Apache POI Introduction

What is Apache POI?

Apache POI is the open source Java library for Microsoft Office formats. The components you will use most are HSSF/XSSF (Excel .xls/.xlsx), HWPF/XWPF (Word .doc/.docx) and HSLF/XSLF (PowerPoint). This tutorial focuses on Excel, the 90% use case: reports, exports, imports and data migration.

HSSF vs XSSF vs SXSSF

• HSSF — old binary .xls (max 65,536 rows). Legacy only.
• XSSF — modern .xlsx (OOXML). Default choice; whole workbook lives in memory.
• SXSSF — streaming writer for huge .xlsx files; keeps only a sliding window of rows in memory.

poi vs poi-ooxml

The poi artefact covers the old binary formats; poi-ooxml adds the XML-based .xlsx/.docx support (plus xmlbeans and friends transitively). For new projects declare poi-ooxml — it pulls in poi automatically.

What You Will Build

Every lesson has runnable code in the companion poi-examples repo: writing workbooks, reading them back, styling, formulas, and a million-row streaming export.