Thread Priority

Java threads support priority configurations and execution roles (daemon vs. user threads). Relying on priorities for execution ordering is a major concurrency anti-pattern, whereas setting daemon status correctly is critical for clean application termination.

Daemon Threads vs. User Threads

Every Java thread is either a user thread or a daemon thread:

• User (Non-Daemon) Threads: Spawned to execute main business logic. The JVM will wait for user threads to complete before exiting.
• Daemon Threads: Spawned to run background services (e.g. Garbage Collector, monitor loops). The JVM will terminate daemon threads immediately when all user threads exit, without running any finally blocks on them.

Thread Priority Pitfall

You can assign a priority to a thread using setPriority(int) with values from 1 (MIN_PRIORITY) to 10 (MAX_PRIORITY). However:

Warning: Java thread priorities are only hints to the underlying OS scheduler. The OS scheduler can ignore priorities completely, and prioritize low-priority threads due to scheduler heuristics. Relaing on priorities for thread synchronization results in thread starvation bugs and platform-dependent failures.

Code Walkthrough

This program shows how setting a thread as daemon changes JVM exit behavior.

public class DaemonThreadDemo {
    public static void main(String[] args) {
        Thread daemonWorker = new Thread(() -> {
            while (true) {
                try {
                    System.out.println("Daemon processing background stats...");
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    break;
                }
            }
        });
daemonWorker.setDaemon(true); // Must be set BEFORE starting the thread
        daemonWorker.start();
System.out.println("Main user thread finishes execution. JVM exits now.");
        // The JVM exits immediately and kills daemonWorker, ignoring finally blocks inside it.
    }
}

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