Kotlin Room Database Singleton Pattern-CodePudding

I am trying to create singleton room database. I found 2 solutions but I don't know what's diffrence between them.

According to this document https://developer.android.com/codelabs/kotlin-android-training-room-database?hl=en&continue=https://codelabs.developers.google.com/?cat=android#5

companion object {
    @Volatile private var INSTANCE:AppDatabase? = null
    fun getInstance(context: Context):AppDatabase {
        synchronized(this){
            var instance = INSTANCE

            if (instance == null){
                instance = Room.databaseBuilder(context.applicationContext,AppDatabase::class.java,"user_table")
                    .fallbackToDestructiveMigration()
                    .build()
                INSTANCE = instance
            }
            return instance
        }
 }

And this one according to Singleton class in Kotlin

companion object {
fun getInstance(context: Context):AppDatabase{
        return Room.databaseBuilder(context.applicationContext,AppDatabase::class.java,"user_table")
            .fallbackToDestructiveMigration()
            .build()
    }
}

I tried these and they all give me same instance. Is there any diffrence between them? In terms of performance or whatever

CodePudding user response：

The differences is the first solution is multi-thread safe instantiation. These will help to prevent different thread re-instantiating your database instance

If you notice, there is @Volatile and synchronized(this) block there.

@Volatile here helps to immediately made the changes to var INSTANCE:AppDatabase visible to other threads
synchronized(this) will ensure only one thread that accessing this block

Found several source explaining about multi-thread safe and race condition, and I think this one also found may be helpful to understand in what kind of condition that should using multi-thread safe way