使用現有的數據庫通過SD卡中的Android

Question

我必須使用Sqlite數據庫的應用程序。我的問題是資產文件夾的數據庫大小太大，所以我必須把它放在SD卡中。我自己嘗試過，但無法理解如何從中獲取數據，導致我在以前的應用程序中使用資產文件夾成功實現的任何方法，它們都發生錯誤。

11-03 21:23:32.426: E/Database(13645): sqlite3_open_v2("/mnt/sdcard/myfolder/LeoTemplate.db", &handle, 1, NULL) failed 

我搜索了很多，所以如果任何人都可以建議我的教程或任何其他建議只關於SD卡，它將是可觀的。

Answer 1

首先，你必須創建一個繼承SQLiteOpenHelper databaseHelper類，然後寫下來的代碼複製到數據庫/數據/數據/包/數據庫如下的..

public class DataBaseHelper extends SQLiteOpenHelper{ 

    //The Android's default system path of your application database. 
     private static String DB_PATH = "/data/data/com.project.examplepackage/databases/"; 
    private static String DB_NAME = "mydatabase.db"; 
    private SQLiteDatabase myDatabase; 
    private final Context myContext; 

    /** 
    * Constructor 
    * Takes and keeps a reference of the passed context in order to access to the application assets and resources. 
    * @param context 
    */ 
    public DataBaseHelper(Context context) { 
     super(context, DB_NAME, null, 1); 
     this.myContext = context; 
    } 

    /** 
    * Creates a empty database on the system and rewrites it with your own database. 
    * */ 
    public 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.getReadableDatabase(); 
      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.OPEN_READONLY); 
     }catch(SQLiteException e){ 
      //database does't exist yet. 
     } 
     if(checkDB != null){ 
      checkDB.close(); 
     } 
     return checkDB != null ? true : 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 = new FileInputStream(new File(database_path_to_sdcard)); 

     // 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[1024]; 
     int length; 
     while ((length = myInput.read(buffer))>0){ 
      myOutput.write(buffer, 0, length); 
     } 
     //Close the streams 
     myOutput.flush(); 
     myOutput.close(); 
     myInput.close(); 

     //Toast.makeText(myContext, "Copy Done", 300).show(); 
    } 

創建該類寫下來之後這個代碼用於將數據庫複製到應用程序的啓動器Activity中。

DataBaseHelper myDBH = new DataBaseHelper(this); 
     myDBH = new DataBaseHelper(this); 
     try { 
      myDBH.createDataBase(); 
      myDBH.close(); 
     } 
     catch (IOException ioe) { 
      throw new Error("Unable to create database"); 
     } 

這種方式可以從SD卡到您的應用程序的數據庫文件夾複製現有的數據庫。