Writing to an excel sheet in a JAR file

I made a Java project in netbeans that uses data from an excel sheet. There is functionality in my project that allows the user to read and write to the excel sheet. I have used the Apache POI library to do this. The program works well in Netbeans IDE 17 but when I try to do the same in a JAR file, I cannot write to the excel sheet.

this is what my code initially looked like:

if(addordel.equals("add")){
            File file = new File("./src/Pick4list.xlsx");
            Workbook workbook;
            try {
                workbook = new XSSFWorkbook(new FileInputStream(file));
                Sheet sheet = workbook.getSheetAt(0);
                int rowNum = -1;
                for (Row row : sheet) {
                    if (row.getCell(0).getStringCellValue().equals(datenum)) {
                        rowNum = row.getRowNum();
                        break;
                    }
                }
                if (rowNum >= 0) {
                    for (int i = 1; i < 10000; i++) {
                        Cell cell = sheet.getRow(rowNum).getCell(i);
                        if (cell == null || cell.getCellType() == CellType.BLANK) {
                            if (i - 1 < dateslist.size()) {
                                String dateStr = dateslist.get(i - 1);
                                cell = sheet.getRow(rowNum).createCell(i);
                                cell.setCellValue(dateStr);
                            } else {
                                break;
                            }
                        }
                    }
                }
                FileOutputStream out = new FileOutputStream(file);
                workbook.write(out);
                out.close();
                workbook.close();
            } catch (FileNotFoundException ex) {
                Logger.getLogger(Pick4gen.class.getName()).log(Level.SEVERE, null, ex);
            } catch (IOException ex) {
                Logger.getLogger(Pick4gen.class.getName()).log(Level.SEVERE, null, ex);
            }
        }

I've read answers from stackoverflow and changed my code to the following:

if(addordel.equals("add")){
            InputStream file = getClass().getResourceAsStream("/data/Pick4list.xlsx");
            Workbook workbook;
            try {
                workbook = new XSSFWorkbook(file);
                Sheet sheet = workbook.getSheetAt(0);
                int rowNum = -1;
                for (Row row : sheet) {
                    if (row.getCell(0).getStringCellValue().equals(datenum)) {
                        rowNum = row.getRowNum();
                        break;
                    }
                }
                if (rowNum >= 0) {
                    for (int i = 1; i < 10000; i++) {
                        Cell cell = sheet.getRow(rowNum).getCell(i);
                        if (cell == null || cell.getCellType() == CellType.BLANK) {
                            if (i - 1 < dateslist.size()) {
                                String dateStr = dateslist.get(i - 1);
                                cell = sheet.getRow(rowNum).createCell(i);
                                cell.setCellValue(dateStr);
                            } else {
                                break;
                            }
                        }
                    }
                }
                FileOutputStream out = new FileOutputStream(file);
                workbook.write(out);
                out.close();
                workbook.close();
            } catch (FileNotFoundException ex) {
                Logger.getLogger(Pick4gen.class.getName()).log(Level.SEVERE, null, ex);
            } catch (IOException ex) {
                Logger.getLogger(Pick4gen.class.getName()).log(Level.SEVERE, null, ex);
            }
        }

I've build the project after this. Opened the jar file using WinRAR, created a folder called "data" and placed my Excel sheet there. This seems to be working since I have another snippet of code that simply reads data from the excel and stores the data in a HashMap (I could print the data from the HashMap so I understood that the program could read the excel sheet). I need help as to how I can change the code so that I can write to the excel sheet in the JAR.

I want to create this program in such a way that no matter on which computer I run the JAR file, I want to be able to read from the excel sheet and also be able to make changes to it (the changes must be saved). Please guide me.

Whilst some OSes (cough windows cough) are, or at least used to be, fundamentally unsafely designed, in general the notion that an app can write to its own data files is a very bad idea, and thus non-broken OSes don't let you. Even windows which tended to go with the deployment model of 'you have e.g. C:\Program Files\MyApp and myapp stores user data and its config file (that is editable in the app itself) in there', is now moving away from this, with a 'My Documents' folder where docs and settings go instead of the app's own 'program files' folder.

The conclusion is that 'can I write to my own jar' is a definitely NO. On some OSes you might be able to, but writing your app such that 'I need to modify my own jar or I do not work' means your app only works on unsafe OSes / now-safe OSes but only if you use the unsafe legacy deployment model.

So what's the real solution?

Templating.

When the user wants an excel sheet and edit it in place, the flow goes like this:

• User indicates they want to do this. Possibly, 'start your app for the first time' is enough of an indication. Possibly they click some button someplace.

• You read in an excel sheet as you do now - using .getResource. Which you didn't quite use correctly, it's MyClass.class.getResource, not getClass().getResource (the latter breaks if you ever subclass, the former wouldn't).

• You then save it not to your jar file but to the documents folder. Unfortunately finding the right place is somewhat tricky, in that you'll end up having to write some platform-dependent code (if user is on windows, save here. If on mac, save there, and so on). But the basic gist is:

• Use System.getProperty("user.home") to find the user's home dir. Save there, or possibly be 'nice' and save in a subdir. For example on mac you optimally would do:

Path p = Paths.get(System.getProperty("user.home"), "Documents");

and save there. That "Documents" thing is more specific.

• Alternatively, pop open a save dialog and let the user choose where to save it.

The 'template' XLS in your jar file never changes. (It can't; its in a file your process cannot write to, or at least should not write to).
Hence the name 'template'.