我正在阅读 Android SQLite NotePad 教程，该教程引用了创建数据库适配器类来创建和访问数据库表。在处理多表 SQLite 数据库时，最佳做法是为每个表创建不同的适配器类还是为整个 Android 应用程序创建单个 DB 适配器类？

我的应用程序使用多个表，我希望不必有一个庞大的适配器类。但是，问题是每个适配器中的每个记事本示例都有一个嵌套的 SQLiteOpenHelper 子类。访问第一个表时，一切正常。然后当我尝试访问第二个 tble(来自不同的 Activity )时，我的应用程序崩溃了。

起初，我认为崩溃是由版本控制问题引起的，但现在两个适配器具有相同的数据库版本，并且仍然崩溃。

以下是表的其中一个 DB 适配器的示例。其他适配器都遵循相同的格式，但实现不同。

public class InfoDBAdapter {
    public static final String ROW_ID = "_id";
    public static final String NAME = "name";

    private static final String TAG = "InfoDbAdapter";
    private static final String DATABASE_NAME = "myappdb";
    private static final String DATABASE_TABLE = "usersinfo";
    private static final int DATABASE_VERSION = 1;


    private static final String DATABASE_CREATE = "create table usersinfo (_id integer primary key autoincrement, "
            + NAME
            + " TEXT," + ");";

    private DatabaseHelper mDbHelper;
    private SQLiteDatabase mDb;

    private final Context mCtx;

    private static class DatabaseHelper extends SQLiteOpenHelper {

        DatabaseHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {

            db.execSQL(DATABASE_CREATE);
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            Log.w(TAG, "Upgrading database from version " + oldVersion + " to " //$NON-NLS-1$//$NON-NLS-2$
                    + newVersion + ", which will destroy all old data"); //$NON-NLS-1$
            //db.execSQL("DROP TABLE IF EXISTS usersinfo"); //$NON-NLS-1$
            onCreate(db);
        }
    }


    public InfoDBAdapter(Context ctx) {
        this.mCtx = ctx;
    }


    public InfoDBAdapter open() throws SQLException {
        this.mDbHelper = new DatabaseHelper(this.mCtx);
        this.mDb = this.mDbHelper.getWritableDatabase();
        return this;
    }

    /**
     * close return type: void
     */
    public void close() {
        this.mDbHelper.close();
    }


    public long createUser(String name) {
        ContentValues initialValues = new ContentValues();
        initialValues.put(NAME, name);
        return this.mDb.insert(DATABASE_TABLE, null, initialValues);
    }


    public boolean deleteUser(long rowId) {

        return this.mDb.delete(DATABASE_TABLE, ROW_ID + "=" + rowId, null) > 0; //$NON-NLS-1$
    }


    public Cursor fetchAllUsers() {

        return this.mDb.query(DATABASE_TABLE, new String[] { ROW_ID,
                NAME}, null, null, null, null, null);
    }


    public Cursor fetchUser(long rowId) throws SQLException {

        Cursor mCursor =

        this.mDb.query(true, DATABASE_TABLE, new String[] { ROW_ID, NAME}, ROW_ID + "=" + rowId, null, //$NON-NLS-1$
                null, null, null, null);
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;

    }


    public boolean updateUser(long rowId, String name) {
        ContentValues args = new ContentValues();
        args.put(NAME, name);
        return this.mDb
                .update(DATABASE_TABLE, args, ROW_ID + "=" + rowId, null) > 0; //$NON-NLS-1$
    }
}

当访问第一个适配器(在本例中为 usersinfo)时，一切都按预期工作。假设我有另一个遵循与上述相同结构的 friend 信息适配器，当它被不同的 Activity 访问时，在我看来 SQLiteOpenHelper 的嵌套子类将尝试再次创建数据库。显然出了点问题，因为在那种情况下，我的应用程序崩溃了。

那么，Android 中的标准做法是创建一个庞大的数据库适配器而不是每个表单独的适配器？

最佳答案

这是我最终实现的解决方案。这是从 Commonsware 书籍中获得的信息的一种混搭，以及我希望我收藏的网络上的一些内容，因为我想给予信任:

对于我需要从数据库中提取的每种类型的数据，我都创建了一个“适配器”类(不是任何东西的子类)。这些适配器类包含访问该信息的数据库所需的所有方法。例如，如果我的数据库中有三个表:

1. 汽车
2. 船
3. 摩托车

我将拥有三个看起来类似于以下的适配器(我只是将其中一个作为演示，但每个的想法都是相同的):

public class CarsDBAdapter {
    public static final String ROW_ID = "_id";
    public static final String NAME = "name";
    public static final String MODEL = "model";
    public static final String YEAR = "year";

    private static final String DATABASE_TABLE = "cars";

    private DatabaseHelper mDbHelper;
    private SQLiteDatabase mDb;

    private final Context mCtx;

    private static class DatabaseHelper extends SQLiteOpenHelper {

        DatabaseHelper(Context context) {
            super(context, DBAdapter.DATABASE_NAME, null, DBAdapter.DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        }
    }

    /**
     * Constructor - takes the context to allow the database to be
     * opened/created
     * 
     * @param ctx
     *            the Context within which to work
     */
    public CarsDBAdapter(Context ctx) {
        this.mCtx = ctx;
    }

    /**
     * Open the cars database. If it cannot be opened, try to create a new
     * instance of the database. If it cannot be created, throw an exception to
     * signal the failure
     * 
     * @return this (self reference, allowing this to be chained in an
     *         initialization call)
     * @throws SQLException
     *             if the database could be neither opened or created
     */
    public CarsDBAdapter open() throws SQLException {
        this.mDbHelper = new DatabaseHelper(this.mCtx);
        this.mDb = this.mDbHelper.getWritableDatabase();
        return this;
    }

