Added a follow-up commit that improves observability when the JRE release file is missing or unreadable:

• DetermineJavaVersion now returns an error instead of silently returning 17 when the release file is absent
• BaseJRE.Finalize() and Tomcat.Supply() both log a **WARNING** when falling back to Java 17

This means a broken or incomplete JRE installation will now produce a visible warning in buildpack output rather than silently proceeding with Java 17 assumptions.

a quick ai review. (with my own context/skilss)
gave me

🔴 Behavioral Change: tomcat.go — version selection now runs unconditionally when JAVA_HOME is set, even if version detection fails
File: src/java/containers/tomcat.go
Before: If DetermineJavaVersion returned an error (e.g., missing/unreadable release file), the entire Tomcat version selection block was skipped. The code fell through to the manifest default version.
After: On error, javaMajorVersion is silently defaulted to 17, and Tomcat version selection proceeds. The fallback to manifest default (dep.Version == "") is now only reached when JAVA_HOME is not set at all.
Concrete failure scenario: An app using IBM JRE (typically Java 8) where the release file is absent or unreadable. Previously: manifest default Tomcat version used. Now: Tomcat 10.x selected (Java 17 default → javaMajorVersion >= 11 → versionPattern = "10.x"). Tomcat 10.x is Jakarta EE 9+ and is incompatible with Java EE 8 apps. The app will fail at runtime, not at staging.
The javaMajorVersion < 11 guard on line 89 (Tomcat 10.x requires Java 11+) is also bypassed in this scenario because the defaulted version is 17.

🔴 Bug: BaseJRE.Supply() now calls WriteEnvFile, AddBinDependencyLink, and LinkDirectoryInDepDir for ALL JREs — previously only OpenJDK did this
Files: src/java/jres/base_jre.go (new), vs old ibm.go, zulu.go, sapmachine.go, oracle.go, graalvm.go
The old per-JRE implementations of Supply() for IBM, Zulu, SapMachine, Oracle, and GraalVM did not call:

• ctx.Stager.WriteEnvFile("JAVA_HOME", javaHome) — writes JAVA_HOME to the deps env dir for subsequent buildpacks
• ctx.Stager.AddBinDependencyLink(javaBin, "java") — symlinks java binary into the shared bin/ dir (puts it on PATH for subsequent buildpacks)
• ctx.Stager.LinkDirectoryInDepDir(libDir, "lib") — links the JRE lib/ dir into deps (adds native libs to LD_LIBRARY_PATH)
  Only openjdk.go had these three calls. BaseJRE.Supply() now applies them to all six JREs.
  Impact: This is a behavioral expansion, not just a refactor. For multi-buildpack staging scenarios, Zulu/IBM/Oracle/SapMachine/GraalVM JREs will now expose JAVA_HOME, java on PATH, and native libs to subsequent buildpacks — they did not before. This is likely the correct behavior (it was probably an oversight in the original per-JRE implementations), but it is an undocumented behavioral change that could affect existing multi-buildpack setups if a subsequent buildpack was relying on JAVA_HOME not being set.

🟡 DetermineJavaVersion no longer handles missing release file silently
Previously returned (17, nil) for missing files; now returns (0, error). All callers in this PR handle it, but the safe default is now scattered — future callers could get 0 instead of a safe default if they forget to handle the error.

🟡 Finalize() may use empty b.version
If Finalize() is called without a prior Supply() (multi-buildpack staging model), b.version will be "". Not a regression from the old code, but the consolidation makes it more visible.

🟡 BaseJRE.Finalize() — WriteJavaOpts with base opts now applies to ALL JREs
File: src/java/jres/base_jre.go
The old openjdk.go Finalize() wrote base JAVA_OPTS (-Djava.io.tmpdir=$TMPDIR -XX:ActiveProcessorCount=$(nproc)). The old IBM, Zulu, SapMachine, Oracle, and GraalVM Finalize() implementations did not write these base opts.
BaseJRE.Finalize() now writes these opts for all JREs. This is likely correct behavior, but it is an undocumented behavioral expansion. For IBM JRE specifically, -XX:ActiveProcessorCount is a HotSpot flag and may not be recognized by IBM J9 JVM, potentially causing a startup warning or error depending on the IBM JRE version.

i' am okay with the things below.
🟢 IBM JRE default Java version changed
Old IBM JRE defaulted to Java 8 on version detection failure; BaseJRE now defaults to 17 for all JREs. Low severity since IBM JRE is rarely used, but it's a silent behavioral change.

i will still do a full review later on

Thanks for the review! @ramonskie can you have another view?

A general note on approach: we tried to match the Ruby 4.x buildpack behaviour as the baseline, only deviating where there are concrete issues or PR comments that justify it.

1. tomcat.go — version detection failure (commit 2c5754b)

The Ruby buildpack has no Java-version-aware Tomcat selection — it always defers to the manifest default (9.+). I think on detection failure the safe fallback is DefaultVersion("tomcat") rather than assuming Java 17 → Tomcat 10.x (see #1155). An explicitly configured version (e.g. JBP_CONFIG_TOMCAT: '{tomcat: {version: "9.+"}}') is still always honoured.

2. WriteEnvFile / AddBinDependencyLink / LinkDirectoryInDepDir for all JREs

I assume this is fine as-is. Ruby never set JAVA_HOME at all — setting it for all JREs is intentional new behaviour in the Go migration (see #1151). I don't think a per-JRE flag is needed here, but open to arguments if there's a specific case where it causes problems.

3. -XX:ActiveProcessorCount for all JREs (commit bf92b62)

In Ruby open_jdk_like_jre.rb adds it for HotSpot only; ibm_jre_initializer.rb never does. Moved it out of baseOpts and into extraFinalizeOpts on each HotSpot JRE (OpenJDK, Oracle, SapMachine, Zulu, GraalVM). IBM keeps its existing opts unchanged.

4. DetermineJavaVersion returns 0 on error (commit 40e4490)

Returns 17 on all error paths so callers that don't check the error still get a safe value.

Also added some notes in the migration md so things that do change do not go unnoticed.

@kiril-keranov can you have a look please for a review? thanks

@tnikolova82 thanks!