BellSoft Liberica JRE 21 used although BP_JVM_VERSION is set to 17

[OLibutzki]

Hi everyone,

I am using Spring Boot 3.2.0 with Java 17 and want to build an Image using Buildpacks.

Although BP_JVM_VERSION is set to 17, a BellSoft Liberica JRE 21 is used.

Here is the log:

[INFO]  > Pulling builder image 'docker.io/paketobuildpacks/builder-jammy-base:latest' 100%
[INFO]  > Pulled builder image 'paketobuildpacks/builder-jammy-base@sha256:228383e5ab6381b50cd59db7043a8f0ff3fb2fa552d070ba110ae0fbaf83e79b'
[INFO]  > Pulling run image 'docker.io/paketobuildpacks/run-jammy-base:latest' 100%
[INFO]  > Pulled run image 'paketobuildpacks/run-jammy-base@sha256:c8dcf96b50a5a1a9b418cb86c5e92866024b8affd87bb015b5b76ee5209b8576'
[INFO]  > Executing lifecycle version v0.18.2
[INFO]  > Using build cache volume 'pack-cache-e389a3329fd4.build'
[INFO]
[INFO]  > Running creator
[INFO]     [creator]     ===> ANALYZING
[INFO]     [creator]     Restoring data for SBOM from previous image
[INFO]     [creator]     ===> DETECTING
[INFO]     [creator]     7 of 26 buildpacks participating
[INFO]     [creator]     paketo-buildpacks/ca-certificates       3.6.6
[INFO]     [creator]     paketo-buildpacks/bellsoft-liberica     10.4.4
[INFO]     [creator]     paketo-buildpacks/syft                  1.41.1
[INFO]     [creator]     paketo-buildpacks/executable-jar        6.8.3
[INFO]     [creator]     paketo-buildpacks/dist-zip              5.6.8
[INFO]     [creator]     paketo-buildpacks/spring-boot           5.27.7
[INFO]     [creator]     paketo-buildpacks/environment-variables 4.5.6
[INFO]     [creator]     ===> RESTORING
[INFO]     [creator]     Restoring metadata for "paketo-buildpacks/ca-certificates:helper" from app image
[INFO]     [creator]     Restoring metadata for "paketo-buildpacks/bellsoft-liberica:java-security-properties" from app image
[INFO]     [creator]     Restoring metadata for "paketo-buildpacks/bellsoft-liberica:jre" from app image
[INFO]     [creator]     Restoring metadata for "paketo-buildpacks/bellsoft-liberica:helper" from app image
[INFO]     [creator]     Restoring metadata for "paketo-buildpacks/syft:syft" from cache
[INFO]     [creator]     Restoring metadata for "paketo-buildpacks/spring-boot:helper" from app image
[INFO]     [creator]     Restoring metadata for "paketo-buildpacks/spring-boot:spring-cloud-bindings" from app image
[INFO]     [creator]     Restoring metadata for "paketo-buildpacks/spring-boot:web-application-type" from app image
[INFO]     [creator]     Restoring metadata for "paketo-buildpacks/environment-variables:environment-variables" from app image
[INFO]     [creator]     Restoring data for "paketo-buildpacks/syft:syft" from cache
[INFO]     [creator]     Restoring data for SBOM from cache
[INFO]     [creator]     ===> BUILDING
[INFO]     [creator]
[INFO]     [creator]     Paketo Buildpack for CA Certificates 3.6.6
[INFO]     [creator]       https://github.com/paketo-buildpacks/ca-certificates
[INFO]     [creator]       Launch Helper: Reusing cached layer
[INFO]     [creator]
[INFO]     [creator]     Paketo Buildpack for BellSoft Liberica 10.4.4
[INFO]     [creator]       https://github.com/paketo-buildpacks/bellsoft-liberica
[INFO]     [creator]       Build Configuration:
[INFO]     [creator]         $BP_JVM_JLINK_ARGS           --no-man-pages --no-header-files --strip-debug --compress=1  configure custom link arguments (--output must be omitted)
[INFO]     [creator]         $BP_JVM_JLINK_ENABLED        false                                                        enables running jlink tool to generate custom JRE
[INFO]     [creator]         $BP_JVM_TYPE                 JRE                                                          the JVM type - JDK or JRE
[INFO]     [creator]         $BP_JVM_VERSION              17                                                           the Java version
[INFO]     [creator]       Launch Configuration:
[INFO]     [creator]         $BPL_DEBUG_ENABLED           false                                                        enables Java remote debugging support
[INFO]     [creator]         $BPL_DEBUG_PORT              8000                                                         configure the remote debugging port
[INFO]     [creator]         $BPL_DEBUG_SUSPEND           false                                                        configure whether to suspend execution until a debugger has attached
[INFO]     [creator]         $BPL_HEAP_DUMP_PATH                                                                       write heap dumps on error to this path
[INFO]     [creator]         $BPL_JAVA_NMT_ENABLED        true                                                         enables Java Native Memory Tracking (NMT)
[INFO]     [creator]         $BPL_JAVA_NMT_LEVEL          summary                                                      configure level of NMT, summary or detail
[INFO]     [creator]         $BPL_JFR_ARGS                                                                             configure custom Java Flight Recording (JFR) arguments
[INFO]     [creator]         $BPL_JFR_ENABLED             false                                                        enables Java Flight Recording (JFR)
[INFO]     [creator]         $BPL_JMX_ENABLED             false                                                        enables Java Management Extensions (JMX)
[INFO]     [creator]         $BPL_JMX_PORT                5000                                                         configure the JMX port
[INFO]     [creator]         $BPL_JVM_HEAD_ROOM           0                                                            the headroom in memory calculation
[INFO]     [creator]         $BPL_JVM_LOADED_CLASS_COUNT  35% of classes                                               the number of loaded classes in memory calculation
[INFO]     [creator]         $BPL_JVM_THREAD_COUNT        250                                                          the number of threads in memory calculation
[INFO]     [creator]         $JAVA_TOOL_OPTIONS                                                                        the JVM launch flags
[INFO]     [creator]         Using Java version 21 extracted from MANIFEST.MF
[INFO]     [creator]       BellSoft Liberica JRE 21.0.1: Reusing cached layer
[INFO]     [creator]       Launch Helper: Reusing cached layer
[INFO]     [creator]       Java Security Properties: Reusing cached layer

