The java version checker requires being written with java 7 APIs.
In order to use java 8 apis in other launcher utilities, this commit
moves the java version checker back to its own jar.
JDK 9 has removed JVM options that were valid in JDK 8 (e.g., GC logging
flags) and replaced them with new flags that are not available in JDK
8. This means that a single JVM options file can no longer apply to JDK
8 and JDK 9, complicating development, complicating our packaging story,
and complicating operations. This commit extends the JVM options syntax
to specify the range of versions the option applies to. If the running
JVM matches the range of versions, the flag will be used to start the
JVM otherwise the flag will be ignored.
We implement this parser in Java for simplicity, and with this we start
our first step towards a Java launcher.
Relates #27675
Today when users start Elasticsearch with their Java configuration
pointing to a pre-Java 8 install, they encounter a cryptic message:
Exception in thread "main" java.lang.UnsupportedClassVersionError:
org/elasticsearch/bootstrap/Elasticsearch : Unsupported major.minor
version 52.0
They often think that they have Java 8 installed but if their JAVA_HOME
or other configuration is causing them to start with a pre-Java 8
install, this error message does not help them.
We introduce a Java version checker that runs on Java 6 as part of the
startup scripts. If the Java version is pre-Java 8, we can display a
helpful error message to the user informing them of the Java version
that the runtime was started with. Otherwise, Elasticsearch starts as it
does today.
Ryan Ernst
Jason Tedor