Technologies used:   Kotlin 1.2.10 | Spring 5 | Maven 3.3.9 | Spring-boot 2.0.0.M7

In this first post, we will look into the simple Dependency Injection with Spring. There is nothing new, some tips to make it smooth.

For the easy of this example, I have created a Spring Service with an Interface GreetingService and a class GreetingServiceImpl

package vn.finixasia.didemo.services

interface GreetingService {

    fun sayGreeting(): String
}

package vn.finixasia.didemo.services

import org.springframework.stereotype.Service

@Service
class GreetingServiceImpl : GreetingService {
    private val HELLO_GURUS = "Hello !!!"

    override fun sayGreeting(): String {
        return HELLO_GURUS
    }
}

Boot Application

To work, Spring need a context, for the example, we will use a SpringBootApplication

package vn.finixasia.didemo

import org.springframework.boot.SpringApplication
import org.springframework.boot.autoconfigure.SpringBootApplication
import vn.finixasia.didemo.controllers.ConstructorInjectedController
import vn.finixasia.didemo.controllers.MyController
import vn.finixasia.didemo.controllers.PropertyInjectedController

@SpringBootApplication
class DiDemoApplication

fun main(args: Array<String>) {
    var ctx = SpringApplication.run(DiDemoApplication::class.java, *args)

    var controller = ctx.getBean("myController") as MyController

    controller.hello()
    println(ctx.getBean(PropertyInjectedController::class.java).sayHello())
    println(ctx.getBean(ConstructorInjectedController::class.java).sayHello())

}

Injection by Properties

Injection by Property is possible with Kotlin, but not recommended. I just show here how to make it work as it is a little bit tricky.

package vn.finixasia.didemo.controllers

import org.springframework.beans.factory.annotation.Autowired
import org.springframework.stereotype.Controller
import vn.finixasia.didemo.services.GreetingService

@Controller
class PropertyInjectedController {

    @Autowired
    lateinit var greetingService: GreetingService

    fun sayHello(): String? {
        return greetingService.sayGreeting()
    }

}

As greetingService is a non-null value, we should initialize his value at the constructor, and this is non-convenient, so we use the key lateinit, to notice Kotlin, the property will be non-null, but it will be set later, not at the constructor.

Injected by Constructor

package vn.finixasia.didemo.controllers

import org.springframework.beans.factory.annotation.Autowired
import org.springframework.stereotype.Controller
import vn.finixasia.didemo.services.GreetingService

@Controller
class ConstructorInjectedController @Autowired constructor(val greetingService: GreetingService) {

    fun sayHello(): String? {
        return greetingService.sayGreeting()
    }
}

The @Autowired annotation is not anymore mandatory since Spring 4.3, I like to keep it has it is self-explanatory.