public class DBHelper extends OrmLiteSqliteOpenHelper {    public static final String DB_NAME = "yaotuo.db";    public static final int DB_VERSION = 4;   //更新时候仅仅须要改动这里就能够了    private static DBHelper mDbHelper;    private Dao
     
       userDao = null;    private Dao
      
        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);      //全部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();        }    }}
      
     

当数据库版本号号发生变化的时候的调用onUpdate。就是你曾经的版本号是1，然后升级成为2了。会调用这个。让你做一些操作，例如说将曾经的表删除掉， 或者其它的