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Spring事务与锁的正确使用姿势(上)
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Bug Maker
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2018-06-10
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<p><img src="http://pa3mctxwu.bkt.clouddn.com/cat.jpg" alt="Markdown"><br><a id="more"></a><br>最近学习spring<strong>事务</strong>的时候,自己弄了一个demo项目,其涉及到<strong>并发</strong>方面的知识,在线程同步的时候使用<strong><strong>Lock</strong></strong>(在并发小的情况下可以采用synchronized,关于synchronized与lock的详解,<a href="https://blog.csdn.net/u012403290/article/details/64910926?locationNum=11&fps=1" target="_blank" rel="noopener"><strong>推荐一篇博文</strong></a>),测试期间出现了了问题.该demo模拟抢购场景,在每次测试的时候都会出现卖多的情况,例如商品只有1000件,会出现销售超过1000件的情况。 </p>
<p>由于spring的bean<strong>默认是单例</strong>的(此项目的bean配置是默认配置),所以,可以保证下面代码中的lock是单例的.<br><strong>代码如下</strong>: </p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Service</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ShopingServiceImpl</span> <span class="keyword">implements</span> <span class="title">ShopingService</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> Lock lock = <span class="keyword">new</span> ReentrantLock(<span class="keyword">true</span>);<span class="comment">//互斥锁 参数默认false，不公平锁</span></span><br><span class="line"> <span class="meta">@Autowired</span></span><br><span class="line"> <span class="keyword">private</span> ShoppingDao dao;</span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="meta">@Transactional</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> Result <span class="title">shopping</span><span class="params">(String goodId, String userId)</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> lock.lock();</span><br><span class="line"> <span class="keyword">int</span> num=dao.selectNumber(goodId);</span><br><span class="line"> <span class="keyword">if</span>(num><span class="number">0</span>){<span class="comment">//当前商品还有库存</span></span><br><span class="line"> dao.setNumber(num-<span class="number">1</span>);</span><br><span class="line"> }<span class="keyword">else</span>{<span class="comment">//当前商品没有库存</span></span><br><span class="line"> <span class="keyword">return</span> Result.unsuccess(<span class="string">"商品已经被抢购完了"</span>);</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }<span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> Result.success(<span class="string">"抢购成功"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>后来终于发现出现问题的原因，在代码中，事务是通过<strong>注解</strong>的形式实现，而lock在方法里面就已经释放，<strong>即可能出现锁已经释放而事务还未提交的情况</strong>。按照正常操作来说，应该由锁来包住事务，而不应在事务中加锁。为了让锁包住事务，我采取另一种方案，<strong>==使用切面处理锁==</strong></p>
<h4 id="使用AOP"><a href="#使用AOP" class="headerlink" title="使用AOP"></a>使用AOP</h4><p>(1)创建锁注解<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Target</span>({ElementType.TYPE,ElementType.METHOD})</span><br><span class="line"><span class="meta">@Retention</span>(RetentionPolicy.RUNTIME)</span><br><span class="line"><span class="keyword">public</span> <span class="meta">@interface</span> ShoppingLock {</span><br><span class="line"> <span class="function">String <span class="title">value</span><span class="params">()</span> <span class="keyword">default</span> ""</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p>(2)声明切面<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Aspect</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ShoppingAspect</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Lock lock = <span class="keyword">new</span> ReentrantLock(<span class="keyword">true</span>);<span class="comment">//互斥锁</span></span><br><span class="line"> </span><br><span class="line"> <span class="comment">/*</span></span><br><span class="line"><span class="comment"> * 声明切点,表示切点为ShoppingLock注解的代码</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="meta">@Pointcut</span>(<span class="string">"@annotation(com.bugmaker.shopping.ShoppingLock)"</span>)</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">ShoppingAspect</span><span class="params">()</span></span>{</span><br><span class="line"> </span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Around</span>(<span class="string">"ShoppingAspect()"</span>)</span><br><span class="line"> <span class="function"><span class="keyword">public</span> Object <span class="title">around</span><span class="params">(ProceedingJoinPoint joinPoint)</span> </span>{ </span><br><span class="line"> lock.lock();<span class="comment">//加锁</span></span><br><span class="line"> Object obj = <span class="keyword">null</span>;</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> obj = joinPoint.proceed();</span><br><span class="line"> } <span class="keyword">catch</span> (Throwable e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span>{</span><br><span class="line"> lock.unlock();<span class="comment">//释放锁</span></span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> obj;</span><br><span class="line"> } </span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p>(3)service代码<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Service</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ShopingServiceImpl</span> <span class="keyword">implements</span> <span class="title">ShopingService</span> </span>{</span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Autowired</span></span><br><span class="line"> <span class="keyword">private</span> ShoppingDao dao;</span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="meta">@ShoppingLock</span><span class="comment">//添加注解,注解为切点,切面中添加了锁处理</span></span><br><span class="line"> <span class="meta">@Transactional</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> Result <span class="title">shopping</span><span class="params">(String goodId, String userId)</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">int</span> num=dao.selectNumber(goodId);</span><br><span class="line"> <span class="keyword">if</span>(num><span class="number">0</span>){<span class="comment">//当前商品还有库存</span></span><br><span class="line"> dao.setNumber(num-<span class="number">1</span>);</span><br><span class="line"> <span class="keyword">return</span> Result.success(<span class="string">"抢购成功"</span>);</span><br><span class="line"> }<span class="keyword">else</span>{<span class="comment">//当前商品没有库存</span></span><br><span class="line"> <span class="keyword">return</span> Result.unsuccess(<span class="string">"商品已经被抢购完了"</span>);</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<h4 id="这样就大功告成了-事情好像没那么简单"><a href="#这样就大功告成了-事情好像没那么简单" class="headerlink" title="这样就大功告成了?事情好像没那么简单!"></a>这样就大功告成了?事情好像没那么简单!</h4><p>ShoppingLock与Transactional的作用范围跟其注解添加的顺序有没有关系呢? </p>
<h5 id="欲知后事-请听下集揭晓。"><a href="#欲知后事-请听下集揭晓。" class="headerlink" title="欲知后事,请听下集揭晓。"></a>欲知后事,请听下集揭晓。</h5>
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