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.wulinfeng.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 wulinfeng
 * @version C10 2018年12月21日
 * @since SDP V300R003C10
 */
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.wulinfeng.test.testpilling;

import java.util.concurrent.CountDownLatch;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import com.wulinfeng.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.
", 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.