多线程例子程序：两个线程累加共享变量，结果正确

public class Test {

    static int count = 0;
    static ReentrantLock lock = new ReentrantLock();

    public static void main(String[] args) throws InterruptedException {

        Runnable runnable = new Runnable() {
            @Override
            public void run () {
                try {
                    lock.lock();
                    for (int i = 0; i < 10000; i++) {
                        count++;
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                } finally {
                    lock.unlock();
                }
            }
        };

        Thread thread1 = new Thread(runnable);
        Thread thread2 = new Thread(runnable);
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();

        System.out.println(count);
    }
}

非公平锁加锁（即 lock.lock();）过程

1. ReentrantLock的lock方法 走到 同步器的 acquire(1)方法
2. 走到aqs的acquire的方法，而其tryAcquire方法实现则是在NonfairSync类中
3. NonfairSync执行tryAcquire方法又回到Sync的nonfairTryAcquire方法
4. Sync的nonfairTryAcquire方法则如下

相关代码 java.util.concurrent.locks.ReentrantLock.Sync#nonfairTryAcquire

abstract static class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = -5179523762034025860L;

    /**
     * Performs non-fair tryLock.  tryAcquire is implemented in
     * subclasses, but both need nonfair try for trylock method.
     */
    @ReservedStackAccess
    final boolean nonfairTryAcquire(int acquires) {
        final Thread current = Thread.currentThread();
        int c = getState();
        if (c == 0) {
            if (compareAndSetState(0, acquires)) {
                setExclusiveOwnerThread(current);
                return true;
            }
        }
        else if (current == getExclusiveOwnerThread()) {
            int nextc = c + acquires;
            if (nextc < 0) // overflow
                throw new Error("Maximum lock count exceeded");
            setState(nextc);
            return true;
        }
        return false;
    }

1. 进行cas自旋获取，成功则设置当前线程获取到锁(此时state=1），否则一直自旋。
2. 如果当前线程就是获取到锁的线程，则为可重入，此时 state > 1
3. 其中 private volatile int state; volatile保证可见性、有序性，不保证原子性（CAS操作保证原子性）

可以看到

1. 当一个线程lock获取到锁后成功返回，并执行自己的代码
2. 其它线程获取不到锁，则一直自旋在哪里

非公平锁解锁（ lock.unlock();）过程

释放锁则相对简单，如下图所示

相关代码java.util.concurrent.locks.ReentrantLock.Sync#tryRelease

@ReservedStackAccess
 protected final boolean tryRelease(int releases) {
       int c = getState() - releases;
       if (Thread.currentThread() != getExclusiveOwnerThread())
           throw new IllegalMonitorStateException();
       boolean free = false;
       if (c == 0) {
           free = true;
           setExclusiveOwnerThread(null);
       }
       setState(c);
       return free;
   }

公平锁

从上面非公平锁可以看到，加锁操作时看哪个线程cas抢到就行，至于谁抢到无所谓。可能有线程永远抢不到，即线程饥饿。这就出现了不公平的现象了

当new ReentrantLock(true);就得到了一个公平锁

public ReentrantLock(boolean fair) {
      sync = fair ? new FairSync() : new NonfairSync();
}

公平锁的加锁逻辑

体现在java.util.concurrent.locks.ReentrantLock.FairSync#tryAcquire

protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (!hasQueuedPredecessors() &&
            compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0)
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

1. 会先判断当前线程前面是否有等待的线程，即利用asq的等待队列进行判断

   • 如果有，则自己加入到链表尾部；返回false
   • 如果队列为空或者自己就是链表头；返回true

2. hasQueuedPredecessors返回false，则可以进行cas获取锁操作了

3. hasQueuedPredecessors返回true，那么自己则要排队了，先来后到(公平)，不用cas操作了，直接加锁失败

公平锁的释放锁逻辑

同非公平锁