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一个应用程序。我正在尝试检查数据库是否存在,如果不存在,请在您的应用程序数据库的Android默认系统路径中创建一个新的db,然后将数据从db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
有人可以给我一些关于我做错了什么的建议吗?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;
}
}
最佳答案
您不需要这样做。
SQLiteOpenHelper父类的优点在于它会自动检查数据库是否已经存在。
在返回可读或可写的SQLiteDatabase对象之前,对getReadableDatabase()或getWritableDatabase()的调用将执行以下操作:
如果数据库不存在,它将调用OnCreate()。在这里您应该编写代码来创建表并在需要时进行初始化。
如果数据库需要升级,则将调用onUpgrade()方法。在这里,您应该根据版本的差异(例如,删除/添加列)执行所需的升级表的操作