[go-migration] Compatibility issue with existing apps due to JRE relocation in the Go-based Java Buildpack

Issue で言及されている buildpack の staging フローと /profile.d/0_java.sh エントリポイントから始め、次に新しい JAVA_HOME がどこで設定されるかを追跡します。.java-buildpack/open_jdk_jre/bin/java をハードコードしたパスで staging を再現し、両方のレガシー JRE パスが正規の場所に解決される一方で、新しい JAVA_HOME も引き続き利用可能であることを確認します。

Describe the bug
With the new Go-based Java Buildpack, the location of the JRE has changed:
for example, from /home/vcap/app/.java-buildpack/open_jdk_jre to the new location /home/vcap/deps/0/jre/jre-17.0.15/.
There are applications that invoke Java via a hardcoded relative path inside the droplet: .java-buildpack/open_jdk_jre/bin/java so those hardcoded paths no longer exist, which breaks apps and CF tasks that rely on that path. For example, an app used to execute CF Task to execute e.g. the DB schema (so the java is currently being invoked via the full path .java-buildpack/open_jdk_jre/bin/java). Or usages like JAVA_HOME=$PWD/.java-buildpack/open_jdk_jre JAVA_OPTS="-Djava.io.tmpdir=$TMPDIR

The new Go-based buildpack has a way to introduce $JAVA_HOME which will do the job but only for new apps (the existing apps pointing to /home/vcap/app/.java-buildpack/open_jdk_jre will fail).

Reproduction steps

• Use an app/script that calls Java using the relative path inside the droplet, e.g.:
  .java-buildpack/open_jdk_jre/bin/java -Xms123M org.springframework.boot.loader.launch.JarLauncher
• Stage the app using the current or the Go-migration branch of the buildpack.
• Run the app or run a CF task that invokes the hardcoded path.
• Observe that the app fails with No such file or directory

Expected behavior
Existing apps/CF Tasks could continue to run without modification, even if the buildpack internal JRE layout changes.

Suggested solution
Introduce compatibility shims during staging:

export JAVA_HOME=<the-new-java-location> (available in /profile.d/0_java.sh)
mkdir -p /home/vcap/app/.java-buildpack
ln -s $JAVA_HOME /home/vcap/app/.java-buildpack/open_jdk_jre
ln -s $JAVA_HOME /home/vcap/app/.java-buildpack/sap_machine_jre

• This preserves .java-buildpack/open_jdk_jre and .java-buildpack/sap_machine_jre as a symlink to the actual JRE location.
• $JAVA_HOME points to the canonical new location.
• Apps with hard-coded paths will continue working.