RFR: 8311302: Allow for jlinking a custom runtime without packaged modules being present [v18]

[Mandy Chung]

On Tue, 27 Feb 2024 15:23:09 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:
> 
>   Only show runtime image suffix for JDK modules

This PR is about having jlink to produce a run-time image without needing packaged modules (jmod files).   The motivation is that jmod files are huge in particular when native debug symbols are included.

The previous proposal to support jlink to create a run-time image without jmod files got too complicated and how plugins should revert/apply the transformation and how the users know what transformations are done in the resulting image.  The primary motivation for this is to produce OpenJDK run-time image without packaged modules (right now we called it _linkable JDK image_ but need to ponder on the name more) and jlink from such _linkable JDK image_ is capable of creating custom run-time image.   Per our offline discussion, the revised proposal is to add an "option" in the JDK build that can produce the linkable JDK image that does not include the packaged modules. 

The build option can be a configure option or a make target that would need advice from the build team.   Such build option would create a run-time image adjacent to `<build>/images/jdk` (for example `<build>/images/linkable-jdk`).   No proposal to change the default, i.e. the linkable image is not built by default.   This PR is to place the infrastructure and support in place.

@magicus is a make target more appropriate for this?

-------------

PR Comment: https://git.openjdk.org/jdk/pull/14787#issuecomment-1986252706