The POI project consists of APIs for manipulating various file formats based upon Microsoft's OLE 2 Compound Document format using pure Java.
OLE 2 Compound Document Format based files include most Microsoft Office files such as XLS and DOC.
As a general policy we try to collaborate as much as possible with other projects to provide this functionality. Examples include: Cocoon for which you'll soon find generators and serializers for our projects; Open Office.org with whom we collaborate in documenting the XLS format; and Lucene for which we'll soon have file format interpretors. When practical, we donate components directly to those projects for POI-enabling them.
A common misconception is that POI writes Excel files. POI is the name of the project. POI contains several components, one of which, HSSF, writes Excel files. The following are components of the entire POI project and a brief summary of their purpose.
POIFS (POI Filesystem) : 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 (Horrible Spreadsheet Format) : 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.
HDF (Horrible Document Format) : HDF is our port of the Microsoft Word 97 file format to pure Java. It supports read and write capability. Please see the HDF project page for more information. This component is in the early stages of design. Jump in!
HPSF (Horrible Property Set Format) : 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. Currently HPSF supports read functionality only. Please see the HPSF project page for more information.