《利用CAS实现自旋锁》

利用CAS实现自旋锁

1.自旋锁概念2.编写自旋锁3.测试自旋锁

1.自旋锁概念

自旋锁（spinlock）：是指当一个线程在获取锁的时候，如果锁已经被其它线程获取，那么该线程将循环等待，然后不断的判断锁是否能够被成功获取，直到获取到锁才会退出循环。

2.编写自旋锁

import java.util.concurrent.atomic.AtomicBoolean; //自旋锁 public class SpinLock { //CAS private AtomicBoolean cas = new AtomicBoolean(false); //锁持有者线程 private Thread spinLockOwnerThread = null; public void lock(){ while (!cas.compareAndSet(false,true)) { //CAS自旋 } spinLockOwnerThread = Thread.currentThread(); } public void unLock(){ //当前线程等于锁持有着线程才允许解锁 if (Thread.currentThread()==spinLockOwnerThread) { spinLockOwnerThread = null; cas.set(false); //最后设置CAS的值为false，为什么？ //如果先执行cas.set(false)，此时又来了一个新的线程， //成功过了CAS的compareAndSet， //将 spinLockOwnerThread 指向Thread.currentThread(); //最后再执行spinLockOwnerThread = null; //这会导致最后持有锁的线程无法解锁 //也就是无法通过Thread.currentThread()==spinLockOwnerThread判断 //从而导致程序一直阻塞 } } }

3.测试自旋锁

class Test { //引入自旋锁 private static final SpinLock spinLock = new SpinLock(); public static void main(String[] args) { Thread t1 = new Thread(Test::run,"t1"); Thread t2 = new Thread(Test::run,"t2"); Thread t3 = new Thread(Test::run,"t3"); Thread t4 = new Thread(Test::run,"t4"); t1.start(); t2.start(); t3.start(); t4.start(); } public static void run(){ spinLock.lock(); System.out.println(Thread.currentThread().getName()); try { Thread.sleep(5000); } catch (InterruptedException e) { e.printStackTrace(); } finally { spinLock.unLock(); } } }