Room db：數據庫問題

Question

好吧，檢查了所有關於房間數據庫的帖子，沒有人解決我的問題。 看起來我的數據庫從未創建過。 我只是使用標准的代碼示例。

public class LocalRepository {
    private static AppDatabase database;
    private static final Object LOCK = new Object();
    private static Context ctx;

    public synchronized static AppDatabase getDB(Context context) {
        if (database == null) {
            ctx = context;
            synchronized (LOCK) {
                if (database == null) {
                    database = Room.databaseBuilder(context,
                            AppDatabase.class, "goDetectDB")
                            .fallbackToDestructiveMigration()
                            .addCallback(dbCallback).build();

                }
            }
        }
        return database;
    }

    private static RoomDatabase.Callback dbCallback = new RoomDatabase.Callback() {
        public void onCreate(SupportSQLiteDatabase db) {

            Executors.newSingleThreadScheduledExecutor().execute(new Runnable() {
                @Override
                public void run() {
                    populateDB(ctx);
                }
            });
        }
    };

    private static void populateDB(Context ctx){
        getDB(ctx).discoveryDao().insertAll(DiscoveryContent.ITEMS.toArray(new Discovery[DiscoveryContent.ITEMS.size()]));
    }

}

這是我的數據庫的倉庫。

@Database(entities = {Discovery.class}, version = 1, exportSchema = true)
public abstract class AppDatabase extends RoomDatabase {

    public abstract DiscoveryDAO discoveryDao();
}

然后我用所有的 getter 和 setter 映射我的 dao。

@Entity(tableName = "discovery") public class Discovery {
    @PrimaryKey
    private int id; ....

使用的道。

@Dao
public interface DiscoveryDAO {
    @Insert
  public void insert(Discovery... items);
    @Update
  public void update(Discovery... items);
    @Delete
  public void delete(Discovery item);

  @Query("SELECT * FROM discovery")
  List<Discovery> getAll();

  @Query("SELECT * FROM discovery WHERE id IN (:listIds)")
  List<Discovery> loadAllByIds(int[] listIds);

  @Query("SELECT * FROM discovery WHERE categorie LIKE :categorie AND " +
          "nom LIKE :nom LIMIT 1")
  Discovery findByName(String categorie, String nom);

  @Insert
  void insertAll(Discovery... items);

  @Query("SELECT * FROM discovery WHERE id = :id")
  public Discovery getItemById(int id);
}

然后我在一個帶有異步任務的片段中調用我的 dao。

public class HomeFragment extends Fragment {

    private HomeViewModel homeViewModel;

    public View onCreateView(@NonNull LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {
        homeViewModel =
                ViewModelProviders.of(this).get(HomeViewModel.class);
        View root = inflater.inflate(R.layout.fragment_home, container, false);
        final TextView textView = root.findViewById(R.id.text_home);

        // ajout db

        new AsyncTask<Void, Void, Void>() {

            String message = "none";

            @Override
            protected Void doInBackground(Void... voids) {
                AppDatabase database = LocalRepository.getDB(getContext());
                final DiscoveryDAO discoveryDAO = database.discoveryDao();
                message = "failure";
                if(discoveryDAO.getAll() != null && discoveryDAO.getAll().size() > 0){
                    message = "read data ";
                    message += discoveryDAO.getAll().size();
                }
                return null;
            }

            @Override
            protected void onPostExecute(Void aVoid) {
                int duration = Toast.LENGTH_SHORT;
                Toast toast = Toast.makeText(getContext(), message, duration);
                toast.show();
            }
        }.execute();


        homeViewModel.getText().observe(this, new Observer<String>() {
            @Override
            public void onChanged(@Nullable String s) {
                textView.setText(s);
            }
        });
        return root;
    }
}

主要問題是我從來沒有收到“讀取數據”消息，它總是失敗。 看起來有問題，但在 logcat 或類似的東西中沒有問題。

我在這里查看了一些關於房間數據庫的帖子，主要錯誤是沒有調用 dao 或類似啟用填充方法的東西。 這里不是這種情況，或者有什么問題。

你能幫助我嗎？

編輯：請注意，在創建回調中使用 dao 的一些示例無法工作，因為您將收到異常或沒有消息，就像我的情況一樣，這很奇怪。 我的答案中的最終代碼:)

