This document discusses different approaches to dependency injection in Scala, including Google Guice, implicit parameters, the cake pattern, Reader monad, MacWire, and Airframe. It compares runtime DI approaches like Guice and Airframe to compile-time approaches. The best approach depends on whether auto-wiring is needed, whether compile-time checking or dynamic binding is preferred, and whether life-cycle management is required.

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Revisit Dependency
Injection in Scala
Naoki Takezoe
@takezoen
at Airframe Meetup #3

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What’s Dependency Injection?
● Inversion of Control
● Externalize components dependency and make it configurable
● Encourage clean components design
● Components can be replaced (e.g. for testing)
● Good collaboration with Aspect Oriented Programming

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Dependency Injection in Java
Explicit configuration to Less configuration

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Dependency Injection in Scala
● Java based DI container
○ Google Guice
● Pure Scala approach
○ Implicit parameter
○ Cake Pattern
○ Reader Monad, etc
● Scala native DI library
○ MacWire
○ Airframe

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Google Guice
● Widely used in Scala because of Play Framework
● Easy to use and understand
● Lack of Scala specific functionality
Define binding in Module
Use @Inject annotation for DI

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Implicit parameter
● No external library is required
● Easy to use and understand
● All necessary components must be in the same scope
Take necessary components as implicit parameter
Define necessary implicit val or def in the execution scope

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Cake Pattern
● No external library is required
● Boilerplate
Combine by mix-in
Declare dependency as self-type annotation

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Reader Monad
● Carry components around as an environment
● Need to put all necessary components together into the environment
Run with environment

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MacWire
● Generate instance creation code with constructor injection by macro
● Rich features (e.g. multi binding, tagged binding, intercepter, etc)
● No boilerplate
● All necessary components must be in the same scope
Create instance using wire[T] macro

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Airframe
● Constructor injection and In-trait injection
● Rich features (e.g. tagged binding, type alias binding, config binding, etc)
● Less configuration by automatic instance creation
● Life-cycle management
In-trait injection
Define binding on Design if you need to
override default binding

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Runtime DI vs Compile-time DI
● Runtime DI
○ Google Guice
○ Airframe
● Compile-time ID
○ Pure Scala approaches
○ MacWire

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Runtime DI vs Compile-time DI
Runtime DI Compile-time DI
Binding error Runtime error Compilation error
Configuration Less configuration Boilerplate is necessary
Life-cycle management Yes No
Overhead Runtime Compile-time
External library Required Not required / Required
Scala specific features Yes / No Yes

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What approach is the best?
● Do you need auto-wiring?
● Which do you need? compile-time dependency check or dynamic-type
binding?
● Do you need object life-cycle management?

14. Thank You!
Danke!
Merci!
谢谢!
ありがとう!
Gracias!
Kiitos!
Copyright 1995-2019 Arm Limited (or its affiliates). All rights reserved.