Java Daemon Thread

A daemon thread is a low-priority background thread that does not prevent the JVM from exiting when the program finishes execution

• Daemon threads run in the background to support user threads.
• The JVM exits automatically when all user (non-daemon) threads complete.
• They are created using the same Thread class but are marked as daemon using the setDaemon(true) method.
• The setDaemon(true) method must be called before the thread starts.
• Common examples include Garbage Collector and Finalizer Thread.

Syntax

thread.setDaemon(true);

Use Cases

1. Garbage Collection: The Garbage Collector (GC) in Java runs as a daemon thread.
2. Background Monitoring: Daemon threads can monitor the state of application components, resources, or connections.
3. Logging and Auditing Services: Daemon threads can be used to log background activities continuously.
4. Cleanup Operations: Daemon threads may periodically clear temporary files, release unused resources, or perform cache cleanup.
5. Scheduler or Timer Tasks: Background schedulers often use daemon threads to trigger tasks at fixed intervals.

Methods Used

• void setDaemon(boolean on): Marks a thread as daemon or user thread. Must be called before start().
• boolean isDaemon(): Checks whether a thread is daemon.

Creating a Daemon Thread

public class DaemonExample extends Thread {
    public void run() {
        if (Thread.currentThread().isDaemon()) {
            System.out.println("Daemon thread running...");
        } else {
            System.out.println("User thread running...");
        }
    }

    public static void main(String[] args) {
        DaemonExample t1 = new DaemonExample();
        DaemonExample t2 = new DaemonExample();

        t1.setDaemon(true);  // must be set before start()

        t1.start();
        t2.start();
    }
}

Daemon thread running...
User thread running...

Behavior of Daemon Thread

public class DaemonBehavior extends Thread {
    public void run() {
        while (true) {
            System.out.println("Daemon thread running...");
        }
    }

    public static void main(String[] args) {
        DaemonBehavior t = new DaemonBehavior();
        t.setDaemon(true);
        t.start();

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

Main (user) thread ends...

The JVM ends immediately after the main thread finishes, even though the daemon thread is still running.

Important Notes

• A thread inherits the daemon status of the thread that creates it.
• The setDaemon(true) method throws IllegalThreadStateException if called after start().
• Daemon threads should not be used for tasks requiring completion, such as writing to a file or updating a database.
• JVM terminates all daemon threads abruptly without performing cleanup operations.

Difference Between User Thread and Daemon Thread