Set Output Stream of specific JShell

I have a code :

public class App {
    public static void main(String[] args) {
        System.out.println("Hello World from main!");
        JShell shell = JShell.builder().build();
        shell.eval("System.out.println("Hello World from JShell!");");

    }
}

Now I want that I can set the output stream for the JShell only and not the normal code.

I tried :

shell.eval("System.setOut(new Printer());");
shell.eval("System.out.println("Hello World!");");

But does not work!

My Printer class:

class Printer extends PrintStream{
    public Printer() {
        super(System.out);
    }

    @Override
    public void print(String s) {
        super.print("Message: - " + s);
    }
}

Answer

First, you need to make JShell use the “local” executionEngine, so that you have access to your Printer:

JShell shell = JShell.builder().executionEngine("local").build();

This basically means “the same JVM”.

Second, remember to import the Printer class, or use its fully qualified name, because Printer is probably not in the the same package as where JShell code is running (a package called REPL from my experiments).

It seems like your Printer class is not public, so you should make it public as well.