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JEP 386: Alpine Linux Port

Summary​

Port the JDK to Alpine Linux, and to other Linux distributions that use musl as their primary C library, on both the x64 and AArch64 architectures,

Motivation​

Musl is an implementation, for Linux-based systems, of the standard library functionality described in the ISO C and POSIX standards. Several Linux distributions including Alpine Linux and OpenWrt are based on musl, while some others provide an optional musl package (e.g., Arch Linux).

The Alpine Linux distribution is widely adopted in cloud deployments, microservices, and container environments due to its small image size. A Docker base image for Alpine Linux, for example, is less than 6 MB. Enabling Java to run out-of-the-box in such settings will allow Tomcat, Jetty, Spring, and other popular frameworks to work in such environments natively.

By using jlink (JEP 282) to reduce the size of the Java runtime, a user will be able to create an even smaller image targeted to run a specific application. The set of modules required by an application can be determined via the jdeps command. For example, if a target application depends only on the java.base module then a Docker image with Alpine Linux and a Java runtime with just that module and the server VM fits in 38 MB.

The same motivation applies to embedded deployments, which also have size constraints.

Description​

This JEP intends to integrate the Portola Project upstream.

This port will not support the attach mechanism of the HotSpot Serviceability Agent.

To build a musl variant of the JDK on Alpine Linux, the following packages are required:

alpine-sdk alsa-lib alsa-lib-dev autoconf bash cups-dev cups-libs fontconfig fontconfig-dev freetype freetype-dev grep libx11 libx11-dev libxext libxext-dev libxrandr libxrandr-dev libxrender libxrender-dev libxt libxt-dev libxtst libxtst-dev linux-headers zip

Once these packages are installed, the JDK build process works as usual.

Musl ports for other architectures may be implemented in follow-up enhancements, if there is demand.

Alternatives​

  • Keep the musl port downstream in the Portola Project.

  • Use the glibc portability layer provided by the system, or build and bundle glibc with the OS image. This alternative has the drawback of increasing image footprint and adding a dependency which is not part of the base OS image. For a cloud deployment scenario, assuming the base Alpine Linux 3.11 musl image is 5.6 MB, the additional glibc layer is 26 MB, and the Java runtime with the java.base module and the server VM is 38 MB, the static footprint overhead of having the glibc portability layer in the image is 30%.

Testing​

  • Since the port affects shared code, we will run JCK and JTreg tests on glibc-based Linux distributions (x64, AArch64, and ARM32), Windows, and Mac before integration to ensure the absence of regressions.

  • We will verify the quality of the musl port by running a relevant subset of JTreg tests on Alpine Linux. We will make sure that a downport of Portola to a current production JDK release passes the JCK on Alpine Linux. We will run sanity testing on OpenWrt.

  • We will define a new subset of JTreg tests suitable for musl, in a vm.musl test group.