Daemon Threads & JVM Shutdown

A daemon thread is a background service thread that does not prevent the JVM from exiting; when all non-daemon threads finish, the JVM terminates and kills all daemon threads abruptly without running their finally blocks.

The JVM stays alive as long as any non-daemon thread is running, so forgetting to call shutdown() on a thread pool causes the process to hang forever even after main() returns.

A shutdown hook runs on an orderly stop (SIGTERM, System.exit, Ctrl-C) but never on kill -9, so use it for fast graceful cleanup, not guaranteed work.