前言

上一篇博客我们讲到秒杀问题的一人一单在单机模式下使用synchronized添加悲观锁能解决并发问题。

但是在集群模式下，我们使用悲观锁就无法解决并发问题，因为集群中每个java虚拟机不是共用一个锁，而是每个java虚拟机都拥有属于自己的锁，因而就无法保证在并发模式下的一人一单问题。

怎么解决呢？

那就要引入我们今天的要学习的内容—分布式锁。

我们这里先简要介绍一下分布式锁原理：集群模式悲观锁失效是因为每个JVM有其独自的锁，而集群模式就必须共享同一个锁，因此，我们取消每个JVM中的锁，让所有的JVM都去共用同一个锁监视器，这就是分布式锁的基本思想。

1. 什么是分布式锁

分布式锁：满足分布式系统或者集群模式下多进程可见并且互斥的锁。

它需要满足以下特点：

实现效果：同一时刻只有一个线程能拥有互斥锁，只有该线程释放互斥锁，其他线程才能获得互斥锁的使用权。

2. 分布式锁的实现

这里我们使用redis提供的分布式锁功能。

2.1 基于Redis的分布式锁实现方法

这里我们要明确一个点，因为我们学习redis需要考虑在高并发模式下，很多线程共享资源的问题。资源共享就会涉及进程阻塞问题，因此每一步操作就需要尽量满足原子性，减少进程阻塞导致的并发问题。

因此，我们在获取锁的方式可以修改为如下方式

将设置互斥锁和超时时间同步完成，是为了解决在设置锁的时候发生进程阻塞导致锁未设置超时时间，导致锁无法释放的问题。

OK，接下来我们就根据以上方法设置我们的第一个分布式锁。

2.2 基于redis实现分布式锁的初级版本

1. 需求分析
   

2. 梳理流程
   

3. 代码实现

public class SimpleRedisLock implements ILock{
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">String</span> KEY_PREFIX <span class="token operator">=</span> <span class="token string">"lock:"</span><span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">String</span> ID_PREFIX <span class="token operator">=</span> UUID<span class="token punctuation">.</span><span class="token function">randomUUID</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token string">"-"</span><span class="token punctuation">;</span>

<span class="token keyword">private</span> <span class="token class-name">StringRedisTemplate</span> stringRedisTemplate<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token class-name">SimpleRedisLock</span><span class="token punctuation">(</span><span class="token class-name">StringRedisTemplate</span> stringRedisTemplate<span class="token punctuation">,</span> <span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>stringRedisTemplate <span class="token operator">=</span> stringRedisTemplate<span class="token punctuation">;</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">tryLock</span><span class="token punctuation">(</span><span class="token keyword">long</span> timeSec<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token comment">//获取线程id</span>
    <span class="token class-name">String</span> threadId <span class="token operator">=</span> ID_PREFIX<span class="token operator">+</span><span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment">//获取锁</span>
    <span class="token class-name">Boolean</span> success <span class="token operator">=</span> stringRedisTemplate<span class="token punctuation">.</span><span class="token function">opsForValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">setIfAbsent</span><span class="token punctuation">(</span>KEY_PREFIX <span class="token operator">+</span> name<span class="token punctuation">,</span> threadId<span class="token punctuation">,</span> timeSec<span class="token punctuation">,</span> <span class="token class-name">TimeUnit</span><span class="token punctuation">.</span>SECONDS<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment">//返回的是boolean类型，是基本数据类型，而我们返回的Boolean类型是包装类，避免拆箱出现空指针。</span>
    <span class="token keyword">return</span> <span class="token class-name">Boolean</span><span class="token punctuation">.</span>TRUE<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>success<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">unLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token comment">//删除锁</span>
    stringRedisTemplate<span class="token punctuation">.</span><span class="token function">delete</span><span class="token punctuation">(</span>KEY_PREFIX <span class="token operator">+</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token punctuation">}</span>

}

这里说明两点

• 我们设置的锁需要满足互斥以及具有超时时间，我们使用setIfAbsent()方法，它不仅满足互斥条件，还能同时设置超时时间，满足需要。
  

• 拆箱与装箱

1）什么是拆箱
• 拆箱就是自动将包装器类型转换为基本数据类型
• 拆箱调用Integer.intValue方法

2）什么是装箱
• 装箱就是自动将基本数据类型转换为包装器类型
• 装箱调用的Integer.valueOf方法

我们在尝试获取锁的方法中返回值时基本数据类型，而设置锁返回的时包装器类型，如果我们将其直接返回会涉及拆箱和装箱过程，我们在开发过程中要尽量避免拆箱和装箱，因为可能会出现空指针问题。

