Turning an ExecutorService to daemon in Java

Question

I am using an ExecutoreService in Java 1.6, started simply by

ExecutorService pool = Executors.newFixedThreadPool(THREADS). 

When my main

Answer 1

You can use Guava's ThreadFactoryBuilder. I didn't want to add the dependency and I wanted the functionality from Executors.DefaultThreadFactory, so I used composition:

class DaemonThreadFactory implements ThreadFactory {
    final ThreadFactory delegate;

    DaemonThreadFactory() {
        this(Executors.defaultThreadFactory());
    }

    DaemonThreadFactory(ThreadFactory delegate) {
        this.delegate = delegate;
    }

    @Override
    public Thread newThread(Runnable r) {
        Thread thread = delegate.newThread(r);
        thread.setDaemon(true);
        return thread;
    }
}

Answer 2

If you only want to use it in one place, then you can inline the java.util.concurrent.ThreadFactory implementation, e.g. for a pool with 4 threads you would write (example shown as a lambda assuming Java 1.8 or newer):

ExecutorService pool = Executors.newFixedThreadPool(4,
        (Runnable r) -> {
            Thread t = new Thread(r);
            t.setDaemon(true);
            return t;
        }
);

But I usually want all of my Thread factories to produce daemon threads, so I add a utility class as follows:

import java.util.concurrent.ThreadFactory;

public class DaemonThreadFactory implements ThreadFactory {

    public final static ThreadFactory instance = 
                    new DaemonThreadFactory();

    @Override
    public Thread newThread(Runnable r) {
        Thread t = new Thread(r);
        t.setDaemon(true);
        return t;
    }
}

That allows me to easily pass DaemonThreadFactory.instance to the ExecutorService, e.g.

ExecutorService pool = Executors.newFixedThreadPool(
    4, DaemonThreadFactory.instance
);

or use it to easily start a daemon Thread from a Runnable, e.g.

DaemonThreadFactory.instance.newThread(
    () -> { doSomething(); }
).start();