SQLite 数据库备份和恢复

Question

我在将 SQLite 数据库从我的应用程序导出到本地文件时遇到问题。 我花了很多时间使用来自现有主题的代码，例如： is it possible backup and RESTORE a database file in android? non root devices 或Backup and restore SQLite database to sdcard。

在我的情况下，按下导出按钮后，应用程序停止工作。

我的活动：

NaprawyDB.java (DBHelper)

public class NaprawyDB extends SQLiteOpenHelper {

//Database Version
private static final int DATABASE_VERSION = 1;

private static String dbName = "NaprawyDB";
private static String tableName = "naprawy";
private static String idColumn = "id";
private static String pojazdColumn = "pojazd";
private static String periodColumn = "period";
private static String przebiegColumn = "przebieg";
private static String kwotaColumn = "kwota";
private static String warsztatColumn = "warsztat";
private static String opisColumn = "opis";
private Context context;

public NaprawyDB(Context context) {

    super(context, dbName, null, DATABASE_VERSION);
}


private static final String CREATE_TABLE_NAPRAWY = "create table " + tableName + "(" +
        idColumn + " integer primary key autoincrement, " +
        pojazdColumn + " text, " +
        periodColumn + " text, " +
        przebiegColumn + " text, " +
        kwotaColumn + " text, " +
        warsztatColumn + " text, " +
        opisColumn + " text " +
        ")";

@Override
public void onCreate(SQLiteDatabase sqLiteDatabase) {
    sqLiteDatabase.execSQL(CREATE_TABLE_NAPRAWY);

}

@Override
public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
    sqLiteDatabase.execSQL("drop table if exists " + tableName);
    onCreate(sqLiteDatabase);

}

public List<Naprawy> findAll() {
    try {
        List<Naprawy> naprawies = new ArrayList<Naprawy>();
        SQLiteDatabase sqLiteDatabase = getWritableDatabase();
        Cursor cursor = sqLiteDatabase.rawQuery("select * from " + tableName, null);
        if (cursor.moveToFirst()) {
            do {
                Naprawy naprawy = new Naprawy();
                naprawy.setId(cursor.getInt(0));
                naprawy.setPojazd(cursor.getString(1));
                naprawy.setPeriod(cursor.getString(2));
                naprawy.setPrzebieg(cursor.getString(3));
                naprawy.setKwota(cursor.getString(4));
                naprawy.setWarsztat(cursor.getString(5));
                naprawy.setOpis(cursor.getString(6));
                naprawies.add(naprawy);


            } while (cursor.moveToNext());

        }
        sqLiteDatabase.close();
        return naprawies;
    } catch (Exception e) {
        return null;
    }

}
public boolean create(Naprawy naprawy){
    try {
        SQLiteDatabase sqLiteDatabase = getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put(pojazdColumn, naprawy.getPojazd());
        contentValues.put(periodColumn, naprawy.getPeriod());
        contentValues.put(przebiegColumn, naprawy.getPrzebieg());
        contentValues.put(kwotaColumn, naprawy.getKwota());
        contentValues.put(warsztatColumn, naprawy.getWarsztat());
        contentValues.put(opisColumn, naprawy.getOpis());
        long rows = sqLiteDatabase.insert(tableName, null, contentValues);
        sqLiteDatabase.close();
        return rows > 0;
    }catch (Exception e) {
        return false;
    }
}
public boolean delete(int id) {
    try {
      SQLiteDatabase sqLiteDatabase = getWritableDatabase();
      int rows = sqLiteDatabase.delete(tableName,idColumn + " = ?", new String[] {String.valueOf(id)});
      sqLiteDatabase.close();
      return rows > 0;

    }catch (Exception e) {
        return false;
    }
}
public void backup(String outFileName) {

    //database path
    final String inFileName = context.getDatabasePath(dbName).toString();

    try {

        File dbFile = new File(inFileName);
        FileInputStream fis = new FileInputStream(dbFile);

        // Open the empty db as the output stream
        OutputStream output = new FileOutputStream(outFileName);

        // Transfer bytes from the input file to the output file
        byte[] buffer = new byte[1024];
        int length;
        while ((length = fis.read(buffer)) > 0) {
            output.write(buffer, 0, length);
        }

        // Close the streams
        output.flush();
        output.close();
        fis.close();

        Toast.makeText(context, "Backup Completed", Toast.LENGTH_SHORT).show();

    } catch (Exception e) {
        Toast.makeText(context, "Unable to backup database. Retry", Toast.LENGTH_SHORT).show();
        e.printStackTrace();
    }
}

Dziennik napraw.java（类似于主要活动）

public class Dziennik_napraw extends AppCompatActivity {


private ListView listViewNaprawy;
FloatingActionButton fab_add;
private Button buttonimport;
private Button buttonexport;
private Button buttonGexport;
private Button buttonGimport;


