What is the exact meaning / purpose of the J and R flags in jshell?

Question

From the help information:

-J              Pass  directly to the runtime system.
                      Use one -J for each runtime fl         


        

Answer 1

As I understand it, JShell has 3 main 'places' to execute code:

• In the current process (see DirectExecutionControl)

• In the same JVM as the JShell client (see LocalExecutionControl)

• On the remote agent (see JdiDefaultExecutionControl)

Using jshell tool, we have no current process before launch, so we have only two options - use one JVM (locally), or use two JVMs - one for JShell client (locally) and another for the execution engine (possibly remotely).

The interesting thing is, JShell always launch two JVMs by default, as the hard-coded --execution key is "failover:0(jdi:hostname(" + loopback + ")),1(jdi:launch(true)), 2(jdi)" (see JShell class source code).

Closer to the point. I've made a couple of experiments with -verbose option and checked JVM options in runtime with ManagementFactory.getRuntimeMXBean().getInputArguments().

• jshell -J-verbose command

  Printed -verbose output in the console.

  No -verbose option in the input arguments: [-agentlib:jdwp=transport=dt_socket,address=127.0.0.1:63305]

• jshell -R-verbose command

  No -verbose output in the console.

  Printed -verbose option in the input arguments: [-agentlib:jdwp=transport=dt_socket,address=127.0.0.1:63339, -verbose]

• jshell --execution="local" -J-verbose command

  Printed -verbose output in the console.

  Printed -verbose option in the input arguments: [-Dapplication.home=C:\Program Files\Java\jdk-9, -Xms8m, -verbose, -Djdk.module.main=jdk.jshell]

• jshell --execution="local" -R-verbose

  No -verbose output in the console.

  No -verbose option in the input arguments: [-Dapplication.home=C:\Program Files\Java\jdk-9, -Xms8m, -Djdk.module.main=jdk.jshell]

TL;DR

Remote execution (default case, execution over JDI)

-J passes option to the JShell client JVM

-R passes option to the execution engine JVM

Local execution (--execution="local")

-J passes option to the only present JVM

-R does nothing