JEP 238: Multi-Version JAR Files

[Markus Keller]

I've discussed this with the Eclipse JDT team. We're very skeptic and we 
think that JEP 238 goes into a wrong direction. Eclipse JDT does not 
intend to add special support for it.

Building such MVJARs will be quite complicated, since it requires having 
separate classpaths and compiling against multiple JDKs to produce the 
release-specific class files that should end up in the same MVJAR. That 
means you either need a preprocessor that would allow to keep 
JDK-release-specific code in the same *.java file, or you need to 
maintain and sync separate *.java files in separate source folders. Both 
options are completely counter Java's "write once, run anywhere" idea, 
whose foundation is to have only one set of source files and no 
platform-specific compilation switches. This is one of the greatest 
features of Java. It's what makes reliable code analysis, dependency 
tracking, and refactoring possible.

Furthermore, the general problem is not specific to the JDK, but also 
occurs for other libraries. Therefore, a JDK-specific solution would be 
at the wrong level.

A better solution for such cases is to provide an adapter JAR that makes 
the new APIs available when running on an older JDK. That way, the 
problem is solved at the origin, and not again and again for each 
client. And with "the problem", I mean the problem that the old JDK 
release didn't offer an API for something that clients should use ASAP, 
even if they can't require the whole new JDK release right away.

Thanks,
Markus Keller
IBM Rational Zurich Research Lab
Eclipse JDT UI