I tried to traverse all the sheets in a xlsx file with this:

XSSFReader.SheetIterator iter = (XSSFReader.SheetIterator) reader.getSheetsData();
while (iter.hasNext())
{
    InputStream stream = iter.next();
}

When the xlsx file contains a chartsheet, a NullPointerException is thrown.

I noticed there is a kindof inconsistence in XSSFReader.SheetIterator.

In the constructor of XSSFReader.SheetIterator only WORKSHEETs are added to the sheetMap.

But in XSSFReader.SheetIterator.next(), it is like this:
public InputStream next() {
    ctSheet = sheetIterator.next();
    String sheetId = ctSheet.getId();
    try {
        // if we have sheetId of ChartSheet, we will get nullpointer exception.
	PackagePart sheetPkg = sheetMap.get(sheetId);
	return sheetPkg.getInputStream();
    } catch(IOException e) {
        throw new POIXMLException(e);
    }
}