Dependency Injection in Scala

Transcript

1. Dependency Injection in Scala Vlastimil Menčík, Jakub Janeček, Michal Příhoda

   Czech Scala Enthusiasts

2. Java DI frameworks with Scala • important for existing Java-based

   projects/platforms • Java-Scala interoperability on JVM-level bears fruits • some issues may arise ◦ we will show some later

3. Spring • most popular • well-designed • a lot more

   than just an IoC container • a lot of people have experimented with Spring + Scala • SpringSource is working on the Spring Scala project

4. The coffee-making domain

5. Spring (annotations)

6. Spring (the same in Java)

7. Spring (application context)

8. Spring observations • actual differences between Java and Scala were

   minimal • constructor injection works well with immutability • setter injection requires some additional work

9. Setter injection in Java

10. Setter injection in Scala

11. Spring Scala project • developed by SpringSource • targets Spring

    3.2 • work in progres (1.0.0.M1) • wiring beans using functional syntax • functional wrappers for Spring templates ◦ functions instead of anonymous callbacks

12. Spring Scala (no annotations)

13. Spring Scala (wiring)

14. Spring Scala (application context)

15. Google Guice • focuses only on the DI • leaner,

    easier to get into • less magic • explicit bindings ◦ defined in module classes • just-in-time bindings ◦ automatic discovery

16. Vanilla Guice

17. Guice with Scala wrapper

18. Guice Observations • simplicity • interesting if you don't need

    (or want) Spring • easy to integrate with other frameworks (even the Scala ones)

19. Contexts and Dependency Injection • the standard (JSR-299) • part

    of Java EE 6 (and every compliant container) • created with lessons from previous DI solutions in mind

20. CDI annotation(s)

21. CDI (beans.xml)

22. Scopes and Contexts • singleton • prototype (aka dependent) •

    session • request • application • ...

23. Scopes cont. • in CDI, only singleton and dependent are

    injected as direct references ◦ different lifecycles, serialization • DI framework will inject a proxy instead ◦ thread-local magic

24. Proxies • restrictions on the proxied class ◦ might vary

    between DI frameworks • needs a default constructor • cannot be final or have final methods

25. Proxy problems • Scala compiler emits final methods ◦ even

    if there are none in the source code • access to a private val from a closure • such class is "unproxyable"

26. Java DI Frameworks Summary • you can use them if

    you want ◦ Spring is not just DI

27. Java DI Frameworks Summary • maybe with some additional wrappers

    to make them more Scala-friendly

28. Java DI Frameworks Summary • problems may arise if there

    are additional restrictions on classes (setters, proxies, ...) ◦ these can be avoided with some effort

29. SubCut "Scala Uniquely Bound Classes Under Traits" https://github.com/dickwall/subcut • very

    small and simple DI library • pure and idiomatic Scala • binding DSL • uses implicits to cut down on boilerplate JavaPosse ScalaWags

30. Step 1: Define Your Components

31. Step 2: Wire'em All

32. Step 3: Define Your Injectable Class

33. Step 4: Run or Test

34. Pros&Cons • Pros ◦ straightforward and simple ◦ idiomatic •

    Cons ◦ no automatic injection ◦ possible runtime errors (not static as Cake)