This document discusses dependency injection in Scala without using Spring. It introduces the "Cake pattern" which uses nested inner classes to achieve dependency injection. It provides an example of implementing dependency injection for a UserService and UserRepository using the Cake pattern with traits. It also shows how to write unit tests using a TestingEnvironment that mocks dependencies. Other approaches like structural types, implicits, and functional currying are mentioned as alternatives to dependency injection in Scala.

1. Hvordan vi klarte oss uten Spring,
dependency injection i Scala
Alf Kristian Støyle
Know IT Objectnet

2. Dependency Injection
Transactions

3. Cake pattern
‘Cake Pattern’ because “(...) beyond my
appreciation of cake, [a] cake is made of a
number of layers (separated by jam), and
can be sliced. The layers represent the
different levels of inner class nesting. It is
conceivable that you would reach the
bottom layer by working your way down
from the top.
http://scala.sygneca.com/patterns/component-mixins

4. http://lamp.epfl.ch/~odersky/papers/ScalableComponent.pdf
http://jonasboner.com/2008/10/06/real-world-scala-
dependency-injection-di.html

5. case class User(username: String, password: String)

6. class UserRepository {
def authenticate(user: User): User = {
println("authenticating user: " + user)
user
}
def create(user: User): User = {
println("creating user: " + user)
user
}
}

7. class UserService {
val userRepository = new UserRepository
def authenticate(username: String, password: String): User =
userRepository.authenticate(new User(username, password))
def create(username: String, password: String): User =
userRepository.create(new User(username, password))
}

8. class UserRepository {
def authenticate(user: User): User = {
println("authenticating user: " + user)
user
}
def create(user: User): User = {
println("creating user: " + user)
user
}
}

9. trait UserRepositoryComponent {
val userRepository: UserRepository
class UserRepository {
def authenticate(user: User): User = {
println("authenticating user: " + user)
user
}
def create(user: User): User = {
println("creating user: " + user)
user
}
}
}

10. class UserService {
def authenticate(username: String, password: String): User =
userRepository.authenticate(new User(username, password))
def create(username: String, password: String): User =
userRepository.create(new User(username, password))
}

11. trait UserServiceComponent extends
UserRepositoryComponent {
//val userRepository: UserRepository
val userService: UserService
class UserService {
def authenticate(username: String, password: String): User =
userRepository.authenticate(new User(username, password))
def create(username: String, password: String): User =
userRepository.create(new User(username, password))
}
}

12. trait UserServiceComponent {
self: UserRepositoryComponent =>
//val userRepository: UserRepository
val userService: UserService
class UserService {
def authenticate(username: String, password: String): User =
userRepository.authenticate(new User(username, password))
def create(username: String, password: String): User =
self.userRepository.create(new User(username, password))
}
}

13. object ComponentRegistry extends
UserServiceComponent with
UserRepositoryComponent {
val userRepository = new UserRepository
val userService = new UserService
}
val userService = ComponentRegistry.userService
val user = userService.authenticate("user", "password")
// => User(user,password)

14. import org.mockito.Mockito._
class TestingEnvironment extends
UserServiceComponent with
UserRepositoryComponent {
val userRepository = mock(classOf[UserRepository])
val userService = new UserService
}
val testEnv = new TestingEnvironment
when(testEnv.userRepository
.authenticate(new User("user", "password")))
.thenReturn(new User("mock", "mockpwd"))
val userService = testEnv.userService
val user = userService.authenticate("user", "password")
// => User(mock,mockpwd)

15. Gotchas
• Class vs Object
• Typenavn

16. Andre måter
• Structural types
• Implicit declarations
• Functional currying
• Spring
• Guice

17. Transactions
import org.springframework.transaction.annotation.Transactional
class UserService {
@Transactional def create(username: String, password: String) = {
userRepository.create(new User(username, password))
}
}

18. Transactions
import TransactionManager._
class UserService {
def create(username: String, password: String) = transactional {
userRepository.create(new User(username, password))
}
}

19. object TransactionManager {
def transactional[A](work: => A): A = ...
}

20. Scala DI
http://jonasboner.com/2008/10/06/real-world-scala-
dependency-injection-di.html
http://programming-scala.labs.oreilly.com/
ch13.html#DependencyInjectionInScala
http://debasishg.blogspot.com/2011/03/pushing-envelope-
on-oo-and-functional.html

21. alf.kristian@gmail.com
http://slideshare.net/stoyle/di-scala
http://github.com/stoyle/di-scala