To be considered open source, the license for use must permit at least modification and reuse of the source, perhaps subject to certain conditions, but not costing money. The source must be either explicitly or implictly part of every distribution, and the source itself is either included or readily available at no cost. You may charge for the software, but you may not charge extra for the source.

Legally it would be software distributed under one of the OSI opensource licenses.

Some of the better known Java open source projects include: JOS, Kaffe (free virtual machine), Japhar (free virtual machine), GNU and the Java language, GCC for Java , GNU Classpath (free Java libraries), J-Emacs, Intel Open Runtime Platform , XML-projects of the Apache group (mostly Java based), Tomcat.

The advantages to the customer of open source are:

• The customer can fix bugs even when the official vendor will not or cannot.
• If the original vendor goes out of business, the customer can carry on, doing development himself, or letting some third party do it.
• The customer can judge the quality of the code.
• Because the code is publicly viewable, developers are less likely to be sloppy.

• There are more eyes looking for and fixing bugs.
• Customers need less hand-holding when they can answer their own questions by looking at the source code.

• It is easy for a competitor to study your source and then write something better based on your hard won experience.
• Many people will often elect to you use your code without paying you anything. You normally have to make money through consulting. As time progresses, and your product becomes better known, there is less and less need for your consulting services to get it to work, though to compensate, there are more customers.