自旋鎖淺析

時間 2019-12-10

標籤淺析简体版

原文原文鏈接

　　自旋鎖的洋名叫spin lock，是一種比較有個性的鎖，由於它站在傳統的互斥鎖的對立面。若是併發時，互斥鎖的作法是讓線程阻塞，但自旋鎖卻不這麼作，而是原地打轉，不停的去搶鎖，搶不到誓不罷休。簡而言之，互斥鎖是重量級（悲觀）鎖，自旋鎖是輕量級（樂觀）鎖。自旋鎖使用場景是：一、多核處理器，二、線程等待鎖的時間很短，短到比線程兩次上下文切換時間還少，說白了就是鎖裏操做的事情很簡單。java

　　如何實現自旋鎖呢？唯有CAS。何謂CAS？它的洋名叫Compare And Swap，簡單來講就是比較並交換。該算法涉及三個數：內存值V，舊的預期值A，新的預期值B。當且僅當舊的預期值A和內存值V相同時，將內存值改成B，不然什麼也不作。算法

　　CAS 是實現自旋鎖的基礎（也是實現樂觀鎖的基礎），CAS 利用 CPU 指令保證了操做的原子性，以達到鎖的效果。先看個JDK實例：多線程

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() + "]";
    }
}

　　看上面標紅那裏，就是自旋鎖實現的關鍵：一、無限循環；二、CAS。接下來再看自旋鎖的實現與應用場景：併發

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;

/**
 * 自旋鎖
 *
 * @author wulinfeng
 * @version C10 2018年12月21日
 * @since SDP V300R003C10
 */
public class SpinLock implements Lock
{
    // 利用AtomicBoolean來調用CAS，ab初始（內存）值是false
    private AtomicBoolean ab = new AtomicBoolean(false);
    
    @Override
    public void lock()
    {
        /*
         * getAndSet將ab設置爲true，並返回ab以前（內存）的值。 
         * 由於ab的初始（內存）值就是false，因此第一個線程不會進入循環，也就是說它搶到了鎖
         * 然後面的線程來的時候，內存值已是true，將進入循環自旋
         */
        while (ab.getAndSet(true))
        {
        }
        
    }
    
    @Override
    public void unlock()
    {
        // 將內存值從新設置爲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;
    }
    
}

　　測試代碼：app

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
{
    // 開始時間
    private long startTime = 0L;
    
    // 計數器
    private int count = 0;
    
    // 讓Junit支持多線程，10個線程就先初始化10
    private CountDownLatch latch = new CountDownLatch(10);
    
    @Before
    public void before()
    {
        startTime = System.currentTimeMillis();
    }
    
    @After
    public void after()
    {
        System.out.printf("count值：%d, 耗時：%d毫秒.\n", count, System.currentTimeMillis() - startTime);
    }
    
    @Test
    public void testSpinLock()
    {
        // 初始化自旋鎖
        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++)
                    {
　　　　　　　　　　　　　　 // 加鎖
                    　　 sl.lock();

                        // 自增
                        count++;

                    　　 // 解鎖
                    　　 sl.unlock();
                    }                    // 一個線程執行完了就減1,10個線程執行完了就變成0，執行主線程
                    latch.countDown();
                }
            }).start();
        }
        
        // 主線程等待
        try
        {
            latch.await();
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        
        TestCase.assertEquals(count, 10000);
    }
}

　　輸出結果：less

count值：10000, 耗時：9毫秒.

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