其他锁： StampedLock（戳）、Semaphore(信号量)、CountdownLatch（倒计时）

1. StampedLock（戳）

该类自 JDK 8 加入，是为了进一步优化读性能，它的特点是在使用读锁、写锁时都必须配合【戳】使用

加锁解锁:读锁

long stamp = lock.readLock();
lock.unlockRead(stamp);

加锁解锁:写锁

long stamp = lock.writeLock();
lock.unlockWrite(stamp);

乐观读，StampedLock 支持tryOptimisticRead() 方法（乐观读），读取完毕后需要做一次 戳校验 如果校验通
过，表示这期间确实没有写操作，数据可以安全使用，如果校验没通过，需要重新获取读锁，保证数据安全。

long stamp = lock.tryOptimisticRead();
// 验戳
if(!lock.validate(stamp)){
// 锁升级
}

提供一个数据容器类内部分别使用读锁保护数据的 read() 方法，写锁保护数据的 write() 方法

class DataContainerStamped {
private int data;
private final StampedLock lock = new StampedLock();
public DataContainerStamped(int data) {
this.data = data;
}
public int read(int readTime) {
//这里是乐观读，不是真正意义上的加锁
long stamp = lock.tryOptimisticRead();
log.debug("optimistic read locking...{}", stamp);
sleep(readTime);
//对乐观读取后，需要做一次戳校验，校验通过，表示这期间确实没有写操作，数据可以安全使用返回data
//校验没通过，则从无锁的乐观读，锁升级 - 读锁
if (lock.validate(stamp)) {
log.debug("read finish...{}, data:{}", stamp, data);
return data;
}
// 锁升级 - 读锁
log.debug("updating to read lock... {}", stamp);
try {
//带戳的读锁
stamp = lock.readLock();
log.debug("read lock {}", stamp);
sleep(readTime);
log.debug("read finish...{}, data:{}", stamp, data);
return data;
} finally {
log.debug("read unlock {}", stamp);
lock.unlockRead(stamp);
}
}
public void write(int newData) {
//带戳的写锁
long stamp = lock.writeLock();
log.debug("write lock {}", stamp);
try {
sleep(2);
this.data = newData;
} finally {
log.debug("write unlock {}", stamp);
lock.unlockWrite(stamp);
}
}
}

测试 读-读 可以优化

public static void main(String[] args) {
DataContainerStamped dataContainer = new DataContainerStamped(1);
new Thread(() -> {
dataContainer.read(1);
}, "t1").start();
sleep(0.5);
new Thread(() -> {
dataContainer.read(0);
}, "t2").start();
}
结果：输出结果，可以看到实际没有加读锁，因为读读校验通过，不走锁升级--读锁
15:58:50.217 c.DataContainerStamped [t1] - optimistic read locking...256
15:58:50.717 c.DataContainerStamped [t2] - optimistic read locking...256
15:58:50.717 c.DataContainerStamped [t2] - read finish...256, data:1
15:58:51.220 c.DataContainerStamped [t1] - read finish...256, data:1

测试 读-写 时优化读补加读锁

public static void main(String[] args) {
DataContainerStamped dataContainer = new DataContainerStamped(1);
new Thread(() -> {
dataContainer.read(1);
}, "t1").start();
sleep(0.5);
new Thread(() -> {
dataContainer.write(100);
}, "t2").start();
}
结果：有写锁，校验不通过，升级到读锁，等到t2线程释放写锁，t1才能加读锁
15:57:00.219 c.DataContainerStamped [t1] - optimistic read locking...256
15:57:00.717 c.DataContainerStamped [t2] - write lock 384
15:57:01.225 c.DataContainerStamped [t1] - updating to read lock... 256
15:57:02.719 c.DataContainerStamped [t2] - write unlock 384
15:57:02.719 c.DataContainerStamped [t1] - read lock 513
15:57:03.719 c.DataContainerStamped [t1] - read finish...513, data:1000
15:57:03.719 c.DataContainerStamped [t1] - read unlock 513

注意

• StampedLock 不支持条件变量
• StampedLock 不支持可重入

2. Semaphore(信号量)

信号量，用来限制能同时访问共享资源的线程上限。

1.自定义例子

public static void main(String[] args) {
// 1. 创建 semaphore 对象,每次都有3个线程能同时工作，
//和线程池类似
Semaphore semaphore = new Semaphore(3);
// 2. 10个线程同时运行
for (int i = 0; i < 10; i++) {
new Thread(() -> {
// 3. 获取许可
try {
semaphore.acquire();
} catch (InterruptedException e) {
e.printStackTrace();
}
try {
log.debug("running...");
sleep(1);
log.debug("end...");
} finally {
// 4. 释放许可
semaphore.release();
}
}).start();
}
}

