How to rotate text in a spreadsheet cell using Apache POI?

CellStyle cssVertical = wb.createCellStyle();
cssVertical.setFont(f);
cssVertical.setRotation((short)90);

Use HSSFCellStyle, that class has a method called setRotation(short rotation) which will rotate the text. All you do is apply the cell style to a cell:

HSSFCellStyle myStyle = workbook.createCellStyle();
myStyle.setRotation((short)90);

HSSFCell c = row.createCell(columnNumber);
c.setCellStyle(myStyle);