Question

我知道它曾多次被问到stackoverflow。这些帖子也有答案，但它们都不适合我。

我有一个数据库文件mydb.sqlite将它放在assets文件夹中。但它在.openDatabase行失败了。我究竟做错了什么？在其他帖子中，我看到dbpath指向/data/data，但我很难在项目中找到该路径。

这是我的第一个Android应用程序。我几年来一直是iphone的开发者。

错误：android.database.sqlite.SQLiteCantOpenDatabaseException：未知错误（代码14）：无法打开数据库

    try {
        SQLiteDatabase db = SQLiteDatabase.openDatabase("/android_asset/mydb.sqlite", null, SQLiteDatabase.NO_LOCALIZED_COLLATORS|SQLiteDatabase.OPEN_READONLY);
        Cursor cursor = db.rawQuery("SELECT * FROM mytable", null);
        cursor.moveToFirst();
    } catch (Exception e) {
        e.printStackTrace();
    }

Answer 1

这些帖子也有答案，但它们都不适合我。

将来，请解释您的尝试以及遇到的具体问题。

在其他帖子中，我看到dbpath指向/ data / data，但我很难在项目中找到该路径

那是因为没有路径。资产不是Android设备上文件系统上的文件。它们是ZIP文件中的条目，即APK，它们不能由SQLite直接使用。相反，它们需要首先解压缩到本地文件中。

使用打包为资产的SQLite数据库的最简单方法是use Jeff Gilfelt's SQLiteAssetHelper。

Answer 2

问题是，我没有调用createDatabase（）。这个createDatabase（）方法是在我从某个地方下载的DatabaseHelper类中，我不知道从哪里来。我把代码放在这里。

ItemListFragment.java :(这是我正在读取数据库的类）

DataBaseHelper dataBaseHelper = new DataBaseHelper(getActivity());
public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

    DataBaseHelper dataBaseHelper = new DataBaseHelper(getActivity());
        try {
            dataBaseHelper.createDataBase();
            SQLiteDatabase db = dataBaseHelper.openDataBase();
            Cursor cursor = db.rawQuery("SELECT * FROM mydb", null);
            cursor.moveToFirst();
            do {
                String data = cursor.getString(cursor.getColumnIndex("name"));
            }while (cursor.moveToNext());
        } catch (Exception e) {
            e.printStackTrace();
    }
}

DatabaseHelper.java：

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteFullException;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.sql.SQLException;

public class DataBaseHelper extends SQLiteOpenHelper{

    //The Android's default system path of your application database.
    private static String DB_PATH = "/data/data/com.example.myapp.myapplication/databases/";

    private static String DB_NAME = "mydb.sqlite";

    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 = myContext.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[1024];
        int length;
        while ((length = myInput.read(buffer))>0){
            myOutput.write(buffer, 0, length);
        }

        //Close the streams
        myOutput.flush();
        myOutput.close();
        myInput.close();

    }

    public SQLiteDatabase openDataBase() throws SQLException{

        //Open the database
        String myPath = DB_PATH + DB_NAME;
        myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
        return myDataBase;
    }

    @Override
    public synchronized void close() {

        if(myDataBase != null)
            myDataBase.close();

        super.close();

    }

    @Override
    public void onCreate(SQLiteDatabase db) {

    }

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

    }

    // Add your public helper methods to access and get content from the database.
    // You could return cursors by doing "return myDataBase.query(....)" so it'd be easy
    // to you to create adapters for your views.

}

Answer 3

您只能从资产文件夹中读取数据库。如果你需要做更多的操作，比如创建，更新，删除，你可以做一些技巧。将数据库从assets文件夹复制到存储，然后您可以执行任何操作。

以下是Working with Android Pre Built Database.

的简要示例

还有一个易于使用的库，用于从assets文件夹访问数据库。您可以在此处查看[Android SQLiteAssetHelper]。2

访问assets文件夹中的.sqlite数据库

3 个答案: