daemon thread

1️⃣ What is a Daemon Thread?

• A background thread that performs auxiliary tasks.
• JVM automatically terminates all daemon threads once all non-daemon threads finish.
• Example: Garbage Collector (GC), logging, automatic backup.

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2️⃣ What is a Non-Daemon Thread?

• A regular thread that handles main application tasks.
• JVM keeps running until all non-daemon threads are finished.
• Example: Handling user requests, running main applications.

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3️⃣ Comparison Table

Feature Daemon Thread Non-Daemon Thread

Purpose	Background tasks	Main application tasks
JVM Termination	Stops when all non-daemon threads exit	JVM waits until it finishes
Examples	GC, logging, monitoring	User input handling, main application

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4️⃣ Code Example for Comparison

class MyDaemonThread extends Thread {
    public void run() {
        while (true) { // Infinite loop (daemon thread)
            System.out.println("Daemon thread running...");
            try { Thread.sleep(1000); } catch (InterruptedException e) {}
        }
    }
}

class MyNonDaemonThread extends Thread {
    public void run() {
        System.out.println("Non-Daemon thread started.");
        try { Thread.sleep(3000); } catch (InterruptedException e) {}
        System.out.println("Non-Daemon thread finished.");
    }
}

public class Main {
    public static void main(String[] args) {
        MyDaemonThread daemonThread = new MyDaemonThread();
        daemonThread.setDaemon(true); // ✅ Set as daemon thread
        daemonThread.start();

        MyNonDaemonThread nonDaemonThread = new MyNonDaemonThread();
        nonDaemonThread.start(); // Start non-daemon thread

        try { Thread.sleep(4000); } catch (InterruptedException e) {}
        System.out.println("Main thread exiting...");
    }
}

🛠 Expected Output

Daemon thread running...
Non-Daemon thread started.
Daemon thread running...
Daemon thread running...
Non-Daemon thread finished.
Main thread exiting...

✅ nonDaemonThread runs until completion.
✅ When the main thread exits, daemonThread also stops automatically.

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5️⃣ How to Set a Daemon Thread in Java

📌 Daemon thread must be set before calling start(). Otherwise, it throws IllegalThreadStateException.

✅ Ways to Set a Daemon Thread

1. Set inside the constructor of a Thread subclass

   class MyThread extends Thread {
       public MyThread() {
           setDaemon(true); // ✅ Set daemon in constructor
       }
   
       public void run() {
           System.out.println("Running as daemon: " + isDaemon());
       }
   }
   

2. Set before calling start() in main()

   MyThread t1 = new MyThread();
   t1.setDaemon(true); // ✅ Set daemon before starting
   t1.start();
   

3. Use an anonymous thread

   Thread t = new Thread(() -> System.out.println("Daemon running"));
   t.setDaemon(true); // ✅ Must set before start()
   t.start();
   

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6️⃣ Conclusion

📌 Daemon Thread → Runs in the background, stops when all non-daemon threads exit.
📌 Non-Daemon Thread → Main application tasks, JVM waits for its completion.
📌 To create a daemon thread, use setDaemon(true) before calling start().

📌 In run method, you can't setDeamon().