- The document discusses when and how to use mocking in unit tests, as there is an ongoing debate between "mockists" and "classicists" on this issue. - It argues that trade-offs should be considered rather than strict ideologies, and mocking is most useful for integrating with external systems or testing conditional interactions. - Principles for testing discussed include separating integration operations, pushing logic down the stack, outside-in design, and avoiding overspecification of tests. Listen to what your tests are telling you about potential refactors. - In the end, there are no absolute rules and the focus should be on finding the sweet spot based on each specific use case.

1. Wann soll ich mocken?
XP Days Germany
David Völkel
21.11.2016

2. @davidvoelkel
@softwerkskammer
@codecentric
TDD & Design

3. SCHICHTEN TESTEN?

4. INTEGRIERTER TEST

5. Unittest
Mock
MOCKING

6. Unittest
Mock
MOCKING

7. Unittest
Mock
MOCKING

8. aufwendiges Setup
Fehlerfindung
# Testfällen
Langsames Feedback
Refactorability sinkt
Isolationsaufwand
Zu wenig Nutzen
Lesbarkeit
TRADE-OFF
ZU GROSS vs ZU KLEIN

9. Woran
orientieren
?

10. IDEOLOGIEN
MOCKISTS CLASSICISTS
"Mocks Aren't Stubs", Martin Fowler

11. TRADE-OFFS
statt
IDEOLOGIEN!

12. USE CASES
MOCKISTS CLASSICISTS
Anbindung Drittsysteme

13. USE CASES
MOCKISTS CLASSICISTS
Anbindung Drittsysteme
Bedingte
Interaktionen

14. USE CASES
MOCKISTS CLASSICISTS
SWEETSPOT
Anbindung Drittsysteme
Bedingte
Interaktionen

15. USE CASES
MOCKISTS CLASSICISTS
SWEETSPOT
Anbindung Drittsysteme
Bedingte
Interaktionen
Geringer
Nutzen

16. VERMEIDEN
USE CASES
MOCKISTS CLASSICISTS
SWEETSPOT
Anbindung Drittsysteme
Bedingte
Interaktionen
Geringer
Nutzen

17. VERMEIDEN
MOCKING
SWEETSPOT
„BEST PRACTICES“

18. VERMEIDEN

19. „DONT MOCK VALUES!

20. „DON‘T MOCK VALUES!
new Value()

21. „DON‘T MOCK VALUES!
new Value()
TestDataBuilderForValue
.withDefaults()
.withField(“1“)

22. INTEGRATION OPERATION
SEGREGATION PRINCIPLE
"Integration Operation Segregation Principle", Ralf Westphal
"Die kniffligen Fälle beim Testen - Sichtbarkeit", Stefan Lieser

23. INTEGRATION OPERATION
SEGREGATION PRINCIPLE
public void sendMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
String title = customer.getSex() == Sex.MALE ? "Mr" :
customer.getMaritialStatus() == MaritialStatus.MARRIED ?
"Mrs" : "Ms";
String content = "Hello " + title + ". " + customer.getName() + ",nn"
+ "We have a special offer for you.nn"
+ "Best regards,n"
+ "ACME Customer Service";
mailService.sendMail(email, content);
}

24. public void sendMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
String title = customer.getSex() == Sex.MALE ? "Mr" :
customer.getMaritialStatus() == MaritialStatus.MARRIED ?
"Mrs" : "Ms";
String content = "Hello " + title + ". " + customer.getName() + ",nn"
+ "We have a special offer for you.nn"
+ "Best regards,n"
+ "ACME Customer Service";
mailService.sendMail(email, content);
}
INTEGRATION OPERATION
SEGREGATION PRINCIPLE

25. public void sendMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
String title = customer.getSex() == Sex.MALE ? "Mr" :
customer.getMaritialStatus() == MaritialStatus.MARRIED ?
"Mrs" : "Ms";
String content = "Hello " + title + ". " + customer.getName() + ",nn"
+ "We have a special offer for you.nn"
+ "Best regards,n"
+ "ACME Customer Service";
mailService.sendMail(email, content);
}
INTEGRATION OPERATION
SEGREGATION PRINCIPLE

