使用SQLiteOpenHelper对数据库进行版本管理
getWritableDatabase()和getReadableDatabase()方法都可以获取一个用于操作数据库的SQLiteDatabase实例。但getWritableDatabase() 方法以读写方式打开数据库,一旦数据库的磁盘空间满了,数据库就只能读而不能写,倘若使用getWritableDatabase()打开数据库就会出错。getReadableDatabase()方法先以读写方式打开数据库,如果数据库的磁盘空间满了,就会打开失败,当打开失败后会继续尝试以只读方式打开数据库。
package cn.itcast.service;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DBOpenHelper extends SQLiteOpenHelper {
private static final String DATABASENAME = "itcast.db"; //数据库名称
private static final int DATABASEVERSION = 1;//数据库版本
public DBOpenHelper(Context context) {
super(context, DATABASENAME, null, DATABASEVERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {//创建的时候就调用,且只调用一次
db.execSQL("CREATE TABLE person (personid integer primary key autoincrement, name varchar(20), amount integer)");//执行有更改的sql语句
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {//当软件升级以及版本改变的时候调用
db.execSQL("DROP TABLE IF EXISTS person");
onCreate(db);
}
}
首先通过上面的DBOpenHelper创建数据库,再实现操作数据库对象,代码如下:
package cn.itcast.service;
import java.util.ArrayList;
import java.util.List;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import cn.itcast.domain.Person;
public class PersonService {
private DBOpenHelper dbOpenHelper;
public PersonService(Context context) {
this.dbOpenHelper = new DBOpenHelper(context);
}
public void save(Person person){
//如果要对数据进行更改,就调用此方法得到用于操作数据库的实例,该方法以读和写方式打开数据库
SQLiteDatabase db = dbOpenHelper.getWritableDatabase();
db.execSQL("insert into person (name) values(?)", new Object[]{person.getName()});
}
public void update(Person person){
SQLiteDatabase db = dbOpenHelper.getWritableDatabase();
db.execSQL("update person set name=? where personid=?",
new Object[]{person.getName(),person.getId()});
}
public void delete(Integer id){
SQLiteDatabase db = dbOpenHelper.getWritableDatabase();
db.execSQL("delete from person where personid=?", new Object[]{id.toString()});
}
public Person find(Integer id){
//如果只对数据进行读取,建议使用此方法
SQLiteDatabase db = dbOpenHelper.getReadableDatabase();
Cursor cursor = db.rawQuery("select * from person where personid=?", new String[]{id.toString()});
if(cursor.moveToFirst()){
int personid = cursor.getInt(cursor.getColumnIndex("personid"));
String name = cursor.getString(cursor.getColumnIndex("name"));
return new Person(personid, name);
}
return null;
}
public List<Person> getScrollData(Integer offset, Integer maxResult){
List<Person> persons = new ArrayList<Person>();
SQLiteDatabase db = dbOpenHelper.getReadableDatabase();
Cursor cursor = db.rawQuery("select * from person limit ?,?",
new String[]{offset.toString(), maxResult.toString()});
while(cursor.moveToNext()){
int personid = cursor.getInt(cursor.getColumnIndex("personid"));
String name = cursor.getString(cursor.getColumnIndex("name"));
Person person = new Person(personid, name);
persons.add(person);
}
cursor.close();
return persons;
}
public long getCount() {
SQLiteDatabase db = dbOpenHelper.getReadableDatabase();
Cursor cursor = db.rawQuery("select count(*) from person", null);
cursor.moveToFirst();
return cursor.getLong(0);
}
}