XSSF

XSSF is a component of the Apache POI project that provides a Java API for reading and writing Microsoft Excel 2007 and later workbook formats based on the Office Open XML (OOXML) standard. It is the POI portion that handles .xlsx files, complementing HSSF, which works with the older binary .xls format. The core API classes in XSSF include XSSFWorkbook, XSSFSheet, XSSFRow, and XSSFCell, which mirror the structure of an Excel workbook.

XSSF relies on the Open Packaging Conventions to store content in a ZIP container and uses OOXML schemas for workbooks, styles, shared strings, and other parts. This enables manipulation of sheets, rows, cells, and their properties, including data types, formulae, and formatting. The API also supports rich text strings, comments, hyperlinks, merged regions, data validation, and some drawing or picture features associated with a workbook. Formula evaluation can be performed via a FormulaEvaluator.

For performance with large workbooks, POI provides SXSSF, a streaming API built on XSSF that reduces memory usage by writing data in a streaming fashion rather than loading the entire workbook into memory.

XSSF is widely used in Java applications for generating reports, processing existing spreadsheets, or templating Excel workbooks. While feature coverage is extensive, not every Excel feature is implemented with equal parity, and some advanced components such as certain charting or macro-related features may be limited in XSSF.