Whenever I write a XSSFWorkbook out using XSSFWorkbook.write, the book afterwards is left in a state where calling getColumnWidth on one of its sheet causes an IndexOutOfBoundsException. Below is a unit test that isolates the problem (in this case it creates a new Workbook and constucts a sheet and cells, but I also have the same problem when starting from an existing workbook loaded through WorkbookFactory).

@Test
public void testColumnWidthPOI() throws Exception {
       XSSFWorkbook workbook = new XSSFWorkbook();
       XSSFSheet sheet = workbook.createSheet();
       XSSFRow row = sheet.createRow(0);
       XSSFCell cell = row.createCell(0);
       cell.setCellValue("hello world");
       assertEquals("hello world",workbook.getSheetAt(0).getRow(0).getCell(0).getStringCellValue());
       assertEquals(2048,workbook.getSheetAt(0).getColumnWidth(0)); //<-works

       //gets a UUID based temporary file
       File tmpDir = new File(System.getProperty("java.io.tmpdir"));
       String uuid = UUID.randomUUID().toString();
       File f = new File(tmpDir,uuid+".xlsx");

       BufferedOutputStream stream = new BufferedOutputStream(new FileOutputStream(f));
       workbook.write(stream);
       stream.close();
       assertTrue(f.exists());

       assertEquals("hello world",workbook.getSheetAt(0).getRow(0).getCell(0).getStringCellValue());
       assertEquals(2048,workbook.getSheetAt(0).getColumnWidth(0)); //<- throws IndexOutOfBoundsException
   }