Answer 1

我相信您不能使用 Dao ，因為這需要一個已構建數據庫的實例。

回調在構建完成之前作為構建的一部分被調用。 您需要使用 SupportSQLiteDatabase 來完成您的工作。

例子

根據您的一些代碼考慮這個濃縮/簡化版本：-

發現.java

@Entity
public class Discovery {

    @PrimaryKey
    Long id;
    String name;

    public Discovery(){}

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

DiscoveryDao.java

@Dao
public interface DiscoveryDao {

    @Insert
    long[] insertAll(Discovery... discoveries);

    @Query("SELECT * FROM Discovery")
    List<Discovery> getAll();
}

AppDatabase.java

@Database(version = 1, entities = Discovery.class)
public abstract class AppDatabase extends RoomDatabase {

    public abstract DiscoveryDao discoveryDao();
}

MainActivity.java

public class MainActivity extends AppCompatActivity {

    AppDatabase mAppDB;
    DiscoveryDao mDao;
    String[] DiscoveryNamesToInsert = new String[]{"Found","Discovered","Located"};

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mAppDB = Room.databaseBuilder(
                this,
                AppDatabase.class,
                "goDetectDB"
        )
                .allowMainThreadQueries()
                .fallbackToDestructiveMigration()
                .addCallback(dbCallback)
                .build();
        mDao = mAppDB.discoveryDao();
        List<Discovery> discoveryList = mDao.getAll();
        for (Discovery d: discoveryList) {
            Log.d("DISCOVERYINFO","DiscoveryName is " + d.getName() + ", ID is " + d.getId());
        }
    }

    private RoomDatabase.Callback dbCallback = new RoomDatabase.Callback() {

        @Override
        public void onCreate(@NonNull SupportSQLiteDatabase db) {
            super.onCreate(db);
            long[] results = new long[DiscoveryNamesToInsert.length];
            db.beginTransaction();
            for (int i = 0; i < DiscoveryNamesToInsert.length; i++) {
                results[i] = insert(db,DiscoveryNamesToInsert[i]);
            }
            db.setTransactionSuccessful();
            db.endTransaction();
        }


        @Override
        public void onOpen(@NonNull SupportSQLiteDatabase db) {
            super.onOpen(db);
        }


        @Override
        public void onDestructiveMigration(@NonNull SupportSQLiteDatabase db) {
            super.onDestructiveMigration(db);
        }

        private long insert(SupportSQLiteDatabase db,String name) {
            ContentValues cv = new ContentValues();
            cv.put("name",name);
            return db.insert("Discovery", OnConflictStrategy.IGNORE,cv);
        }
    };
}

• insert方法使用 SupportSQLiteDatabase 進行插入， onCreate方法調用 insert 方法 x 次（示例中為 3 次）。 結果數組將具有結果，即對於一個添加的 id 的 id，對於一個被忽略的 -1。

結果（經過 2 次運行以確保僅添加一次數據）：-

2019-10-05 14:11:37.135 D/DISCOVERYINFO: DiscoveryName is Found, ID is 1
2019-10-05 14:11:37.135 D/DISCOVERYINFO: DiscoveryName is Discovered, ID is 2
2019-10-05 14:11:37.135 D/DISCOVERYINFO: DiscoveryName is Located, ID is 3

Answer 2

編輯：請注意，在創建回調中使用 dao 的一些示例無法工作，因為您將收到異常或沒有消息，就像我的情況一樣，這很奇怪。

數據庫 object 中的最終代碼（使用奶酪對象）。

@Entity(tableName = Cheese.TABLE_NAME)
public class Cheese {

    /** The name of the Cheese table. */
    public static final String TABLE_NAME = "cheeses";

    /** The name of the ID column. */
    public static final String COLUMN_ID = BaseColumns._ID;

    /** The name of the name column. */
    public static final String COLUMN_NAME = "name";

