线程池ExecutorService空闲线程的个数

• keepAliveTime：表示线程没有任务执行时最多保持多久时间会终止。默认情况下，只有当线程池中的线程数大于corePoolSize时，keepAliveTime才会起作用，直到线程池中的线程数不大于corePoolSize，即当线程池中的线程数大于corePoolSize时，如果一个线程空闲的时间达到keepAliveTime，则会终止，直到线程池中的线程数不超过corePoolSize。但是如果调用了allowCoreThreadTimeOut(boolean)方法，在线程池中的线程数不大于corePoolSize时，keepAliveTime参数也会起作用，直到线程池中的线程数为0；
• allowCoreThreadTimeOut默认是false，即核心线程不会超时关闭，除非手动关闭线程池才会销毁线程

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Main {
    public static void main(String[] args) {

        ThreadPoolExecutor executor = new ThreadPoolExecutor(5, 10, 10, TimeUnit.MILLISECONDS,
                new ArrayBlockingQueue(5), new ThreadPoolExecutor.CallerRunsPolicy());
        System.out.println("核心线程空闲超时是否关闭:" + executor.allowsCoreThreadTimeOut());//核心线程空闲超时是否关闭:false
        for (int i = 0; i < 15; i++) {
            MyTask myTask = new MyTask(i);
            executor.execute(myTask);
            System.out.println("线程池中线程数目：" + executor.getPoolSize() + "，队列中等待执行的任务数目：" +
                    executor.getQueue().size() + "，已执行完的任务数目：" + executor.getCompletedTaskCount());
        }
        executor.shutdown();//手动关闭线程池
        while (!executor.isTerminated()) {
        }
        if (executor.isTerminated()) {
            System.out.println("线程池中线程数目：" + executor.getPoolSize() + "，队列中等待执行的任务数目：" +
                    executor.getQueue().size() + "，已执行完的任务数目：" + executor.getCompletedTaskCount());
        }
    }
}

class MyTask implements Runnable {
    private int taskNum;

    public MyTask(int num) {
        this.taskNum = num;
    }

    @Override
    public void run() {
        System.out.println("线程名称：" + Thread.currentThread().getName() + "，正在执行task " + taskNum);
        try {
            Thread.currentThread().sleep(4000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("task " + taskNum + "执行完毕");
    }
}

输出内容的最后是：线程池中线程数目：0，队列中等待执行的任务数目：0，已执行完的任务数目：15

 

allowCoreThreadTimeOut设置为true，不手动关闭

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Main {
    public static void main(String[] args) {

        ThreadPoolExecutor executor = new ThreadPoolExecutor(5, 10, 10, TimeUnit.MILLISECONDS,
                new ArrayBlockingQueue(5), new ThreadPoolExecutor.CallerRunsPolicy());
        System.out.println("核心线程空闲超时是否关闭:" + executor.allowsCoreThreadTimeOut());//核心线程空闲超时是否关闭:false
        executor.allowCoreThreadTimeOut(true);
        for (int i = 0; i < 15; i++) {
            MyTask myTask = new MyTask(i);
            executor.execute(myTask);
            System.out.println("线程池中线程数目：" + executor.getPoolSize() + "，队列中等待执行的任务数目：" +
                    executor.getQueue().size() + "，已执行完的任务数目：" + executor.getCompletedTaskCount());
        }
        
        while (true) {
            System.out.println("线程池中线程数目：" + executor.getPoolSize() + "，队列中等待执行的任务数目：" +
                    executor.getQueue().size() + "，已执行完的任务数目：" + executor.getCompletedTaskCount());
        }
    }
}

class MyTask implements Runnable {
    private int taskNum;

    public MyTask(int num) {
        this.taskNum = num;
    }

    @Override
    public void run() {
        System.out.println("线程名称：" + Thread.currentThread().getName() + "，正在执行task " + taskNum);
        try {
            Thread.currentThread().sleep(4000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("task " + taskNum + "执行完毕");
    }
}

输出内容的最后是：线程池中线程数目：0，队列中等待执行的任务数目：0，已执行完的任务数目：15

 

allowCoreThreadTimeOut默认为false，不手动关闭

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Main {
    public static void main(String[] args) {

        ThreadPoolExecutor executor = new ThreadPoolExecutor(5, 10, 10, TimeUnit.MILLISECONDS,
                new ArrayBlockingQueue(5), new ThreadPoolExecutor.CallerRunsPolicy());
        System.out.println("核心线程空闲超时是否关闭:" + executor.allowsCoreThreadTimeOut());//核心线程空闲超时是否关闭:false
//        executor.allowCoreThreadTimeOut(true);
        for (int i = 0; i < 15; i++) {
            MyTask myTask = new MyTask(i);
            executor.execute(myTask);
            System.out.println("线程池中线程数目：" + executor.getPoolSize() + "，队列中等待执行的任务数目：" +
                    executor.getQueue().size() + "，已执行完的任务数目：" + executor.getCompletedTaskCount());
        }

        while (true) {
            System.out.println("线程池中线程数目：" + executor.getPoolSize() + "，队列中等待执行的任务数目：" +
                    executor.getQueue().size() + "，已执行完的任务数目：" + executor.getCompletedTaskCount());
        }
    }
}

class MyTask implements Runnable {
    private int taskNum;

    public MyTask(int num) {
        this.taskNum = num;
    }

    @Override
    public void run() {
        System.out.println("线程名称：" + Thread.currentThread().getName() + "，正在执行task " + taskNum);
        try {
            Thread.currentThread().sleep(4000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("task " + taskNum + "执行完毕");
    }
}

输出内容的最后是：线程池中线程数目：5，队列中等待执行的任务数目：0，已执行完的任务数目：15

 

总结：

1 如果手动shutdown，则空闲线程数为0
2 如果allowCoreThreadTimeOut默认为false，不手动shutdown，则空闲线程数为是核心线程数

3 如果allowCoreThreadTimeOut设置为true，不手动shutdown，则空闲线程数为0

参考：

https://blog.csdn.net/u010002184/article/details/80847398
https://www.cnblogs.com/dolphin0520/p/3932921.html         Java并发编程：线程池的使用
https://blog.csdn.net/lmj623565791/article/details/27250059    Java并发专题 带返回结果的批量任务执行 CompletionService ExecutorService.invokeAll

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