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Michael Zucchi

 B.E. (Comp. Sys. Eng.)

  also known as zed
  & handle of notzed

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Sunday, 05 November 2017, 13:22

java 9

Yesterday I had a quick look at java 9 - i hadn't installed it earlier as I was waiting for GA and I didn't really have a need. After a long silent spell I don't get many hits these days to this site so I don't know if anyone will read this but whatevers.

I guess the main new thing is the module system. It probably has some warts but overall it looks quite decent. maven and `aficionados' of other modularisation systems seem to be upset about some things with it but I mean, maven?

I did a bit of playing with zcl to see how it could be modularised. At least on paper it's a very good fit for this project due to the native code used. In practice it seems a little clumsy, at least at my first attempt.

I decided to separate it into two parts - the main reusable library and the tools package. This required adding another top-level directory for each module (as the module name is used by the compiler), and a couple of simple module-info.java files.

au.notzed.zcl/module-info.java

module au.notzed.zcl { // module name
    exports au.notzed.zcl; // package name

    requires java.logging; // module name
}
au.notzed.zcl/tools/module-info.java
module au.notzed.zcl.tools { // module name
    exports au.notzed.zcl.tools; // package name

    requires au.notzed.zcl; // module name
}

With the source moved from src/au to au.notzed.zcl/au or au.notzed.zcl.tools/au as appropriate (sigh, yuck). Note that the name is enforced and must match the module name although the rest of the structure and where the module name exists in the path is quite flexible; here i obviously chose the simplest/shortest possible because I much prefer it that way.

The filenames in the Makefile were updated and a tiny change added is all that is needed to create both java modules at once. Makefile

zcl_JAVAC_FLAGS=-h build/include/zcl --module-source-path .

Yes I also moved to using java -h to create the jni header files as I noticed javah is now deprecated.

Ok that was easy. Now what?

The next part is to create a jmod file. This can be platform specific and include native libraries (and other resources - although i'm not sure how flexible that is).

The manual commands are fairly simple. After i've had a bit of play with it I will incorporate it into java.make with a new _jmods target mechanism.

build/jmods/zcl.jmod: build/zcl_built
  -rm $@
  mkdir -p build/jmods
  jmod create \
    --class-path build/zcl/au.notzed.zcl \
    --libs jni/bin/gnu-amd64/lib \
    --target-platform linux-amd64 \
    --module-version $(zcl_VERSION) \
    $@

As an aside it's nice to see them finally moving to gnu-style command switches.

Now this is where thing kind of get weird and I had a little misreading of the documentation at first (as a further aside I must say the documentation for jdk 9 is not up to it's normal standards at all, it doesn't even come with man pages). While a modularised jar can be used like any other at runtime whilst adding the benefits of encapsulation and dependency checking a jmod really only has one purpose - to create a custom JRE instance. As such I initially went the jmod route for both zcl and zcl.tools and found it was a bit clumsy to use (generating a whole jre for a test app? at least it was "only" 45MB!). The whole idea seems to somewhat fight against the 'write once run anywhere' aspect of java, even though i've had to create the same functionality separately for delivering desktop applications myself. For example it would be nice if jlink could be used to create a multi-platform distribution package for your components without including the jre as well (i.e. lib/linux-amd64, lib/windows-amd64 for native libraries and package up all the jars etc), but I guess that isn't the purpose of the tool and it will be useful for me nevertheless. There is definitely some merit to precise versioning of validated software (aka configuration management) although these days with regular security updates it spreads the task of keeping up to date a bit further out.

One nice thing is that the module path is actually a path of directories and not modules. No need to add every single jar file to the classpath, you just dump them in a directory. This wasn't possible with the classpath because the classpath was what defined the dependencies (albeit in a pretty loose way).

X-platform?

As I only cross-compile my software for toy platforms I was also curious how this was supposed to work ...

I found a question/answer on stack overflow that stated unequivocally that jlink was platform specific and that was that. This is incorrect. jlink is platform agnostic and you must supply the location of the system JDK jmods. The only problem is right now the only microsoft windows jdk available is an executable installer and no tar is available, one only hopes this is a temporary situation. So the binary must first be run inside some microsoft windows instance and then I believe I can just copy the files around. Maybe it will work in wine, but either way I haven't checked this yet.

GNU GPL?

One issue I do see as a free software developer is that once you jlink your modules, they are no longer editable (or cross platform). By design the modules are stored in an undocumented and platform-specific binary format. So here's the question ... how does this affect the GNU General Public License, and particularly the GNU Lesser General Public License? The former perhaps isn't much different from any statically linked binary -because GPL means all source must be GPL compatible. But in the case of the LGPL it is possible to link with non-GPL components - only in the case that the LGPL components may be replaced by the receiver of the software. In dynamic linking this can be achieved by ensuring all LGPL components are isolated in their own library and simply changing the load path or library file, but for static linking this requires that all object files are available for re-linking (hah lol, like anyone gives a shit, but that's the contract). So anyone distributing a binary will have to distribute all the modules that were used to build it together with the source of the LGPL modules. Yeah I can see that happening. Or perhaps the module path might be enough, and there is a mechanism for patching (albeit intended for development purposes).

Still, I think an article may be required from The Free Software Foundation to clarify the new java situation.

It would also be nice if jlink has support for bundling source which addresses much of the GPL distribution issue. Obviously it does because the jdk itself includes it's own source-code but it gets put into lib/ which seems an odd place to put it (again i don't know how flexible this structure is although it appears to be quite limited).

Update: Ahh covered by the classpath exception I guess, at least for the JRE. If one builds a custom JRE though any third party modules would also require the classpath exception? Or would including the source and all modules used to create the JRE suffice? Hmmm.

JDK 9

I would guess that the modularisation will have slow uptake because it's quite a big change and locks your code into java 9+, and it may evolve a little over the next jdk or two. I'm in two minds about using it myself just yet because of this reason and also because NetBeans has failed to deliver any support for it so far that i can tell (I was dissapointed to see Oracle abandon NetBeans to apache which is most probably part of it, and they're too busy changing license headers to get any real work done). There will also likely be blowback from those invested in existing systems, merits or not. And then there's dealing with the fuckup of a situation that android/"java" is in.

I myself will poke around with it for a while and merge the functionality into java.make, it's actually a pretty close fit to everything i've done (which is a nice validation that my solution wasn't far off) and will simplify it even if i might have to make a few minor changes like platform names.

Its a pretty good fit for my work but will require a bit of setup so I wont rush into it (a couple dozen lines of shell is doing a good enough job for me). Besides it's probably worth getting some experience with it before committing to a particular design. The NetBeans situation will also be a bit of a blocker and i'll probably wait for 9 to be released first.

Tagged java.
JNI and garbage collection | zcl 0.6
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