How to check the number of currently running threads in Java?

Question

I\'m looking for a way to see the number of currently running threads

1. Through Windows first
2. Programmatically

Answer 1

Code snippet:

import java.util.Set;
import java.lang.Thread.State;
public class ActiveThreads {
    public static void main(String args[]) throws Exception{
        for ( int i=0; i< 5; i++){
            Thread t = new Thread(new MyThread());
            t.setName("MyThread:"+i);
            t.start();
        }
        int threadCount = 0;
        Set threadSet = Thread.getAllStackTraces().keySet();
        for ( Thread t : threadSet){
            if ( t.getThreadGroup() == Thread.currentThread().getThreadGroup() && 
                t.getState() == Thread.State.RUNNABLE){
                System.out.println("Thread :"+t+":"+"state:"+t.getState());
                ++threadCount;
            }
        }
        System.out.println("Thread count started by Main thread:"+threadCount);
    }
}

class MyThread implements Runnable{
    public void run(){
        try{
            Thread.sleep(2000);
        }catch(Exception err){
            err.printStackTrace();
        }
    }
}

output:

Thread :Thread[main,5,main]:state:RUNNABLE
Thread count started by Main thread:1

Explanation:

Thread.getAllStackTraces().keySet() provides you list of all Threads, which have been started by both Program and System. In absence of ThreadeGroup condition, you will get System thread count if they are active.

Reference Handler, Signal Dispatcher,Attach Listener and Finalizer