Trying to understand the saving of changes to a Word doc from Apache poi

Question

I have a Word document (docx); I want to make changes to that document and save the result as another file, leaving the original in place. I have the following code illustrating my current problem:

package sandbox.word.doccopy;

import java.io.FileOutputStream;
import java.io.IOException;
import java.util.List;

import org.apache.poi.openxml4j.exceptions.InvalidFormatException;
import org.apache.poi.openxml4j.opc.OPCPackage;
import org.apache.poi.xwpf.usermodel.XWPFDocument;
import org.apache.poi.xwpf.usermodel.XWPFParagraph;
import org.apache.poi.xwpf.usermodel.XWPFRun;

public class CopyTest
{
  public static void main(String[] args) throws Exception
  {
    String sourceFilename      = "CopyTestSource.docx";
    String destinationFilename = "CopyTestResult.docx";
    
    CopyTest docCopy = new CopyTest();
    docCopy.copyTesting(sourceFilename, destinationFilename);
    System.out.println("done");
  }
  
  public void copyTesting(String source, String destination)
      throws IOException, InvalidFormatException
  {
    XWPFDocument doc = new XWPFDocument(OPCPackage.open(source));
    // for each paragraph that has runs, 
    // put an exclamation at the end of the first run.
    for (XWPFParagraph par : doc.getParagraphs())
    {
      List<XWPFRun> runs = par.getRuns();
      if (runs.size() > 0) 
      { XWPFRun run = par.getRuns().get(0);
        String text = run.getText(0);
        text = text + "!";
        run.setText(text, 0);
      }
    }
    
//    FileOutputStream fos = new FileOutputStream(destination);
//    doc.write(fos);
//    fos.close();
    doc.close();
  }
  
}

There are three ways I've run this, changing commented lines at the bottom of the class file. As you see, there are three lines that create a file output stream with the destination filename, write to it, and close it, and one line that just closes the current document.

If I comment out the 3 lines and leave the 1 line, no changes are written to the current document (and, of course, the copy document is not created).

If I leave all 4 lines uncommented, the copy document is created with changes, and the changes are also written to the source document.

If I comment out the 4th line, I get a destination document with changes, and the source document is left unchanged.

The last one is what I want, I can write my code to do that. But I would expect that closing the document after it is changed would either change it or not change it, and that changing it wouldn't depend on whether I had written the changes to another file.

Can anyone shed any light on this?