• 重入锁ReentrantLock，支持重进入的锁，能支持一个线程对资源的重复加锁
• 支持获取锁时的公平和非公平的选择，默认非公平
• 锁的公平与否：在绝对时间上，先对锁进行获取的请求一定先被满足，则锁是公平的，反之不公平。
• 重进入是指任意线程在获取到锁之后能够再次获取该锁，需要解决两个问题：
  • 识别获取锁的线程是否为占据当前锁的线程。
  • 线程重复n次获取锁，随后第n次释放锁后，其他线程能获取到锁。
  • ReentrantLock通过组合自定义同步�器实现锁的获取和释放：
  • nonfairTryAcquire方法非公平获取
  • tryAcquire()方法公平获取，比nonfairTryAcquire多了hasQueuedPredecessors()方法判断同步队列当前节点是否有前驱节点的判断。 公平锁和非公平锁的代码示例：

import org.junit.Test;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * @author pengjunzhe
 */
public class FairAndUnfairTest {
    private static Lock fairLock = new ReentrantLock2(true);
    private static Lock unFairLock = new ReentrantLock2(false);

    @Test
    public void fair() {
        testLock(fairLock);
    }

    @Test
    public void unfair() {
        testLock(unFairLock);
    }

    private void testLock(Lock lock) {
        //TODO:启动5个Job
        for (int i = 0; i < 5; i++) {
            Thread thread = new Thread(new Job(lock), String.valueOf(i));
            thread.start();
        }
        SleepUtils.second(2);
    }

    private static class Job extends Thread {
        private Lock lock;

        public Job(Lock lock) {
            this.lock = lock;
        }

        @Override
        public void run() {
            // TODO：连续两次打印当前的Thread和等待队列中的Thread
            for (int i = 0; i < 2; i++) {
                lock.lock();
                try {
                    System.out.println("Lock by["
                            + Thread.currentThread().getName()
                            + "]"
                            + "Waiting by " + ((ReentrantLock2) lock).getQueuedThreadNames());
                } finally {
                    lock.unlock();
                }
            }
        }
    }

    private static class ReentrantLock2 extends ReentrantLock {
        public ReentrantLock2(boolean fair) {
            super(fair);
        }

        @Override
        public Collection<Thread> getQueuedThreads() {
            List<Thread> threads = new ArrayList<>(super.getQueuedThreads());
            Collections.reverse(threads);
            return threads;
        }

        public Collection<String> getQueuedThreadNames() {
            List<String> names = new ArrayList<>(8);
            getQueuedThreads().forEach((s) -> names.add(s.getName()));
            return names;
        }
    }
}

输出分别为： Fair：

Lock by[0]Waiting by []
Lock by[1]Waiting by [2, 3, 4, 0]
Lock by[2]Waiting by [3, 4, 0, 1]
Lock by[3]Waiting by [4, 0, 1, 2]
Lock by[4]Waiting by [0, 1, 2, 3]
Lock by[0]Waiting by [1, 2, 3, 4]
Lock by[1]Waiting by [2, 3, 4]
Lock by[2]Waiting by [3, 4]
Lock by[3]Waiting by [4]
Lock by[4]Waiting by []

Unfair：

Lock by[0]Waiting by []
Lock by[0]Waiting by [1, 2, 3, 4]
Lock by[1]Waiting by [2, 3, 4]
Lock by[1]Waiting by [2, 3, 4]
Lock by[2]Waiting by [3, 4]
Lock by[2]Waiting by [3, 4]
Lock by[3]Waiting by [4]
Lock by[3]Waiting by [4]
Lock by[4]Waiting by []
Lock by[4]Waiting by []