@Override
protected void onCreate (Bundle saveInstanceState) {
    super.onCreate(saveInstanceState);
    setContentView(R.layout.activity_dziennik_napraw);

    this.fab_add = (FloatingActionButton) findViewById(R.id.fab_dodaj);

    this.fab_add.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent intent1 = new Intent(Dziennik_napraw.this, AddNaprawyActivity.class);
            startActivity(intent1);

        }
    });

    final NaprawyDB naprawyDB = new NaprawyDB(getApplicationContext());
    this.listViewNaprawy = (ListView) findViewById(R.id.listViewNaprawy);
    this.listViewNaprawy.setAdapter(new NaprawyListAdapter(this,  naprawyDB.findAll()));
    this.listViewNaprawy.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int i, long l) {
            Naprawy naprawy = naprawyDB.findAll().get(i);
            Intent intent1 = new Intent(Dziennik_napraw.this, NaprawyDetailActivity.class);
            intent1.putExtra("naprawy", naprawy);
            startActivity(intent1);



        }
    });
    this.buttonexport = (Button) findViewById(R.id.button_export);
    this.buttonexport.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
        String outFileName = Environment.getExternalStorageDirectory() + File.separator + getResources().getString(R.string.app_name) + File.separator;
            performBackup(naprawyDB, outFileName);
        }

    });



}
private void performBackup(final NaprawyDB naprawyDB, final String outFileName) {

    verifyStoragePermissions(this);

    File folder = new File(Environment.getExternalStorageDirectory() + File.separator + getResources().getString(R.string.app_name));

    boolean success = true;
    if (!folder.exists())
        success = folder.mkdirs();
    if (success) {

        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Backup Name");
        final EditText input = new EditText(this);
        input.setInputType(InputType.TYPE_CLASS_TEXT);
        builder.setView(input);
        builder.setPositiveButton("Save", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                String m_Text = input.getText().toString();
                String out = outFileName + m_Text + ".db";
                naprawyDB.backup(out);
            }
        });
        builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                dialog.cancel();
            }
        });

        builder.show();
    } else
        Toast.makeText(this, "Unable to create directory. Retry", Toast.LENGTH_SHORT).show();
}
// Storage Permissions variables
private static final int REQUEST_EXTERNAL_STORAGE = 1;
private static String[] PERMISSIONS_STORAGE = {
        android.Manifest.permission.READ_EXTERNAL_STORAGE,
        Manifest.permission.WRITE_EXTERNAL_STORAGE
};
public static void verifyStoragePermissions(Activity activity) {
    // Check if we have read or write permission
    int writePermission = ActivityCompat.checkSelfPermission(activity, android.Manifest.permission.WRITE_EXTERNAL_STORAGE);
    int readPermission = ActivityCompat.checkSelfPermission(activity, android.Manifest.permission.READ_EXTERNAL_STORAGE);

    if (writePermission != PackageManager.PERMISSION_GRANTED || readPermission != PackageManager.PERMISSION_GRANTED) {
        // We don't have permission so prompt the user
        ActivityCompat.requestPermissions(
                activity,
                PERMISSIONS_STORAGE,
                REQUEST_EXTERNAL_STORAGE
        );
    }
}

}

你知道我会在哪里犯错吗？ 所有数据库工作都很好。我可以创建新项目/编辑/删除，结果以列表视图的形式显示。 感谢您的任何回复:)

Logcat：

05-16 15:48:30.341 27280-27280/com.example.jacek.hondadiagnostic E/AndroidRuntime: 致命异常: main 进程：com.example.jacek.hondadiagnostic，PID：27280 java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“java.io.File android.content.Context.getDatabasePath(java.lang.String)” 在 database.NaprawyDB.backup(NaprawyDB.java:166) 在 com.example.jacek.hondadiagnostic.Dziennik_napraw$4.onClick(Dziennik_napraw.java:109) 在 android.support.v7.app.AlertController$ButtonHandler.handleMessage(AlertController.java:166) 在 android.os.Handler.dispatchMessage(Handler.java) 在 android.os.Looper.loop(Looper.java) 在 android.app.ActivityThread.main(ActivityThread.java) 在 java.lang.reflect.Method.invoke（本机方法） 在 java.lang.reflect.Method.invoke(Method.java:372) 在 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java)

从 Logcat 我有信息要从空对象引用进行备份。

Answer 1

异常清楚地表明，您的上下文字段为空。这是真的，因为你从来没有给它赋值。

private Context context;

public NaprawyDB(Context context) {
    super(context, dbName, null, DATABASE_VERSION);
    this.context = context; //<--- this line is missing.
}