Jetpack App Startup: keep your startup logic organized

I doubt that there's an app in production that does not initialize a number of objects.

This usually takes place in the Application class. You create a class that extends Application and you declare it in your manifest. Easy, right? When there's only a small number of things to initialize, this makes sense. Application is created only once and remains in context for the lifetime of your app. But what happens when you need to initialize a lot of things and comments such as // DO NOT CHANGE ORDERING! start appearing?

Lately, Jetpack introduced a new library just for this called App Startup. The concept is that you define an Initializer for each item you want to initialize and then define its dependencies (for the framework to figure out the ordering). Following this approach, your initialization logic will remain organized and in order, decoupled from the Application class.

Setup  

Firstly, add this to your build.gradle.

implementation "androidx.startup:startup-runtime:1.0.0-alpha01"

Check out the official doc for the latest version.

Initializers

Now you need to define an initializer for each component you want to initialize. A common use case is for the dependency injection framework (e.g. Dagger, Koin) to be initialized during application startup. This can have its own Initializer with no dependencies.

class DependencyInjectionInitializer : Initializer<Unit> {

    override fun create(context: Context) {
        // Init logic for the D.I. framework you use
    }

    override fun dependencies(): List<Class<out Initializer<*>>> = emptyList()
}

Any other component that needs to be initialized, can declare DependencyInjectionInitializer as a dependency. This ensures that when something is requested from the D.I. framework, it will be available.

class BillingControllerInitializer : Initializer<Unit> {

    private val billingController by inject(BillingController::class.java)

    override fun create(context: Context) {
        // Init logic for the component that uses the D.I. framework
        billingController.connect()
    }

    override fun dependencies(): List<Class<out Initializer<*>>> =
        listOf(DependencyInjectionInitializer::class.java)
}

Declare the initializers

The last step for these Initializers to run automatically is to declare them in your AndroidManifest.xml.

<application>
[...]
    <provider
        android:name="androidx.startup.InitializationProvider"
        android:authorities="${applicationId}.androidx-startup"
        android:exported="false"
        tools:node="merge">
        <meta-data android:name="my.java.package.BillingControllerInitializer"
              android:value="androidx.startup" />
    </provider>
</application>

Note that there is no need to declare all the Initializers. Only the "leafs" in the dependency graph. In this example, we only need to declare BillingControllerInitializer because it depends on DependencyInjectionInitializer. The App Startup library will find it and initialize it as well.

Manual initialization

The library provides an option for the Initializers not to run automatically. You can make a manual call when the initialization should happen. Further calls to the initialization method will return the same object (i.e. the library will keep a cache internally). Personally, I don't find this approach very useful, but if it fits your use case, consult the official doc.

Happy initializing!