自旋锁浅析
自旋锁的洋名叫spin lock，是一种比较有个性的锁，因为它站在传统的互斥锁的对立面。如果并发时，互斥锁的做法是让线程阻塞，但自旋锁却不这么做，而是原地打转，不停的去抢锁，抢不到誓不罢休。简而言之，互斥锁是重量级（悲观）锁，自旋锁是轻量级（乐观）锁。自旋锁使用场景是：1、多核处理器，2、线程等待锁的时间很短，短到比线程两次上下文切换时间还少，说白了就是锁里操作的事情很简单。
如何实现自旋锁呢？惟有CAS。何谓CAS？它的洋名叫Compare And Swap，简单来说就是比较并交换。该算法涉及三个数：内存值V，旧的预期值A，新的预期值B。当且仅当旧的预期值A和内存值V相同时，将内存值改为B，否则什么也不做。
CAS 是实现自旋锁的基础（也是实现乐观锁的基础），CAS 利用 CPU 指令保证了操作的原子性，以达到锁的效果。先看个JDK实例：
public class CountDownLatch { /** * Synchronization control For CountDownLatch. * Uses AQS state to represent count. */ private static final class Sync extends AbstractQueuedSynchronizer { private static final long serialVersionUID = 4982264981922014374L; Sync(int count) { setState(count); } int getCount() { return getState(); } protected int tryAcquireShared(int acquires) { return (getState() == 0) ? 1 : -1; } protected boolean tryReleaseShared(int releases) { // Decrement count; signal when transition to zero for (;;) { int c = getState(); if (c == 0) return false; int nextc = c-1; if (compareAndSetState(c, nextc)) return nextc == 0; } } } private final Sync sync; /** * Constructs a {@code CountDownLatch} initialized with the given count. * * @param count the number of times {@link #countDown} must be invoked * before threads can pass through {@link #await} * @throws IllegalArgumentException if {@code count} is negative */ public CountDownLatch(int count) { if (count < 0) throw new IllegalArgumentException("count < 0"); this.sync = new Sync(count); } /** * Causes the current thread to wait until the latch has counted down to * zero, unless the thread is {@linkplain Thread#interrupt interrupted}. * * <p>If the current count is zero then this method returns immediately. * * <p>If the current count is greater than zero then the current * thread becomes disabled for thread scheduling purposes and lies * dormant until one of two things happen: * <ul> * <li>The count reaches zero due to invocations of the * {@link #countDown} method; or * <li>Some other thread {@linkplain Thread#interrupt interrupts} * the current thread. * </ul> * * <p>If the current thread: * <ul> * <li>has its interrupted status set on entry to this method; or * <li>is {@linkplain Thread#interrupt interrupted} while waiting, * </ul> * then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * * @throws InterruptedException if the current thread is interrupted * while waiting */ public void await() throws InterruptedException { sync.acquireSharedInterruptibly(1); } /** * Causes the current thread to wait until the latch has counted down to * zero, unless the thread is {@linkplain Thread#interrupt interrupted}, * or the specified waiting time elapses. * * <p>If the current count is zero then this method returns immediately * with the value {@code true}. * * <p>If the current count is greater than zero then the current * thread becomes disabled for thread scheduling purposes and lies * dormant until one of three things happen: * <ul> * <li>The count reaches zero due to invocations of the * {@link #countDown} method; or * <li>Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or * <li>The specified waiting time elapses. * </ul> * * <p>If the count reaches zero then the method returns with the * value {@code true}. * * <p>If the current thread: * <ul> * <li>has its interrupted status set on entry to this method; or * <li>is {@linkplain Thread#interrupt interrupted} while waiting, * </ul> * then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * * <p>If the specified waiting time elapses then the value {@code false} * is returned. If the time is less than or equal to zero, the method * will not wait at all. * * @param timeout the maximum time to wait * @param unit the time unit of the {@code timeout} argument * @return {@code true} if the count reached zero and {@code false} * if the waiting time elapsed before the count reached zero * @throws InterruptedException if the current thread is interrupted * while waiting */ public boolean await(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); } /** * Decrements the count of the latch, releasing all waiting threads if * the count reaches zero. * * <p>If the current count is greater than zero then it is decremented. * If the new count is zero then all waiting threads are re-enabled for * thread scheduling purposes. * * <p>If the current count equals zero then nothing happens. */ public void countDown() { sync.releaseShared(1); } /** * Returns the current count. * * <p>This method is typically used for debugging and testing purposes. * * @return the current count */ public long getCount() { return sync.getCount(); } /** * Returns a string identifying this latch, as well as its state. * The state, in brackets, includes the String {@code "Count ="} * followed by the current count. * * @return a string identifying this latch, as well as its state */ public String toString() { return super.toString() + "[Count = " + sync.getCount() + "]"; } }
看上面标红那里，就是自旋锁实现的关键：1、无限循环；2、CAS。接下来再看自旋锁的实现与应用场景：
package com.wulf.test.testpilling.util; import java.util.concurrent.TimeUnit; import java.util.concurrent.atomic.AtomicBoolean; import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.Lock; /** * 自旋锁 * * @author wulf * @since 2018年12月21日*/ public class SpinLock implements Lock { // 利用AtomicBoolean来调用CAS，ab初始（内存）值是false private AtomicBoolean ab = new AtomicBoolean(false); @Override public void lock() { /* * getAndSet将ab设置为true，并返回ab之前（内存）的值。 * 因为ab的初始（内存）值就是false，所以第一个线程不会进入循环，也就是说它抢到了锁 * 而后面的线程来的时候，内存值已经是true，将进入循环自旋 */ while (ab.getAndSet(true)) { } } @Override public void unlock() { // 将内存值重新设置为false ab.set(false); } @Override public void lockInterruptibly() throws InterruptedException { // TODO Auto-generated method stub } @Override public boolean tryLock() { // TODO Auto-generated method stub return false; } @Override public boolean tryLock(long time, TimeUnit unit) throws InterruptedException { // TODO Auto-generated method stub return false; } @Override public Condition newCondition() { // TODO Auto-generated method stub return null; } }
测试代码：
package com.wulf.test.testpilling; import java.util.concurrent.CountDownLatch; import org.junit.After; import org.junit.Before; import org.junit.Test; import com.wulf.test.testpilling.util.SpinLock; import junit.framework.TestCase; public class SpinLockTest { // 开始时间 private long startTime = 0L; // 计数器 private int count = 0; // 让Junit支持多线程，10个线程就先初始化10 private CountDownLatch latch = new CountDownLatch(10); @Before public void before() { startTime = System.currentTimeMillis(); } @After public void after() { System.out.printf("count值：%d, 耗时：%d毫秒.\n", count, System.currentTimeMillis() - startTime); } @Test public void testSpinLock() { // 初始化自旋锁 SpinLock sl = new SpinLock(); for (int i = 0; i < 10; i++) { new Thread(new Runnable() { @Override public void run() { for (int j = 0; j < 1000; j++) {
// 加锁
sl.lock();
// 自增 count++;
// 解锁
sl.unlock(); } // 一个线程执行完了就减1,10个线程执行完了就变成0，执行主线程 latch.countDown(); } }).start(); } // 主线程等待 try { latch.await(); } catch (InterruptedException e) { e.printStackTrace(); } TestCase.assertEquals(count, 10000); } }
输出结果：
count值：10000, 耗时：9毫秒.
