Sun has launched a fresh programme designed to spur open-source developer interest in its Java application server software, placing the code under a licence the company says is in the "spirit" of open source.

However, the project - called GlassFish, after a fish so transparent its skeleton is visible - places severe restrictions on the way in which source code can be used. The Java System Application Server has been made available under the Java Research Licence, which lets developers manipulate code only for academic and research use.

Sun has long offered the application server and other Java code under the Sun Community Source Licence (SCSL), which has provisions for research use; the JRL, a new licence, was designed to simplify and relax the terms of the SCSL's "research" section, Sun said.

"There is greater flexibility in working with the code for research use and the license is worldwide, excluding Restricted Parties List (embargoes countries,etc.)," Sun said on its JRL site. "The language has been clarified and separated from the commercial terms so researchers and teachers should feel more confident with their usage rights." Any productive use, even within an organisation, means developers must obtain a commercial licence and agree to Java compatibility requirements.

Sun characterised GlassFish as a compromise between developers' interest in contributing to Java in an open-source manner, and ensuring that Java doesn't split into incompatible "forks".

"Sun's community development process and JRL and JDL licenses meet the spirit of the Open Source definition, but we retain some control of the code to ensure compatibility of J2EE - a key requirement of our licensees," Sun said on the GlassFish site.

Critics said GlassFish is just another example of Sun's effort to benefit from the open-source development model - getting free bug-fixes and development effort - without offering much to developers in return. Sun has made it clear it is not interested in Linux-style open-source licensing. In April, Sun president Jonathan Schwartz characterised the GNU General Public License (GPL), which covers Linux, as a tool allowing United States businesses to pillage developing countries of their intellectual property.

Sun has taken a more open approach with OpenSolaris, the open-source version of its flagship Unix operating system, largely because the increasing popularity of Linux has forced the company's hand, say analysts. Sun began releasing parts of Solaris as open source last week.

Sun's Java application server lags behind more popular alternatives from BEA Systems, WebLogic and IBM, and an JBoss' open-source application server has already become widely used. JBoss CEO Marc Fleury criticised GlassFish and Sun's application server as "irrelevant". "It is irrelevant what kind of licences they use since the whole thing is irrelevant anyway," he wrote in a blog post. "What are they going to do now? Fight it out for second place? With IBM?"

Sun will discuss GlassFish at its JavaOne conference in San Francisco next week.