26. public void sendMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
String content = renderContent(customer);
mailService.sendMail(email, content);
}
private String renderContent(Customer customer) {
String title = customer.getSex() == Sex.MALE ? "Mr" :
customer.getMaritialStatus() == MaritialStatus.MARRIED ?
"Mrs" : "Ms";
return "Hello " + title + ". " + customer.getName() + ",nn"
+ "We have a special offer for you.nn"
+ "Best regards,n"
+ "ACME Customer Service";
}
INTEGRATION OPERATION
SEGREGATION PRINCIPLE

27. public void sendMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
String content = renderContent(customer);
mailService.sendMail(email, content);
}
private String renderContent(Customer customer) {
String title = customer.getSex() == Sex.MALE ? "Mr" :
customer.getMaritialStatus() == MaritialStatus.MARRIED ?
"Mrs" : "Ms";
return "Hello " + title + ". " + customer.getName() + ",nn"
+ "We have a special offer for you.nn"
+ "Best regards,n"
+ "ACME Customer Service";
}
TESTS?
N Unittests

28. public void sendMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
String content = renderContent(customer);
mailService.sendMail(email, content);
}
private String renderContent(Customer customer) {
String title = customer.getSex() == Sex.MALE ? "Mr" :
customer.getMaritialStatus() == MaritialStatus.MARRIED ?
"Mrs" : "Ms";
return "Hello " + title + ". " + customer.getName() + ",nn"
+ "We have a special offer for you.nn"
+ "Best regards,n"
+ "ACME Customer Service";
}
TESTS?
1 Integrierter Test
N Unittests

29. PUSH LOGIC
DOWN THE STACK
Siehe "The Failures of “Intro to TDD”" - Justin Searls

30. PUSH LOGIC
DOWN THE STACK

31. SWEETSPOT

32. public String signup(String username) throws Exception {
if(userDB.findUserBy(username) == null) {
userDB.createUser(new User(username));
return "Welcome " + username;
} else {
return "Username ' " + username + "' "
+ "already taken, please choose another";
}
}
BEDINGTE
INTERAKTION

33. SYSTEM GRENZEN
3rd
Party
Service
Adapter

34. SYSTEM GRENZEN
3rd
Party
Service
Mock

35. OUTSIDE-IN
DESIGN

36. OUTSIDE-IN
DESIGN
Alternative “Fake it“

37. BEST
PRACTICES

38. NO
OVERSPECIFICATION!

39. NO
OVERSPECIFICATION!
“Specify exactly what should happen
but no more”

40. REIHENFOLGE?

41. COMMAND & QUERY
SEPARATION
REIHENFOLGE
IMMER NÖOTWENDIG?
public String signup(String username) throws Exception {
if(userDB.findUserBy(username) == null) {
userDB.createUser(new User(username));
...
} else {
...
}
}

42. COMMAND & QUERY
SEPARATION
REIHENFOLGE
IMMER NÖOTWENDIG?
public String signup(String username) throws Exception {
if(userDB.findUserBy(username) == null) {
userDB.createUser(new User(username));
...
} else {
...
}
}

43. COMMAND & QUERY
SEPARATION
REIHENFOLGE
IMMER NÖOTWENDIG?
public String signup(String username) throws Exception {
if(userDB.findUserBy(username) == null) {
userDB.createUser(new User(username));
...
} else {
...
}
}

44. COMMAND & QUERY
SEPARATION
REIHENFOLGE
IMMER NÖOTWENDIG?
public String signup(String username) throws Exception {
if(userDB.findUserBy(username) == null) {
userDB.createUser(new User(username));
...
} else {
...
}
}
@Test public void signup() throws Exception {
...
when(userDB.findUserBy(anyString())).thenReturn(null);
mailingService.signup(username);
verify(userDB).createUser(new User(username));
}

