[调优]Future超时阻塞与线程池拒绝策略

Future超时阻塞与线程池拒绝策略

• 一、拒绝策略DiscardOldestPolicy
• 二、拒绝策略DiscardPolicy

一、拒绝策略DiscardOldestPolicy

/**
 * @author: guanjian
 * @date: 2020/06/30 14:39
 * @description:
 */
public class TestThreadPoolRejectDiscardOldestPolicy {
 
    /**
     * 声明一个线程池
     */
    private final static ThreadPoolExecutor executorService = new ThreadPoolExecutor(
            //初始工作线程是1
            1,
            //最大线程也是1，意味着同一时刻只有一个线程可以在执行
            1,
            1L, TimeUnit.MINUTES,
            //阻塞队列只允许一个线程排队
            new ArrayBlockingQueue<>(1),
            //队列排队溢出时，丢弃队首线程
            new ThreadPoolExecutor.DiscardOldestPolicy());
 
    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {
 
 
        Future f1 = executorService.submit(() -> {
            try {
                System.out.println("f1 thread executing");
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
 
        Future f2 = executorService.submit(() -> {
            try {
                System.out.println("f2 thread executing");
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
 
        Future f3 = executorService.submit(() -> {
            try {
                System.out.println("f3 thread executing");
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
 
        //f1、f2、f3三个线程按顺序提交请求线程池执行
        //f1线程提交，获取执行权，经过5s执行时间，f1.get()返回结果
        //f2线程提交，由于f1执行需要5s，直接进入阻塞队列等待
        //f3线程提交，由于f1在执行，f2在阻塞队列且队列已满，且当前拒绝策略是DiscardOldestPolicy，会把队首排队线程f2弹出
        //f3被提交到阻塞队列中，当f1执行完，f3继续执行；
        //但是f2被弹出，f2.get()同样会一直阻塞，f3即时获取到了执行权也完成了执行，但按照代码顺序f3.get()根本不会得到执行
        //由于future.get()会一直阻塞但线程已经被抛弃没有机会变更状态，因此会一直阻塞
        System.out.printf("f1=%s",f1.get());
        System.out.printf("f2=%s",f2.get());
        System.out.printf("f3=%s",f3.get());
        System.out.println("================end================");
 
        //当使用DiscardOldestPolicy作为线程池拒绝策略时，要进行超时获取避免hang住线程一直阻塞自旋无法释放cpu
        System.out.printf("f1=%s",f1.get(10,TimeUnit.SECONDS));
        System.out.printf("f2=%s",f2.get(10,TimeUnit.SECONDS));
        System.out.printf("f3=%s",f3.get(10,TimeUnit.SECONDS));
        System.out.println("================end================");
    }
}

二、拒绝策略DiscardPolicy

/**
 * @author: guanjian
 * @date: 2020/06/30 14:39
 * @description: 模拟线程池队列满
 */
public class TestThreadPoolRejectDiscardPolicy {
 
    /**
     * 声明一个线程池
     */
    private final static ThreadPoolExecutor executorService = new ThreadPoolExecutor(
            //初始工作线程是1
            1,
            //最大线程也是1，意味着同一时刻只有一个线程可以在执行
            1,
            1L, TimeUnit.MINUTES,
            //阻塞队列只允许一个线程排队
            new ArrayBlockingQueue<>(1),
            //队列排队溢出时，直接抛弃溢出的线程
            new ThreadPoolExecutor.DiscardPolicy());
 
    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {
 
 
        Future f1 = executorService.submit(() -> {
            try {
                System.out.println("f1 thread executing");
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
 
        Future f2 = executorService.submit(() -> {
            try {
                System.out.println("f2 thread executing");
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
 
        Future f3 = executorService.submit(() -> {
            try {
                System.out.println("f3 thread executing");
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
 
        //f1、f2、f3三个线程按顺序提交请求线程池执行
        //f1线程提交，获取执行权，经过5s执行时间，f1.get()返回结果
        //f2线程提交，由于f1执行需要5s，直接进入阻塞队列等待，f1执行完，f2开始执行
        //f3线程提交，由于f1在执行，f2在阻塞队列且队列已满，f3被当前拒绝策略抛弃，
        //由于future.get()会一直阻塞但线程已经被抛弃没有机会变更状态，因此会一直阻塞
        System.out.printf("f1=%s",f1.get());
        System.out.printf("f2=%s",f2.get());
        System.out.printf("f3=%s",f3.get());
 
        //当使用DiscardPolicy作为线程池拒绝策略时，要进行超时获取避免hang住线程一直阻塞自旋无法释放cpu
        System.out.printf("f1=%s",f1.get(10,TimeUnit.SECONDS));
        System.out.printf("f2=%s",f2.get(10,TimeUnit.SECONDS));
        System.out.printf("f3=%s",f3.get(10,TimeUnit.SECONDS));
    }
}