问题描述
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以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 Adapter Class 还是为整个 Android 应用程序创建单个 DB Adapter 类?
I was reading the Android SQLite NotePad tutorial that referenced creating a DB Adapter class to create and access a DB table. When dealing with a multi-table SQLite Database, is it best practice to create a different Adapter Class for each table or create a single DB Adapter class for the entire Android Application?
我的应用程序使用多个表，我希望不必拥有一个庞大的适配器类.然而，问题是每个适配器中的每个 NotePad Example 都有一个 SQLiteOpenHelper 的嵌套子类.访问第一个表时，一切正常.当我尝试访问第二个表(来自不同的活动)时，我的应用程序崩溃了.
My application uses multiple tables and I was hoping not to have to have a single massive adapter class. the problem, however, is that I have a nested subclass of SQLiteOpenHelper per the NotePad Example within each adapter. When the first table is accessed, everything works fine. When I then try to access the second tble(from a different activity) my app crashes.
起初，我认为崩溃是由版本问题引起的，但现在两个适配器都具有相同的数据库版本，但它仍然崩溃.
At first, I thought the crash was being caused by a versioning issue, but both adapters now have the same database version and it's still crashing.
这是该表的数据库适配器之一的示例.其他适配器都遵循相同的格式，但实现方式不同.
Here's an example of one of the DB Adapters for the table. The other adapters all follow the same format with varying implementations.
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; //$NON-NLS-1$
}
}
当第一个适配器(在本例中为 usersinfo)被访问时，一切都按预期进行.假设我有另一个用于朋友信息的适配器，它遵循与上述相同的结构，当它被不同的活动访问时，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，因为在这种情况下，我的应用程序崩溃了.
When the first adapter, in this case usersinfo, is accessed, everything works as expected. Let's say I have another adapter for friend info that follows the same structure as above, when it is accessed by a different activity, it would seem to me that the nested subclass of SQLiteOpenHelper would attempt to create the database again. Obviously something is wrong because in that scenario, my app crashes.
Android 中的标准做法是创建单个庞大的数据库适配器而不是每个表的单个适配器吗?
So is the standard practice within Android to create a single mammoth db adapter instead of individual adapters per table?
推荐答案
这是我最终实施的解决方案.这是从 Commonsware 书籍中获得的信息以及网络上的一些内容的混搭，我希望我将其添加为书签，因为我想给予信任:
Here is the solution I eventually ended up implementing. It's kind of a mash-up from info gained in the Commonsware books, and some stuff around the web that I wish I bookmarked cause I want to give credit:
对于我需要从数据库中提取的每种类型的数据，我创建了一个适配器"类(不是从任何东西子类化的).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:
For each type of data that I need to pull from the db, I create an "adapter" class (not subclassed from anything). These adapter classes hold all of the methods necessary for accessing the db for that piece of info. For example, if I had three tables in my db:
- 汽车
- 船
- 摩托车
我将拥有三个与以下类似的适配器(我只放了一个作为演示，但每个的想法都是一样的):
I would have three adapters that would look similar to the following(I'm only putting in one as a demo, but the idea is the same for each):
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.
So if you imagine I have one of these classes "adapters" for each table.
当我的应用程序启动画面启动时，我使用了Android 初学者:创建适用于 Android 的多个 SQLite 表
When my app splash screen starts, I use the technique presented Android For Beginners: Creating multiple SQLite Tables for Android
所以我的主要 DBAdapter(负责在单个数据库中创建我的所有表)看起来像这样:
So my main DBAdapter (which is responsible for creating all of my tables in a single db) looks like this:
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 类仅在应用程序首次启动时被调用，它的唯一职责是创建/升级表.对数据的所有其他访问都是通过单独的适配器"类完成的.我发现这非常有效，并且不会产生我之前提到的版本控制问题.
The DBAdapter class only gets called when the app first starts and its only responsibility is to create/upgrade the tables. All other access to the data is done through the individual "adapter" class. I've found that this works perfectly and does not create the versioning issues that I mentioned earlier.
希望这会有所帮助.
这篇关于Android 中的多表 SQLite 数据库适配器?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！