在秒杀券的一人一单代码实现之前获取我们设置的分布式互斥锁，添加基本的分布式锁,由于后面执行代码可能抛异常，因此需要try/finally释放锁

 //添加基本的分布式锁,由于后面执行代码可能抛异常，因此需要try/finally释放锁
        SimpleRedisLock redisLock = new SimpleRedisLock(redisTemplate, "order:" + userId);
        boolean flag = redisLock.tryLock(5);
        if(!flag){
            return Result.fail("一人只能购买一张优惠券");
        }
    <span class="token keyword">try</span> <span class="token punctuation">{<!-- --></span>
        <span class="token comment">//一人一单</span>
        <span class="token keyword">int</span> count <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">query</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">eq</span><span class="token punctuation">(</span><span class="token string">"user_id"</span><span class="token punctuation">,</span> userId<span class="token punctuation">)</span>
                <span class="token punctuation">.</span><span class="token function">eq</span><span class="token punctuation">(</span><span class="token string">"voucher_id"</span><span class="token punctuation">,</span> voucherId<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">count</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span>count<span class="token operator">&gt;</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
            <span class="token keyword">return</span> <span class="token class-name">Result</span><span class="token punctuation">.</span><span class="token function">fail</span><span class="token punctuation">(</span><span class="token string">"一个用户只能购买一个优惠券"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        <span class="token comment">//当更新时查询的库存大于0时进行库存减一</span>
        <span class="token keyword">boolean</span> success <span class="token operator">=</span> seckillVoucherService<span class="token punctuation">.</span><span class="token function">update</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
                <span class="token punctuation">.</span><span class="token function">setSql</span><span class="token punctuation">(</span><span class="token string">"stock=stock-1"</span><span class="token punctuation">)</span>
                <span class="token punctuation">.</span><span class="token function">gt</span><span class="token punctuation">(</span><span class="token string">"voucher_id"</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span>
                <span class="token punctuation">.</span><span class="token function">eq</span><span class="token punctuation">(</span><span class="token string">"stock"</span><span class="token punctuation">,</span> voucher<span class="token punctuation">.</span><span class="token function">getStock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">update</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>success<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
            <span class="token keyword">return</span> <span class="token class-name">Result</span><span class="token punctuation">.</span><span class="token function">fail</span><span class="token punctuation">(</span><span class="token string">"优惠券已被抢完"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>

        <span class="token comment">//6. 创建订单</span>
        <span class="token comment">//6.1 设置id</span>
        <span class="token class-name">VoucherOrder</span> voucherOrder <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">VoucherOrder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">Long</span> voucherOrderId <span class="token operator">=</span> <span class="token class-name">RedisIdWorker</span><span class="token punctuation">.</span><span class="token function">nextId</span><span class="token punctuation">(</span><span class="token string">"voucherOrder"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        voucherOrder<span class="token punctuation">.</span><span class="token function">setId</span><span class="token punctuation">(</span>voucherOrderId<span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token comment">//6.2 设置user_id</span>
        voucherOrder<span class="token punctuation">.</span><span class="token function">setUserId</span><span class="token punctuation">(</span>userId<span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token comment">//6.3 设置优惠券id</span>
        voucherOrder<span class="token punctuation">.</span><span class="token function">setVoucherId</span><span class="token punctuation">(</span>voucherId<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">save</span><span class="token punctuation">(</span>voucherOrder<span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">return</span> <span class="token class-name">Result</span><span class="token punctuation">.</span><span class="token function">ok</span><span class="token punctuation">(</span>voucherOrderId<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{<!-- --></span>
        redisLock<span class="token punctuation">.</span><span class="token function">unLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

2.3 改进分布式锁

我们结合下面的图进行分析一下使用上述分布式锁可能发生的情况：

1. 当线程1获取redis锁后执行业务，这时业务阻塞，因而导致redis锁超时释放。
2. 线程2此时获取到redis锁，执行业务，线程2执行业务过程中，线程1被唤醒，完成业务释放了redis锁，而业务2还在执行。
3. 线程3获取redis锁，也开始执行业务，此时，线程2和线程3都在执行业务，我们的redis锁在并发模式下又形同虚设了。

怎么解决呢？

我们想想之前是如何解决超卖问题呢？是不是使用了CAS方法。即扣减库存的时候判断库存是否跟之前查询的库存相等，相等再去扣减库存。

方法类似，即我们在删除锁的时候可以设置一个线程标识，标识此时是哪个线程获取了锁。假如一旦获取锁的线程1阻塞，锁超时释放，此时线程2获取了锁，将锁的标识更换了，线程2执行业务的时候线程1唤醒，想要去释放锁，结果发现锁已经不是自己的，就不去释放。问题就解决了。

ok，我们看一下解决的流程图

我们只需要在删除锁的代码进行修改即可。

	public void unLock() {
    <span class="token comment">//获取当前线程ID</span>
    <span class="token class-name">String</span> currentId <span class="token operator">=</span> stringRedisTemplate<span class="token punctuation">.</span><span class="token function">opsForValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>KEY_PREFIX <span class="token operator">+</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment">//线程标识</span>
    <span class="token class-name">String</span> threadId <span class="token operator">=</span> ID_PREFIX<span class="token operator">+</span><span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment">//判断当前线程与线程标识是否相同，相同再进行释放</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>threadId<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>currentId<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
        <span class="token comment">//删除锁</span>
        stringRedisTemplate<span class="token punctuation">.</span><span class="token function">delete</span><span class="token punctuation">(</span>KEY_PREFIX <span class="token operator">+</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token comment">//删除锁</span>
    <span class="token comment">//stringRedisTemplate.delete(KEY_PREFIX + name);</span>

