Is a Running Thread Eligible for Garbage Collection?

Is a Thread Garbage Collected?

This example showcases a running thread that remains active despite losing its reference in the main thread.

<code class="java">public class TestThread {
    public static void main(String[] s) {
        Thread t = new Thread() {
            public void run() {
                while (true) {
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                    }
                    System.out.println("thread is running...");
                }
            }
        };
        t.start(); // Line A
        t = null; // Line B
        while (true) {
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
            }
            System.gc();
            System.out.println("Executed System.gc()");
        } // The program will run forever until you use ^C to stop it
    }
}</code>

Understanding the Garbage Collection Mechanism

The reference to the thread t is removed on Line B, but the thread continues to execute because it's considered a "garbage collection root." The garbage collector determines whether an object is reachable by using roots as reference points.

Since the running thread is a root, it keeps itself, and potentially other objects it's referencing, from being garbage collected. This parallels the behavior of the main thread, which also lacks direct references and yet is not eligible for collection.

Implications

This behavior has implications for memory management and potential memory leaks. If a thread is designed to be used indefinitely, its removal from the application code should not be treated as a signal to garbage collect it. It's important to account for such scenarios in memory usage analysis and performance tuning.

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