Turning an ExecutorService to daemon in Java

I would use Guava's ThreadFactoryBuilder class.

ExecutorService threadPool = Executors.newFixedThreadPool(THREADS, new ThreadFactoryBuilder().setDaemon(true).build());

If you're not already using Guava, I'd go with a ThreadFactory subclass like described at the top of Pshemo's answer

There already is a built-in functionality for creating an ExecutorService that terminates all threads after a certain period of inactivity: You can create a ThreadPoolExecutor, pass it the desired timing information, and then call allowCoreThreadTimeout(true) on this executor service:

/**
 * Creates an executor service with a fixed pool size, that will time 
 * out after a certain period of inactivity.
 * 
 * @param poolSize The core- and maximum pool size
 * @param keepAliveTime The keep alive time
 * @param timeUnit The time unit
 * @return The executor service
 */
public static ExecutorService createFixedTimeoutExecutorService(
    int poolSize, long keepAliveTime, TimeUnit timeUnit)
{
    ThreadPoolExecutor e = 
        new ThreadPoolExecutor(poolSize, poolSize,
            keepAliveTime, timeUnit, new LinkedBlockingQueue<Runnable>());
    e.allowCoreThreadTimeOut(true);
    return e;
}

EDIT Referring to the remarks in the comments: Note that this thread pool executor will not automatically shut down when the application exits. The executor will continue to run after the application exits, but no longer than the keepAliveTime. If, depending on the precise application requirements, the keepAliveTime has to be longer than a few seconds, the solution in the answer by Pshemo may be more appropriate: When the threads are set to be daemon threads, then they will end immediately when the application exits.

Probably simplest and preferred solution is in Marco13's answer so don't get fooled by vote difference (this answer is few years older) or acceptance mark (it just means that this solution was appropriate for OP circumstances, not that it is best in general).

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You can use ThreadFactory to set threads inside Executor to daemons. This will affect executor service in a way that it will also become daemon thread so it (and threads handled by it) will stop if there will be no other non-daemon thread. Here is simple example:

ExecutorService exec = Executors.newFixedThreadPool(4,
        new ThreadFactory() {
            public Thread newThread(Runnable r) {
                Thread t = Executors.defaultThreadFactory().newThread(r);
                t.setDaemon(true);
                return t;
            }
        });

exec.execute(YourTaskNowWillBeDaemon);

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But if you want to get executor which will let its task finish, and at the same time will automatically call its shutdown() method when application is complete, you may want to wrap your executor with Guava's MoreExecutors.getExitingExecutorService.

ExecutorService exec = MoreExecutors.getExitingExecutorService(
        (ThreadPoolExecutor) Executors.newFixedThreadPool(4), 
        100_000, TimeUnit.DAYS//period after which executor will be automatically closed
                             //I assume that 100_000 days is enough to simulate infinity
);
//exec.execute(YourTask);
exec.execute(() -> {
    for (int i = 0; i < 3; i++) {
        System.out.println("daemon");
        try {
            TimeUnit.SECONDS.sleep(1);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
});

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();