45. COMMAND & QUERY
SEPARATION
REIHENFOLGE
IMMER NÖOTWENDIG?
public String signup(String username) throws Exception {
if(userDB.findUserBy(username) == null) {
userDB.createUser(new User(username));
...
} else {
...
}
}
@Test public void signup() throws Exception {
...
when(userDB.findUserBy(anyString())).thenReturn(null);
mailingService.signup(username);
verify(userDB).createUser(new User(username));
}

46. COMMAND & QUERY
SEPARATION
REIHENFOLGE
IMMER NÖOTWENDIG?
public String signup(String username) throws Exception {
if(userDB.findUserBy(username) == null) {
userDB.createUser(new User(username));
...
} else {
...
}
}
@Test public void signup() throws Exception {
...
when(userDB.findUserBy(anyString())).thenReturn(null);
mailingService.signup(username);
verify(userDB).createUser(new User(username));
}

47. COMMAND & QUERY
SEPARATION
REIHENFOLGE
IMMER NÖOTWENDIG?
public String signup(String username) throws Exception {
if(userDB.findUserBy(username) == null) {
userDB.createUser(new User(username));
...
@Test public void signup() throws Exception {
...
when(userDB.findUserBy(anyString())).thenReturn(null);
mailingService.signup(username);
verify(userDB).createUser(new User(username));
}
„Allow Queries, expect Commands!“

48. LISTEN TO YOUR TESTS!
Image by M.J. Moneymaker

49. LISTEN TO YOUR TESTS!
# Verifications
Image by M.J. Moneymaker

50. LISTEN TO YOUR TESTS!
# Verifications
Overspecification!
Image by M.J. Moneymaker

51. LISTEN TO YOUR TESTS!
# Verifications
Overspecification!
# Dependencies
Image by M.J. Moneymaker

52. LISTEN TO YOUR TESTS!
# Verifications
Overspecification!
# Dependencies
Extract Class!
Image by M.J. Moneymaker

53. LISTEN TO YOUR TESTS!
# Verifications
Overspecification!
# Dependencies
Extract Class!
# Interactions
Image by M.J. Moneymaker

54. LISTEN TO YOUR TESTS!
# Verifications
Overspecification!
# Dependencies
Extract Class!
# Interactions
Tell, don‘t ask!
Image by M.J. Moneymaker

55. TELL DONT ASK
=> Tell don't ask
public void volumeUpClicked() {
int volume = speaker.getVolume();
if (volume < speaker.getMaximumVolume()) {
speaker.setVolume(volume++);
}
}

56. TELL DONT ASK
=> Tell don't ask
public void volumeUpClicked() {
int volume = speaker.getVolume();
if (volume < speaker.getMaximumVolume()) {
speaker.setVolume(volume++);
}
}

57. TELL DONT ASK
=> Tell don't ask
public void volumeUpClicked() {
int volume = speaker.getVolume();
if (volume < speaker.getMaximumVolume()) {
speaker.setVolume(volume++);
}
}
public void volumeUpClicked() {
speaker.putUpVolume();
}
class Speaker {
public void putUpVolume() {
if (this.volume < this.maximum
this.volume++;
}
}
...

58. VERMEIDEN

59. SWEETSPOT

60. BEST
PRACTICES

61. MOCKIST
ODER
CLASSICIST?

62. MOCKIST
ODER
CLASSICIST?
TRADE-OFFS!

63. QUELLEN
• „Growing Object Oriented Systems“,
Nat Pryce, Steve Freeman
• "Mocks Aren't Stubs",
Martin Fowler
• "Integration Operation Segregation
Principle",
Ralf Westphal
• "Die kniffligen Fälle beim Testen -
Sichtbarkeit",
Stefan Lieser

64. Q&A ?!

65. Licence
Creative Commons
Attribution-ShareAlike 3.0