The no-framework Scala Dependency Injection Framework

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Using a DI framework/container may seem obvious. But when was the last time you considered *why* do you really need one? After all, "dependency injection" is just a fancy name for passing arguments to a constructor. In the talk we'll walk through some of the features of DI containers and see if we can replace them with pure Scala code. We'll start with "manual" DI, followed with using MacWire to generate the wiring code for us. Then we'll proceed to a no-framework scopes implementation (e. g. request or session), which are very useful in web applications. We will also discuss possibilities of adding interceptors using macros. And finally, we'll see how to use traits to create and compose modules (similar to the module concept known from Guice), which can be viewed as a simplified cake pattern. As MacWire heavily uses macros, as a bonus, I'll explain how Scala Macros work and when they can be useful.

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The no-framework Scala Dependency Injection Framework

  1. 1. The no-framework Scala Dependency Injection framework 11/12/2013 BuildStuff 2013 Adam Warski
 @adamwarski BuildStuff 2013
  2. 2. You don’t need anything special to do Dependency Injection 11/12/2013 BuildStuff 2013 @adamwarski
  3. 3. We often over-complicate 11/12/2013 BuildStuff 2013 @adamwarski
  4. 4. Who Am I? ❖ Day: coding @ SoftwareMill! ❖ Afternoon: playgrounds, Duplos, etc.! ❖ Evenings: blogging, open-source! ❖ Original author of Hibernate Envers! ❖ ElasticMQ, Veripacks, MacWire! ❖ http://www.warski.org 11/12/2013 BuildStuff 2013 @adamwarski
  5. 5. What is Dependency Injection? 11/12/2013 BuildStuff 2013 @adamwarski
  6. 6. What is DI? class PresentPackager {! ! def wrap() {! ! ! new RibbonSelector().selectRandom()! ! ! …! } ! } 11/12/2013 BuildStuff 2013 @adamwarski
  7. 7. What is DI? class PresentPackager(rs: RibbonSelector) {! ! def wrap() {! ! ! rs.selectRandom()! ! ! …! } ! } 11/12/2013 BuildStuff 2013 @adamwarski
  8. 8. Yes, DI is just using parameters 11/12/2013 BuildStuff 2013 @adamwarski
  9. 9. Why? ❖ Restrict the knowledge of the class class PresentPackager {
 
 class PresentPackager
 ! (rs: RibbonSelector) {
 ! ! ! ! ! } def wrap() {
 ! new RibbonSelector()
 ! ! .selectRandom()
 ! …
 }
 11/12/2013 BuildStuff 2013 
 ! ! ! ! } def wrap() {
 ! rs.selectRandom()
 ! …
 }
 @adamwarski
  10. 10. But still … ❖ We need to have the news somewhere 11/12/2013 BuildStuff 2013 @adamwarski
  11. 11. Let’s create a DI container! a.k.a. framework 11/12/2013 BuildStuff 2013 @adamwarski
  12. 12. DI in Java ❖ Many frameworks! ❖ Configuration via:! ❖ XML! ❖ annotations! ❖ Java 11/12/2013 BuildStuff 2013 @adamwarski
  13. 13. What’s wrong with that? ❖ Do I really need a DI framework? 11/12/2013 BuildStuff 2013 @adamwarski
  14. 14. Let’s go back … ❖ … and just use our host language! ❖ in this case, Scala! ❖ mapping DI framework concepts to native language constructs 11/12/2013 BuildStuff 2013 @adamwarski
  15. 15. Manual DI! ! object PresentWrapper extends App {! ! ! val ribbonSelector = 
 ! ! ! new RibbonSelector()! ! ! val wrappingPaperFeeder = 
 ! ! ! new WrappingPaperFeeder()! ! ! ! ! val presentPackager = 
 ! ! ! new PresentPackager(
 ! ! ! ! ribbonSelector, 
 ! ! ! ! wrappingPaperFeeder)! } 11/12/2013 BuildStuff 2013 @adamwarski
  16. 16. Manual DI! ! object PresentWrapper extends App {! ! ! lazy val ribbonSelector = 
 ! ! ! new RibbonSelector()! ! ! lazy val wrappingPaperFeeder = 
 ! ! ! new WrappingPaperFeeder()! ! ! ! ! lazy ! ! ! ! ! ! val presentPackager = 
 ! new PresentPackager(
 ! ! ribbonSelector, 
 ! ! wrappingPaperFeeder)! } 11/12/2013 BuildStuff 2013 @adamwarski
  17. 17. MacWire import com.softwaremill.macwire.MacwireMacros._! object PresentWrapper extends App {! ! ! lazy val ribbonSelector = 
 ! ! ! wire[RibbonSelector]! ! ! lazy val wrappingPaperFeeder =
 ! ! ! wire[WrappingPaperFeeder]! ! ! lazy val presentPackager =
 ! ! ! wire[PresentPackager]
 
