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<h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>用了Spring这么多年，以为我对循环依赖的问题已经了如指掌，但是现实还是给我了一巴掌。</p>
<p>最近线上服务报了错误：</p>
<blockquote>
<p>Bean with name xx has been injected into other beans [ x ] in its raw version as part of a circular reference, </p>
<p>but has eventually been wrapped. This means that said other beans do not use the final version of the bean. </p>
<p>This is often the result of over-eager type matching - consider using ‘getBeanNamesOfType’ with the ‘allowEagerInit’ flag turned off, for example.</p>
</blockquote>
<p>Exception是BeanCurrentlyInCreationException，看起来是个循环依赖的问题，但是日志的报错是我从来没见过的。</p>
<h2 id="正常的循环依赖"><a href="#正常的循环依赖" class="headerlink" title="正常的循环依赖"></a>正常的循环依赖</h2><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></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">A</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> B b;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="title">A</span><span class="params">(B b)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.b = b;</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"></span><br><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">B</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> A a;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="title">B</span><span class="params">(A a)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.a = a;</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>如上，我们创建了两个Service，并且通过构造函数的方式注入。</p>
<p>启动时，Spring会报错：</p>
<blockquote>
<p>Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name ‘a’: Requested bean is currently in creation: Is there an unresolvable circular reference?</p>
</blockquote>
<p>并且还会贴心的会你画个图：</p>
<blockquote>
<p>The dependencies of some of the beans in the application context form a cycle:</p>
<p>┌─────┐<br>| a defined in file [A.class]<br>↑ ↓<br>| b defined in file [B.class]<br>└─────┘</p>
</blockquote>
<p>这种也是我理解中的循环依赖问题，解决方案也是比较简单，我们换成set方法注入，或者成员变量注入就行。</p>
<h2 id="复现上文的异常"><a href="#复现上文的异常" class="headerlink" title="复现上文的异常"></a>复现上文的异常</h2><p>上文的异常，我一开始以为是AOP导致的问题，于是在A上加了个Aop的方法，发现Aop能够很好的解决这种循环依赖。</p>
<p>解决方案是三级缓存：</p>
<p>参考文章： <a href="https://segmentfault.com/a/1190000039134606" target="_blank" rel="noopener">https://segmentfault.com/a/1190000039134606</a></p>
<p>那我就疑惑了，到底是什么情况导致的呢?</p>
<p>后来搜到了文章：<a href="https://segmentfault.com/a/1190000018835760" target="_blank" rel="noopener">https://segmentfault.com/a/1190000018835760</a></p>
<p>发现@Async可以复现，于是我修改了代码：</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></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">A</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> B b;</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Autowired</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setB</span><span class="params">(B b)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.b = b;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Async</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">doAsync</span><span class="params">()</span> </span>{</span><br><span class="line"> System.out.println(<span class="string">"hello"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><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">B</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> A a;</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Autowired</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setA</span><span class="params">(A a)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.a = a;</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>发现确实可以复现了，这就暴露出我的知识盲区了，难道@Async不是通过Aop去解决的吗？</p>
<p>为什么@Aspect注解切的方法，不会报循环依赖，但是@Async的方法会呢？</p>
<p>带着疑惑，我又尝试了@Transectional会不会导致循环依赖问题，发现并不会。</p>
<p>所以，为啥@Async如此特殊呢？</p>
<h2 id="对比"><a href="#对比" class="headerlink" title="对比"></a>对比</h2><p>于是我得对比，@Async和@Transectional在实现上的区别</p>
<p>为此，我们把A的方法声明称这样：</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></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">A</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> B b;</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Autowired</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setB</span><span class="params">(B b)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.b = b;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Transactional</span></span><br><span class="line"> <span class="meta">@Async</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">doAsync</span><span class="params">()</span> </span>{</span><br><span class="line"> System.out.println(<span class="string">"hello"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>同时配上一个手写的Aspect：</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></pre></td><td class="code"><pre><span class="line"><span class="meta">@Aspect</span></span><br><span class="line"><span class="meta">@Component</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">AspectA</span> </span>{</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Before</span>(<span class="string">"execution(public A.doAsync())"</span>)</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">beforeA</span><span class="params">()</span> </span>{</span><br><span class="line"> System.out.println(<span class="string">"beforeA 执行"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>在DefaultAdvisorChainFactory的getInterceptorsAndDynamicInterceptionAdvice方法中</p>
<p>我们可以查到这个方法所有的Advisor。</p>
<p><img src="http://log.lovezhy.cc/hVSBxt.png" alt=""></p>
<p>可以看到一共有三个</p>
