共享锁：线程可以同时持有。

实现代码如下：

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
/**
 * 共享锁
 */
public class SharedLock implements Lock {
    //同步器，初始化创建放在本类中的构造方法中
    private Sync sync;

    //构造方法，初始化同步器的计数值
    public SharedLock(int count) {
        sync = new Sync(count);
    }

    // 静态内部类，同步器！继承AQS，是实现锁的核心
    private class Sync extends AbstractQueuedSynchronizer {

        //构造方法，先判断值是否为正，正则将值放入放入计数中。
        Sync(int count){
            if (count < 0){
                throw new IllegalArgumentException();
            }
            setState(count);
        }

        /**
         * 获取锁 真正的实现方法
         *      通过for 死循环来确保CAS的正常完成
         *      或者剩余数量如果小于0 也直接返回
         * @param acquire 请求锁的个数
         * @return
         */
        @Override
        protected int tryAcquireShared(int acquire) {
            for (;;){
                // 当前可用的锁数量
                int available = getState();
                // 减去使用掉的锁，剩余的锁数量
                int remaining = available - acquire;
                // compareAndSetState(available,remaining)保证线程安全，将剩余锁设置为剩余的数量
                if (remaining < 0 || compareAndSetState(available,remaining)){
                    return remaining;
                }
            }
        }

        /**
         * 释放锁 真正的实现方法
         * @param release 释放锁的个数
         * @return
         */
        @Override
        protected boolean tryReleaseShared(int release) {
            for (;;){
                // 当前剩余
                int current = getState();
                // 总计
                int count = current + release;
                // 判断释放的是否为负值！
                if (count < current){
                    throw new Error("Maximum permit count exceeded");
                }
                if (compareAndSetState(current,count)){
                    return true;
                }
            }
        }

        protected final Condition newCondition(){
            return new ConditionObject();
        }
    }


    /**
     * 面向使用者，获取一个共享锁
     */
    @Override
    public void lock() {
        sync.acquireShared(1);
    }

    @Override
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * 尝试获取一个共享锁
     * @return true 获取成功
     *         false 获取失败
     */
    @Override
    public boolean tryLock() {
        return sync.tryAcquireShared(1) >= 0;
    }

    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        return sync.tryAcquireSharedNanos(1,unit.toNanos(time));
    }

    /**
     * 面向使用者，释放一个共享锁
     */
    @Override
    public void unlock() {
        sync.releaseShared(1);
    }

    @Override
    public Condition newCondition() {
        return sync.newCondition();
    }
}

测试：

public class SharedLockTest {
    private SharedLock lock = new SharedLock(3);
    private static CountDownLatch countDownLatch = new CountDownLatch(1);
    private class Test extends Thread{
        @Override
        public void run() {
            try {
                countDownLatch.await();
                if (lock.tryLock()){
                    System.out.println(Thread.currentThread().getName() + "获取了一个锁！" + System.currentTimeMillis());
                    Thread.sleep(3000);
                    lock.unlock();
                }else {
                    System.out.println(Thread.currentThread().getName());
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }


    public static void main(String[] args) {
        SharedLockTest test = new SharedLockTest();
        for (int i = 0;i<10;i++){
            Test t = test.new Test();
            t.start();
        }
        countDownLatch.countDown();
    }
}