    /** The unique ID of the cheese. */
    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(index = true, name = COLUMN_ID)
    public long id;

    /** The name of the cheese. */
    @ColumnInfo(name = COLUMN_NAME)
    public String name;

    /**
     * Create a new {@link Cheese} from the specified {@link ContentValues}.
     *
     * @param values A {@link ContentValues} that at least contain {@link #COLUMN_NAME}.
     * @return A newly created {@link Cheese} instance.
     */
    public static Cheese fromContentValues(ContentValues values) {
        final Cheese cheese = new Cheese();
        if (values.containsKey(COLUMN_ID)) {
            cheese.id = values.getAsLong(COLUMN_ID);
        }
        if (values.containsKey(COLUMN_NAME)) {
            cheese.name = values.getAsString(COLUMN_NAME);
        }
        return cheese;
    }

    /** Dummy data. */
    public static final String[] CHEESES = {
            "Abbaye de Belloc", "Abbaye du Mont des Cats", "Abertam", "Abondance", "Ackawi",
            "Acorn", "Adelost", "Affidelice au Chablis", "Afuega'l Pitu", "Airag", "Airedale",
            "Aisy Cendre", "Allgauer Emmentaler", "Alverca", "Ambert", "American Cheese",
            "Ami du Chambertin", "Anejo Enchilado", "Anneau du Vic-Bilh", "Anthoriro", "Appenzell",
            "Aragon", "Ardi Gasna", "Ardrahan", "Armenian String", "Aromes au Gene de Marc",
            "Asadero", "Asiago", "Aubisque Pyrenees", "Autun", "Avaxtskyr", "Baby Swiss",
            "Babybel", "Baguette Laonnaise", "Bakers", "Baladi", "Balaton", "Bandal", "Banon",
            "Barry's Bay Cheddar", "Basing", "Basket Cheese", "Bath Cheese", "Bavarian Bergkase",
            "Baylough", "Beaufort", "Beauvoorde", "Beenleigh Blue", "Beer Cheese", "Bel Paese",
            "Bergader", "Bergere Bleue", "Berkswell", "Beyaz Peynir", "Bierkase", "Bishop Kennedy",
            "Blarney", "Bleu d'Auvergne", "Bleu de Gex", "Bleu de Laqueuille",
            "Bleu de Septmoncel", "Bleu Des Causses", "Blue", "Blue Castello", "Blue Rathgore",
            "Blue Vein (Australian)", "Blue Vein Cheeses", "Bocconcini", "Bocconcini (Australian)",
            "Boeren Leidenkaas", "Bonchester", "Bosworth", "Bougon", "Boule Du Roves",
            "Boulette d'Avesnes", "Boursault", "Boursin", "Bouyssou", "Bra", "Braudostur",
            "Breakfast Cheese", "Brebis du Lavort", "Brebis du Lochois", "Brebis du Puyfaucon",
            "Bresse Bleu", "Brick", "Brie", "Brie de Meaux", "Brie de Melun", "Brillat-Savarin",
            "Brin", "Brin d' Amour", "Brin d'Amour", "Brinza (Burduf Brinza)",
            "Briquette de Brebis", "Briquette du Forez", "Broccio", "Broccio Demi-Affine",
            "Brousse du Rove", "Bruder Basil", "Brusselae Kaas (Fromage de Bruxelles)", "Bryndza",
            "Buchette d'Anjou", "Buffalo", "Burgos", "Butte", "Butterkase", "Button (Innes)",
            "Buxton Blue", "Cabecou", "Caboc", "Cabrales", "Cachaille", "Caciocavallo", "Caciotta",
            "Caerphilly", "Cairnsmore", "Calenzana", "Cambazola", "Camembert de Normandie",
            "Canadian Cheddar", "Canestrato", "Cantal", "Caprice des Dieux", "Capricorn Goat",
