This is a question that stems out from here: Do I have to release my code as GPL when a Java Library is GPLIt seems that the Java System libraries are licensed with a class path exception. After [attempting] to read what the class path exception is, it remains very unclear.What is a class path exception?
What is a class path exception?
licensing;exceptions
Some of the OpenJDK code is covered by the classpath exception, documented as follows (the second paragraph constitutes the exception):Linking this library statically or dynamically with other modules is making      a combined work based on this library.  Thus, the terms and conditions of      the GNU General Public License cover the whole combination.As a special exception, the copyright holders of this library give you      permission to link this library with independent modules to produce an      executable, regardless of the license terms of these independent modules,      and to copy and distribute the resulting executable under terms of your      choice, provided that you also meet, for each linked independent module,      the terms and conditions of the license of that module.  An independent      module is a module which is not derived from or based on this library.  If      you modify this library, you may extend this exception to your version of      the library, but you are not obligated to do so.  If you do not wish to do      so, delete this exception statement from your version.The exception means that you can link the relevant code with other code and use the license you wish, as long as you respect the license of code (modules, libraries etc.) which isn't covered by the classpath exception. As I understand it this effectively means that the license covering the code placed under the classpath exception has no effect on its use (and redistribution once linked), only on its modification.The classpath exception is just a way of allowing users of this implementation of the Java platform (compiler, VM and libraries) to use it for projects without affecting the resulting licenses. This is typical of compilers and platforms, since most compiler and platform authors don't want their use to restrict the licenses which can apply to projects using them.The OpenJDK itself is covered by the OpenJDK assembly exception:Linking this OpenJDK Code statically or dynamically with other code      is making a combined work based on this library.  Thus, the terms      and conditions of GPL2 cover the whole combination.As a special exception, Sun gives you permission to link this      OpenJDK Code with certain code licensed by Sun as indicated at      http://openjdk.java.net/legal/exception-modules-2007-05-08.html      (Designated Exception Modules) to produce an executable,      regardless of the license terms of the Designated Exception Modules,      and to copy and distribute the resulting executable under GPL2,      provided that the Designated Exception Modules continue to be      governed by the licenses under which they were offered by Sun.This just means that you can use the OpenJDK under the terms of the GPL version 2, including code in the OpenJDK which isn't covered by the GPL.