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<flag> passes option to the JShell client JVM

-R<flag> passes option to the execution engine JVM

Local execution (--execution="local")

-J<flag> passes option to the only present JVM

-R<flag> does nothing

Answer 2

I still find the scope of explaining the usage of the flags used in both the attributes for the question to be answered completely, hence putting that in words here.

➜ -J flag is used to provide a runtime argument to the JShell which is similar to the way one provides while performing via an IDE under Run -> Configuration to specify arguments as -Dkey=value.

The usage of the attribute is documented and quite similar to ones illustrated for the flag -C just that the -J flags are instead the java command line options. For example, using the -XX:+PrintCommandLineFlags would detail out the runtime flags used by the current JVM.

So, the default values of command line flags used by your JShell instance(without setting any additional flag) could be as:-

But let's say you don't want to make use of the CompactStrings of Java9 in your JShell execution you can tell the JVM using the -J flag and -XX:-CompactStrings attribute to do so, as -

jshell -R-XX:+PrintCommandLineFlags -R-XX:-CompactStrings

would list out the following output:

➜ Similar java command line options/flags, when attached with and used to start JShell on a remote JVM, are linked to -R attribute of the JShell during remote execution.

jshell -R-XX:+PrintCommandLineFlags -R-XX:-CompactStrings

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Anatoly's answer has got some good amount of research attached to it and I would suggest that to be read to understand about control and execution engine of the JShell for local Vs remote execution.