Apache License

The Apache License (Apache Software License previous to version 2.0) is a free-software license authored by the Apache Software Foundation (ASF). The Apache License (versions 1.0, 1.1, and 2.0) requires preservation of the copyright notice and disclaimer, but it is not a copyleft license — it allows use of the source code for the development of proprietary software as well as free and open source software.

All software produced by the ASF or any of its projects or subjects is licensed according to the terms of the Apache License. Some non-ASF software is licensed using the Apache License as well: as of July 2009, over 5000 non-ASF projects located at SourceForge.net are available under the terms of the Apache License.

Licensing conditions

Like any free-software license, the Apache License allows the user of the software the freedom to use the software for any purpose, to distribute it, to modify it, and to distribute modified versions of the software.

The Apache License does not require modified versions of the software to be distributed using the same license nor even that it be distributed as free/open-source software. The Apache license only requires that a notice is kept informing recipients that Apache licensed code has been used. Thus, in contrast to copyleft licenses, recipients of modified versions of Apache licensed code do not necessarily also get the above freedoms. Or considering the situation from the Apache licensees perspective, they receive the freedom to use the code in any way they want, including using it in closed source products (cf Paragraph 4).

Two files that must be put at the top directory of redistributed software packages:

• LICENSE - a copy of the license itself.
• NOTICE - A "notice" text document listing the names of licensed libraries used, together with their developers.

In every licensed file, any original copyright or patent notices in redistributed code must be preserved, and in every licensed file changed a notification must be added stating that changes have been made to that file.

GPL compatibility

The Apache Software Foundation and the Free Software Foundation (FSF) agree that the Apache License 2.0 is a free software licence, compatible with version 3 of the GNU General Public License (GPL). ^[5]

However, the Free Software Foundation considers all versions of the Apache License (as of 2007) to be incompatible with the previous GPL versions 1 and 2.