1. Overview

Daemon threads in Java are background threads that support other tasks, like system garbage collection, logging, system monitoring, and more. They run with lower priority and the Java Virtual Machine (JVM) terminates them after all user threads finish.  Many JVM threads are daemon threads by default.

In this short article, we’ll explore the main uses of daemon threads, and compare them to user threads. Additionally, we’ll demonstrate how to programmatically create, run, and verify if a thread is a daemon thread.

2. Creating Daemon Threads

To create daemon threads, all we need to do is to call setDaemon() of the Thread API. In this example, we’ll use the NewThread class which extends the Thread class:

NewThread daemonThread = new NewThread();
daemonThread.setDaemon(true);
daemonThread.start();

Any thread inherits the daemon status of the thread that created it. Since the main thread is a user thread, any thread created inside the main method is by default a user thread.

The method setDaemon() can only be called after the Thread object has been created and the thread hasn’t been started. An attempt to call setDaemon() while a thread is running throws an IllegalThreadStateException:

@Test(expected = IllegalThreadStateException.class)
public void whenSetDaemonWhileRunning_thenIllegalThreadStateException() {
 NewThread daemonThread = new NewThread();
 daemonThread.start();
 daemonThread.setDaemon(true);
}

Additionally, we can use the isDaemon() method to check if a thread is a daemon thread:

@Test
public void whenCallIsDaemon_thenCorrect() {
 NewThread daemonThread = new NewThread();
 daemonThread.setDaemon(true);
 daemonThread.start();
 assertTrue(daemonThread.isDaemon());

 NewThread userThread = new NewThread();
 userThread.start();
 assertFalse(userThread.isDaemon());
}

3. Differences Between Daemon and User Threads

So, we learned that Java offers two types of platform threads: user threads and daemon threads. While user threads are high-priority threads, daemon threads are low-priority threads whose only role is to provide services to user threads.

Let’s highlight other differences between daemon and user threads:

Since daemon threads are meant to serve user threads and are only needed while they are running, they won’t prevent the JVM from exiting once all user threads have finished their execution.

Infinite loops typically exist in daemon threads and won’t cause problems. This is happening because any code, including the finally blocks, won’t be executed once all user threads have finished their execution. As a result, daemon threads aren’t  recommended for I/O tasks.

However, there are exceptions to this rule. Poorly designed code in daemon threads can prevent the JVM from exiting. For example, calling Thread.join() on a running daemon thread can block the application’s shutdown.

4. Conclusion

In this quick tutorial, we’ve seen what daemon threads are and what they can be used for in a few practical scenarios. Following that, we compared daemon threads to user threads, focusing on their differences.