Daemon vs User Threads

The difference is about JVM life dependency

User Thread → Keeps the application alive
Daemon Thread → Runs in background, does NOT keep JVM alive

JVM exits when all user threads finish, even if daemon threads are still running.

Aspect	User Thread	Daemon Thread
Purpose	Main work (business logic)	Background support tasks
JVM dependency	JVM waits for it	JVM does NOT wait
Lifecycle	Independent	Ends when JVM ends
Examples	API requests, DB calls	Garbage Collector, monitoring
Default type	Yes	No

Example to Understand Clearly

public class Main {
    public static void main(String[] args) {

        Thread t = new Thread(() -> {
            while (true) {
                System.out.println("Daemon running...");
            }
        });

        t.setDaemon(true); // mark as daemon
        t.start();

        System.out.println("Main thread finished");
    }
}

What happens?

Main thread finishes quickly
Only daemon thread remains
JVM exits immediately

👉 Output:

Main thread finished
Daemon running... (maybe few times or not at all)

Same Example WITHOUT Daemon

t.setDaemon(false); // default

Now:

JVM will NOT exit
Program keeps running forever

🧵 User Threads

Used for:

Handling HTTP requests (Spring Boot)
Business logic
Kafka consumers

👉 These must complete → JVM waits

Daemon Threads

Used for:

Garbage Collection (GC)
Background cleanup
Monitoring

👉 These are supporting tasks only

Important Rules

Must set daemon BEFORE start

t.setDaemon(true);
t.start();

❌ This will fail:

t.start();
t.setDaemon(true); // IllegalThreadStateException