Java thread equivalent of Python thread daemon mode

Question

In python I can set a thread to be a daemon, meaning if parent dies, the child thread automatically dies along with it.

Is there an equivalent in Java?

Currently I am starting a thread like this in Java, but the underlying child thread does not die and hang even if main thread exits

         executor = Executors.newSingleThreadExecutor();
         executor.submit(() -> {
             while (true) {
                 //Stopwatch stopwatch = Stopwatch.createStarted();

                 String threadName = Thread.currentThread().getName();
                 System.out.println("Hello " + threadName);
                 try {
                     Thread.sleep(1*1000);
                 } catch (InterruptedException e) {
                     break;
                 }   

             }       
         });

In Java, the only difference between a daemon thread and a non-daemon thread is that the former won't keep the JVM alive. A daemon thread won't die because the thread that started it has died (unless, of course, the parent thread dying means there's no more non-daemon threads running). — Slaw
– Slaw, Commented Apr 3, 2019 at 8:31
In addition to @Slaw's comment; there is also no way to focibly kill a Thread in Java so such a thing could not be implemented. — Boris the Spider
– Boris the Spider, Commented Apr 3, 2019 at 8:35

Maciej Dobrowolski · Accepted Answer · 2019-04-03 08:59:05Z

1

When you're interacting with bare Thread you can use:

Thread thread = new Thread(new MyJob());
thread.setDaemon(true);

thread.start();

In your example, there's ExecutorService that needs to be provided with ThreadFactory which should do the similar job - like this:

Executors.newSingleThreadExecutor(new ThreadFactory() {
    @Override
    public Thread newThread(Runnable r) {
        Thread thread = new Thread(r);
        thread.setDaemon(true);

        return thread;
    }
});

I would also recommend using Guavas ThreadFactoryBuilder:

Executors.newSingleThreadExecutor(
        new ThreadFactoryBuilder()
                .setDaemon(true)
                .build()
);

It eases setting the most common thread properties and allows for chaining multiple thread factories

update

As Slaw and Boris the Spider rightfully noticed - you have mentioned the behavior that would cause killing child-thread when parent-thread dies. There's nothing like that either in Python or Java. Daemon threads will be killed when all other non-daemon threads exited.

edited Apr 3, 2019 at 8:59

answered Apr 3, 2019 at 8:29

Maciej Dobrowolski

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7 Comments

Boris the Spider Over a year ago

These are not the same thing. The OP says in Python that if the parent dies, the child is killed.

Maciej Dobrowolski Over a year ago

@BoristheSpider I don't think there's something like that in Python :) Thread's parent is the process spawning it

Boris the Spider Over a year ago

That is exactly my point. This answer is not only wrong it is misleading. Your comment about the thread's parents is of course incorrect.

ealeon Over a year ago

it looks like my submit() isnt being called anymore after changing to using new ThreadFactory(). am i missing something?

Maciej Dobrowolski Over a year ago

@ealeon In a thread factory, you should pass Runnable to a Thread constructor (like that: Thread thread = new Thread(r);) I have updated the code

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