Understanding ThreadPoolExecutor keepAliveTime=0 in Java

In the book "The Art of Concurrency Programming" a passage explains that when the number of threads in a pool exceeds corePoolSize, keepAliveTime is the maximum time that the excess idle threads wait for new tasks before being terminated; setting keepAliveTime to 0L means they are terminated immediately.

Many blog posts incorrectly claim that a keepAliveTime of 0 means idle threads live forever; this article demonstrates the correct behavior with a verification program.

Below is the verification code that creates a ThreadPoolExecutor with corePoolSize 1, maximumPoolSize 2, keepAliveTime 0 seconds, and a queue of size 1. Three tasks are submitted, and the pool size and active thread count are printed every second.

public static void main(String[] args) throws InterruptedException {<br/>        // Create a thread pool<br/>        ThreadPoolExecutor executor = new ThreadPoolExecutor(1,<br/>                2,<br/>                0, TimeUnit.SECONDS,<br/>                new LinkedBlockingQueue<Runnable>(1));<br/><br/>        for (int i = 0; i < 3; i++) {<br/>            executor.execute(new DemoTask(i));<br/>        }<br/><br/>        while (true) {<br/>            System.out.println("Total threads: " + executor.getPoolSize() + ", Active threads: " + executor.getActiveCount());<br/>            TimeUnit.SECONDS.sleep(1);<br/>        }<br/>    }<br/><br/>    static class DemoTask implements Runnable {<br/>        int num;<br/><br/>        public DemoTask(int i) {<br/>            this.num = i;<br/>        }<br/><br/>        @Override<br/>        public void run() {<br/>            System.out.println("num=" + num + " Thread = " + Thread.currentThread().getName());<br/>            if (num >= 1) {<br/>                try {<br/>                    TimeUnit.SECONDS.sleep(1);<br/>                    System.out.println("num=" + num + " sleep 1 s finished");<br/>                } catch (InterruptedException e) {<br/>                    e.printStackTrace();<br/>                }<br/>            } else {<br/>                try {<br/>                    TimeUnit.SECONDS.sleep(3);<br/>                    System.out.println("num=" + num + " sleep 3 s finished");<br/>                } catch (InterruptedException e) {<br/>                    e.printStackTrace();<br/>                }<br/>            }<br/>        }<br/>    }<br/>

The printed result shows that after the first task completes, the extra thread (beyond corePoolSize) is terminated immediately, confirming that keepAliveTime=0 does not keep threads alive indefinitely.

This concludes the article.