ex in Domino. I get below exception if file is in class path and start with '/'
java.lang.NullPointerException
java.io.File.(File.java:222)
org.zkoss.zss.ui.Spreadsheet.getBook(Spreadsheet.java:316)
org.zkoss.zss.ui.Spreadsheet.getSelectedSheet(Spreadsheet.java:413)
org.zkoss.zss.ui.Spreadsheet.getColumnfreeze(Spreadsheet.java:689)
org.zkoss.zss.ui.Spreadsheet.getMaxcolumnsInJSON(Spreadsheet.java:632)
org.zkoss.zss.ui.Spreadsheet.setMaxcolumns(Spreadsheet.java:624)

I think the code in Spreadsheet.getBook should check
Executions.getCurrent().getDesktop().getWebApp().getRealPath(_src) return null or not first.