While exploring Jenkins 2.60.3 on Oracle Java 9 build 181 (pre-release), it reports illegal access exceptions and warns that a future release of Java 9 will forbid illegal access.
This was from a Windows agent using JDK 9 that was started from a batch file. The batch file included the line java -jar slave.jar -jnlpUrl http://jenkins.markwaite.net:8080/computer/cb-pc/slave-agent.jnlp -secret my-elided-secret. The same message does not appear when the Windows agent is running JDK 8, whether the master is running JDK 8 or JDK 9.
Stack trace shows:
INFO: Remote identity confirmed: 23:d0:c4:ac:00:ef:27:66:df:6c:b7:dd:ba:05:6f:6e
Sep 04, 2017 6:59:21 AM hudson.remoting.jnlp.Main$CuiListener status
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by hudson.remoting.RemoteClassLoader (file:/C:/J/slave.jar) to method java.lang.ClassLoader.getClassLoadingLock(java.lang.String)
WARNING: Please consider reporting this to the maintainers of hudson.remoting.RemoteClassLoader
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release