RFR: 8311302: Allow for jlinking a custom runtime without packaged modules being present [v32]

[Alan Bateman]

On Thu, 6 Jun 2024 09:47:30 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

>> Please review this patch which adds a jlink mode to the JDK which doesn't need the packaged modules being present. A.k.a run-time image based jlink. Fundamentally this patch adds an option to use `jlink` even though your JDK install might not come with the packaged modules (directory `jmods`). This is particularly useful to further reduce the size of a jlinked runtime. After the removal of the concept of a JRE, a common distribution mechanism is still the full JDK with all modules and packaged modules. However, packaged modules can incur an additional size tax. For example in a container scenario it could be useful to have a base JDK container including all modules, but without also delivering the packaged modules. This comes at a size advantage of `~25%`. Such a base JDK container could then be used to `jlink` application specific runtimes, further reducing the size of the application runtime image (App + JDK runtime; as a single image *or* separate bundles, depending on the app 
 being modularized).
>> 
>> The basic design of this approach is to add a jlink plugin for tracking non-class and non-resource files of a JDK install. I.e. files which aren't present in the jimage (`lib/modules`). This enables producing a `JRTArchive` class which has all the info of what constitutes the final jlinked runtime.
>> 
>> Basic usage example:
>> 
>> $ diff -u <(./bin/java --list-modules --limit-modules java.se) <(../linux-x86_64-server-release/images/jdk/bin/java --list-modules --limit-modules java.se)
>> $ diff -u <(./bin/java --list-modules --limit-modules jdk.jlink) <(../linux-x86_64-server-release/images/jdk/bin/java --list-modules --limit-modules jdk.jlink)
>> $ ls ../linux-x86_64-server-release/images/jdk/jmods
>> java.base.jmod            java.net.http.jmod       java.sql.rowset.jmod      jdk.crypto.ec.jmod         jdk.internal.opt.jmod                     jdk.jdi.jmod         jdk.management.agent.jmod  jdk.security.auth.jmod
>> java.compiler.jmod        java.prefs.jmod          java.transaction.xa.jmod  jdk.dynalink.jmod          jdk.internal.vm.ci.jmod                   jdk.jdwp.agent.jmod  jdk.management.jfr.jmod    jdk.security.jgss.jmod
>> java.datatransfer.jmod    java.rmi.jmod            java.xml.crypto.jmod      jdk.editpad.jmod           jdk.internal.vm.compiler.jmod             jdk.jfr.jmod         jdk.management.jmod        jdk.unsupported.desktop.jmod
>> java.desktop.jmod         java.scripting.jmod      java.xml.jmod             jdk.hotspot.agent.jmod     jdk.i...
>
> Severin Gehwolf has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix default description of keep-packaged-modules

I've read through all src changes. I think Sundar is looking at the code changes too.

The overall design seems solid. I know it took a long time to get there but this is nature of a feature like this. At this point I regret that there isn't a JEP. A JEP would have captured the motivation, outlined the design, the reasoning for the restrictions, and document the design choices/directions that have been prototyped. If there isn't a JEP then I suppose it can come later if the feature is progressed and there is discussion about making it the default rather than opt-in at build configure time.

As cleanup, I think we will need to bring some consistency to the terminology and phrasing in documentation, code and comments. Right now there is "run-time linking", "linkable run-time", "run-time linkable JDK image", "linkable jimage".

Also as cleanup, I think the code needs more comments. There is no JEP right now with an authoritive description of the feature so anyone maintaining this code will have to figure out a lot of details. It feels like there should be somehting that documents the effect of --enable-runtime-link-image, how the diffs are generated and saved, and how they are used by jlink. There is also a lot of new code in ImageFileCreator and JlinkTask that is asking for some method descriptions so that anyone touching this code can quickly understand what these methods are doing. I don't want to get into code style issues now but I think it would be helpful for future maintainers to avoid more of the overfly long lines if possible (some of them are 150, 160, 170+ and really hard to look at code side-by-side).

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PR Comment: https://git.openjdk.org/jdk/pull/14787#issuecomment-2151964298