Bug 56563

Created attachment 31660 [details]
A file for which the problem reproduces

When reading two copies of the same file (doesn't necessarily need to be the exact same file, but do need to contain the same styles or something along those lines),
from two threads simultaneously (each thread processes its own file), the format string of cell styles get mixed up.

The following code demonstrates this, printing to System.out every time a non date cell is mistakenly recognized as date. Note that starting only one of the threads yields no System.out messages.

public class CellFormatBugExample implements Runnable {

	public static void main(String[] args) {
		new Thread(new CellFormatBugExample("C:/temp/file_1.xls")).start();
		new Thread(new CellFormatBugExample("C:/temp/file_2.xls")).start();
	}

	String filePath;

	public CellFormatBugExample(String filePath) {
		this.filePath = filePath;
	}

	@Override
	public void run() {

		File inputFile = new File(filePath);
		try (FileInputStream stream = new FileInputStream(inputFile)) {

			Workbook wb = WorkbookFactory.create(stream);
			Sheet sheet = wb.getSheetAt(0);

			for (Row row : sheet) {
				for (Integer idxCell = 0; idxCell < row.getLastCellNum(); idxCell++) {

					Cell cell = row.getCell(idxCell);
					cell.getCellStyle().getDataFormatString();
					if (cell.getCellType() == HSSFCell.CELL_TYPE_NUMERIC) {
						boolean isDate = HSSFDateUtil.isCellDateFormatted(cell);
						if (idxCell > 0 && isDate) {
							System.out.println("cell " + idxCell + " is not a date!");
						}
					}
				}
			}
		}
		catch (Exception e) {
			e.printStackTrace();
		}
	}
}

Make another copy of the attached file - "file_2.xls" and run the code to reproduce.
Digging around a bit, seems the cause for this is a caching bug in the HSSFCellStyle.getDataFormatString() method.
As far as I could understand a simple resolution would be to synchronize access to that method.