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Dependency Injection in Scala
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Jakub Janeček
January 29, 2013
Programming
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Dependency Injection in Scala
Presented at Czech Scala Enthusiasts meetup in January 2013.
Jakub Janeček
January 29, 2013
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Transcript
Dependency Injection in Scala Vlastimil Menčík, Jakub Janeček, Michal Příhoda
Czech Scala Enthusiasts
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
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
The coffee-making domain
Spring (annotations)
Spring (the same in Java)
Spring (application context)
Spring observations • actual differences between Java and Scala were
minimal • constructor injection works well with immutability • setter injection requires some additional work
Setter injection in Java
Setter injection in Scala
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
Spring Scala (no annotations)
Spring Scala (wiring)
Spring Scala (application context)
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
Vanilla Guice
Guice with Scala wrapper
Guice Observations • simplicity • interesting if you don't need
(or want) Spring • easy to integrate with other frameworks (even the Scala ones)
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
CDI annotation(s)
CDI (beans.xml)
Scopes and Contexts • singleton • prototype (aka dependent) •
session • request • application • ...
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
Proxies • restrictions on the proxied class ◦ might vary
between DI frameworks • needs a default constructor • cannot be final or have final methods
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"
Java DI Frameworks Summary • you can use them if
you want ◦ Spring is not just DI
Java DI Frameworks Summary • maybe with some additional wrappers
to make them more Scala-friendly
Java DI Frameworks Summary • problems may arise if there
are additional restrictions on classes (setters, proxies, ...) ◦ these can be avoided with some effort
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
Step 1: Define Your Components
Step 2: Wire'em All
Step 3: Define Your Injectable Class
Step 4: Run or Test
Pros&Cons • Pros ◦ straightforward and simple ◦ idiomatic •
Cons ◦ no automatic injection ◦ possible runtime errors (not static as Cake)