Questions about the Hermetic Java project

David Holmes david.holmes at oracle.com
Thu May 29 05:36:03 UTC 2025 

Hi Jiangli,

<trimming>

On 29/05/2025 3:27 am, Jiangli Zhou wrote:
>>> This is unfortunately quite complex, and I have started a discussion with Alan if it is possible to update the JNI spec so that both static and dynamic entry points can have the form "JNI_OnLoad_<library-name>". Ideally, I'd like to see us push for this with as much effort as possible. If we got this in place, static builds would be much easier, and the changes required for Hermetic Java even smaller.
>>
>> Thumbs up! That seems to be a good direction. Currently in the leyden
>> branch, it first looks up the unique
>> JNI_OnLoad<_lib_name>|Agent_OnLoad<_lib_name> etc for built-in
>> libraries, then search for the dynamic libraries using the
>> conventional naming when necessary. e.g.:
>>
>> https://github.com/openjdk/leyden/commit/a5c886d2e85a0ff0c3712a5488ae61d8c9d7ba1a
>> https://github.com/openjdk/leyden/commit/1da8e3240e0bd27366d19f2e7dde386e46015135
>>
>> When spec supports JNI_OnLoad_<library-name> and etc. for dynamic
>> libraries, we may still need to support the conventional naming
>> without the <_lib_name> part for existing libraries out there.
> 
> Resuming the conversation on using
> JNI_OnLoad_L|JNI_OnUnload_L|Agent_OnLoad_L|Agent_OnUnLoad_L|Agent_OnAttach_L
> for dynamically linked JNI & agent libraries. It is related to
> JDK-8350450 [1]: Compile object files once for both static and dynamic
> builds. We have recently enabled building & tier1 testing for
> static-jdk with release binary in GHA on linux-x64. The debug build
> however cannot be enabled in GHA due to space/resource limit (please
> see more details in JDK-8350450). So it's a good time to pick up
> JDK-8350450 related work. Based on discussions with Magnus in
> JDK-8350450 bug comments and separate emails from last year, I'll
> extract the runtime changes from the leyden/hermetic-java-runtime
> branch [2] for supporting JNI_OnLoad_L (and etc) for dynamically
> linked JNI/agent libraries. The work has been broadly tested in our
> internal prototype on JDK 11 and newer versions (linux-x64).
> 
> Regarding the spec part, Ron, Alan, Magnus and myself had several
> discussions last year during hermetic Java meetings. The general
> understanding was that using JNI_OnLoad_L (and etc) for dynamically
> linked JDK native libraries requires no JNI/JVMTI spec change. I
> wonder if the following languages should be relaxed a bit to address
> potential questions. Any thoughts?

I don't know exactly what was discussed, but I don't see how you can not 
update the JNI (and other) specifications to do what you want to do, if 
that involves invoking JNI_OnLoad_L in the dynamic case instead of 
JNI_OnLoad. ??