2. Semaphore加锁解锁流程

Semaphore 有点像一个停车场，permits 就好像停车位数量，当线程获得了 permits 就像是获得了停车位，然后
停车场显示空余车位减一
刚开始，permits（state）为 3，这时 5 个线程来获取资源

注意：这里的state表示可以加进来的线程数，0就是没有线程空位了，与之前的state正好相反。

假设其中 Thread-1，Thread-2，Thread-4 cas 竞争成功，
而 Thread-0 和 Thread-3 竞争失败，进入 AQS 队列park 阻塞

这时 Thread-4 释放了 permits，状态如下

接下来 Thread-0 竞争成功，permits 再次设置为 0，
设置自己为 head 节点，断开原来的 head 节点，unpark 
接下来的 Thread-3 节点，但由于 permits 是 0，
因此 Thread-3 在尝试不成功后再次进入 park 状态

3. Semaphore源码

其实和之前的源码分析差不多，这里直接上源码了

static final class NonfairSync extends Sync {
private static final long serialVersionUID = -2694183684443567898L;
NonfairSync(int permits) {
// permits 即 state
super(permits);
}
// Semaphore 方法, 方便阅读, 放在此处
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
// AQS 继承过来的方法, 方便阅读, 放在此处
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
// 尝试获得共享锁
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
// Sync 继承过来的方法, 方便阅读, 放在此处
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (
// 如果许可已经用完, 返回负数, 表示获取失败, 进入 doAcquireSharedInterruptibly
remaining < 0 ||
// 如果 cas 重试成功, 返回正数, 表示获取成功
compareAndSetState(available, remaining)
) {
return remaining;
}
}
}
// AQS 继承过来的方法, 方便阅读, 放在此处
private void doAcquireSharedInterruptibly(int arg) throws InterruptedException {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
// 再次尝试获取许可
int r = tryAcquireShared(arg);
if (r >= 0) {
// 成功后本线程出队（AQS）, 所在 Node设置为 head
// 如果 head.waitStatus == Node.SIGNAL ==> 0 成功, 下一个节点 unpark
// 如果 head.waitStatus == 0 ==> Node.PROPAGATE
// r 表示可用资源数, 为 0 则不会继续传播
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
// 不成功, 设置上一个节点 waitStatus = Node.SIGNAL, 下轮进入 park 阻塞
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
// Semaphore 方法, 方便阅读, 放在此处
public void release() {
sync.releaseShared(1);
}
// AQS 继承过来的方法, 方便阅读, 放在此处
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
// Sync 继承过来的方法, 方便阅读, 放在此处
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}
}

3. CountdownLatch（倒计时）

用来进行线程同步协作，等待所有线程完成倒计时。
其中构造参数用来初始化等待计数值，await() 用来等待计数归零，countDown() 用来让计数减一

public static void main(String[] args) throws InterruptedException {
//3代表倒计时从3开始到0结束
CountDownLatch latch = new CountDownLatch(3);
new Thread(() -> {
log.debug("begin...");
sleep(1);
//countDown() 用来让计数减一
latch.countDown();
log.debug("end...{}", latch.getCount());
}).start();
new Thread(() -> {
log.debug("begin...");
sleep(2);
//countDown() 用来让计数减一
latch.countDown();
log.debug("end...{}", latch.getCount());
}).start();
new Thread(() -> {
log.debug("begin...");
sleep(1.5);
//countDown() 用来让计数减一
latch.countDown();
log.debug("end...{}", latch.getCount());
}).start();
log.debug("waiting...");
//当计数减为0的时候，开始运行在这之后的程序
latch.await();
log.debug("wait end...");
}

结果：
18:44:00.778 c.TestCountDownLatch [main] - waiting...
18:44:00.778 c.TestCountDownLatch [Thread-2] - begin...
18:44:00.778 c.TestCountDownLatch [Thread-0] - begin...
18:44:00.778 c.TestCountDownLatch [Thread-1] - begin...
18:44:01.782 c.TestCountDownLatch [Thread-0] - end...2
18:44:02.283 c.TestCountDownLatch [Thread-2] - end...1
18:44:02.782 c.TestCountDownLatch [Thread-1] - end...0
18:44:02.782 c.TestCountDownLatch [main] - wait end...