Dagger Dependency Injection in main function

Question

I am calling a function in a class S3FileOperationsAdapter from the main function in my kotlin code. I am injecting the class S3FileOperationsAdapter in the main function file. So it looks like

class Runner {
    @set:Inject
    lateinit var s3FileOperationsAdapter: S3FileOperationsAdapter

    fun main(args: Array<String>) {
       s3FileOperationsAdapter.readFunction()
    }
}

Now the issue is:

• When I try to run the above code, I get the error Error: Main method is not static in class com.amazon.bram.sim.BatchJobRunner, please define the main method as:. This is understandable.

And we can only make a static function within an object in kotlin. So upon doing that, I cannot Inject the dependency, because Dagger does not support injection into Kotlin objects. So it feels like a deadlock.

My question is, I want to inject the dependency in this file anyhow so that I can call the respective function. And I am calling this function from the "fun main()" in kotlin. How can I achieve this? Has anyone ever faced this before?

Answer 1

In order to inject anything in Dagger, you must first create an instance of your component. Since no code at all will run before fun main(), this needs to be done during main itself (or in a field initializer).

After creating an instance of the component, you can ask it for an instance of S3FileOperationsAdapter directly.

fun main(args: Array<String>) {
    // Create the component.
    val component = DaggerMyComponent.create()
    // or use the Component.Builder or Component.Factory you defined for MyComponent.

    // Get an object from the component.
    // This method should be defined in your component interface.
    val adapter = component.s3FileOperationsAdapter()

    // Use the object.
    adapter.readFunction()
}

If your actual code is more complicated, with multiple injected objects and a longer main() function, this may be a bit unwieldy. In that case, you can extract your current main() into its own class and let Dagger provide that class in main().