David
-----


>  From JNI spec [3]:
> 
> - JNI_OnLoad/JNI_OnUnload
>    Optional function defined by dynamically linked libraries.
> 
>    LINKAGE:
>    Exported from dynamically linked native libraries that contain
> native method implementations.
> 
> - JNI_OnLoad_L
>    Mandatory function that must be defined by statically linked libraries .
> 
>    LINKAGE:
>    Exported from statically linked native libraries that contain native
> method implementations.
> 
> - JNI_OnUnload_L
>    Optional function defined by statically linked libraries.
> 
>  From JVMTI spec [4]:
> 
> An agent L whose image has been combined with the VM is defined as
> statically linked if and only if the agent exports a function called
> Agent_OnLoad_L.
> 
> [1]: https://bugs.openjdk.org/browse/JDK-8350450
> [2]: https://github.com/openjdk/leyden/tree/hermetic-java-runtime
> [3]: https://docs.oracle.com/en/java/javase/21/docs/specs/jni/
> [4]: https://docs.oracle.com/en/java/javase/24/docs/specs/jvmti.html
> 
> Best,
> Jiangli
> 
>>
>>>
>>> And finally, on top of all of this, is the question of widening the platform support. To support linux/gcc with objcopy is trivial, but the question about Windows still remain. I have two possible ways forward, one is to check if there is alternative tooling to use (the prime candidate is the clang-ldd), and the other is to try to "fake" a partial linking by concatenating all source code before compiling. This is not ideal, though, for many reasons, and I am not keen on implementing it, not even for testing. And at this point, I have not had time to investigate any of these options much further, since I have been focusing on 1) above.
>>>
>>> A third option is of course to just say that due to toolchain limitations, static linking is not available on Windows.
>>
>> Thank you for taking this on! Potentially we could consider taking the
>> objcopy to localizing hotspot symbols on unix-like platforms, based on
>> https://github.com/openjdk/jdk/pull/17456 discussions. Additional
>> testing is still needed to verify the solution.
>>
>>>
>>> My recommendation is that you keep on working to resolve the (much more thorny) issues of resource access in Hermetic Java in your branch, where you have a prototype static build that works for you. In the meantime, I will make sure that there will be a functioning, stable and robust way of creating static builds in the mainline, that can be regularly tested and not bit-rot, like the static build hacks that has gone in before.
>>
>> Most of the JDK resources are now supported as hermetic jimage
>> (lib/modules) bundled in the
>> https://github.com/openjdk/leyden/tree/hermetic-java-runtime branch.
>> The remaining sound.properties, ct.sym and .jfc files can be handled
>> later. Overally, that part of the work has confirmed the hermetic
>> jimage bundled solution is robust and helps resolve some of the
>> difficult start-up sequence issues observed when the hermetic resource
>> was implemented using JAR file based solution.
>>
>> It might be a good idea to follow up on the static linking discussion
>> in tomorrow's zoom meeting (hope you'll be able to join tomorrow).
>>
>> Thanks!
>>
>> Jiangli
>>>
>>> /Magnus
>>>
>>>
>>>
>>> Thanks!
>>> Jiangli
>>>
>>> On Thu, Feb 15, 2024 at 12:01 PM Jiangli Zhou <jianglizhou at google.com> wrote:
>>>>
>>>> On Wed, Feb 14, 2024 at 5:07 PM Jiangli Zhou <jianglizhou at google.com> wrote:
>>>>>
>>>>> Hi Magnus,
>>>>>
>>>>> Thanks for looking into this from the build perspective.
>>>>>
>>>>> On Wed, Feb 14, 2024 at 1:00 AM Magnus Ihse Bursie
>>>>> <magnus.ihse.bursie at oracle.com> wrote:
>>>>>>
>>>>>> First some background for build-dev: I have spent some time looking at
>>>>>> the build implications of the Hermetic Java effort, which is part of
>>>>>> Project Leyden. A high-level overview is available here:
>>>>>> https://cr.openjdk.org/~jiangli/hermetic_java.pdf and the current source
>>>>>> code is here: https://github.com/openjdk/leyden/tree/hermetic-java-runtime.
>>>>>
>>>>> Some additional hermetic Java related references that are also useful:
>>>>>
>>>>> - https://bugs.openjdk.org/browse/JDK-8303796 is an umbrella bug that
>>>>> links to the issues for resolving static linking issues so far
>>>>> - https://github.com/openjdk/jdk21/pull/26 is the enhancement for
>>>>> building the complete set of static libraries in JDK/VM, particularly
>>>>> including libjvm.a
>>>>>
>>>>>>
>>>>>> Hermetic Java faces several challenges, but the part that is relevant
>>>>>> for the build system is the ability to create static libraries. We've
>>>>>> had this functionality (in three different ways...) for some time, but
>>>>>> it is rather badly implemented.
>>>>>>
>>>>>> As a result of my investigations, I have a bunch of questions. :-) I
>>>>>> have gotten some answers in private discussion, but for the sake of
>>>>>> transparency I will repeat them here, to foster an open dialogue.
>>>>>>
>>>>>> 1. Am I correct in understanding that the ultimate goal of this exercise
>>>>>> is to be able to have jmods which include static libraries (*.a) of the
