Question

为什么每次我的应用程序启动时SQLiteOpenHelper都会调用onCreate（）。这是我的onCreate（）

的代码

@Override
  public void onCreate(SQLiteDatabase db) {
      Log.i("onCreate()", "Enter");
      //create cards table
     db.execSQL(         
     "create table circles" + 
     "("+
     "id integer primary key,"+
     "x integer not null," +
     "y integer not null"+
     ")"
     );     


  Log.i("onCreate()", "Exit");
  }

我的扩展SQLiteOpenHelper类周围有一个外部类，当我查询时，我这样做：

Cursor cursor = openHelper.getWritableDatabase().rawQuery("select * from circles", null);

并因为if语句

而跳过此块

if (cursor.moveToFirst()) {...}

这是我的整个数据库包装类：

package db.main;

import java.util.ArrayList;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
import testing.main.Circle;

public class DBWrapper {

private static final String DATABASE_NAME = "circles.db";
   private static final int DATABASE_VERSION = 1;
   private static final String[] TABLES = new String[] { "circles"};

private Context context;
   private OpenHelper openHelper;

public DBWrapper(Context context) {
       context.deleteDatabase(DATABASE_NAME);
      this.context = context;
      this.openHelper = new OpenHelper(this.context);
   }

public void insertCircle(Circle c)
   {
       String sql = "insert into circles (x, y) values (" + c.getX() + ", " + c.getY() + ")";
       Log.i("DBWrapper::insertCircle()", "Executing sql: " + sql);
       openHelper.getWritableDatabase().execSQL(sql);
   }

public void clearCircles()
   {
       String sql = "delete * from circles";
       Log.i("DBWrapper::clearCircles()", "Executing sql: " + sql);
       openHelper.getWritableDatabase().execSQL(sql);
   }

public ArrayList getCircles()
   {
       ArrayList circles = new ArrayList();
       Cursor cursor = openHelper.getWritableDatabase().query(TABLES[0], null, null, null, null, null, null);
       //Cursor cursor = openHelper.getWritableDatabase().rawQuery("select * from circles", null);
       Log.i("DBWrapper::getCircles()", "move to first1");
       if (cursor.moveToFirst()) {
           Log.i("DBWrapper::getCircles()", "move to first");
           do {
               Log.i("DBWrapper::getCircles()", "Creating circle: " + cursor.getString(1) + ", " + cursor.getString(2));
               circles.add(new Circle(Integer.parseInt(cursor.getString(1)),
                                    Integer.parseInt(cursor.getString(2)))); 
           } while (cursor.moveToNext());
       }
       if (cursor != null && !cursor.isClosed()) {
           cursor.close();
       }
       return circles;
   }
   private static class OpenHelper extends SQLiteOpenHelper {

  OpenHelper(Context context) {
     super(context, DATABASE_NAME, null, DATABASE_VERSION);
  }    


  @Override
  public void onCreate(SQLiteDatabase db) {
      Log.i("OpenHelper::onCreate()", "Enter");
      //create cards table
     db.execSQL(         
     "create table circles" + 
     "("+
     "id integer primary key,"+
     "x integer not null," +
     "y integer not null"+
     ")"
     );       


  Log.i("OpenHelper::onCreate()", "Exit");
  }

  @Override
  public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
     Log.w("Example", "Upgrading database, this will drop tables and recreate.");
     for(String s: TABLES)
     {
         db.execSQL("DROP TABLE IF EXISTS " + s);
     }
     onCreate(db);
  }

} }

Answer 1

查看你的DBWrapper构造函数，

你正在打电话

context.deleteDatabase(DATABASE_NAME);

每次调用它时都会删除数据库文件。强制SQLHelper重新创建数据库。

Answer 2

以下链接有望帮助。

stackoverflow.com -> Query if Adroid DB exists!

希望有所帮助

上面的海报还提到你正在删除数据库

content.deleteDabase(DATABASE_NAME)

你在DBWrapper（）构造函数

中的

Answer 3

试试这个：

public class DataBaseHelper extends SQLiteOpenHelper {
    private static final String DATENBANK_NAME = "yourdatabase.db";
    private static final int DATENBANK_VERSION = 1;

    public DataBaseHelper(Context context) {
        super(context, DATENBANK_NAME, null, DATENBANK_VERSION);
    }

    public void onCreate(SQLiteDatabase db) {
        db.execSQL(PartialTripTbl.SQL_CREATE);
    }

    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + PartialTripTbl.TABLE_NAME);
        onCreate(db);
    }

}

Answer 4

如果您想在第一次启动应用时创建一次数据库，则需要修改该查询。

您应该检查数据库是否已存在，如果不是，则创建数据库，否则不创建数据库。

CREATE TABLE如果不存在TABLE_NAME（key data_type）;

在onCreate方法中调用此查询。

每次启动应用程序时都会重新创建Android数据库

4 个答案: