Counting Threads

[J Mangual]

How does one count the number of threads in a program? I want to modify the method below so that when a new socket connects (and a new thread is opened) the count increases by 1 and when the thread ends it decreases by 1. Alternatively, I would like to store all the active threads in an array. Is there a standard way of doing this?

import java.net.*;
import java.io.*;

public class FileServerM {
	public static void main(String[] args) throws IOException {
		// create socket
		ServerSocket servsock = null;
		boolean listening = true;
		try{
			servsock = new ServerSocket(13267);
		} catch (IOException e){
			System.err.println("Could not listen on port: 4444");
			System.exit(-1);
		}
		
		// keep count of how many threads are open
		FileServerThread[] fileServerThreads = new FileServerThread[10]; 
		int numberOfThreads;
		
		while (listening){
			new FileServerThread(servsock.accept()).start();
		}
		
		servsock.close();
	}
}

[Campbell Ritchie]

Sounds a more complicated question than we usually get here on beginners'. Moving.

[Steve Luke]

You can use ThreadGroups to do this. Your best bet would be to make a new ThreadGroup that you would add all the Threads of interest in:

ThreadGroup fileServerThreadGroup = new ThreadGroup("File Server");
while (listenening {
    new FileServerThread(serversock.accept(), fileServerThreadGroup).start();
}

//.... inside the FileServerThread ...
public class FileServerThread extends Thread {
  //...
  public FileServerThread(Socket socket, ThreadGroup group) {
      //Call Thread's CTOR
      super(group, null, "Some name to give...");
      //...
  }
}

Then you can get the count from the group like:

    int activeSockets = fileServerThreadGroup.activeCount();