Android - 打开 SQLite 数据库

Question

我想打开一个数据库并从中获取数据。我阅读了有关此的教程并提出了申请。我正在尝试检查数据库是否存在，如果不存在，请在应用程序数据库的 Android 默认系统路径中创建一个新数据库，并将数据从数据库复制到那里(位于 Assets 文件夹中)。但是当我运行应用程序时出现错误:

SQLiteDatabase.rawQueryWithFactory(SQLiteDatabase$CursorFactory, String, String[], String) line: 1568   

有人可以就我做错了什么给我一些建议吗？这是代码:

package thesis.app.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 android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

public class DBHelperForQuiz extends SQLiteOpenHelper {

    //The Android's default system path of your application database.
    private static String DB_PATH = "/data/data/thesis.app.quiz/databases/";
    private static String DB_NAME = "QuizDB";
    private SQLiteDatabase myDataBase;
    private final Context myContext;

    public DBHelperForQuiz(Context context) {
        super(context, DB_NAME, null, 1);
        this.myContext = context;
    }

    @Override
    public void onCreate(SQLiteDatabase db) {

    }

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

    }

    public void openDataBase() {


        String myDbPath = DB_PATH + DB_NAME;
        myDataBase = SQLiteDatabase.openDatabase(myDbPath, null,
                SQLiteDatabase.OPEN_READONLY);
    }

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

    public List<Question> getQuestionsList(int difficulty, int numberOfQuestions) {



        List<Question> questionsList = new ArrayList<Question>();

        Cursor c = myDataBase.rawQuery("SELECT * FROM questions", 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;

    }

    public void createDataBase() throws IOException {

        if (!checkIfDBAlredyExist()) {

            this.getReadableDatabase(); 

            try {
                InputStream myInput = myContext.getAssets().open(DB_NAME);

                String newDbPath = DB_PATH + DB_NAME; 
                OutputStream myOutput = new FileOutputStream(newDbPath);

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

                myOutput.flush();
                myOutput.close();
                myInput.close();

            } catch (IOException e) {

                e.printStackTrace();
                throw new Error("Error copying database");
            }

        }

    }

    public boolean checkIfDBAlredyExist() {


        SQLiteDatabase tmpDb = null;
        String dbPath = DB_PATH + DB_NAME;
        try {
            tmpDb = SQLiteDatabase.openDatabase(dbPath, null,
                    SQLiteDatabase.OPEN_READONLY);
        } catch (SQLException e) {
            e.printStackTrace();
        }

        if (tmpDb != null) {
            tmpDb.close();
            //return true;
        }

        return tmpDb != null ? true : false;
    }
}

Answer 1

你不需要那样做。

SQLiteOpenHelper 父类的美妙之处在于它会自动检查数据库是否已经存在。

调用 getReadableDatabase() 或 getWritableDatabase() 将在返回可读或可写 SQLiteDatabase 对象之前执行以下操作:

1. 如果数据库不存在，它将调用 OnCreate()。您应该在此处编写代码来创建表并在需要时进行初始化。
2. 如果数据库需要升级，会调用onUpgrade()方法。在这里，您应该根据版本的不同(即删除/添加列)进行任何升级表所需的操作