 } 11/12/2013 BuildStuff 2013 @adamwarski
  18. 18. Side-note: 
 Scala Macros 11/12/2013 BuildStuff 2013 @adamwarski
  19. 19. Side-note: Scala Macros ❖ Scala code executed at compile time! ❖ Operate on trees! ❖ Can inspect the environment, generate code! ❖ the code is type-checked 11/12/2013 BuildStuff 2013 @adamwarski
  20. 20. Side-note: Scala Macros ❖ E.g. debug macro! def debug(params: Any*) = macro debug_impl! def debug_impl
 ! ! ! (c: Context)
 ! ! ! (params: c.Expr[Any]*): c.Expr[Unit]! ! debug(presentCount) ⟹! ! println(“presentCount = “ + presentCount) 11/12/2013 BuildStuff 2013 @adamwarski
  21. 21. Side-note: Scala Macros ❖ Debug macro implementation! import c.universe._! val paramRep = show(param.tree)
 val paramRepTree = Literal(Constant(paramRep))
 val paramRepExpr = c.Expr[String](paramRepTree)! reify { println(
 ! paramRepExpr.splice + 
 ! " = " + 
 ! param.splice) } 11/12/2013 BuildStuff 2013 @adamwarski
  22. 22. Side-note: Scala Macros ❖ MacWire! def wire[T] = macro wire_impl[T]! def wire_impl
 ! ! ! [T: c.WeakTypeTag]
 ! ! ! (c: Context): c.Expr[T] 11/12/2013 BuildStuff 2013 @adamwarski
  23. 23. MacWire import com.softwaremill.macwire.MacwireMacros._! object PresentWrapper extends App {! ! lazy val ribbonSelector = 
 ! ! ! ! wire[RibbonSelector]! ! lazy val wrappingPaperFeeder =
 ! ! ! ! wire[WrappingPaperFeeder]! ! lazy val presentPackager =
 ! ! ! ! wire[PresentPackager]
 } 11/12/2013 BuildStuff 2013 @adamwarski
  24. 24. Scopes ❖ How long will an object (instance) live? 11/12/2013 BuildStuff 2013 @adamwarski
  25. 25. Singleton & dependent object NorthPole extends App {! ! // Singleton! lazy val santaClaus = wire[SantaClaus]! ! ! // Dependent! def gnome = wire[Gnome]! } 11/12/2013 BuildStuff 2013 @adamwarski
  26. 26. Arbitrary scopes trait WebFrontEnd {
 ! lazy val loggedInUser = 
 ! ! session(new LoggedInUser)
 
 ! def session: Scope
 }! ! trait Scope {
 ! def apply(factory: => T): T
 } 11/12/2013 BuildStuff 2013 @adamwarski
  27. 27. Arbitrary scopes object MyApp extends WebFrontEnd {
 ! val session: Scope = 
 ! ! ! ! new ThreadLocalScope()! ! val filter = new ScopeFilter(session)
 
