This commit introduces the ability for the core Elasticsearch JAR to be
a multi-release JAR containing code that is compiled for JDK 8 and code
that is compiled for JDK 9. At runtime, a JDK 8 JVM will ignore the JDK
9 compiled classfiles, and a JDK 9 JVM will use the JDK 9 compiled
classfiles instead of the JDK 8 compiled classfiles. With this work, we
utilize the new JDK 9 API for obtaining the PID of the running JVM,
instead of relying on a hack.
For now, we want to keep IDEs on JDK 8 so when the build is in an IDE we
ignore the JDK 9 source set (as otherwise the IDE would give compilation
errors). However, with this change, running Gradle from the command-line
now requires JAVA_HOME and JAVA_9_HOME to be set. This will require
follow-up work in our CI infrastructure and our release builds to
accommodate this change.
Relates #28051
This is related to #27933. It introduces a jar named elasticsearch-core
in the lib directory. This commit moves the JarHell class from server to
elasticsearch-core. Additionally, PathUtils and some of Loggers are
moved as JarHell depends on them.
Alexander Reelsen
Jason Tedor
Tim Brooks