GPL with the Classpath exception - clarification needed

John Cowan cowan at
Thu Mar 26 22:30:05 UTC 2009

Wilson, Andrew scripsit:

> A cautious person would probably prefer, if possible, a license with a
> black-letter permission that use of a class library does not create a
> derivative work subject to the copyleft license of the library itself.

The relevant distinction for LGPL purposes is not whether something is
a derivative work or not: every program that incorporates the library
into itself is a derivative work.  The question is whether the derivative
work is a "work based on the Library" (LGPL 2.1) / "Modified Version of
the Library" (LGPL 3.0) or not.

Now subclassing a class plainly does not *modify* the class.  So programs
that contain subclasses of an LGPLed class, or instantiations of an
LGPLed interface, or what have you, are "works that use the Library"
(LGPL 2.1) / "Applications" (LGPL 3.0) and may be under any license
provided it is possible to replace the library.  In Java, or any language
normally delivered either in source form or in component-by-component
binary form with run-time linkage, this is very easy, meaning that the
LGPL and the GPL+CP are essentially equivalent.  The GPL+CP matters
only for a library written in a language where delivery by way of static
executables is the norm, and replacement would require the proprietary
part of the application to be delivered as .o files or something similar.

After fixing the Y2K bug in an application:     John Cowan
        WELCOME TO <censored>                   cowan at
        DATE: MONDAK, JANUARK 1, 1900 

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