Why won't Apache POI OPCPackage close() method save/write contents to open writable file?

Question

Using the following code, cell value changes made to my Excel Spreadsheet are not saved:

OPCPackage pkg = OPCPackage.open(inputFile);
XSSFWorkbook wb = new XSSFWorkbook(pkg);

ModifyWorkbook();

pkg.close();

The following is a workaround I wrote, but I don't understand why it's necessary.

OPCPackage pkg = OPCPackage.open(inputFile);
XSSFWorkbook wb = new XSSFWorkbook(pkg);

ModifyWorkbook();

File tmp = File.createTempFile("tempSpreadsheet", ".xlsx");
FileOutputStream newExcelFile = new FileOutputStream(tmp);
wb.write(newExcelFile);
newExcelFile.close();
tmp.deleteOnExit();
pkg.close();

The javadocs and guides on the subject indicate that the .close() method should save and close. Checks on the values of the cells modified, prior to closing, indicate that the changes ARE made, but the pkg.close() method alone is not sufficient to write those changes to the file when closed.

Answer 1

Calling close on an (XSSF)Workbook will call OPCP.close which javadoc states:

Calling myWorkbook.close() should write the changes to file it was open from.

However, the current version (3.15) seems to have problems doing so. The reason itself is unknown to me, but I discovered that calling myWorkbook.write() will commit not only the changes to the output stream, but also to your current instance after calling close.

A reasonable workaround could therefore be:

try (OutputStream bos = ByteStreams.nullOutputStream()){
    workbook.write(bos);
    workbook.close();
} catch (IOException e) {
    log.error("Could not write Output File", e);
}

Here done with Guavas ByteStreams. Feel free to use other null output stream implementations.