Android 无法从 Assets 复制数据库

Question

我有一个问题。我有 2 个安卓项目。一个是“主要”项目。另一个项目是我尝试根据在 Internet 上找到的教程实现基本的测验应用程序。

测验应用程序运行正常，所以我想在“主”项目上实现它。 (在“主”项目中，我有 2 个包，一个用于测验的类)我创建了类、布局，将数据库复制到 Assets 文件夹。一切都和第二个项目一样。但它不能复制数据库。我有找不到表的错误。

它生成了数据库，我用 ddms 检查了它(拉出数据库)但无法复制表格。 “主项目”中的代码与第二个项目相同(!)。 (在第二个项目中一切都很好)

代码 fragment 如下:

package com.thesis.Quiz;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;

import com.thesis.eliminations.R;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class DBHelperForQuiz extends SQLiteOpenHelper{

    //The Androids default system path of your application database.
    private static String DB_PATH = "/data/data/com.thesis.eliminations/databases/";
    private static String DB_NAME = "QuizDB";
    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 DBHelperForQuiz(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();
        //SQLiteDatabase db_Read = null;
        //SQLiteDatabase db = null;
        if(dbExist){
            //do nothing - database already exist
            //db = super.getReadableDatabase();
        }else{
            this.getReadableDatabase();
            //db_Read = this.getReadableDatabase();
            //db_Read.close();
            try {
                copyDataBase();
            //  db = super.getReadableDatabase();
            } catch (IOException e) {
                //throw new Error("Error copying database");
                e.printStackTrace();
            }
        }
    }

    /**
     * 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);
        InputStream myInput = myContext.getResources().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 void openDataBase() throws SQLException{
        //Open the database
        String myPath = DB_PATH + DB_NAME;
        myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
    }

    @Override
    public synchronized void close() {
        if(myDataBase != null)
            myDataBase.close();
        super.close();
    }

    public List<Question> getQuestionsList(int difficulty, int numberOfQuestions) {
        Log.d("NOQ",Integer.toString(numberOfQuestions));
        List<Question> questionsList = new ArrayList<Question>();
        Cursor c = myDataBase
                .rawQuery("SELECT * FROM QUESTIONS WHERE DIFFICULTY="
                        + difficulty + " ORDER BY RANDOM() LIMIT "
                        + numberOfQuestions, null);

        while (c.moveToNext()) {
            Question q = new Question();
            q.setQuestionText(c.getString(1));
            q.setRightAnswer(c.getString(2));
            q.setAnswerOption1(c.getString(3));
            q.setAnswerOption2(c.getString(4));
            q.setAnswerOption3(c.getString(5));
            q.setQuestionDiffLevel(difficulty);
            questionsList.add(q);
        }
        return questionsList;
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub
    }
}

和:

public class QuizActivityMain extends Activity implements OnClickListener {

    Button bStart, bQuit, bSetDiff;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.quizmenu);
        initialize();
    }

    private void initialize() {
        bStart = (Button) findViewById(R.id.bStartQuiz);
        bQuit = (Button) findViewById(R.id.bExitQuiz);
        bSetDiff = (Button) findViewById(R.id.bSetDiff);
        bSetDiff.setOnClickListener(this);
        bQuit.setOnClickListener(this);
        bStart.setOnClickListener(this);
        Log.d("init", "initialize..");
    }

    @Override
    public void onClick(View v) {
        Intent i;
        switch (v.getId()) {
            case R.id.bStartQuiz:
                List<Question> questionsList = getQuestionsFromDB();
                QuizGame currentQuizGame = new QuizGame();
                currentQuizGame.setNumberOfRounds(this.getNumOfQuestions());
                currentQuizGame.setQuestions(questionsList);
                Log.d("ql set", "ql set");
    /*  ((QuizApplication) getApplication())
                .setCurrentQuizGame(currentQuizGame);*/

                ((QuizApplication)getApplication()).setCurrentQuizGame(currentQuizGame); 
                Log.d("cqg set", "cqg set");
                i = new Intent(this, AskedQuestion.class);
                Log.d("i started", "intent");
                startActivity(i);
                Log.d("i started", "intent started");
                break;

            case R.id.bExitQuiz:
                finish();
                break;

            case R.id.bSetDiff:
                i = new Intent(this, QuizSettings.class);
                startActivity(i);
                break;
        }
    }

    private List<Question> getQuestionsFromDB() throws Error{
        int tmpDifficulty = getDifficulty();
        int tmpNumberOfQuestions = getNumOfQuestions();
        DBHelperForQuiz myDbHelper = new DBHelperForQuiz(this.getApplicationContext());
        myDbHelper = new DBHelperForQuiz(this);

        try {
            myDbHelper.createDataBase();
        } catch (IOException e) {
            throw new Error("Unable to create database");
        }
        try {
            myDbHelper.openDataBase();
            Log.d("openeddb", "open db success");
        } catch (SQLException sqle) {
            throw sqle;
        }

        Log.d("ql try", "ql making");
        List<Question> qL = myDbHelper.getQuestionsList(tmpDifficulty, tmpNumberOfQuestions);

        Log.d("TMPNOQ",Integer.toString(tmpNumberOfQuestions));
        Log.d("ql rdy", "ql done");
        myDbHelper.close();
        Log.d("db cls", "closed");
        return qL;
    }

    private int getNumOfQuestions() {
        return 5;
    }

    private int getDifficulty() {
        return 2;
    }
}

Answer 1

在 createDataBase() 中有以下检查；

this.getReadableDatabase();

这会检查是否已经存在具有所提供名称的数据库，如果没有，则创建一个空数据库，以便可以用 Assets 文件夹中的数据库覆盖它。在较新的设备上，这可以完美运行，但有些设备无法运行。主要是旧设备。我不知道为什么，但似乎 getReadableDatabase() 函数不仅获取数据库而且打开它。如果您随后将数据库从 Assets 文件夹复制到它上面，它仍然具有指向空数据库的指针，您将收到表不存在的错误。

因此，为了使其适用于所有设备，您应该将其修改为以下几行:

SQLiteDatabase db = this.getReadableDatabase();
if (db.isOpen()){
    db.close();
}

即使在检查时打开了数据库，之后关闭了，也不会再给你添麻烦。