            "Capriole Banon", "Carre de l'Est", "Casciotta di Urbino", "Cashel Blue", "Castellano",
            "Castelleno", "Castelmagno", "Castelo Branco", "Castigliano", "Cathelain",
            "Celtic Promise", "Cendre d'Olivet", "Cerney", "Chabichou", "Chabichou du Poitou",
            "Chabis de Gatine", "Chaource", "Charolais", "Chaumes", "Cheddar",
            "Cheddar Clothbound", "Cheshire", "Chevres", "Chevrotin des Aravis", "Chontaleno",
            "Civray", "Coeur de Camembert au Calvados", "Coeur de Chevre", "Colby", "Cold Pack",
            "Comte", "Coolea", "Cooleney", "Coquetdale", "Corleggy", "Cornish Pepper",
            "Cotherstone", "Cotija", "Cottage Cheese", "Cottage Cheese (Australian)",
            "Cougar Gold", "Coulommiers", "Coverdale", "Crayeux de Roncq", "Cream Cheese",
            "Cream Havarti", "Crema Agria", "Crema Mexicana", "Creme Fraiche", "Crescenza",
            "Croghan", "Crottin de Chavignol", "Crottin du Chavignol", "Crowdie", "Crowley",
            "Cuajada", "Curd", "Cure Nantais", "Curworthy", "Cwmtawe Pecorino",
            "Cypress Grove Chevre", "Danablu (Danish Blue)", "Danbo", "Danish Fontina",
            "Daralagjazsky", "Dauphin", "Delice des Fiouves", "Denhany Dorset Drum", "Derby",
            "Dessertnyj Belyj", "Devon Blue", "Devon Garland", "Dolcelatte", "Doolin",
            "Doppelrhamstufel", "Dorset Blue Vinney", "Double Gloucester", "Double Worcester",
            "Dreux a la Feuille", "Dry Jack", "Duddleswell", "Dunbarra", "Dunlop", "Dunsyre Blue",
            "Duroblando", "Durrus", "Dutch Mimolette (Commissiekaas)", "Edam", "Edelpilz",
            "Emental Grand Cru", "Emlett", "Emmental", "Epoisses de Bourgogne", "Esbareich",
            "Esrom", "Etorki", "Evansdale Farmhouse Brie", "Evora De L'Alentejo", "Exmoor Blue",
            "Explorateur", "Feta", "Feta (Australian)", "Figue", "Filetta", "Fin-de-Siecle",
            "Finlandia Swiss", "Finn", "Fiore Sardo", "Fleur du Maquis", "Flor de Guia",
            "Flower Marie", "Folded", "Folded cheese with mint", "Fondant de Brebis",
            "Fontainebleau", "Fontal", "Fontina Val d'Aosta", "Formaggio di capra", "Fougerus",
            "Four Herb Gouda", "Fourme d' Ambert", "Fourme de Haute Loire", "Fourme de Montbrison",
            "Fresh Jack", "Fresh Mozzarella", "Fresh Ricotta", "Fresh Truffles", "Fribourgeois",
            "Friesekaas", "Friesian", "Friesla", "Frinault", "Fromage a Raclette", "Fromage Corse",
            "Fromage de Montagne de Savoie", "Fromage Frais", "Fruit Cream Cheese",
            "Frying Cheese", "Fynbo", "Gabriel", "Galette du Paludier", "Galette Lyonnaise",
            "Galloway Goat's Milk Gems", "Gammelost", "Gaperon a l'Ail", "Garrotxa", "Gastanberra",
            "Geitost", "Gippsland Blue", "Gjetost", "Gloucester", "Golden Cross", "Gorgonzola",
            "Gornyaltajski", "Gospel Green", "Gouda", "Goutu", "Gowrie", "Grabetto", "Graddost",
            "Grafton Village Cheddar", "Grana", "Grana Padano", "Grand Vatel",
            "Grataron d' Areches", "Gratte-Paille", "Graviera", "Greuilh", "Greve",
            "Gris de Lille", "Gruyere", "Gubbeen", "Guerbigny", "Halloumi",
            "Halloumy (Australian)", "Haloumi-Style Cheese", "Harbourne Blue", "Havarti",