 ! // bootstrap the web server 
 ! // using the filter
 } 11/12/2013 BuildStuff 2013 @adamwarski
  28. 28. Arbitrary scopes class ScopeFilter(sessionScope: ThreadLocalScope) 
 ! ! ! extends Filter {! ! def doFilter(request: ServletRequest) {
 ! ! sessionScope
 ! ! ! .withStorage(request.getSession()) {! ! ! ! request.proceed()! ! ! }
 ! }
 } 11/12/2013 BuildStuff 2013 @adamwarski
  29. 29. Modules ❖ Pre-wired! ❖ Composable! ❖ Dependencies! ❖ Module per package?! ❖ Veripacks :) 11/12/2013 BuildStuff 2013 @adamwarski
  30. 30. Modules ❖ Module: trait! ❖ Pre-wired: new, MacWire! ❖ Composable: extends/with! ❖ Dependencies: extends/with / abstract members 11/12/2013 BuildStuff 2013 @adamwarski
  31. 31. Modules trait PresentWrapper {! ! lazy val ribbonSelector = 
 ! ! ! ! wire[RibbonSelector]! ! lazy val wrappingPaperFeeder =
 ! ! ! ! wire[WrappingPaperFeeder]! lazy val presentPackager =
 ! ! ! ! wire[PresentPackager]! } 11/12/2013 BuildStuff 2013 @adamwarski
  32. 32. Modules trait PresentFactory extends PresentWrapper {! ! lazy val teddyBearProvider = 
 ! ! ! ! wire[TeddyBearProvider]! ! lazy val toyTrainProvider =
 ! ! ! ! wire[ToyTrainProvider]! lazy val presentAssembly =
 ! ! ! ! wire[PresentAssembly]! } 11/12/2013 BuildStuff 2013 @adamwarski
  33. 33. Modules trait HomeOfSanta {! ! lazy val santaClaus = wire[SantaClaus]! ! lazy val rudolf = wire[Rudolf]! ! lazy val fireplace = wire[Fireplace]! ! def presentAssembly: PresentAssembly! } 11/12/2013 BuildStuff 2013 @adamwarski
  34. 34. Modules trait PresentWrapper { … }
 trait PresentFactory extends PresentWrapper { }
 trait HomeOfSanta { … }! ! object NorthPole 
 ! extends PresentWrapper 
 ! with PresentFactory 
 ! with HomeOfSanta {! ! santaClaus.deliver()! } 11/12/2013 BuildStuff 2013 @adamwarski
  35. 35. Testing Santa’s Home class HomeOfSantaTest extends FlatSpec {! ! it should “deliver presents” in {! ! ! val mockPresentAssembly = …! ! ! new HomeOfSanta {! !! lazy val presentAssembly = 
 ! ! ! ! mockPresentAssembly }! ! ! …! ! }! } 11/12/2013 BuildStuff 2013 @adamwarski
  36. 36. Cake Pattern trait PresentPackagerModule {! ! ! ! ! ! ! class PresentPackager {
 ! def wrap() {
 ! ! ribbonSelector.selectRandom()
 ! ! …
 ! }
 }
 ! lazy val presentPackager = new PresentPackager()
 ! def ribbonSelector: RibbonSelector
 } 11/12/2013 BuildStuff 2013 @adamwarski
  37. 37. Cake Pattern val cake = new PresentPackagerModule
 ! with RibbonSelectorModule
 ! with WrappingPaperFeederModule
 ! with TeddyBearProviderModule
 ! with ToyTrainProviderModule
 ! with PresentAssemblyModule
 ! with … { } 11/12/2013 BuildStuff 2013 @adamwarski
  38. 38. Other features ❖ Interceptors! ! trait Chimney {! ! lazy val presentTransferer =
 ! ! transactional(wire[PresentTransferer])! ! def transactional: Interceptor! } 11/12/2013 BuildStuff 2013 @adamwarski
  39. 39. Other features ❖ Factories! ❖ a dedicated object or …! ! trait PresentBoxer {! ! def box(size: Size) = wire[Box]! } 11/12/2013 BuildStuff 2013 @adamwarski
  40. 40. Other features ❖ Instance maps! ❖ for integrating e.g. with Play! ❖ Factories! ❖ In-method wiring! ❖ More coming, someday :) 11/12/2013 BuildStuff 2013 @adamwarski
  41. 41. Summing up ❖ Reconsider using a framework! ❖ Native Scala gives a lot of power! ❖ ❖ ❖ use it! wisely! More flexibility (less constraints) 11/12/2013 BuildStuff 2013 @adamwarski
  42. 42. Links ❖ http://www.warski.org! ❖ https://github.com/adamw/macwire! ❖ http://springsource.com/ 11/12/2013 BuildStuff 2013 @adamwarski
  43. 43. Thanks! ❖ Questions?! ❖ Stickers ->! ❖ adam@warski.org 11/12/2013 BuildStuff 2013 @adamwarski

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