Android 房间数据库是 singleton。为什么没有私有构造函数？

Question

Below is my AppDatabase class. which is singleton

    import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
import androidx.room.TypeConverters;

@Database(entities = {NoteEntity.class}, version =1,exportSchema = false)
@TypeConverters(DateConverter.class)
public abstract class AppDatabase extends RoomDatabase {
    private static AppDatabase instance;
//android will automatically generate code for this abstract method. so we dont need to implement this
    public abstract NoteDao noteDao();

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

    }
}

Above code work fine. but when i include private constructor

private AppDatabase(){}

this error shows

error: AppDatabase() has private access in AppDatabase public final class AppDatabase_Impl extends AppDatabase {

My question is if this is singleton pattern why i cant include private constructor

Answer 1

It's because Room cannot implement your AppDatabase class (AppDatabase_Impl) when you give it a private constructor. AppDatabase_Impl needs to be able to call the super constructor.

You can change the constructor's access modifier to protected:

protected AppDatabase(){}