            "Heidi Gruyere", "Hereford Hop", "Herrgardsost", "Herriot Farmhouse", "Herve",
            "Hipi Iti", "Hubbardston Blue Cow", "Hushallsost", "Iberico", "Idaho Goatster",
            "Idiazabal", "Il Boschetto al Tartufo", "Ile d'Yeu", "Isle of Mull", "Jarlsberg",
            "Jermi Tortes", "Jibneh Arabieh", "Jindi Brie", "Jubilee Blue", "Juustoleipa",
            "Kadchgall", "Kaseri", "Kashta", "Kefalotyri", "Kenafa", "Kernhem", "Kervella Affine",
            "Kikorangi", "King Island Cape Wickham Brie", "King River Gold", "Klosterkaese",
            "Knockalara", "Kugelkase", "L'Aveyronnais", "L'Ecir de l'Aubrac", "La Taupiniere",
            "La Vache Qui Rit", "Laguiole", "Lairobell", "Lajta", "Lanark Blue", "Lancashire",
            "Langres", "Lappi", "Laruns", "Lavistown", "Le Brin", "Le Fium Orbo", "Le Lacandou",
            "Le Roule", "Leafield", "Lebbene", "Leerdammer", "Leicester", "Leyden", "Limburger",
            "Lincolnshire Poacher", "Lingot Saint Bousquet d'Orb", "Liptauer", "Little Rydings",
            "Livarot", "Llanboidy", "Llanglofan Farmhouse", "Loch Arthur Farmhouse",
            "Loddiswell Avondale", "Longhorn", "Lou Palou", "Lou Pevre", "Lyonnais", "Maasdam",
            "Macconais", "Mahoe Aged Gouda", "Mahon", "Malvern", "Mamirolle", "Manchego",
            "Manouri", "Manur", "Marble Cheddar", "Marbled Cheeses", "Maredsous", "Margotin",
            "Maribo", "Maroilles", "Mascares", "Mascarpone", "Mascarpone (Australian)",
            "Mascarpone Torta", "Matocq", "Maytag Blue", "Meira", "Menallack Farmhouse",
            "Menonita", "Meredith Blue", "Mesost", "Metton (Cancoillotte)", "Meyer Vintage Gouda",
            "Mihalic Peynir", "Milleens", "Mimolette", "Mine-Gabhar", "Mini Baby Bells", "Mixte",
            "Molbo", "Monastery Cheeses", "Mondseer", "Mont D'or Lyonnais", "Montasio",
            "Monterey Jack", "Monterey Jack Dry", "Morbier", "Morbier Cru de Montagne",
            "Mothais a la Feuille", "Mozzarella", "Mozzarella (Australian)",
            "Mozzarella di Bufala", "Mozzarella Fresh, in water", "Mozzarella Rolls", "Munster",
            "Murol", "Mycella", "Myzithra", "Naboulsi", "Nantais", "Neufchatel",
            "Neufchatel (Australian)", "Niolo", "Nokkelost", "Northumberland", "Oaxaca",
            "Olde York", "Olivet au Foin", "Olivet Bleu", "Olivet Cendre",
            "Orkney Extra Mature Cheddar", "Orla", "Oschtjepka", "Ossau Fermier", "Ossau-Iraty",
            "Oszczypek", "Oxford Blue", "P'tit Berrichon", "Palet de Babligny", "Paneer", "Panela",
            "Pannerone", "Pant ys Gawn", "Parmesan (Parmigiano)", "Parmigiano Reggiano",
            "Pas de l'Escalette", "Passendale", "Pasteurized Processed", "Pate de Fromage",
            "Patefine Fort", "Pave d'Affinois", "Pave d'Auge", "Pave de Chirac", "Pave du Berry",
            "Pecorino", "Pecorino in Walnut Leaves", "Pecorino Romano", "Peekskill Pyramid",
            "Pelardon des Cevennes", "Pelardon des Corbieres", "Penamellera", "Penbryn",
            "Pencarreg", "Perail de Brebis", "Petit Morin", "Petit Pardou", "Petit-Suisse",
            "Picodon de Chevre", "Picos de Europa", "Piora", "Pithtviers au Foin",
