Thread Termination & Daemon Threads

1. Why is Termination Tricky?

Threads consume resources (Memory, CPU, File Handles). If a thread finishes its task but stays alive, it’s a Resource Leak.

• We cannot use the stop() method (it is deprecated and dangerous).
• We must use Cooperative Termination.

2. Method 1: The Interrupt Signal

Interrupting is a way for one thread to signal another: “Please stop what you are doing.”

Scenario A: Thread is Blocking (Sleeping/Waiting)

If a thread is executing a method like Thread.sleep() or wait(), it will throw an InterruptedException when interrupted.

public void run() {
    try {
        Thread.sleep(500000); // Long task
    } catch (InterruptedException e) {
        System.out.println("Thread was interrupted externally. Closing resources...");
        return; // Exit the run method cleanly
    }
}

Scenario B: Thread is Busy (Hard Calculation)

If the thread is in a loop doing heavy math, it won’t “feel” the interrupt unless it manually checks its status.

public void run() {
    for (int i = 0; i < 1000000; i++) {
        if (Thread.currentThread().isInterrupted()) {
            System.out.println("Interrupted during calculation!");
            return;
        }
        // Do heavy work
    }
}

3. Daemon Threads

By default, a Java application will not exit as long as there is at least one “User Thread” still running. Even if main finishes, the app stays alive.

Daemon Threads are background “helper” threads.

• The Rule: If only Daemon threads are left running, the JVM will shut down automatically.
• Use Case: Garbage collection, background monitoring, or auto-save features.
• How to set: You must set this before calling .start().

Thread bgThread = new Thread(new LongTask());
bgThread.setDaemon(true); // Now this won't block the app from closing
bgThread.start();

4. Key Differences Table

5. Summary Checklist

• Do I understand that interrupt() is a request, not a command?
• Can I identify when to use isInterrupted() vs catching InterruptedException?
• Do I know that a Daemon thread’s code might suddenly stop mid-execution when the app exits?