[OLibutzki]

Started the Maven build using Java 21. That caused that Build-Jdk-Spec: 21 is written into the MANIFEST.MF file. Buildpacks seems to read this property in order to determine the correct JDK-version.

I am not sure if it's a good strategy to derive the Runtime Java version from the build Java version.

[dmikusa]

Yes, that's correct.

If you build with Java 21, that generally means you need Java 21 to run because the class files will be written in the Java 21 format. You can compile with an older Java version and run on newer versions, but the other way around doesn't work without some fiddling.

Can you build your JAR with Java 21 and run it with Java 17? Does that work locally?

[OLibutzki]

If you build with Java 21, that generally means you need Java 21 to run because the class files will be written in the Java 21 format.

Afaik, that's not the case anymore. Starting with Java 9, it's possible to set the --release option to configure the Java release version. Therefore it's possible to build a Java 17 application with Java 21.

That's why I wonder if it's correct to derive the Java runtime version from the MANIFEST.MF. I assume that using the BP_JVM_VERSION would make more sense.

This (very old) issue might be interesting, too: https://issues.apache.org/jira/browse/MJAR-62

[dmikusa]

A big problem that we've seen is people compiling locally and then the wrong version being used in the container, which results in the infamous "class file is wrong version" error. So in that context, this trick of looking at the MANIFEST.MF file is useful for a lot of users.

I'm not sure I'd want to remove it, but I was under the impression that BP_JVM_VERSION has a higher precedence so if you're setting that env variable, I think it should pick the version you set. Sounds like that's not the case, so if you could open a bug in https://github.com/paketo-buildpacks/libjvm/issues we can take a look.

[OLibutzki]

Sounds like that's not the case, so if you could open a bug in https://github.com/paketo-buildpacks/libjvm/issues we can take a look.

Done: paketo-buildpacks/libjvm#350