Java.Multithreading.What are “daemon threads”?

👻 What Is a Daemon Thread?

A daemon thread is a background thread that does not prevent the JVM from exiting.

In simpler terms:

• If only daemon threads are running, the JVM will shut down automatically.
• They’re considered non-essential to the program’s main logic.

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🧪 Real-Life Analogy

Think of daemon threads like assistants:

• They do background tasks (like garbage collection, monitoring, etc.)
• But when the main work (non-daemon threads) is done, the JVM says: “Okay, assistants, we’re closing shop — time to go!”
  And it shuts down, killing the daemon threads.

🧰 How to Make a Thread Daemon

Thread t = new Thread(() -> {
    while (true) {
        System.out.println("Background work...");
    }
});
t.setDaemon(true); // must be called BEFORE start()
t.start();

❗ If you forget to call setDaemon(true) before start(), you’ll get an IllegalThreadStateException.

⚠️ JVM Behavior

• If at least one non-daemon thread is running → JVM stays alive.
• If only daemon threads are running → JVM exits without waiting for them to finish.

✅ When to Use Daemon Threads

Use Case	Example
Background tasks	Logging, Metrics, Auto-saving
System-level support	JVM garbage collector threads
Non-critical monitoring	Heartbeat pings, cleanup tasks

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❌ When NOT to Use Daemon Threads

• For anything critical: file writes, user operations, database updates.
• Because daemon threads can be terminated abruptly when the JVM exits.

📌 Quick Demo

public class DaemonDemo {
    public static void main(String[] args) {
        Thread daemon = new Thread(() -> {
            while (true) {
                System.out.println("I'm a daemon thread.");
                try { Thread.sleep(1000); } catch (InterruptedException e) {}
            }
        });

        daemon.setDaemon(true);
        daemon.start();

        System.out.println("Main thread sleeping 2 seconds...");
        try { Thread.sleep(2000); } catch (InterruptedException e) {}

        System.out.println("Main thread done.");
        // JVM exits here — daemon thread is killed
    }
}

🧨 Output will stop after main thread finishes — even if daemon is still looping.

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🧠 Summary

Feature	Description
Daemon thread	Background thread — doesn’t block JVM exit
Set with	thread.setDaemon(true)
Use for	Monitoring, housekeeping
JVM kills it when	No non-daemon threads are running