Why is the Java 11 base Docker image so large? (openjdk:11-jre-slim)
Why is
alpinenot used any more as a base image for Java 11 slim images?
That's because, sadly, there is no official stable OpenJDK 11 build for Alpine currently.
Alpine uses musl libc, as opposed to the standard glibc used by most Linuxes out there, which means that a JVM must be compatible with musl libc for supporting vanilla Alpine. The musl OpenJDK port is being developed under OpenJDK's Portola project.
The current status is summarized on the OpenJDK 11 page:
The Alpine Linux build previously available on this page was removed as of JDK 11 GA. It’s not production-ready because it hasn’t been tested thoroughly enough to be considered a GA build. Please use the early-access JDK 12 Alpine Linux build in its place.
The only stable OpenJDK versions for Alpine currently are 7 and 8, provided by the IcedTea project.
However - if you're willing to consider other than the official OpenJDK, Azul's Zulu OpenJDK offers a compelling alternative:
- It supports Java 11 on Alpine musl (version 11.0.2 as of the time of writing);
- It is a certified OpenJDK build, verified using the OpenJDK TCK compliance suite;
- It is free, open source and docker ready (Dockerhub).
For support availability and roadmap, see Azul support roadmap.
Update, 3/6/19: As of yesterday, openjdk11 is available in Alpine repositories! It could be grabbed on Alpine using:
apk --no-cache add openjdk11
The package is based on the jdk11u OpenJDK branch plus ported fixes from project Portola, introduced with the following PR. Kudos and huge thanks to the Alpine team.
Why is the unstable sid version used for LTS Java images?
That's a fair question / request. There's actually an open ticket for providing Java 11 on a stable Debian release:
https://github.com/docker-library/openjdk/issues/237
Update, 26/12/18: The issue has been resolved, and now the OpenJDK 11 slim image is based on stretch-backports OpenJDK 11 which was recently made available (PR link).
Why is the slim/headless/JRE package for OpenJDK 11 so large compared to the similar OpenJDK 8 package? What is this modules file which brings 135 MB in OpenJDK 11?
Java 9 introduced the module system, which is a new and improved approach for grouping packages and resources, compared to jar files. This article from Oracle gives a very detailed introduction to this feature:
https://www.oracle.com/corporate/features/understanding-java-9-modules.html
The modules file bundles all modules shipped with the JRE. The complete list of modules could be printed with java --list-modules. modules is indeed a very large file, and as commented, it contains all standard modules, and it is therefore quite bloated.
One thing to note however is that it replaces rt.jar and tools.jar which became deprecated, among other things, so when accounting for the size of modules when comparing to pre-9 OpenJDK builds, the sizes of rt.jar and tools.jar should be subtracted (they should take up some 80MB combined).
as for 07.2019 https://adoptopenjdk.net/ has official Alpine support for Java 11:
https://github.com/AdoptOpenJDK/openjdk-docker/blob/master/11/jdk/alpine/
https://hub.docker.com/r/adoptopenjdk/openjdk11
However, modules (jmods, jlink) still shall be considered when one assembles minimal application.
Note: slim images don't contain some modules (like java.sql) - they are excluded explicitly (https://github.com/AdoptOpenJDK/openjdk-docker/blob/21b8393b9c23f94d6921a56cce27b026537c6ca2/11/jdk/alpine/slim-java.sh#L233)
If you are considering only Official Images and your target achievement is to use the smaller JRE image available, I would suggest you to look at the official OpenJDK image openjdk:11-jre-slim-buster which is just 69.2 MB.