This commit modifies the build to require JDK 9 for
compilation. Henceforth, we will compile with a JDK 9 compiler targeting
JDK 8 as the class file format. Optionally, RUNTIME_JAVA_HOME can be set
as the runtime JDK used for running tests. To enable this change, we
separate the meaning of the compiler Java home versus the runtime Java
home. If the runtime Java home is not set (via RUNTIME_JAVA_HOME) then
we fallback to using JAVA_HOME as the runtime Java home. This enables:
- developers only have to set one Java home (JAVA_HOME)
- developers can set an optional Java home (RUNTIME_JAVA_HOME) to test
on the minimum supported runtime
- we can test compiling with JDK 9 running on JDK 8 and compiling with
JDK 9 running on JDK 9 in CI
0a79555a12