>>>>>> native code which the module uses, and that the user can then run a
>>>>>> special jlink command to have this linked into a single executable
>>>>>> binary (which also bundles the *.class files and any additional
>>>>>> resources needed)?
>>>>>>
>>>>>> 2. If so, is the idea to create special kinds of static jmods, like
>>>>>> java.base-static.jmod, that contains *.a files instead of lib*.so files?
>>>>>> Or is the idea that the normal jmod should contain both?
>>>>>>
>>>>>> 3. Linking .o and .a files into an executable is a formidable task. Is
>>>>>> the intention to have jlink call a system-provided ld, or to bundle ld
>>>>>> with jlink, or to reimplement this functionality in Java?
>>>>>
>>>>> I have a similar view as Alan responded in your other email thread.
>>>>> Things are still in the early stage for the general solution.
>>>>>
>>>>> In the https://github.com/openjdk/leyden/tree/hermetic-java-runtime
>>>>> branch, when configuring JDK with --with-static-java=yes, the JDK
>>>>> binary contains the following extra artifacts:
>>>>>
>>>>> - static-libs/*.a: The complete set of JDK/VM static libraries
>>>>> - jdk/bin/javastatic: A demo Java launcher fully statically linked
>>>>> with the selected JDK .a libraries (e.g. it currently statically link
>>>>> with the headless) and libjvm.a. It's the standard Java launcher
>>>>> without additional work for hermetic Java.
>>>>>
>>>>> In our prototype for hermetic Java, we build the hermetic executable
>>>>> image (a single image) from the following input (see description on
>>>>> singlejar packaging tool in
>>>>> https://cr.openjdk.org/~jiangli/hermetic_java.pdf):
>>>>>
>>>>> - A customized launcher (with additional work for hermetic) executable
>>>>> fully statically linked with JDK/VM static libraries (.a files),
>>>>> application natives and dependencies (e.g. in .a static libraries)
>>>>> - JDK lib/modules, JDK resource files
>>>>> - Application classes and resource files
>>>>>
>>>>> Including a JDK library .a into the corresponding .jmod would require
>>>>> extracting the .a for linking with the executable. In some systems
>>>>> that may cause memory overhead due to the extracted copy of the .a
>>>>> files. I think we should consider the memory overhead issue.
>>>>>
>>>>> One possibility (as Alan described in his response) is for jlink to
>>>>> invoke the ld on the build system. jlink could pass the needed JDK
>>>>> static libraries and libjvm.a (provided as part of the JDK binary) to
>>>>> ld based on the modules required for the application.
>>>>>
>>>>
>>>> I gave a bit more thoughts on this one. For jlink to trigger ld, it
>>>> would need to know the complete linker options and inputs. Those
>>>> include options and inputs related to the application part as well. In
>>>> some usages, it might be easier to handle native linking separately
>>>> and pass the linker output, the executable to jlink directly. Maybe we
>>>> could consider supporting different modes for various usages
>>>> requirements, from static libraries and native linking point of view:
>>>>
>>>> Mode #1
>>>> Support .jmod packaged natives static libraries, for both JDK/VM .a
>>>> and application natives and dependencies. If the inputs to jlink
>>>> include .jmods, jlink can extract the .a libraries and pass the
>>>> information to ld to link the executable.
>>>>
>>>> Mode #2
>>>> Support separate .a as jlink input. Jlink could pass the path
>>>> information to the .a libraries and other linker options to ld to
>>>> create the executable.
>>>>
>>>> For both mode #1 and #2, jlink would then use the linker output
>>>> executable to create the final hermetic image.
>>>>
>>>> Mode #3
>>>> Support a fully linked executable as a jlink input. When a linked
>>>> executable is given to jlink, it can process it directly with other
>>>> JDK data/files to create the final image, without native linking step.
>>>>
>>>> Any other thoughts and considerations?
>>>>
>>>> Best,
>>>> Jiangli
>>>>
>>>>>>
>>>>>> 4. Is the intention is to allow users to create their own jmods with
>>>>>> static libraries, and have these linked in as well? This seems to be the
>>>>>> case.
>>>>>
>>>>> An alternative with less memory overhead could be using application
>>>>> modular JAR and separate .a as the input for jlink.
>>>>>
>>>>>> If that is so, then there will always be the risk for name
>>>>>> collisions, and we can only minimize the risk by making sure any global
>>>>>> names are as unique as possible.
>>>>>
>>>>> Part of the current effort includes resolving the discovered symbol
>>>>> collision issues with static linking. Will respond to your other email
>>>>> on the symbol issue separately later.
>>>>>
>>>>>>
>>>>>> 5. The original implementation of static builds in the JDK, created for
>>>>>> the Mobile project, used a configure flag, --enable-static-builds, to
>>>>>> change the entire behavior of the build system to only produce *.a files
>>>>>> instead of lib*.so. In contrast, the current system is using a special
>>>>>> target instead.
>>>>>
