How can I convert POI HSSFWorkbook to bytes?

As that mailing list post said

Invoking HSSFWorkbook.getBytes() does not return all of the data necessary to re- construct a complete Excel file.

You can use the write method with a ByteArrayOutputStream to get at the byte array.

ByteArrayOutputStream bos = new ByteArrayOutputStream();
try {
    workbook.write(bos);
} finally {
    bos.close();
}
byte[] bytes = bos.toByteArray();

(The close call is not really needed for a ByteArrayOutputStream, but imho it is good style to include anyway in case its later changed to a different kind of stream.)

How about:

ByteArrayOutputStream baos = new ByteArrayOutputStream();
workbook.write(baos);
byte[] xls = baos.toByteArray();

In order to get a full excel file out, you must call the write(OutputStream) method. If you want bytes from that, just give a ByteArrayOutputStream