1. Introduction to Threads in Java

In Java, threads are used to perform tasks concurrently. There are two types of threads:

• User Threads: These are the threads that perform application tasks. The JVM will continue running as long as at least one user thread is alive.
• Daemon Threads: These are background service threads that support user threads. The JVM will exit as soon as all user threads are finished, regardless of whether daemon threads are still running.

2. What is a Daemon Thread?

A Daemon Thread in Java is a thread that runs in the background and does not prevent the JVM from exiting.

Examples of daemon threads:

• Garbage Collector
• Finalizer
• Attach Listener
• Timer thread (sometimes)

These threads are typically used to provide services in the background for other threads.

3. Characteristics of Daemon Threads

Property	Description
Lifecycle	Dies automatically when all user threads die
Priority	Usually low
JVM Exit	JVM exits even if daemon threads are running
Use case	Performing background tasks (like logging, cleanup, etc.)
Thread type	Must be explicitly marked as daemon before starting the thread

4. How to Create a Daemon Thread in Java

To make a thread a daemon, use the method:

thread.setDaemon(true);

Important: You must call setDaemon(true) before calling start() on the thread.

5. Example: Creating a Simple Daemon Thread

public class DaemonThreadExample {
    public static void main(String[] args) {
        Thread daemonThread = new Thread(() -> {
            while (true) {
                System.out.println("Daemon thread is running...");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    System.out.println("Daemon thread interrupted");
                }
            }
        });

        daemonThread.setDaemon(true); // Set before start
        daemonThread.start();

        System.out.println("Main thread ends here.");
    }
}

Output:

You may only see the daemon thread run a few times before the JVM exits because the main thread (user thread) ends quickly.

6. Daemon vs User Thread – Comparison

Feature	User Thread	Daemon Thread
JVM Lifecycle	JVM waits for them	JVM does not wait
Purpose	Main application logic	Background service tasks
Shutdown behavior	JVM waits till it’s done	JVM terminates it abruptly
Default daemon?	No	No

7. Inheritance of Daemon Property

If thread A creates thread B:
B is a daemon if and only if A is a daemon at the time of B’s creation unless explicitly changed.

public class InheritDaemon {
    public static void main(String[] args) {
        Thread parent = new Thread(() -> {
            Thread child = new Thread(() -> {
                System.out.println("Child daemon: " + Thread.currentThread().isDaemon());
            });
            child.start();
        });

        parent.setDaemon(true);
        parent.start();
    }
}

Output:

Child daemon: true

8. When NOT to Use Daemon Threads

Avoid daemon threads when:

• The thread must complete its task before shutdown (e.g., writing to a file, network request).
• The task has side effects that should not be lost.
• It is managing resources that need proper cleanup.

9. Daemon Thread Termination Behavior

If the JVM shuts down while daemon threads are running:

• Daemon threads are abruptly terminated.
• Finally blocks, shutdown hooks, and try-with-resources may not run.
• You can use Shutdown Hooks to handle proper cleanup.

Runtime.getRuntime().addShutdownHook(new Thread(() -> {
    System.out.println("Shutdown Hook called!");
}));

10. Check if Thread is Daemon

You can check daemon status using:

System.out.println(Thread.currentThread().isDaemon());