isOpen函式對Room資料庫或SQLite有效嗎？

包 androidx.room有這個功能，如果我連接到資料庫，它應該回傳。但我想確保這是僅適用于 Room 而不是SQLite的正確功能 ，我得到的所有資訊都是錯誤的。

/**
 * Returns true if database connection is open and initialized.
 *
 * @return true if the database connection is open, false otherwise.
 */
public boolean isOpen() {
    // We need to special case for the auto closing database because mDatabase is the
    // underlying database and not the wrapped database.
    if (mAutoCloser != null) {
        return mAutoCloser.isActive();
    }

    final SupportSQLiteDatabase db = mDatabase;
    return db != null && db.isOpen();
}

uj5u.com熱心網友回復：

是的，但是當您打開資料庫時，您很可能會誤解，因為您是錯誤的。

那就是構建 RoomDatabase 不會（除非你強行）打開資料庫。它保持實際開放，直到需要為止。只有在實際嘗試訪問資料庫時才需要。

考慮以下顯示狀態的內容：-

public class MainActivity extends AppCompatActivity {

    TheDatabase db;
    AllDao dao;
    List<Table1> table1List;
    private static final String TAG = "MAININFO";

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

        try {
            Log.d(TAG, "Getting Room instance"   db.isOpen());
        } catch (Exception e) {
            e.printStackTrace();
        }
        db = TheDatabase.getInstance(this);
        try {
            Log.d(TAG,"Getting DAO"   db.isOpen());
        } catch (Exception e) {
            e.printStackTrace();
        }
        dao = db.getAllDao();
        try {
            Log.d(TAG, "Querying Table1 "   db.isOpen());
        } catch (Exception e) {
            e.printStackTrace();
        }
        table1List = dao.getAll();
        try {
            Log.d(TAG, "After Query "   db.isOpen());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

@Database (TheDatabase) 類包含一個回呼，當 onOpen 或 onCreate 被呼叫時，該回呼會寫入日志。

運行時，日志包括以下內容：-

2021-10-07 06:40:49.037 D/MAININFO: Getting DAOfalse
2021-10-07 06:40:49.039 D/MAININFO: Querying Table1 false
2021-10-07 06:40:49.069 D/THEDATABSEINFO: onCreate has been called. For a.a.so69466269roomisopen.TheDatabase$1 a.a.so69466269roomisopen.TheDatabase$1@112c3ea Opencounter = 0
2021-10-07 06:40:49.075 D/THEDATABSEINFO: onOpen has been called. For a.a.so69466269roomisopen.TheDatabase$1 a.a.so69466269roomisopen.TheDatabase$1@112c3ea OPenCounter = 0
2021-10-07 06:40:49.080 D/MAININFO: After Query true

所以在查詢之后 （即在實際訪問資料庫之后） isOpen 回傳 true。在訪問資料庫之前 isOpen 回傳 false。

• 第一次嘗試（在獲取實體之前）發出 NPE（正如預期的那樣），因此訊息不會寫入日志。

強制打開

現在考慮@Database 類TheDatabase，自從運行上面的代碼以來，它添加了以下行（在回傳實體之前）。

instance.getOpenHelper().getWritableDatabase(); //<<<<< FORCE OPEN

這是完整的TheDatabase類（針對力進行了修改）：-

abstract class TheDatabase extends RoomDatabase {
    abstract AllDao getAllDao();
    private static final String DATABASE_NAME = "thedatabase.db";
    private static final String TAG = "THEDATABSEINFO";

    private static volatile TheDatabase instance = null;
    private static int counter = 0;

    static TheDatabase getInstance(Context context) {
        if (instance == null) {
            instance = Room.databaseBuilder(
                    context,
                    TheDatabase.class,
                    DATABASE_NAME
            )
                    .addCallback(callback)
                    .allowMainThreadQueries()
                    .build();
        }
        instance.getOpenHelper().getWritableDatabase(); //<<<<< FORCE OPEN
        return instance;
    }

    static Callback callback = new RoomDatabase.Callback() {
        @Override
        public void onCreate(@NonNull SupportSQLiteDatabase db) {
            super.onCreate(db);
            Log.d(TAG,"onCreate has been called. For "   this.getClass().getName()   " "   this   " Opencounter = "   counter);
        }

        @Override
        public void onOpen(@NonNull SupportSQLiteDatabase db) {
            super.onOpen(db);
            Log.d(TAG,"onOpen has been called. For "   this.getClass().getName()   " "   this   " OPenCounter = "   counter  );
        }

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

現在運行時日志包括：-

2021-10-07 07:07:16.264 D/THEDATABSEINFO: onOpen has been called. For a.a.so69466269roomisopen.TheDatabase$1 a.a.so69466269roomisopen.TheDatabase$1@112c3ea OPenCounter = 0
2021-10-07 07:07:16.264 D/MAININFO: Getting DAOtrue
2021-10-07 07:07:16.266 D/MAININFO: Querying Table1 true
2021-10-07 07:07:16.271 D/MAININFO: After Query true

即isOpen一旦獲得實體就回傳真。

標籤：安卓 数据库 sqlite 科特林 android-room

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