Application performance guru Kirk Pepperdine (pictured) added his two cents to the ongoing Web discussions about Sun's announcement last week that it is open sourcing its implementation of Java. He pinpoints the Classpath exception as being the key valve that "leaves business free to build upon yet protects the underlying implementation."

Pepperdine, a former Enterprise Editor of Java Developer's Journal, identifies two controls that, he says "offer us protection from incompatible forks that come with the Java label."

The first is that the Java specification is still controlled by the JCP and so "you can’t call something Java unless it passes the compatibility test suite (CTS)." The second key element is the choice of license, the GPL.

Pepperdine seeks to clear up a widespread misapprehension about the GPL:

"What people fear is that GPL license requires that anything that touches a piece of GPL’ed code must be published. The implication is that all Java applications that make use of Sun’s JDK/JRE would be copy-left’ed into the world of open source. This is where the Classpath exception comes into play.

Classspath is the GNU implementation of the Java platform specification. In the license agreement that comes it is a specific exception to the GPL license. That exception limits the reach of the GPL license by stating that only code modifications to Classpath must be republished and code extending or using Classpath does not. Let’s consider this in light of an example using java.util.ArrayList.

If all you were to do was subclass ArrayList than the Classpath exception would apply and you would not have to publish your code. However, if you were to alter the code in ArrayList, than you would be required to publish your modifications. As you can see the Classpath exception leaves business free to build upon yet protects the underlying implementation."