🧩 Why?
Because:
- The main thread is created by the JVM before your program code even runs.
- You don’t get a chance to call
setDaemon(true)on it. - Once a thread has started, you can no longer change its daemon status — it must be set before calling
.start().
✅ From the Java Docs
“A daemon thread is a thread that does not prevent the JVM from exiting once all user (non-daemon) threads finish. The main thread is a user thread by default and cannot be made a daemon.”
💥 If You Try…
You can’t even get a reference to the main thread and make it daemon:
Thread.currentThread().setDaemon(true); // ❌ IllegalThreadStateException
It throws:
Exception in thread "main" java.lang.IllegalThreadStateException
Because the thread is already running!
🧠 Fun Fact
- The JVM exits when all non-daemon threads finish, including the main thread.
- So the main thread is the reason daemon threads get killed — not something you’d ever want to mark as daemon.
🔒 Summary
| Can main thread be daemon? | ❌ No | | Why not? | It’s already started by the time your code runs | | Can you change a thread’s daemon status after .start()? | ❌ No | | Daemon threads die when…| All non-daemon threads (like main) finish |