            "Plateau de Herve", "Plymouth Cheese", "Podhalanski", "Poivre d'Ane", "Polkolbin",
            "Pont l'Eveque", "Port Nicholson", "Port-Salut", "Postel", "Pouligny-Saint-Pierre",
            "Pourly", "Prastost", "Pressato", "Prince-Jean", "Processed Cheddar", "Provolone",
            "Provolone (Australian)", "Pyengana Cheddar", "Pyramide", "Quark",
            "Quark (Australian)", "Quartirolo Lombardo", "Quatre-Vents", "Quercy Petit",
            "Queso Blanco", "Queso Blanco con Frutas --Pina y Mango", "Queso de Murcia",
            "Queso del Montsec", "Queso del Tietar", "Queso Fresco", "Queso Fresco (Adobera)",
            "Queso Iberico", "Queso Jalapeno", "Queso Majorero", "Queso Media Luna",
            "Queso Para Frier", "Queso Quesadilla", "Rabacal", "Raclette", "Ragusano", "Raschera",
            "Reblochon", "Red Leicester", "Regal de la Dombes", "Reggianito", "Remedou",
            "Requeson", "Richelieu", "Ricotta", "Ricotta (Australian)", "Ricotta Salata", "Ridder",
            "Rigotte", "Rocamadour", "Rollot", "Romano", "Romans Part Dieu", "Roncal", "Roquefort",
            "Roule", "Rouleau De Beaulieu", "Royalp Tilsit", "Rubens", "Rustinu", "Saaland Pfarr",
            "Saanenkaese", "Saga", "Sage Derby", "Sainte Maure", "Saint-Marcellin",
            "Saint-Nectaire", "Saint-Paulin", "Salers", "Samso", "San Simon", "Sancerre",
            "Sap Sago", "Sardo", "Sardo Egyptian", "Sbrinz", "Scamorza", "Schabzieger", "Schloss",
            "Selles sur Cher", "Selva", "Serat", "Seriously Strong Cheddar", "Serra da Estrela",
            "Sharpam", "Shelburne Cheddar", "Shropshire Blue", "Siraz", "Sirene", "Smoked Gouda",
            "Somerset Brie", "Sonoma Jack", "Sottocenare al Tartufo", "Soumaintrain",
            "Sourire Lozerien", "Spenwood", "Sraffordshire Organic", "St. Agur Blue Cheese",
            "Stilton", "Stinking Bishop", "String", "Sussex Slipcote", "Sveciaost", "Swaledale",
            "Sweet Style Swiss", "Swiss", "Syrian (Armenian String)", "Tala", "Taleggio", "Tamie",
            "Tasmania Highland Chevre Log", "Taupiniere", "Teifi", "Telemea", "Testouri",
            "Tete de Moine", "Tetilla", "Texas Goat Cheese", "Tibet", "Tillamook Cheddar",
            "Tilsit", "Timboon Brie", "Toma", "Tomme Brulee", "Tomme d'Abondance",
            "Tomme de Chevre", "Tomme de Romans", "Tomme de Savoie", "Tomme des Chouans", "Tommes",
            "Torta del Casar", "Toscanello", "Touree de L'Aubier", "Tourmalet",
            "Trappe (Veritable)", "Trois Cornes De Vendee", "Tronchon", "Trou du Cru", "Truffe",
            "Tupi", "Turunmaa", "Tymsboro", "Tyn Grug", "Tyning", "Ubriaco", "Ulloa",
            "Vacherin-Fribourgeois", "Valencay", "Vasterbottenost", "Venaco", "Vendomois",
            "Vieux Corse", "Vignotte", "Vulscombe", "Waimata Farmhouse Blue",
            "Washed Rind Cheese (Australian)", "Waterloo", "Weichkaese", "Wellington",
            "Wensleydale", "White Stilton", "Whitestone Farmhouse", "Wigmore", "Woodside Cabecou",
            "Xanadu", "Xynotyro", "Yarg Cornish", "Yarra Valley Pyramid", "Yorkshire Blue",
            "Zamorano", "Zanetti Grana Padano", "Zanetti Parmigiano Reggiano"
    };

