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 > [2] http://openjdk.java.net/jeps/223 > > Cheers, > Paul
