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getQueueLength()作用是取得等待permits的线程个数

hasQueuedThreads()作用是判断有没有线程在等待permits

这两个方法通常都是在判断当前有没有等待permits的线程信息时使用

Service.java

package org.fool.java.concurrent.semaphore.queue;

import java.util.concurrent.Semaphore;

public class Service {
    private Semaphore semaphore = new Semaphore(1);

    public void testMethod() {
        try {
            semaphore.acquire();

            Thread.sleep(1000);

            System.out.println("Returns an estimate of the number of threads waiting to acquire " + semaphore.getQueueLength());

            System.out.println("Whether any threads are waiting to acquire? " + semaphore.hasQueuedThreads());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            semaphore.release();
        }
    }
}

ThreadA.java

package org.fool.java.concurrent.semaphore.queue;

public class ThreadA implements Runnable {
    private Service service;

    public ThreadA(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        service.testMethod();
    }
}

QueueTest.java

package org.fool.java.concurrent.semaphore.queue;

public class QueueTest {
    public static void main(String[] args) {
        Service service = new Service();

        Thread a = new Thread(new ThreadA(service));
        a.start();

        for (int i = 0; i < 5; i++) {
            new Thread(new ThreadA(service)).start();
        }
    }
}

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