Hermetic Java (static image packaging/formatting) investigation and proposal
Brian Goetz
brian.goetz at oracle.com
Thu Feb 2 17:15:52 UTC 2023
Thanks for sharing this information!
As far as I understand it, this is a _packaging mechanism_ to take a
combination of JDK, application classfiles and resources, and native
libraries, and combine them into a single executable, and the benefits
of this include "one file to distribute" and "can't run the app on the
wrong JDK". Further, the system classloader has been modified to load
classes out of the image rather than from the file system.
My question is about your relationship to the "closed world" assumption
(and the optimizations that can derive from it); I am unsure of whether
you make any closed-world assumptions in your current approach? Is
there any reason why classes cannot be loaded dynamically,
`invokedynamic` sites can't be linked dynamically, etc? Secondarily,
does your current implementation perform any optimizations related to
faster startup and warmup?
My assumption is that the answer is "no restrictions on dynamism, no
specific optimizations for startup/warmup, it's purely a packaging
mechanism", combined with a reminder that the sorts of optimizations
that have been explored elsewhere (Native Image, CraC, SnapStart) could
equally well be combined with your packaging approach.
Do I have it right?
Cheers,
-Brian
On 2/2/2023 11:13 AM, Jiangli Zhou wrote:
> (Resending in plain text formatting)
>
> Hi,
>
> During the last one and a half years, Google has done some extensive
> research on linux-x64 with Java static image, as project Hermetic Java
> [1]. We would like to share our experiences/results with the community
> and present our approach for discussion under the Leyden project. We
> hope to contribute the work to OpenJDK through project Leyden, via the
> JEP [2] process as needed.
>
> With Hermetic Java, our main goal is to create a single executable
> image including the Java runtime environment, Java application and the
> dependencies. This addresses some real-world Java deployment issues
> and challenges that we have encountered over the years. We believe it
> fits very well with the overall goal of project Leyden in the
> following aspects:
>
> - Provide a build-time created static image derived from an
> application and JDK; Image executes as a standalone program.
> - Satisfy closed-world constraints.
> - Is built on top of OpenJDK and can utilize existing OpenJDK
> components including the Hotspot VM, runtime JIT compiler (C1, C2),
> CDS, etc.
>
> Our focus has been on the image packaging and formatting part. This
> works roughly as follows:
>
> 1. The executable image (see slide #10 of [1]) consists of three
> sections: the ELF executable section (see slide #14), the JDK runtime
> section (see slide #20, #21) and the JAR section (see slide #22).
>
> The ELF section is at the beginning of the image and contains the Java
> launcher executable, which allows the image to work as a native
> executable. The JDK runtime section contains the JDK lib/modules image
> starting at a page-aligned file offset. This section can include other
> data that requires special alignment, such as the CDS archive. The JAR
> section holds the Java application classes, dependent library classes,
> and resources. JDK runtime resource files, such as java.security and
> java.policy are also packaged within the JAR section.
>
> 2. The Java launcher executable is statically linked with Hotspot/JDK
> natives and application JNI natives (see slide #15 - #18).
>
> For static native library support, we enhance and complete existing
> OpenJDK work [3, 4, 5]. It provides a flexible solution for loading
> built-in (static) native libraries while still allowing dynamically
> loading shared JNI libraries (if desired).
>
> 3. With a single executable image, we define the image file path as
> the java.home (see slide #23). A JavaHome class is used to provide
> uniform APIs for accessing JDK resources in traditional and Hermetic
> Java (single image) execution modes.
>
> Hermetic Java is an accumulation of wisdom that Google obtained from
> real-world production deployments over many years (years before the
> current project research/experiments). We would love to gather
> feedback from community members. Any input and feedback are welcome
> and appreciated!
>
> We are happy to provide additional information and answer questions
> (open to discussions in any form).
>
> [1] http://cr.openjdk.java.net/~jiangli/hermetic_java.pdf
>
> [2] http://cr.openjdk.java.net/~mr/jep/jep-2.0-02.html
>
> [3] https://bugs.openjdk.org/browse/JDK-8005716
>
> [4] https://bugs.openjdk.org/browse/JDK-8136556
>
> [5] https://bugs.openjdk.org/browse/JDK-8232748
>
> Best regards,
>
> Jiangli
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