Kotlin双空校验式单例

以数据库创建为例
在Java中

@Database(entities = {User.class}, version = 1)
public abstract class UsersDatabase extends RoomDatabase {

    private static volatile UsersDatabase INSTANCE;

    public abstract UserDao userDao();

    public static UsersDatabase getInstance(Context context) {
        if (INSTANCE == null) {
            synchronized (UsersDatabase.class) {
                if (INSTANCE == null) {
                    INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                            UsersDatabase.class, "Sample.db")
                            .build();
                }
            }
        }
        return INSTANCE;
    }

}

在Kotlin中

@Database(entities = arrayOf(User::class), version = 1)
abstract class UsersDatabase : RoomDatabase() {

    abstract fun userDao(): UserDao

    companion object {

        @Volatile private var INSTANCE: UsersDatabase? = null

        fun getInstance(context: Context): UsersDatabase =
                INSTANCE ?: synchronized(this) {
                    INSTANCE ?: buildDatabase(context).also { INSTANCE = it }
                }

        private fun buildDatabase(context: Context) =
                Room.databaseBuilder(context.applicationContext,
                        UsersDatabase::class.java, "Sample.db")
                        .build()
    }
}