " + c.getName() + "!";
sender.sendEmail(c.getEmailAddr(),
new Email(content))
}
}
}

public class SpecialOffersTest {
EmailSender sender = mock(EmailSender.class)
public testSendEmail() {
SpecialOffers offers = new SpecialOffers(sender);
offers.sendSpecialOffers(
new Customer(false, “Bob”, “foo@foo.com”));
verify(sender).send(“foo@foo.com”,
new Email(“Hi, Bob!”));
}
}

EmailSender sender = mock(EmailSender.class)
SpecialOffers offers = new SpecialOffers(sender);
offers.sendSpecialOffers(
new Customer(false, “Bob”, “foo@foo.com”));
verify(sender).send(“foo@foo.com”,
new Email(“Hi, Bob!”));
}
}
Mock

Ok, so it tests we send an
email.
But what is the mock trying
to tell us?

public interface EmailSender {
void sendEmail(String addr, Email email);
}
private final EmailSender sender;
void sendSpecialOffers(Customer c) {
sender.sendEmail(c.getEmailAddr(),
new Email(content))
}
}
}
I only care about the intent to
send an email, not the actual
sending. Can the intent be its
own thing?

public interface SendEmailIntent {
String getAddress();
Email getEmail();
}
public interface Interpreter {
void interpret(SendEmailIntent intent);
}
Optional<SendEmailIntent> sendSpecialOffers(
Customer c) {
return Optional.of(new SendEmailIntent(
c.getEmailAddr(),
new Email(content)));
} else {
return Optional.empty();
}
}
}
We can have
an
interpreter
elsewhere.

SpecialOffers offers = new SpecialOffers();
SendEmailIntent intent = offers.sendSpecialOffers(
new Customer(false, “Bob”,
“foo@foo.com”)).get();
Assert.equals(intent.getAddress(), “foo@foo.com”);
Assert.equals(intent.getEmail().getText(),
“Hi, Bob!”);
}
}

- Separated intent
from execution
- More reusable
- Simpler
- Easier
- Mocks are gone

Test-first
encourages you
to design code
well enough to
test…

Mocks & stubs
set a
looooow bar

This totally
guts TDD’s
value for
design.

Conclusion:
Side effects are
the real killer

All I do is make the
input deterministic. If
the input is already
just immutable values,
then you don’t need
me.
Stub Mock

If you’re just using me
because stuff is hard
to create, you need to
get back and design
harder!
Stub Mock

I make output
deterministic, by
recording method
calls instead of
allowing effects.
Stub Mock

Sometimes, this
means that I test a
pointless web of lies,
that doesn’t touch the
code’s reason for
existence.
Stub Mock

Other times, I am really
testing the intent of the
code, which can be pulled
out as its own structure.
This separates the
concern of choosing the
next thing.
Stub Mock

Stub Mock
If you are using
immutable types and
pure functions, then
you’re home and
hosed.
Forget about
• “collaborators”
• “Tell don’t ask”
• Avoiding static methods
• Avoiding “new”.

Imagine a world without mocks

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