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我必须制作一个使用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大，所以我必须把它放在 sdcard 中。我自己尝试过，但无法理解如何从中获取数据，因为无论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，它们都给出了类似的错误。

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，所以如果有人可以建议我教程或任何其他仅关于 sdcard 的建议，那将是可观的。

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1 回答 1

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首先，您必须创建扩展 SQLiteOpenHelper 的 databaseHelper 类，然后写下用于将数据库复制到 /data/data/package/databases 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();
    }

After creating this class write down this code for copying database into Application's launcher Activity.

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

This way you can copy existing database from sdcard to your application 's database folder.

于 2012-11-03T09:36:09.437 回答