AbstractQueuedSynchronizer

• TwinsLock (自定义组件类)
• MainTest (测试类)
• 输出结果

TwinsLock (自定义组件类)

package test.AbstractQueuedSynchronizerDemo;


import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;

/**
 * @author xuzhihua
 * @date 2019/2/24 10:19 AM
 * 设计一个同步工具(TwinsLock)：该工具在同一时刻，只运行至多两个线程同时访问，超过两个线程的访问将被阻塞
 *
 */
public class TwinsLock implements Lock {

    // 初始状态 status 为2， 当1个线程进行获取，status减1；该线程释放，则status加1
    // status的合理范围是 0、1、2
    private final Sync sync = new Sync(2);


    private static final class Sync extends AbstractQueuedSynchronizer {

        Sync(int count) {
            if (count <= 0) {
                throw new IllegalArgumentException("count must larger than zero.");
            }
            setState(count);
        }

        public int tryAcquireShared(int reduceCount) {
            for (;;) {
                int current = getState();
                int newCount = current - reduceCount;
                if (newCount < 0 || compareAndSetState(current, newCount)) {
                    return newCount;
                }
            }
        }

        public boolean tryReleaseShared(int returnCount) {
            for (;;) {
                int current = getState();
                int newCount = current + returnCount;
                if (compareAndSetState(current, newCount))
                    return true;
            }
        }
    }

    @Override
    public void lock() {
        sync.acquireShared(1);
    }

    @Override
    public void unlock() {
        sync.releaseShared(1);
    }

    @Override
    public void lockInterruptibly() throws InterruptedException {

    }

    @Override
    public boolean tryLock() {
        return false;
    }

    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        return false;
    }

    @Override
    public Condition newCondition() {
        return null;
    }



}

MainTest (测试类)

package test.AbstractQueuedSynchronizerDemo;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;

/**
 * @author xuzhihua
 * @date 2019/2/24 10:30 AM
 * 测试类
 * 测试结果，线程名称成对输出，也就是在同一个时刻只有两个线程能够获取到锁
 */
public class MainTest {

    public static void main(String[] args) {
        test();
    }

    public static void test() {
        final Lock lock = new TwinsLock();
        class Worker extends  Thread {
            public void run() {
                while (true) {
                    lock.lock();
                    try {
                        TimeUnit.SECONDS.sleep(1);
                        System.out.println(Thread.currentThread().getName());
                        TimeUnit.SECONDS.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } finally {
                        lock.unlock();
                    }
                }
            }
        }
        // 启动10个线程
        for (int i = 0; i < 10; i++) {
            Worker w = new Worker();
            w.setDaemon(true);
            w.start();
        }
        // 每隔1秒换行
        for (int i = 0; i < 10; i++) {
            try {
                TimeUnit.SECONDS.sleep(1);
                System.out.println();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

输出结果