HSSF

HSSF is a component of the Apache POI project used for reading and writing Microsoft Excel 97-2003 binary workbook files (.xls). It provides a Java API that maps the elements of a binary Excel file to POI classes such as HSSFWorkbook, HSSFSheet, HSSFRow, and HSSFCell. HSSF files are stored in the POIFS (OLE 2 Compound Document) filesystem, reflecting the structure of older Excel workbooks. The API supports creating and modifying workbooks, including cells, rows, formulas, cell styles, fonts, hyperlinks, and basic formatting.

HSSF covers the core features of the Excel 97-2003 binary format, but is bound by its format's limitations: a single sheet can have up to 65,536 rows and 256 columns. Some features introduced in newer Excel formats or versions—such as OOXML-specific features and newer chart types—are not part of HSSF. For handling newer Excel files (.xlsx), the XSSF component provides a separate API within POI. HSSF typically uses more memory for large workbooks due to its in-memory model, and for very large datasets the POI project recommends alternatives or streaming approaches with the corresponding components for newer formats rather than HSSF.