>>>>> I think we would need both configure flag and special target for the
>>>>> static builds.
>>>>>
>>>>>> In my eyes, this is a much worse solution. Apart from
>>>>>> the conceptual principle (if the build should generate static or dynamic
>>>>>> libraries is definitely a property of what a "configuration" means),
>>>>>> this makes it much harder to implement efficiently, since we cannot make
>>>>>> changes in NativeCompilation.gmk, where they are needed.
>>>>>
>>>>> For the potential objcopy work to resolve symbol issues, we can add
>>>>> that conditionally in NativeCompilation.gmk if STATIC_LIBS is true. We
>>>>> have an internal prototype (not included in
>>>>> https://github.com/openjdk/leyden/tree/hermetic-java-runtime yet) done
>>>>> by one of colleagues for localizing symbols in libfreetype using
>>>>> objcopy.
>>>>>
>>>>>>
>>>>>> That was not as much a question as a statement. 🙂 But here is the
>>>>>> question: Do you think it would be reasonable to restore the old
>>>>>> behavior but with the new methods, so that we don't use special targets,
>>>>>> but instead tells configure to generate static libraries? I'm thinking
>>>>>> we should have a flag like "--with-library-type=" that can have values
>>>>>> "dynamic" (which is default), "static" or "both".
>>>>>
>>>>> If we want to also build a fully statically linked launcher, maybe
>>>>> --with-static-java? Being able to configure either dynamic, static or
>>>>> both as you suggested also seems to be a good idea.
>>>>>
>>>>>> I am not sure if "both" are needed, but if we want to bundle both lib*.so and *.a files
>>>>>> into a single jmod file (see question 2 above), then it definitely is.
>>>>>> In general, the cost of producing two kinds of libraries are quite
>>>>>> small, compared to the cost of compiling the source code to object files.
>>>>>
>>>>> Completely agree. It would be good to avoid recompiling the .o file
>>>>> for static and dynamic builds. As proposed in
>>>>> https://bugs.openjdk.org/browse/JDK-8303796:
>>>>>
>>>>> It's beneficial to be able to build both .so and .a from the same set
>>>>> of .o files. That would involve some changes to handle the dynamic JDK
>>>>> and static JDK difference at runtime, instead of relying on the
>>>>> STATIC_BUILD macro.
>>>>>
>>>>>>
>>>>>> Finally, I have looked at how to manipulate symbol visibility. There
>>>>>> seems many ways forward, so I feel confident that we can find a good
>>>>>> solution.
>>>>>>
>>>>>> One way forward is to use objcopy to manipulate symbol status
>>>>>> (global/local). There is an option --localize-symbol in objcopy, that
>>>>>> has been available in objcopy since at least 2.15, which was released
>>>>>> 2004, so it should be safe to use. But ideally we should avoid using
>>>>>> objcopy and do this as part of the linking process. This should be
>>>>>> possible to do, given that we make changes in NativeCompilation.gmk --
>>>>>> see question 5 above.
>>>>>>
>>>>>> As a fallback, it is also possible to rename symbols, either piecewise
>>>>>> or wholesale, using objcopy. There are many ways to do this, using
>>>>>> --prefix-symbols, --redefine-sym or --redefine-syms (note the -s, this
>>>>>> takes a file with a list of symbols). Thus we can always introduce a
>>>>>> "post factum namespace" by renaming symbols.
>>>>>
>>>>> Renaming or redefining the symbol at build time could cause confusions
>>>>> with debugging. That's a concern raised in
>>>>> https://github.com/openjdk/jdk/pull/17456 discussions.
>>>>>
>>>>> Additionally, redefining symbols using tools like objcopy may not
>>>>> handle member names referenced in string literals. For example, in
>>>>> https://github.com/openjdk/jdk/pull/17456 additional changes are
>>>>> needed in assembling and SA to reflect the symbol change.
>>>>>
>>>>>>
>>>>>> So in the end, I think it will be fully possible to produce .a files
>>>>>> that only has global symbols for the functions that are part of the API
>>>>>> exposed by that library, and have all other symbols local, and make this
>>>>>> is in a way that is consistent with the rest of the build system.
>>>>>>
>>>>>> Finally, a note on Hotspot. Due to debugging reasons, we export
>>>>>> basically all symbols in hotspot as global. This is not reasonable to do
>>>>>> for a static build. The effect of not exporting those symbols will be
>>>>>> that SA will not function to 100%. On the other hand, I have no idea if
>>>>>> SA works at all with a static build. Have you tested this? Is this part
>>>>>> of the plan to support, or will it be officially dropped for Hermetic Java?
>>>>>
>>>>> We have done some testing with jtreg SA related tests for the fully
>>>>> statically linked `javastatic`.
>>>>>
>>>>> If we use objcopy to localize symbols in hotspot, it's not yet clear
>>>>> what's the impact on SA. We could do some tests. The other question
>>>>> that I raised is the supported gcc versions (for partial linking)
>>>>> related to the solution.
>>>>>
>>>>> Best,
>>>>> Jiangli
>>>>>
>>>>>>
>>>>>> /Magnus
>>>>>>

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