public class DBHelper extends OrmLiteSqliteOpenHelper {
    public static final String DB_NAME = "yaotuo.db";
    public static final int DB_VERSION = 4;   //更新时候只需要修改这里就可以了
    private static DBHelper mDbHelper;
    private Dao<User, Integer> userDao = null;
    private Dao<Jw, Integer> jwDao = null;
    public DBHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
    }

    public static DBHelper getInstance(Context context){
        if(mDbHelper == null) {
            mDbHelper = new DBHelper(context);
        }
        return mDbHelper;
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase, ConnectionSource connectionSource) {
        try {
            System.out.println("create table");
            TableUtils.createTable(connectionSource, User.class);
            TableUtils.createTable(connectionSource, Jw.class);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, ConnectionSource connectionSource, int i, int i2) {
        try {
            System.out.println("drop jw class");
            TableUtils.dropTable(connectionSource, Jw.class, true);      //所有的表都必须得删除的， 否则在onCreate方法中会出现建表异常
            TableUtils.dropTable(connectionSource, User.class, true);
            onCreate(sqLiteDatabase, connectionSource);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

当数据库版本号发生变化的时候的调用onUpdate。就是你以前的版本是1，然后升级成为2了，会调用这个，让你做一些操作，比方说将以前的表删除掉， 或者其他的