<span class="token punctuation">}</span>

2.4 基于Lua脚本改善分布式锁

上述改进能否完美解决因线程阻塞而导致的锁错误释放问题呢？

我们不妨来看一下下面这张图。

看不明白没关系，我帮你们读懂一下。

1. 首先，线程1尝试获取锁成功，执行业务，执行完成后，尝试去释放锁，当程序判断完成是自己的锁后刚要去执行释放锁操作，诶，真不巧 ，线程被阻塞了，锁又被超时释放了。
2. 这时，线程2尝试获取锁成功，执行业务，此时，线程1又醒了，醒了还没完，因为线程1已经判断锁是自己的，因此人家醒了二话不说就把锁给释放了
3. 最后，线程3进来了，它也获取锁成功，执行业务，此时线程2和线程3就并行执行业务，我们的梦想又破灭了。。。
   

怎么解决呢？

我们想想问题的关键是不是因为判断锁和删除锁是分开进行的，因而才会出现线程阻塞问题。那我们能不能将这两步操作改成原子级别的操作呢？

单纯靠java实现可能不太行，这里就要引入我们要讲的主角，Lua脚本。

ok，现在我们基于Lua脚本来改进一下我们的释放锁的逻辑。

Lua脚本

-- 比较线程标示与锁中的标示是否一致
if(redis.call('get', KEYS[1]) ==  ARGV[1]) then
    -- 释放锁 del key
    return redis.call('del', KEYS[1])
end
return 0

public class SimpleRedisLock implements ILock{
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">String</span> KEY_PREFIX <span class="token operator">=</span> <span class="token string">"lock:"</span><span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">String</span> ID_PREFIX <span class="token operator">=</span> UUID<span class="token punctuation">.</span><span class="token function">randomUUID</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token string">"-"</span><span class="token punctuation">;</span>

<span class="token comment">//代码开始就加载静态代码块，也就是lua脚本</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">DefaultRedisScript</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Long</span><span class="token punctuation">&gt;</span></span> UNLOCK_SCRIPT<span class="token punctuation">;</span>
<span class="token keyword">static</span> <span class="token punctuation">{<!-- --></span>
    UNLOCK_SCRIPT <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">DefaultRedisScript</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    UNLOCK_SCRIPT<span class="token punctuation">.</span><span class="token function">setLocation</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ClassPathResource</span><span class="token punctuation">(</span><span class="token string">"unlock.lua"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    UNLOCK_SCRIPT<span class="token punctuation">.</span><span class="token function">setResultType</span><span class="token punctuation">(</span><span class="token class-name">Long</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">private</span> <span class="token class-name">StringRedisTemplate</span> stringRedisTemplate<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token class-name">SimpleRedisLock</span><span class="token punctuation">(</span><span class="token class-name">StringRedisTemplate</span> stringRedisTemplate<span class="token punctuation">,</span> <span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>stringRedisTemplate <span class="token operator">=</span> stringRedisTemplate<span class="token punctuation">;</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">tryLock</span><span class="token punctuation">(</span><span class="token keyword">long</span> timeSec<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token comment">//获取线程id</span>
    <span class="token class-name">String</span> threadId <span class="token operator">=</span> ID_PREFIX<span class="token operator">+</span><span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment">//获取锁</span>
    <span class="token class-name">Boolean</span> success <span class="token operator">=</span> stringRedisTemplate<span class="token punctuation">.</span><span class="token function">opsForValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">setIfAbsent</span><span class="token punctuation">(</span>KEY_PREFIX <span class="token operator">+</span> name<span class="token punctuation">,</span> threadId<span class="token punctuation">,</span> timeSec<span class="token punctuation">,</span> <span class="token class-name">TimeUnit</span><span class="token punctuation">.</span>SECONDS<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment">//返回的是boolean类型，是基本数据类型，而我们返回的Boolean类型是包装类，避免拆箱出现空指针。</span>
    <span class="token comment">/*boolean flag = Boolean.TRUE.equals(success);
    return flag;*/</span>
    <span class="token keyword">return</span> <span class="token class-name">Boolean</span><span class="token punctuation">.</span>TRUE<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>success<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">unLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>

    <span class="token comment">//使用lua脚本执行原子级别的操作，不会因为线程阻塞导致释放锁发生错误。</span>
    stringRedisTemplate<span class="token punctuation">.</span><span class="token function">execute</span><span class="token punctuation">(</span>UNLOCK_SCRIPT<span class="token punctuation">,</span>
            <span class="token class-name">Collections</span><span class="token punctuation">.</span><span class="token function">singletonList</span><span class="token punctuation">(</span>KEY_PREFIX <span class="token operator">+</span> name<span class="token punctuation">)</span><span class="token punctuation">,</span>
            ID_PREFIX<span class="token operator">+</span><span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token punctuation">}</span>

}

执行lua脚本调用 stringRedisTemplate.execute的API，传递三个参数


经过三个版本的分布式锁迭代，我们设计的锁就基本能够满足我们的使用需求了。