deliver-assassin.rb

# JRuby
require 'java'

VirtualMachine = com.sun.tools.attach.VirtualMachine

vm = VirtualMachine.attach("4567")      # target JVM process ID
vm.loadAgent("/tmp/ThreadAssassin.jar") # path to JAR with Agent code.
vm.detach

SomeThreadedApp.java

public class SomeThreadedApp {
    public static void main(String[] args) {
        new Thread(new BoringRunner(), "bRunner1").start();
        new Thread(new BoringRunner(), "bRunner2").start();
        new Thread(new BoringRunner(), "bRunner3").start();
    }

    static class BoringRunner implements Runnable {
        public void run() {
            int count = 1;
            while (true) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException ie) {}
                System.out.println("Thread named " + Thread.currentThread().getName()
                                    + " and count is: " + count);
                count++;
            }
        }
    }
}

ThreadAssassin.java

import java.lang.instrument.Instrumentation;

public class ThreadAssassin {
    public static void agentmain(String agentArgs, Instrumentation inst) {
        ThreadGroup root = Thread.currentThread().getThreadGroup();
        while (root.getParent() != null) {
            root = root.getParent();
        }
        Thread[] allThreads = new Thread[root.activeCount()];
        root.enumerate(allThreads);

        System.out.println("The assassin takes aim!");

        for (Thread t : allThreads) {
            if (t.getName().startsWith("bRunner")) {
                System.out.println("Thread named: " + t.getName() + " is gonna die!");
                t.stop(); // I know this is deprecated and bad; just for effect.
            }
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {}
        }
    }
}