Spring Scala Based Sample Bean Configuration

I have been using Spring Scala for a toy project for the last few days and I have to say that it is a fantastic project, it simplifies Spring configuration even further when compared to the already simple configuration purely based on Spring Java Config.

Let me demonstrate this by starting with the Cake Pattern based sample here:

// =======================
// service interfaces
trait OnOffDeviceComponent {
  val onOff: OnOffDevice
  trait OnOffDevice {
    def on: Unit
    def off: Unit
  }
}
trait SensorDeviceComponent {
  val sensor: SensorDevice
  trait SensorDevice {
    def isCoffeePresent: Boolean
  }
}

// =======================
// service implementations
trait OnOffDeviceComponentImpl extends OnOffDeviceComponent {
  class Heater extends OnOffDevice {
    def on = println("heater.on")
    def off = println("heater.off")
  }
}
trait SensorDeviceComponentImpl extends SensorDeviceComponent {
  class PotSensor extends SensorDevice {
    def isCoffeePresent = true
  }
}
// =======================
// service declaring two dependencies that it wants injected
trait WarmerComponentImpl {
  this: SensorDeviceComponent with OnOffDeviceComponent =>
  class Warmer {
    def trigger = {
      if (sensor.isCoffeePresent) onOff.on
      else onOff.off
    }
  }
}

// =======================
// instantiate the services in a module
object ComponentRegistry extends
  OnOffDeviceComponentImpl with
  SensorDeviceComponentImpl with
  WarmerComponentImpl {

  val onOff = new Heater
  val sensor = new PotSensor
  val warmer = new Warmer
}

// =======================
val warmer = ComponentRegistry.warmer
warmer.trigger

Cake pattern is a pure Scala way of specifying the dependencies. 

Now, if we were to specify this dependency using Spring's native Java config, but with Scala as the language, firs to define the components that need to be wired together:

trait SensorDevice {
    def isCoffeePresent: Boolean
  }

  class PotSensor extends SensorDevice {
    def isCoffeePresent = true
  }

  trait OnOffDevice {
    def on: Unit
    def off: Unit
  }

  class Heater extends OnOffDevice {
    def on = println("heater.on")
    def off = println("heater.off")
  }

  class Warmer(s: SensorDevice, o: OnOffDevice) {
    def trigger = {
      if (s.isCoffeePresent) o.on
      else o.off
    }
  }

and the configuration with Spring Java Config and a sample which makes use of this configuration:

import org.springframework.context.annotation.Configuration
  import org.springframework.context.annotation.Bean

  @Configuration
  class WarmerConfig {
    @Bean
    def heater(): OnOffDevice = new Heater

    @Bean
    def potSensor(): SensorDevice = new PotSensor

    @Bean
    def warmer() = new Warmer(potSensor(), heater())
  }

  import org.springframework.context.annotation.AnnotationConfigApplicationContext

  val ac = new AnnotationConfigApplicationContext(classOf[WarmerConfig])

  val warmer = ac.getBean("warmer", classOf[Warmer])
  
  warmer.trigger 

Taking this further to use Spring-Scala project to specify the dependencies, the configuration and a sample look like this:

import org.springframework.context.annotation.Configuration
  import org.springframework.context.annotation.Bean

  @Configuration
  class WarmerConfig {
    @Bean
    def heater(): OnOffDevice = new Heater

    @Bean
    def potSensor(): SensorDevice = new PotSensor

    @Bean
    def warmer() = new Warmer(potSensor(), heater())
  }

  import org.springframework.context.annotation.AnnotationConfigApplicationContext

  val ac = new AnnotationConfigApplicationContext(classOf[WarmerConfig])

  val warmer = ac.getBean("warmer", classOf[Warmer])
  
  warmer.trigger 

The essence of the Spring Scala project as explained in this wiki is the "bean" method derived from the `FunctionalConfiguration` trait, this method can be called to create a bean, passing in parameters to specify, if required, bean name, alias, scope and a function which returns the instantiated bean.

This sample hopefully gives a good appreciation for how simple Spring Java Config is, and how much more simpler Spring-Scala project makes it for Scala based projects.