我无法在数据库 DAO android 中存储值

Question

Hi I can't store values in the database DAO android. I have User.class:

public class User {
    @PrimaryKey @NonNull 
    public String uid;
@ColumnInfo(name="name")
public String name;
 public User(String uid, String name){
        this.uid=uid;
        this.name=name;
    }
}

DAO database:

@Dao
public interface UserDAO {
    @Query("select*from User")
    List<User> getAll();
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void insertAll(User users);
}

My Dtababase class is:

@Database(entities = {User.class},version=1)
public abstract  class AppDatabase extends RoomDatabase {
    public abstract UserDAO userDAO();
}

My code

 new Thread(new Runnable() {
                        @Override
                        public void run() {
                            try {
                                AppDatabase db = Room.databaseBuilder(activity.getApplicationContext(),
                                        AppDatabase.class, "user_db").build();
                                UserDAO userDao = db.userDAO();
                                userDao.insertAll(new User("11","Melany"));
                                Log.d("main_activity_uid",
                                        "ok  you are pretty");

                            } catch (Exception e) {
                                e.printStackTrace();
                            }
                        }
                    }).start();

Now my code don't store new User(i can't print "ok you are pretty").Why? I have another question: i know that when you store data in the database you use threads but what do threads allow you to do?

Answer 1

A problem is that you always create in a new database instance instead of having a singleton database.

Single instance ( source ):

    private static volatile WordRoomDatabase INSTANCE;

     static WordRoomDatabase getDatabase(final Context context) {
            if (INSTANCE == null) {
                synchronized (WordRoomDatabase.class) {
                    if (INSTANCE == null) {
                        INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                                WordRoomDatabase.class, "word_database")
                                .build();
                    }
                }
            }
            return INSTANCE;
        }

---

You need a thread because the Room database does not allow operations on the main thread to prevent blocking the UI ( source )