GContracts is a Groovy library that implements Design by Contract using annotations. It allows specifying preconditions, postconditions, and class invariants using annotations like @Requires, @Ensures, and @Invariant. These annotations take closure expressions that are evaluated at compile-time and run-time to check conditions. GContracts works with Groovy 1.6 and higher and has features like labeling, interface contracts, inheritance support, and integration with Grails.

1. GContracts
Andre Steingress
@sternegross/@gcontracts

3. LINZ, AUSTRIA

10. www.ejug.at

11. ABOUT
GCONTRACTS.

12. GCONTRACTS
DESIGN BY CONTRACT (tm) FOR GROOVY
Groovy 1.7+ (was 1.6 loooong time ago)
BSD LICENSE
https://github.com/andresteingress/gcontracts

13. JUMP START.

14. LET‘S REVIEW THIS
ACCOUNT CLASS.
Googled, found at javacoffeebreak.com

15. PREREQUISITES.

16. @Grab(
group='org.gcontracts',
module='gcontracts-core',
version='[1.2.3,)'
)
no additional dependencies...

17. import org.gcontracts.annotations.*

18. 3 ANNOTATIONS.

19. Commonalities
• special assertion types
• specifying boolean expressions
• closure instances as annotation
parameters
@MyAnnotation({ param?.size() > 0 })

20. @Requires
• precondition
• a method‘s bouncer

21. @Ensures
• postcondition
• a method‘s mother

22. @Invariant
• class-invariant
• an object‘s god.

23. LET‘S REVIEW THIS
ACCOUNT CLASS.
Googled, found at javacoffeebreak.com

24. INTERNALS.

25. @Ensures({ count() == old.count + 1 })
{ count() == old.count + 1 }
{ result, old -> assert count() == old.count + 1 }
compile-time
class $_gc_closure1 extends Closure {
def doCall(...) {
// ...
assert count() == old.count + 1
// ...
}
}
@Requires($_gc_213123123123.class)

26. JVM Arguments: -ea -da
if ($GCONTRACTS_ENABLED) { ... }
org.gcontracts.ViolationTracker.init()
$_gc_result = Putable$_gc_closure1.newInstance(this,
run-time
this).call(element, key)
if (org.gcontracts.ViolationTracker.violationsOccured())
org.gcontracts.ViolationTracker.rethrowFirst()

27. Officially Supported since Groovy 1.8!

28. Works since Groovy 1.6
http://pniederw.wordpress.com/2010/03/04/3/

29. MORE FEATURES.

30. LABELING

31. INTERFACE CONTRACTS

32. INTERFACE CONTRACTS

33. INHERITANCE
✓INTERFACE CONTRACTS
✓ABSTRACT CLASSES
✓CONCRETE CLASSES
✓MIXED

34. INHERITANCE
✓ACROSS COMPILATION UNITS

35. MICRO CONTRACTS
REUSABLE CONTRACT PART

36. MICRO CONTRACTS

37. Grails Integration
• Container extends object construction time
• @Service stereotype causes CI validation in
@PostConstruct

38. GroovyDoc Integration

40. https://github.com/andresteingress/gcontracts/wiki

41. ABOUT
CONTRACTS.

42. MR.
„DESIGN BY CONTRACT“

44. WHY CONTRACTS?

45. SPOCK, JUNIT, et. al. ARE
„THE CHINAMAN IS
NOT THE ISSUE HERE,
DUDE.“

46. External API Changes New Programmers
Changing Requirements
Assumptions Additional Libraries
New Use-Cases More Clients
New Knowledge Code Conventions

47. External API Changes New Programmers
Changing Requirements
Assumptions Additional Libraries
New Use-Cases More Clients
New Knowledge Code Conventions

48. WHAT DID YOU PROGRAM LAST WEEK?
CAN YOU RECALL YOUR THOUGHTS, MODELS?
WHY SHOULD ANYONE ELSE KNOW ABOUT THEM?

49. BUILD YOUR PLAYGROUND.

50. http://about.me/asteingress

51. Thank you!

52. "Design by Contract" is a trademark of Interactive Software Engineering.