Question on MVJAR usage

[Steve Drach]

> Thank you Steve. I have only one follow-up then. If the platform version is *always* a major version, does that imply that minor versions will not add API?

No.  Minor releases can add new JDK specific apis.  We’ve chosen to support only major release for now, a known restriction.

> The big benefit of MRJAR :-) is that I can provide separate code for different targeted runtimes. If 9.1 wouldn't add new API, then I am good. If that's the policy, great. I just want to confirm it is because I don't see too much of a technical difference between the two cases (8 to 9 vs 9.0 to 9.1). Does that make sense?

Yes, it makes sense, but as I said, it’s a known restriction.

> 
> Cheers,
> Paul
> 
> On Tue, Jul 19, 2016 at 1:41 PM, Steve Drach <steve.drach at oracle.com <mailto:steve.drach at oracle.com>> wrote:
> Hi Paul,
> 
> > I have some questions. I believe core-lib is the right place. If not please
> > let me know.
> 
> This is the right place.  First, the name was changed to Multi-Release JAR, so it’s MRJAR (or Mr. Jar) instead of MVJAR.
> 
> >
> > 1) Given a Java 9 runtime, is there any perceptible difference between a
> > non-multiversion jar, and a versioned jar which has placed all its classes
> > under /META-INF/versions/9 ? Pretend each jar has the same identical
> > binaries/resources.
> 
> There is probably a small difference in performance although I haven’t measured it.  Whether the difference is perceptible or not probably depends on the characteristics of the sensor.
> 
> >
> > 2) Does the runtime care if the class version does not match what's under
> > /META-INF/versions/9? For example, what if I have targeted a Java 8 class
> > file format under versions/9?
> 
> The MRJAR runtime does not care.  However if you put a class file targeted for Java 10 in the  /META-INF/versions/9 directory and run it on a Java 9 platform, you’ll probably get an error.
> 
> >
> > 3) Why does the new MVJAR JEP writeup [1] use versions/8 in the example?
> 
> Because we don’t have a real example that uses Java 9 and the JEP was written when we thought we’d target this feature for Java 8.
> 
> > Is
> > it simply for illustration, but I don't see how that's a useful example
> > since it's an impossibility. There is no MVJAR support prior to Java 8 so
> > isn't a better (and realer) example be /9 and /10?
> 
> Yes, it would be better, but as I said, we don’t have a real world example yet, so anything we do would be a contrived example.  We probably need to do something with that part of the JEP.
> 
> >
> > 3) The same MVJAR JEP writeup doesn't clearly indicate what is considered a
> > "platform version". All the examples show a single digit, but I believe
> > Verona [2] has specificed the platform to include both major and minor
> > versions. For example, Verona says the minor version may include "revisions
> > to standard APIs mandated by a Maintenance Release of the relevant Platform
> > Specification". Because it mentions platform, it should be possible to do
> > /9, /9.0, and /9.1. Please advise?
> 
> Platform versions are major versions, i.e. 8, 9, 10, etc.  They are the values derived from Runtime.Version::major
> 
> >
> > 4) Although MVJAR JEP writeup says "JAR metadata, such as that found in the
> > MANIFEST.MF file and the META-INF/services directory, need not be
> > versioned." The keyword here is "need not" which is not the same as "can
> > not" or "may not”.
> 
> Yes, you are right, it’s incorrect.  Perhaps it should say "JAR metadata, such as that found in the MANIFEST.MF file and the META-INF/services directory, are not versioned.”
> 
> > So if it is needed, how does one version different
> > services for different platforms?
> 
> It can’t be done.
> 
> > Can there be /META-INF under the
> > appropriate versioned folder?
> 
> Technically, it can be done but it won’t be interpreted as a “true” META-INF directory, it’ll just be a path component for the jar entry.
> 
> > Maybe /META-INF/versions/9/META-INF?
> 
> You won’t achieve what you expect, depending of course on what you expect. ;-)
> 
> > I do not
> > see anything in the JEP that says it's supported or non-supported. Please
> > advise.
> 
> It’s not supported.
> 
> >
> > [1] http://openjdk.java.net/jeps/238 <http://openjdk.java.net/jeps/238>
> > [2] http://openjdk.java.net/jeps/223 <http://openjdk.java.net/jeps/223>
> >
> > Cheers,
> > Paul
> 
>