Keepalivetime=0 description of ThreadPoolExecutor

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As the title , Here's the picture , explain ThreadPoolExecutor Of keepAliveTime=0.

The above is from 《 The art of concurrent programming 》 A Book , The book has the following description ：

“ “ When the number of threads in the thread pool is greater than corePoolSize when ,keepAliveTime The maximum time to wait for new tasks for redundant idle threads , After this time, the redundant threads will be terminated . Here is the keepAliveTime Set to 0L, It means superfluous The idle thread of will be terminated immediately .”

A lot of blog materials on the Internet say 0 Indicates that the thread survives permanently when it is idle . It's not accurate .

Attach verification code ：

public static void main(String[] args) throws InterruptedException {
        //  Created a thread pool 
        ThreadPoolExecutor executor = new ThreadPoolExecutor(1,
                2,
                0, TimeUnit.SECONDS,
                new LinkedBlockingQueue<Runnable>(1));

        for (int i = 0; i < 3; i++) {
            executor.execute(new DemoTask(i));
        }

        while (true) {
            System.out.println(" Bus number ：" + executor.getPoolSize() + ",  Number of currently active threads ：" + executor.getActiveCount());
            TimeUnit.SECONDS.sleep(1);
        }
    }

    static class DemoTask implements Runnable {
        int num;

        public DemoTask(int i) {
            this.num = i;
        }

        @Override
        public void run() {
            System.out.println("num=" + num + " Thread = " + Thread.currentThread().getName());
            if (num >= 1) {
                try {
                    TimeUnit.SECONDS.sleep(1);
                    System.out.println("num=" + num + " sleep 1 s end ");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            } else {
                try {
                    TimeUnit.SECONDS.sleep(3);
                    System.out.println("num=" + num + " sleep 3 s end ");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

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Print the results ：

num=2 Thread = pool-1-thread-2
 Bus number ：2,  Number of currently active threads ：2
num=0 Thread = pool-1-thread-1
num=2 sleep 1 s end 
num=1 Thread = pool-1-thread-2
 Bus number ：2,  Number of currently active threads ：2
nu

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