    /**
     * close return type: void
     */
    public void close() {
        this.mDbHelper.close();
    }

    /**
     * Create a new car. If the car is successfully created return the new
     * rowId for that car, otherwise return a -1 to indicate failure.
     * 
     * @param name
     * @param model
     * @param year
     * @return rowId or -1 if failed
     */
    public long createCar(String name, String model, String year){
        ContentValues initialValues = new ContentValues();
        initialValues.put(NAME, name);
        initialValues.put(MODEL, model);
        initialValues.put(YEAR, year);
        return this.mDb.insert(DATABASE_TABLE, null, initialValues);
    }

    /**
     * Delete the car with the given rowId
     * 
     * @param rowId
     * @return true if deleted, false otherwise
     */
    public boolean deleteCar(long rowId) {

        return this.mDb.delete(DATABASE_TABLE, ROW_ID + "=" + rowId, null) > 0; //$NON-NLS-1$
    }

    /**
     * Return a Cursor over the list of all cars in the database
     * 
     * @return Cursor over all cars
     */
    public Cursor getAllCars() {

        return this.mDb.query(DATABASE_TABLE, new String[] { ROW_ID,
                NAME, MODEL, YEAR }, null, null, null, null, null);
    }

    /**
     * Return a Cursor positioned at the car that matches the given rowId
     * @param rowId
     * @return Cursor positioned to matching car, if found
     * @throws SQLException if car could not be found/retrieved
     */
    public Cursor getCar(long rowId) throws SQLException {

        Cursor mCursor =

        this.mDb.query(true, DATABASE_TABLE, new String[] { ROW_ID, NAME,
                MODEL, YEAR}, ROW_ID + "=" + rowId, null, null, null, null, null);
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;
    }

    /**
     * Update the car.
     * 
     * @param rowId
     * @param name
     * @param model
     * @param year
     * @return true if the note was successfully updated, false otherwise
     */
    public boolean updateCar(long rowId, String name, String model,
            String year){
        ContentValues args = new ContentValues();
        args.put(NAME, name);
        args.put(MODEL, model);
        args.put(YEAR, year);

        return this.mDb.update(DATABASE_TABLE, args, ROW_ID + "=" + rowId, null) >0; 
    }

}

因此，如果您想象我为每个表都有这些类“适配器”之一。

当我的应用启动画面启动时，我使用了 Android For Beginners: Creating multiple SQLite Tables for Android 提供的技术

所以我的主 DBAdapter(负责在单个数据库中创建我的所有表)如下所示:

public class DBAdapter {

    public static final String DATABASE_NAME = "stuffIOwn"; //$NON-NLS-1$

    public static final int DATABASE_VERSION = 1;

    private static final String CREATE_TABLE_CARS =
       "create table cars (_id integer primary key autoincrement, " //$NON-NLS-1$
    + CarsDBAdapter.NAME+ " TEXT," //$NON-NLS-1$
    + CarsDBAdapter.MODEL+ " TEXT," //$NON-NLS-1$
    + CarsDBAdapter.YEAR+ " TEXT" + ");"; //$NON-NLS-1$ //$NON-NLS-2$

    private static final String CREATE_TABLE_BOATS = "create table boats (_id integer primary key autoincrement, " //$NON-NLS-1$
    +BoatsDBAdapter.NAME+" TEXT," //$NON-NLS-1$
    +BoatsDBAdapter.MODEL+" TEXT," //$NON-NLS-1$
    +BoatsDBAdapter.YEAR+" TEXT"+ ");"; //$NON-NLS-1$  //$NON-NLS-2$

        private static final String CREATE_TABLE_CYCLES = "create table cycles (_id integer primary key autoincrement, " //$NON-NLS-1$
    +CyclesDBAdapter.NAME+" TEXT," //$NON-NLS-1$
    +CyclesDBAdapter.MODEL+" TEXT," //$NON-NLS-1$
    +CyclesDBAdapter.YEAR+" TEXT"+ ");"; //$NON-NLS-1$  //$NON-NLS-2$


    private final Context context; 
    private DatabaseHelper DBHelper;
    private SQLiteDatabase db;

    /**
     * Constructor
     * @param ctx
     */
    public DBAdapter(Context ctx)
    {
        this.context = ctx;
        this.DBHelper = new DatabaseHelper(this.context);
    }

    private static class DatabaseHelper extends SQLiteOpenHelper 
    {
        DatabaseHelper(Context context) 
        {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db) 
        {
            db.execSQL(CREATE_TABLE_CARS);
            db.execSQL(CREATE_TABLE_BOATS);
            db.execSQL(CREATE_TABLE_CYCLES);           
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, 
        int newVersion) 
        {               
            // Adding any table mods to this guy here
        }
    } 

   /**
     * open the db
     * @return this
     * @throws SQLException
     * return type: DBAdapter
     */
    public DBAdapter open() throws SQLException 
    {
        this.db = this.DBHelper.getWritableDatabase();
        return this;
    }

    /**
     * close the db 
     * return type: void
     */
    public void close() 
    {
        this.DBHelper.close();
    }
}

DBAdapter 类仅在应用首次启动时被调用，它的唯一职责是创建/升级表。所有其他对数据的访问都是通过单独的“适配器”类完成的。我发现这可以完美运行，并且不会产生我之前提到的版本控制问题。

希望这会有所帮助。

关于android - Android中的多个表SQLite DB适配器？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/4063510/