What is the difference between SXSSFWorkbook and XSSFWorkbook?

SXSSFWorkbook is designed for writing data into xlsx file (and only appending is supported). This is what you really wanna use when it comes to creating large xlsx files to avoid loading all the data into memory. So you can create SXSSFWorkbook based on XSSFWorkbook, but you can only append data (create new rows). No modifications of existing rows allowed.

XSSFWorkbook can do both reading and any kind of modifications. But it uses memory to store all rows. So it's not very good choice if you wanna deal with larg xlsx files.