在Android中设置数据库

Question

官方Android开发者网站上首先提到，我们必须先“定义架构和合同”，这是这样的：

public static abstract class FeedEntry implements BaseColumns { 
public static final String TABLE_NAME = "entry"; 
public static final String COLUMN_NAME_ENTRY_ID = "entryid"; 
public static final String COLUMN_NAME_TITLE = "title"; 
public static final String COLUMN_NAME_SUBTITLE = "subtitle"; 
... 

}

第二步提到的是'使用SQL助手创建数据库'。

但是我在网上看到的所有教程都直接使用'helper'创建一个类;对于example。 这是正确的方法？都？

另外，我应该在主Activity中定义并创建数据库还是为数据库创建单独的Activity？

Answer 1

在SQLite浏览器（如SQLite Maestro）中创建数据库，将数据库复制到项目的assets文件夹中。在您的MainActivity中调用createDatabase()方法（首先在您的应用程序中，启动器活动）。

private void createDataBase(){ 
    DataBaseHelper db = new DataBaseHelper(this); 
    try { 
     db.createDataBase(); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    }finally{ 
     db.close(); 
    } 
} 

和辅助类：

public class DataBaseHelper extends SQLiteOpenHelper{ 

    private static String DB_PATH = "/data/data/your.package/databases/"; 
    private static String DB_NAME = "YourDataBaseName.sqlite"; 
    private static int DB_VERSION = 1; 

    private static short WRITE_BUFFER_SIZE = 8192; 

    private Context context; 

    /** 
    * Constructor 
    * Takes and keeps a reference of the passed context in order to access to the application assets and resources. 
    * @param context Current application context 
    */ 
    public DataBaseHelper(Context context) { 
     super(context, DB_NAME, null, DB_VERSION); 

     this.context = context; 
    } 

    /** 
    * Creates a empty database on the system and rewrites it with your own database. 
    * */ 
    public synchronized void createDataBase() throws IOException{ 

     boolean dbExist = checkDataBase(); 

     if(dbExist){ 
      //do nothing - database already exist 
     } 
     else 
     { 
      //By calling this method and empty database will be created into the default system path 
       //of your application so we are gonna be able to overwrite that database with our database. 
      this.getWritableDatabase(); 

      try { 
       copyDataBase(); 
      } catch (IOException e) { 
       throw new Error("Error copying database"); 
      } 

     } 
    } 

    /** 
    * Check if the database already exist to avoid re-copying the file each time you open the application. 
    * @return true if it exists, false if it doesn't 
    */ 
    private boolean checkDataBase(){ 
     SQLiteDatabase checkDB = null; 
     try{ 
      String myPath = DB_PATH + DB_NAME; 

      checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.NO_LOCALIZED_COLLATORS); 
     }catch(SQLException e){ 
      e.printStackTrace(); 
      //database does't exist yet. 
     }catch(Exception e){ 
      e.printStackTrace(); 
     } 

     if(checkDB != null){ 
      if(checkDB.getVersion() < DB_VERSION){ 
       checkDB.close(); 
       return false; 
      }else{ 
       checkDB.close(); 
       return true; 
      } 
     } 

     return false; 
    } 

    /** 
    * Copies your database from your local assets-folder to the just created empty database in the 
    * system folder, from where it can be accessed and handled. 
    * This is done by transfering bytestream. 
    */ 
    private void copyDataBase() throws IOException{  
     //Open your local db as the input stream  
     InputStream myInput = context.getAssets().open(DB_NAME); 

     // Path to the just created empty db 
     String outFileName = DB_PATH + DB_NAME; 

     //Open the empty db as the output stream 
     OutputStream myOutput = new FileOutputStream(outFileName); 

     //transfer bytes from the inputfile to the outputfile 
     byte[] buffer = new byte[WRITE_BUFFER_SIZE]; 
     int length; 
     while ((length = myInput.read(buffer)) > 0){ 
      myOutput.write(buffer, 0, length); 
     } 
     //Close the streams 
     myOutput.flush(); 
     myOutput.close(); 
     myInput.close(); 
     this.getWritableDatabase().close(); 
    } 

    @Override 
    public synchronized void close() { 
      super.close(); 
    } 

    @Override 
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
    } 

    @Override 
    public void onCreate(SQLiteDatabase db) { 
    } 

    /** 
    * Delete database and create new one or copy from assets if exists. 
    */ 
    public void clearDatabase(){  
     context.deleteDatabase(DB_NAME); 
     try { 
      createDataBase(); 
     } catch (IOException e) { 
      e.printStackTrace(); 
     } 

     this.getWritableDatabase().close(); 
    } 
}