thenobody / di-test

Example project using vanilla Scala dependency injection and unit tests

Scala dependency injection example

This project demonstrates the usage of pure Scala dependency injection and unit testing. This approach was inspired by DI described at http://www.michaelpollmeier.com/2014/06/29/simple-dependency-injection-scala/.

In this approach, we use constructor dependency passing rather than defining them on class init. This enables compile time checking of dependency mocks in unit tests and promotes dependency usage visibility to the developer in general.

The container

net/thenobody/ditest/container/package.scala

The container defines two types of instantiation:

1. singleton

object RevenueServiceInstance extends RedisRevenueService(
  config.getString("ditest.redis.host"),
  config.getInt("ditest.redis.database"),
  config.getInt("ditest.redis.timeout"),
  UserServiceInstance
)

The instance of RevenueServiceInstance is created only once (when requested for the first time).

2. prototype

object RevenueServiceProvider {
  def apply(): RedisRevenueService = new RedisRevenueService(
    config.getString("ditest.redis.host"),
    config.getInt("ditest.redis.database"),
    config.getInt("ditest.redis.timeout"),
    UserServiceInstance
  )
}

A new instance is created everytime it is requested.

Config

The project uses typesafe/Config and by default reads the configuration parameters from src/main/resources/application.conf. This can be overriden by passing config.file system parameter when running from console:

$ java -jar path/to/di-test.jar net.thenobody.ditest.Main -Dconfig.file=/path/to/application.conf

Usage

First, the container needs to be imported:

import net.thenobody.ditest.container._

Afterwards, the instances can be obtained by:

val revenueService1: RevenueService = RevenueServiceInstance   // returns singleton
val revenueService2: RevenueService = RevenueServiceProvider() // returns new instance on every invocation

Unit tests

Example unit test can be found in src/test/scala/net/thenobody/ditest/service/RedisRevenueServiceTest.scala which uses Mockito. The mocks are passed to the tested class as constructor arguments (compile time dependency check).