1.如果DB已经存在,onCreate
则不会调用。onUpgrade
仅当您更改数据库版本时才会调用。onUpgrade
如果你的APP的数据库发生了一些变化,你应该使用,并且你必须在新的数据结构上顺利迁移。
public class DbInit extends SQLiteOpenHelper {
private static final String DATABASE_NAME = "name";
private static final int DATABASE_VERSION = 3;
private static final String DATABASE_CREATE = "create table connections . .. . ...
public DbInit(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase database) {
database.execSQL(DATABASE_CREATE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
if (isChangeFromToVersion(1, 2, oldVersion, newVersion)) {
//Execute UPDATE here
}
}
private boolean isChangeFromToVersion(int from, int to, int oldVersion, int newVersion ) {
return (from == oldVersion && to == newVersion);
}
....
2.简单示例如何打开与数据库的连接并获取游标对象。
公共类 DAO {
private SQLiteDatabase database;
private DbInit dbHelper;
public ConnectionDAO(Context context) {
dbHelper = new DbInit(context);
}
public void open() throws SQLException {
database = dbHelper.getWritableDatabase();
}
public Connection getConnectionById(long id) {
Cursor cursor = null;
try {
open();
cursor = database.query(DbInit.TABLE_CONNECTIONS, allColumns, DbInit.COLUMN_ID + " = '" + id + "'", null, null, null, null);
if (!cursor.moveToFirst())
return null;
return cursorToConnection(cursor);
} finally {
if (cursor != null)
cursor.close();
close();
}
}
private Connection cursorToConnection(Cursor cursor) {
Connection connection = new Connection();
connection.setId(cursor.isNull(0) ? null : cursor.getInt(0));
connection.setName(cursor.isNull(1) ? null : cursor.getString(1));
.....
.....
return connection;
}