Class Overview
A helper class to manage database creation and version management. You create a subclass implementing onCreate(SQLiteDatabase), onUpgrade(SQLiteDatabase, int, int) and optionally onOpen(SQLiteDatabase), and this class takes care of opening the database if it exists, creating it if it does not, and upgrading it as necessary. Transactions are used to make sure the database is always in a sensible state.
helper类主要用于管理数据库的创建和版本管理。可以实现它的一些函数，如：onCreate()/onUpdate/onOpen()。
下面是一些常用的成员函数：
public SQLiteOpenHelper (Context context, String name, SQLiteDatabase.CursorFactory factory, int version)
Create a helper object to create, open, and/or manage a database. The database is not actually created or opened until one of getWritableDatabase() or getReadableDatabase() is called.
Parameters
| context | to use to open or create the database |
|---|---|
| name | of the database file, or null for an in-memory database |
| factory | to use for creating cursor objects, or null for the default |
| version | number of the database (starting at 1); if the database is older, onUpgrade(SQLiteDatabase, int, int) will be used to upgrade the database |
public synchronized SQLiteDatabase getReadableDatabase ()
Create and/or open a database. This will be the same object returned by getWritableDatabase() unless some problem, such as a full disk, requires the database to be opened read-only. In that case, a read-only database object will be returned. If the problem is fixed, a future call to getWritableDatabase() may succeed, in which case the read-only database object will be closed and the read/write object will be returned in the future.
Returns
- a database object valid until
getWritableDatabase()orclose()is called.
Throws
| SQLiteException | if the database cannot be opened |
|---|
public synchronized SQLiteDatabase getWritableDatabase ()
Create and/or open a database that will be used for reading and writing. Once opened successfully, the database is cached, so you can call this method every time you need to write to the database. Make sure to call close() when you no longer need it.
Errors such as bad permissions or a full disk may cause this operation to fail, but future attempts may succeed if the problem is fixed.
Returns
- a read/write database object valid until
close()is called
Throws
| SQLiteException | if the database cannot be opened for writing |
|---|
public abstract void onCreate (SQLiteDatabase db)
Called when the database is created for the first time. This is where the creation of tables and the initial population of the tables should happen.
Parameters
| db | The database. |
|---|
这个函数在第一次创建数据库时被调用。
public abstract void onUpgrade (SQLiteDatabase db, int oldVersion, int newVersion)
Called when the database needs to be upgraded. The implementation should use this method to drop tables, add tables, or do anything else it needs to upgrade to the new schema version.
The SQLite ALTER TABLE documentation can be found here. If you add new columns you can use ALTER TABLE to insert them into a live table. If you rename or remove columns you can use ALTER TABLE to rename the old table, then create the new table and then populate the new table with the contents of the old table.
Parameters
| db | The database. |
|---|---|
| oldVersion | The old database version. |
| newVersion | The new database version. |
实际编程经验：可以定义一个继承于SQliteOpenHelper的helper子类，在这个子类中完成数据库的建立，更新，以及修改、查询等操作。