[java] How to get the number of threads in a Java process

How can I see the number of threads in a Java process?

java.lang.Thread.activeCount()

It will return the number of active threads in the current thread's thread group.

docs: http://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#activeCount()

Get number of threads using jstack

jstack <PID> | grep 'java.lang.Thread.State' | wc -l

The result of the above code is quite different from top -H -p <PID> or ps -o nlwp <PID> because jstack gets only threads from created by the application.

In other words, jstack will not get GC threads

Using Linux Top command

top -H -p (process id)

you could get process id of one program by this method :

ps aux | grep (your program name)

for example :

ps aux | grep user.py

ManagementFactory.getThreadMXBean().getThreadCount() doesn't limit itself to thread groups as Thread.activeCount() does.

    public class MainClass {

        public static void main(String args[]) {

          Thread t = Thread.currentThread();
          t.setName("My Thread");

          t.setPriority(1);

          System.out.println("current thread: " + t);

          int active = Thread.activeCount();
          System.out.println("currently active threads: " + active);
          Thread all[] = new Thread[active];
          Thread.enumerate(all);

          for (int i = 0; i < active; i++) {
             System.out.println(i + ": " + all[i]);
          }
       }
   }

There is a static method on the Thread Class that will return the number of active threads controlled by the JVM:

Thread.activeCount()

Returns the number of active threads in the current thread's thread group.

Additionally, external debuggers should list all active threads (and allow you to suspend any number of them) if you wish to monitor them in real-time.

I have written a program to iterate all Threads created and printing getState() of each Thread

import java.util.Set;

public class ThreadStatus {
    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<Thread> threadSet = Thread.getAllStackTraces().keySet();
        for ( Thread t : threadSet){
            if ( t.getThreadGroup() == Thread.currentThread().getThreadGroup()){
                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:

java ThreadStatus

Thread :Thread[MyThread:0,5,main]:state:TIMED_WAITING
Thread :Thread[main,5,main]:state:RUNNABLE
Thread :Thread[MyThread:1,5,main]:state:TIMED_WAITING
Thread :Thread[MyThread:4,5,main]:state:TIMED_WAITING
Thread :Thread[MyThread:2,5,main]:state:TIMED_WAITING
Thread :Thread[MyThread:3,5,main]:state:TIMED_WAITING
Thread count started by Main thread:6

If you remove below condition

if ( t.getThreadGroup() == Thread.currentThread().getThreadGroup())

You will get below threads in output too, which have been started by system.

Reference Handler, Signal Dispatcher,Attach Listener and Finalizer.

Generic solution that doesn't require a GUI like jconsole (doesn't work on remote terminals), ps works for non-java processes, doesn't require a JVM installed.

ps -o nlwp <pid>