Get a list of all threads currently running in Java

Question

Is there any way I can get a list of all running threads in the current JVM (including the threads not started by my class)?

Is it also possible to get the Thread and Class objects of all threads in the list?

I want to be able to do this through code.

Answer 1

To get an iterable set:

Set<Thread> threadSet = Thread.getAllStackTraces().keySet();

Performance: 0 ms for 12 threads (Azul JVM 16.0.1, Windows 10, Ryzen 5600X).

Answer 2

Get a handle to the root ThreadGroup, like this:

ThreadGroup rootGroup = Thread.currentThread().getThreadGroup();
ThreadGroup parentGroup;
while ((parentGroup = rootGroup.getParent()) != null) {
    rootGroup = parentGroup;
}

Now, call the enumerate() function on the root group repeatedly. The second argument lets you get all threads, recursively:

Thread[] threads = new Thread[rootGroup.activeCount()];
while (rootGroup.enumerate(threads, true ) == threads.length) {
    threads = new Thread[threads.length * 2];
}

Note how we call enumerate() repeatedly until the array is large enough to contain all entries.

Answer 3

Yes, take a look at getting a list of threads. Lots of examples on that page.

That's to do it programmatically. If you just want a list on Linux at least you can just use this command:

kill -3 processid

and the VM will do a thread dump to stdout.

Answer 4

You can get a lot of information about threads from the ThreadMXBean.

Call the static ManagementFactory.getThreadMXBean() method to get a reference to the MBean.

Answer 5

Have you taken a look at jconsole?

This will list all threads running for a particular Java process.

You can start jconsole from the JDK bin folder.

You can also get a full stack trace for all threads by hitting Ctrl+Break in Windows or by sending kill pid --QUIT in Linux.

Answer 6

You can try something like this:

Thread.getAllStackTraces().keySet().forEach((t) -> System.out.println(t.getName() + "\nIs Daemon " + t.isDaemon() + "\nIs Alive " + t.isAlive()));

and you can obviously get more thread characteristic if you need.

Answer 7

Apache Commons users can use ThreadUtils. The current implementation uses the walk the thread group approach previously outlined.

for (Thread t : ThreadUtils.getAllThreads()) {
      System.out.println(t.getName() + ", " + t.isDaemon());
}

Answer 8

In Groovy you can call private methods

// Get a snapshot of the list of all threads 
Thread[] threads = Thread.getThreads()

In Java, you can invoke that method using reflection provided that security manager allows it.

Answer 9

Code snippet to get list of threads started by main thread:

import java.util.Set;

public class ThreadSet {
    public static void main(String args[]) throws Exception{
        Thread.currentThread().setName("ThreadSet");
        for ( int i=0; i< 3; i++){
            Thread t = new Thread(new MyThread());
            t.setName("MyThread:"+i);
            t.start();
        }
        Set<Thread> threadSet = Thread.getAllStackTraces().keySet();
        for ( Thread t : threadSet){
            if ( t.getThreadGroup() == Thread.currentThread().getThreadGroup()){
                System.out.println("Thread :"+t+":"+"state:"+t.getState());
            }
        }
    }
}

class MyThread implements Runnable{
    public void run(){
        try{
            Thread.sleep(5000);
        }catch(Exception err){
            err.printStackTrace();
        }
    }
}

output:

Thread :Thread[MyThread:2,5,main]:state:TIMED_WAITING
Thread :Thread[MyThread:0,5,main]:state:TIMED_WAITING
Thread :Thread[MyThread:1,5,main]:state:TIMED_WAITING
Thread :Thread[ThreadSet,5,main]:state:RUNNABLE

If you need all threads including system threads, which have not been started by your program, remove below condition.

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

Now output:

Thread :Thread[MyThread:2,5,main]:state:TIMED_WAITING
Thread :Thread[Reference Handler,10,system]:state:WAITING
Thread :Thread[MyThread:1,5,main]:state:TIMED_WAITING
Thread :Thread[ThreadSet,5,main]:state:RUNNABLE
Thread :Thread[MyThread:0,5,main]:state:TIMED_WAITING
Thread :Thread[Finalizer,8,system]:state:WAITING
Thread :Thread[Signal Dispatcher,9,system]:state:RUNNABLE
Thread :Thread[Attach Listener,5,system]:state:RUNNABLE

Answer 10

To get a list of threads and their full states using the terminal, you can use the command below:

jstack -l <PID>

Which <PID> is the id of process running on your computer. To get the process id of your java process you can simply run the jps command.

Also, you can analyze your thread dump that produced by jstack in TDAs (Thread Dump Analyzer) such fastthread or spotify thread analyzer tool.

Answer 11

    public static void main(String[] args) {


        // Walk up all the way to the root thread group
        ThreadGroup rootGroup = Thread.currentThread().getThreadGroup();
        ThreadGroup parent;
        while ((parent = rootGroup.getParent()) != null) {
            rootGroup = parent;
        }

        listThreads(rootGroup, "");
    }


    // List all threads and recursively list all subgroup
    public static void listThreads(ThreadGroup group, String indent) {
        System.out.println(indent + "Group[" + group.getName() + 
                ":" + group.getClass()+"]");
        int nt = group.activeCount();
        Thread[] threads = new Thread[nt*2 + 10]; //nt is not accurate
        nt = group.enumerate(threads, false);

        // List every thread in the group
        for (int i=0; i<nt; i++) {
            Thread t = threads[i];
            System.out.println(indent + "  Thread[" + t.getName() 
                    + ":" + t.getClass() + "]");
        }

        // Recursively list all subgroups
        int ng = group.activeGroupCount();
        ThreadGroup[] groups = new ThreadGroup[ng*2 + 10];
        ng = group.enumerate(groups, false);

        for (int i=0; i<ng; i++) {
            listThreads(groups[i], indent + "  ");
        }
    }

Answer 12

In the java console, hit Ctrl-Break. It will list all threads plus some information about the heap. This won't give you access to the objects of course. But it can be very helpful for debugging anyway.

Answer 13

You can use getAllThreadIds that Returns all live thread IDs. Some threads included in the returned array may have been terminated when this method returns.

ManagementFactory.getThreadMXBean().getAllThreadIds()