The POI project consists of APIs for manipulating various file formats based upon Microsoft's OLE 2 Compound Do(edited)ent format using pure Java. In short, you can read and write MS Excel files using Java. Soon, you'll be able to read and write Word files using Java. POI is your Java Excel solution as well as your Java Word solution. However, we have a complete API for porting other OLE 2 Compound Do(edited)ent formats and welcome others to participate.
The following are components of the entire POI project and a brief summary of their purpose:
POIFS for OLE 2 Documents
POIFS is the oldest and most stable part of the project. It is our port of the OLE 2 Compound Document Format to pure Java. It supports both read and write functionality. All of our components ultimately rely on it by definition. Please see the POIFS project page for more information.
HSSF for Excel Documents
HSSF is our port of the Microsoft Excel 97(-2002) file format (BIFF8) to pure Java. It supports read and write capability. Please see the HSSF project page for more information.
HWPF for Word Documents
HWPF is our port of the Microsoft Word 97 file format to pure Java. It supports read and write capability. Please see the HWPF project page for more information. This component is in the early stages of development. It can already read and write simple files.
HPSF for Document Properties
HPSF is our port of the OLE 2 property set format to pure Java. Property sets are mostly use to store a document's properties (title, author, date of last modification etc.), but they can be used for application-specific purposes as well.
HPSF supports reading and writing of properties. However, with the current POI release only reading is possible. In order to write properties, you'll have to fetch the latest POI version from the CVS repository.
So why should you use POIFS or HSSF?
You'd use POIFS if you had a document written in OLE 2 Compound Document Format, probably written using MFC, that you needed to read in Java. Alternatively, you'd use POIFS to write OLE 2 Compound Document Format if you needed to inter-operate with software running on the Windows platform. We are not just bragging when we say that POIFS is the most complete and correct implementation of this file format to date!
You'd use HSSF if you needed to read or write an Excel file using Java (XLS). You can also read and modify spreadsheets using this API, although right now writing is more mature.