为什么我仍然需要使用带有依赖注入库 (Hilt) 的 singleton 类？

Question

One of the reasons the official android documentation recommends using a dependency injection library such as Hilt instead of manual dependency injection is it's difficult to reuse objects. And I'd have to apply the singleton pattern, which makes testing more difficult because all tests share the same singleton instance. So Hilt should replace singleton classes.

But I was checking the sunflower sample and I noticed it still uses the singleton pattern with Hilt.

The App database is declared as a singleton:

// For Singleton instantiation
@Volatile private var instance: AppDatabase? = null

fun getInstance(context: Context): AppDatabase {
    return instance ?: synchronized(this) {
        instance ?: AppDatabase.buildDatabase(context).also { instance = it }
    }
}

DatabaseModule is also scoped to the application container:

@InstallIn(SingletonComponent::class)
@Module
class DatabaseModule {

    @Singleton
    @Provides
    fun provideAppDatabase(@ApplicationContext context: Context): AppDatabase {
        return AppDatabase.getInstance(context)
    }

    //...
}

Using the singleton pattern here doesn't make sense to me because by annotating @Provides provideDatabase() method with @Singleton . Hilt makes sure only one instance is used across the app lifetime.

Shouldn't Hilt replace the singleton pattern as well?

Answer 1

One of the reasons the official android documentation recommends using a dependency injection library such as Hilt instead of manual dependency injection is it's difficult to reuse objects. And I'd have to apply the singleton pattern, which makes testing more difficult because all tests share the same singleton instance. So Hilt should replace singleton classes.

But I was checking the sunflower sample and I noticed it still uses the singleton pattern with Hilt.

The App database is declared as a singleton:

// For Singleton instantiation
@Volatile private var instance: AppDatabase? = null

fun getInstance(context: Context): AppDatabase {
    return instance ?: synchronized(this) {
        instance ?: AppDatabase.buildDatabase(context).also { instance = it }
    }
}

DatabaseModule is also scoped to the application container:

@InstallIn(SingletonComponent::class)
@Module
class DatabaseModule {

    @Singleton
    @Provides
    fun provideAppDatabase(@ApplicationContext context: Context): AppDatabase {
        return AppDatabase.getInstance(context)
    }

    //...
}

Using the singleton pattern here doesn't make sense to me because by annotating @Provides provideDatabase() method with @Singleton . Hilt makes sure only one instance is used across the app lifetime.

Shouldn't Hilt replace the singleton pattern as well?