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Daemon Thread in Java

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Daemon Thread in Java

Daemon Thread is part of Java concurrency. A thread is a path of work inside a process. Create with Thread or Runnable, then call start() — not run(). run() only runs on the same thread.

For Daemon Thread: if two threads share data, synchronize or the result can be wrong. sleep pauses. join waits for another thread to finish. daemon threads die when all user threads finish.

Daemon Thread deadlock: two threads wait for each other’s lock forever. Avoid by locking in the same order. Don’t guess thread order — it can change every run.

Let's take this on the board with one tiny Main class — no extra files. Daemon Thread in Java — start() schedules the thread; main also prints — order may vary. Start from main, go line by line, and stop at each print. That output is the proof for Daemon Thread.

Exam tip

start() vs run() is a classic interview question.

Example

// Thread demo
public class Main {
  public static void main(String[] args) {
    Thread t = new Thread(() -> System.out.println("Hi from thread"));
    t.start();
    System.out.println("Hi from main");
  }
}

Daemon Thread in Java — start() schedules the thread; main also prints — order may vary.

Short notes

DefDaemon Thread — thread = one path of work. start() schedules it. run() does not.
RuleDaemon Thread — shared data needs synchronization.
RememberDaemon Thread — sleep = pause. join = wait for other thread.
TrapDaemon Thread — calling run() instead of start(). Deadlock = locks in wrong order.

// Thread demo public class Main { public static void main(String[] args) { Thread t = new Thread(() -> System.out.println("Hi from thread")); t.start(); System.out.println("Hi from main"); } }

Daemon Thread in Java

Example

Short notes

Questions

Daemon Thread in Java

Example

Short notes

Questions