Can the Java 9 jshell be used to run code in another JVM?

Question

Java 9 has a read-eval-print loop for Java, called jshell. I've seen it work in it's basic mode, from the command line. Can it also be used in a remote process? In other words, can I connect to another Java process and enter code snippets to run within that runtime? This would be a nice way to change configuration state in an app server without having to write an admin tool with a UI.

Answer 1

Attaching JShell is a project that implements an extension to JShell for exactly this. It uses the Java Agent for communication with the target JVM.

I have not used it so I cannot say how well it works.

Observations after a quick inspection

• The read-me file looks professional.
• The code looks small and pretty simple, as if it hasn't been under development for a long time.
• There are no tickets in the bug tracker, which indicates that it hasn't been used much.

Example from the project read-me

• Start the target JVM with -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=XXXhostname:XXXport (update XXXhostname and XXXport as appropriate) and call new uk.org.cinquin.attaching_jshell.ExistingVMRemoteExecutionControl() from that JVM prior to using JShell.

• call JShell as follows: java -cp lib/attaching_jshell.jar jdk.internal.jshell.tool.JShellToolProvider --execution "attachToExistingVM:hostname(XXXhostname),port(XXXport)" using the same values of XXXhostname and XXXport as above

• Run code in the remote JVM like this:

  import uk.org.cinquin.attaching_jshell.ExistingVMRemoteExecutionControl;
  String s = ExistingVMRemoteExecutionControl.theGoodsForTesting