Question on MVJAR usage
steve.drach at oracle.com
Tue Jul 19 18:41:34 UTC 2016
> 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
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  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.
> 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  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
It’s not supported.
>  http://openjdk.java.net/jeps/238
>  http://openjdk.java.net/jeps/223
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