Question

我必须创建一个使用Sqlite数据库的应用程序。我的问题是资产文件夹的数据库大小太多，所以我必须把它放在SD卡中。我自己尝试过，但无法理解如何从中获取数据，导致我在以前的应用程序中使用资产文件夹成功实现的任何方法，它们都给出了错误。

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

我已经搜索了很多，所以如果有人可以向我推荐教程或任何其他有关sdcard的建议，那将会很明显。

Answer 1

首先，您必须创建扩展SQLiteOpenHelper的databaseHelper类，然后记下将数据库复制到/ data / data / package / databases中的代码，如下所示。

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();
    }

创建此类后，请记下此代码，以便将数据库复制到Application的启动器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");
        }

这样您就可以将现有数据库从SD卡复制到应用程序的数据库文件夹中。

在android中使用现有的数据库通过sdcard

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