Using room as singleton in kotlin

Question

I\'m trying to use Room as singleton so I didn\'t have to invoke Room.databaseBuilder() -which is expensive- more than once.

@Database(entities = ar         


        

Answer 1

In this particular case I would resort to using Dagger 2, or some other dependency injection library like Koin or Toothpick. All three libraries allow to provide dependancies as singletons.

Here's the code for Dagger 2 module:

@Module
class AppModule constructor(private val context: Context) {
    @Provides
    @Singleton
    fun providesDatabase(): AppDatabase {
        return Room.databaseBuilder(
                context,
                AppDatabase::class.java,
                "train.db")
                .build()
    }
}

AppComponent:

@Singleton
@Component(modules = arrayOf(
        AppModule::class
))
interface AppComponent {
    fun inject(viewModel: YourViewModel)
    fun inject(repository: YourRepository)
}

Application class to provide injection:

class App : Application() {
    companion object {
        private lateinit var appComponent: AppComponent
        val component: AppComponent get() = appComponent
    }

    override fun onCreate() {
        super.onCreate()
        initializeDagger()
    }

    private fun initializeDagger() {
        component = DaggerAppComponent.builder()
                .appModule(AppModule(this))
                .build()
    }
}

And then inject your database as singleton to wherever you need it (for example in your app's repository):

@Inject lateinit var appDatabase: AppDatabase

init {
    App.component.inject(this)
}

Answer 2

I found a solution so here is the answer for future me and any one who might have the same problem.

---

After some research, I found that I have two options.

1. use Double-checked locking
2. use Initialization-on-demand holder idiom

so I considered implementing one of them, but this doesn't feel right in kotlin too much boilerplate code :p

---

so after more research, I stumbled upon this great article which provides an excellent solution which uses Double-checked locking but in an eligant way.

my code becomes like this:

companion object : SingletonHolder<AppDatabase, Context>({
       Room.databaseBuilder(it.applicationContext, AppDatabase::class.java, "train.db").build()
})

from the article:

A reusable Kotlin implementation:

We can encapsulate the logic to lazily create and initialize a singleton with argument inside a SingletonHolder class. In order to make that logic thread-safe, we need to implement a synchronized algorithm and the most efficient one — which is also the hardest to get right — is the double-checked locking algorithm.

open class SingletonHolder<T, A>(creator: (A) -> T) {
    private var creator: ((A) -> T)? = creator
    @Volatile private var instance: T? = null

    fun getInstance(arg: A): T {
        val i = instance
        if (i != null) {
            return i
        }

        return synchronized(this) {
            val i2 = instance
            if (i2 != null) {
                i2
            } else {
                val created = creator!!(arg)
                instance = created
                creator = null
                created
            }
        }
    }
}

---

Extra: if you want Singleton with two arguments

open class SingletonHolder2<out T, in A, in B>(creator: (A, B) -> T) {
    private var creator: ((A, B) -> T)? = creator
    @Volatile private var instance: T? = null

    fun getInstance(arg0: A, arg1: B): T {
        val i = instance
        if (i != null) return i

        return synchronized(this) {
            val i2 = instance
            if (i2 != null) {
                i2
            } else {
                val created = creator!!(arg0, arg1)
                instance = created
                creator = null
                created
            }
        }
    }
}

Answer 3

Here's how i figured out...

@Database(entities = [MyEntity::class], version = dbVersion, exportSchema = true)
abstract class AppDB : RoomDatabase() {

// First create a companion object with getInstance method
    companion object {
        fun getInstance(context: Context): AppDB = 
    Room.databaseBuilder(context.applicationContext, AppDB::class.java, dbName).build()
    }

    abstract fun getMyEntityDao(): MyEntityDao
}

// This is the Singleton class that holds the AppDB instance 
// which make the AppDB singleton indirectly
// Get the AppDB instance via AppDBProvider through out the app
object AppDBProvider {

private var AppDB: AppDB? = null

    fun getInstance(context: Context): AppDB {
        if (appDB == null) {
            appDB = AppDB.getInstance(context)
        }
       return appDB!!
    }

}

Answer 4

singleton in kotlin is real easy just do this

companion object {
    @JvmStatic
    val DATABASE_NAME = "DataBase"

    @JvmField
    val database = Room.databaseBuilder(App.context(), DataBase::class.java, DataBase.DATABASE_NAME).build()

}

Answer 5

You could make use of the Kotlin standard library's

fun <T> lazy(LazyThreadSafetyMode.SYNCHRONIZED, initializer: () -> T): Lazy<T>

companion object {
    private lateinit var context: Context
    private val database: AppDatabase by lazy(LazyThreadSafetyMode.SYNCHRONIZED) {
        Room.databaseBuilder(context, AppDatabase::class.java, "trains.db").build()
    }
    fun getDatabase(context: Context): AppDatabase {
        this.context = context.applicationContext
        return database
    }
}

Personally though, I would normally add ApplicationContext-dependent singletons inside the Application, e.g.

<!-- AndroidManifest.xml -->
<manifest>
  <application android:name="MyApplication">
...

class MyApplication : Application() {
    val database: AppDatabase by lazy {
        Room.databaseBuilder(this, AppDatabase::class.java, "train.db").build()
    }
}

You can even define an extension method for easy access as context.database.

val Context.database
    get() =
        generateSequence(applicationContext) {
       (it as? ContextWrapper)?.baseContext
       }.filterIsInstance<MyApplication>().first().database