kotlin + aws lambda requiring empty constructor

Question

I have the following class written in Kotlin+Guice that is invoked with a lambda

class LambdaProcessor @Inject constructor(private val s3Util: S3Util) {

    fun lambdaInvokesThisMethod() {
        s3Util.DoSomething()
    }
}

That works great for unit testing, but lambda requires the class to have an empty constructor.

I can convert this same class to have an empty constructor by doing this:

class LambdaProcessor {

    @Inject lateinit var s3Util: S3Util

    init {
        Guice.createInjector(GuiceDependencyInjector()).injectMembers(this)
    }

    fun lambdaInvokesThisMethod() {
        s3Util.DoSomething()
    }
}

That code now works great on lambda but I can't mock s3Util in my unit tests anymore because the init method gets called.

How can I get both scenarios to work together?

Answer 1

In Kotlin, if you have a default constructor then all your other constructors must call your default. But, you can have multiple constructors without a default constructor. Here is the final code we used.

class LambdaProcessor {

    @Inject private lateinit var s3Util: S3Util

    /**
     * Lambda calls the no-arg constructor, use our IoC library to initialize our dependencies
     */
    constructor() {
        Guice.createInjector(GuiceDependencyInjector()).injectMembers(this)
    }

    /*
     * Unit-testing constructor
     */
    constructor(s3Util: S3Util) {
        this.s3Util = s3Util
    }

    fun lambdaInvokesThisMethod() {
        s3Util.DoSomething()
    }
}