}

奶酪的 DAO。

/**
 * Data access object for Cheese.
 */
@Dao
public interface CheeseDao {

    /**
     * Counts the number of cheeses in the table.
     *
     * @return The number of cheeses.
     */
    @Query("SELECT COUNT(*) FROM " + Cheese.TABLE_NAME)
    int count();

    /**
     * Inserts a cheese into the table.
     *
     * @param cheese A new cheese.
     * @return The row ID of the newly inserted cheese.
     */
    @Insert
    long insert(Cheese cheese);

    /**
     * Inserts multiple cheeses into the database
     *
     * @param cheeses An array of new cheeses.
     * @return The row IDs of the newly inserted cheeses.
     */
    @Insert
    long[] insertAll(Cheese[] cheeses);

    /**
     * Select all cheeses.
     *
     * @return A {@link Cursor} of all the cheeses in the table.
     */
    @Query("SELECT * FROM " + Cheese.TABLE_NAME)
    List<Cheese> selectAll();

    /**
     * Select a cheese by the ID.
     *
     * @param id The row ID.
     * @return A {@link Cursor} of the selected cheese.
     */
    @Query("SELECT * FROM " + Cheese.TABLE_NAME + " WHERE " + Cheese.COLUMN_ID + " = :id")
    Cursor selectById(long id);

    /**
     * Delete a cheese by the ID.
     *
     * @param id The row ID.
     * @return A number of cheeses deleted. This should always be {@code 1}.
     */
    @Query("DELETE FROM " + Cheese.TABLE_NAME + " WHERE " + Cheese.COLUMN_ID + " = :id")
    int deleteById(long id);

    /**
     * Update the cheese. The cheese is identified by the row ID.
     *
     * @param cheese The cheese to update.
     * @return A number of cheeses updated. This should always be {@code 1}.
     */
    @Update
    int update(Cheese cheese);

}

和填充數據的數據庫。

@Database(entities = {Discovery.class, Director.class, Cheese.class}, version = 1, exportSchema = true)
public abstract class AppDatabase extends RoomDatabase {

    public abstract DiscoveryDAO discoveryDao();

    public abstract DirectorDao directorDao();

    public abstract CheeseDao cheese();

    /** The only instance */
    private static AppDatabase sInstance;

    /**
     * Gets the singleton instance of SampleDatabase.
     *
     * @param context The context.
     * @return The singleton instance of SampleDatabase.
     */
    public static synchronized AppDatabase getInstance(final Context context) {
        if (sInstance == null) {
            sInstance = Room
                    .databaseBuilder(context.getApplicationContext(), AppDatabase.class, "goDetectDB")
                    .allowMainThreadQueries()
                    .addCallback(new RoomDatabase.Callback() {

                        @Override
                        public void onCreate(@NonNull SupportSQLiteDatabase db) {
                            super.onCreate(db);
                            db.beginTransaction();
                            Cheese cheese = new Cheese();
                            for (int i = 0; i < Cheese.CHEESES.length; i++) {
                                cheese.name = Cheese.CHEESES[i];
                                ContentValues cv = new ContentValues();
                                cv.put(Cheese.COLUMN_NAME, cheese.name);
                                long id = db.insert(Cheese.TABLE_NAME, OnConflictStrategy.IGNORE, cv);
                                cheese.id = id;
                                Log.d("DISCOVERYINFO","DiscoveryName cheese is " + cheese.name + ", ID is " + cheese.id);
                            }
                            db.setTransactionSuccessful();
                            db.endTransaction();
                        }


                        @Override
                        public void onOpen(@NonNull SupportSQLiteDatabase db) {
                            super.onOpen(db);
                        }


                        @Override
                        public void onDestructiveMigration(@NonNull SupportSQLiteDatabase db) {
                            super.onDestructiveMigration(db);
                        }

                    })
                    .build();
        }
        return sInstance;
    }

}

然后你可以在主活動\o/中調用dao。 謝謝你的幫助 ！

database = AppDatabase.getInstance(getApplicationContext()).cheese().selectAll();