<ol>
<li>ExposeInvocationInterceptor：先不管，和我们的业务无关</li>
<li>BeanFactoryTransactionAttributeSourceAdvisor：就是我们@Transactional的相关Aop</li>
<li>AspectA：自定义的Aop</li>
</ol>
<p>可以发现，并没有@Async相关的Aop代码。</p>
<p>而通过对DefaultAdvisorChainFactory的调用链，可以分析到，这里的执行，对原来类的Aop织入，其实在</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">doCreateBean#</span><br><span class="line">addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));</span><br></pre></td></tr></table></figure>
<p>的逻辑中已经生成</p>
<p>所以也就导致</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><span class="line">26</span><br><span class="line">27</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> (earlySingletonExposure) {</span><br><span class="line"> Object earlySingletonReference = getSingleton(beanName, <span class="keyword">false</span>);</span><br><span class="line"> <span class="keyword">if</span> (earlySingletonReference != <span class="keyword">null</span>) {</span><br><span class="line"> <span class="comment">//这里拿到的exposedObject和Bean其实是一样的，不会抛出异常。</span></span><br><span class="line"> <span class="keyword">if</span> (exposedObject == bean) {</span><br><span class="line"> exposedObject = earlySingletonReference;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (!<span class="keyword">this</span>.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {</span><br><span class="line"> String[] dependentBeans = getDependentBeans(beanName);</span><br><span class="line"> Set<String> actualDependentBeans = <span class="keyword">new</span> LinkedHashSet<>(dependentBeans.length);</span><br><span class="line"> <span class="keyword">for</span> (String dependentBean : dependentBeans) {</span><br><span class="line"> <span class="keyword">if</span> (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {</span><br><span class="line"> actualDependentBeans.add(dependentBean);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span> (!actualDependentBeans.isEmpty()) {</span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> BeanCurrentlyInCreationException(beanName,</span><br><span class="line"> <span class="string">"Bean with name '"</span> + beanName + <span class="string">"' has been injected into other beans ["</span> +</span><br><span class="line"> StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +</span><br><span class="line"> <span class="string">"] in its raw version as part of a circular reference, but has eventually been "</span> +</span><br><span class="line"> <span class="string">"wrapped. This means that said other beans do not use the final version of the "</span> +</span><br><span class="line"> <span class="string">"bean. This is often the result of over-eager type matching - consider using "</span> +</span><br><span class="line"> <span class="string">"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example."</span>);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>那为什么加了@Async就会不一样呢？</p>
<p>通过上文，我们知道了@Async的逻辑和传统的Aop逻辑不太一样。</p>
<p>@Async是通过AsyncAnnotationBeanPostProcessor去织入自己的逻辑。</p>
<p>这个BeanPostProcessor在<code>applyBeanPostProcessorsAfterInitialization</code>方法中被调用</p>
<p>导致生成的是一个新的代理Bean，和原来的Bean就会不一样，也就是走到了抛出异常的逻辑。</p>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>还是不能想当然，Aop是一个广泛的概念，但是在Spring中，其实还是冗余了不同的实现。</p>
<p>并不是所有加了Annotation的实现，都是一样的。</p>
<p>有可能是Aspect的实现，也有可能是类似BeanPostProcessor的实现。</p>
<p>需要看源码具体原因具体分析。</p>
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<p>注：本文讲的不完全是CMS垃圾收集器，其实讲的是并发垃圾收集器的思路。</p>
<p>而Java中的并发垃圾收集器包含CMS和G1。</p>
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<h1 id="一-前言"><a href="#一-前言" class="headerlink" title="一. 前言"></a>一. 前言</h1><p>Disruptor几乎是每个Java开发绕不过去的坎，其实我想学习这个框架很久了，之前打开看了几次，但是有点复杂就放弃了。</p>
<p>这一次看到了Mpsc，心里在构思多生产者多消费者的队列怎么怎么做，自然就想到了RingBuffer。</p>
<p>有了上文的基础，下面我们就Disruptor来看看多生产者多消费者是怎么实现的。</p>
<p>注意：这个文章并不是特别的分析Disruptor是怎么实现高性能的，诸如网上说的那些伪共享之类，</p>
<p>就是带大家看看源码实现。</p>
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<h1 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h1><p>前面我们聊完了Mpsc，在提一下，Mpsc主要是针对的单消费者多生产者的情况。</p>
<p>对于消费者而言，因为只有一个消费者，所以不需要任何同步。</p>
<p>对于生产者而言，为了防止多线程下会出现问题，所以使用CAS操作。</p>
<p>但是上一篇文章中Mpsc使用的是链表的结构，不加以控制容易OOM。</p>
<p>为了避免这个问题，我们可以使用数组来作为底层存储。</p>
<p>原理其实和这边文章的RingBuffer讲的类似。</p>
<p>这里的RingBuffer其实就是Disruptor的实现，不过我单独抽成了一个文章来讲Disruptor的源码。</p>
<p>这里就划一些示意图，解释RingBuffer的原理。</p>
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<h1 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h1><p>其实一开始没有接触过这个，但是看Netty源码的时候，发现Netty的对象池使用了MpscQueue，感觉还挺有意思的。</p>
<p>MpscQueue主要针对的是单消费者，多生产者的情况，实现上是LockFree的，这里的Lock Free，一般都是指用了CAS解决并发问题。</p>
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<p>20200818补充</p>
<ol>
<li><a href="https://zhuanlan.zhihu.com/p/112574579" target="_blank" rel="noopener">LSM Tree的Leveling 和 Tiering Compaction</a> 文章讲了LSM的多种Compact策略，写的很好</li>
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<h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>这一节讲述最复杂的Version以及Compact的部分</p>
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<h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>数据库中基本都有WAL功能，LevelDB也不例外，不过这里的WAL功能实现比较简单</p>
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<p>Manifest文件，简单的看就是当前VersionEdit的持久化信息，其中包含了：</p>
<ol>
<li>Comparator：全局的比较方法</li>
<li>LogNumber：下一个MemTable的WAL日志文件的FileNumber</li>
<li>PreviousLogNumber：已经被废弃，之前版本有用到</li>
<li>NextFileNumber：下一个File的数字</li>
<li>LastSequence：最新的Seq</li>
<li>Compact_Pointer：在Compact一章中统一讲。</li>
<li>Deteted_Files：相对于上一个Version，删除的文件</li>
<li>New_Files：相对于上一个Version，新增的文件</li>
</ol>