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<h1 id="概述"><a href="#概述" class="headerlink" title="概述"></a>概述</h1><p>在对多路IO复用进行实现的时候，ByteBuf这个结构是必不可少的，尽管NIO中也实现了ByteBuf，但使用起来并不是特别方便，而且有可以优化的地方。 因此，Netty在ByteBuf这一块完全是自己的一套方案，不仅使用起来方便，有着丰富的API，而且性能也是非常的出色，本篇将对ByteBuf这一组件进行学习。</p>
<h1 id="ByteBuf结构"><a href="#ByteBuf结构" class="headerlink" title="ByteBuf结构"></a>ByteBuf结构</h1><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></pre></td><td class="code"><pre><span class="line"><span class="comment">/* +-------------------+------------------+------------------+</span></span><br><span class="line"><span class="comment">* | discardable bytes | readable bytes | writable bytes |</span></span><br><span class="line"><span class="comment">* +-------------------+------------------+------------------+</span></span><br><span class="line"><span class="comment">* | | | |</span></span><br><span class="line"><span class="comment">* 0 <= readerIndex <= writerIndex <= capacity</span></span><br><span class="line"><span class="comment">*/</span></span><br></pre></td></tr></table></figure>
<p>源码的注释很形象，这里就直接拿来用了，ByteBuf分为下面三个区间：</p>
<ol>
<li>[0, readerIndex) 这个区间是已经读过的数据， </li>
<li>[readerIndex, writerIndex) 这个区间是可读数据区间</li>
<li>[writeIndex, capacity] 这段区间是可写区间</li>
<li>还有一个变量maxCapacity， 即如果capacity不够写了，则可以扩容，最大不超过maxCapacity，默认Integer.MAX_VALUE</li>
</ol>
<p>ByteBuf的高频API，readXxx, writeXxx, markXxx, resetXxx… 限于篇幅这里就不过多的记录用法了… </p>
<h1 id="ByteBuf的分类"><a href="#ByteBuf的分类" class="headerlink" title="ByteBuf的分类"></a>ByteBuf的分类</h1><p><img src="/2020/10/16/Netty%E6%BA%90%E7%A0%81%E9%98%85%E8%AF%BB%E7%AC%94%E8%AE%B0%EF%BC%88%E4%BA%94%EF%BC%89ByteBuf/bytebuf.png" alt="bytebuf"></p>
<p>上面的继承关系图中间删掉了几层关系，这样看起来比较方便理解（好像也没方便到哪去…名字一个个的都那么长</p>
<h2 id="从三个角度对ByeBuf分类"><a href="#从三个角度对ByeBuf分类" class="headerlink" title="从三个角度对ByeBuf分类"></a>从三个角度对ByeBuf分类</h2><p>下面从三个角度来对ByteBuf进行分类：</p>
<ol>
<li>Pooled/Unpooled(是否池化)</li>
<li>Unsafe(是否使用Unsafe来进行数据读写)</li>
<li>Heap/Direct(是堆中内存还是直接内存) </li>
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<h1 id="Pipline的创建"><a href="#Pipline的创建" class="headerlink" title="Pipline的创建"></a>Pipline的创建</h1><p>经过前面几篇的分析之后，可以知道Pipline的创建是在创建Channel的时候被创建的，在<code>AbstractChannel</code>的构造方法中，如下：</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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">protected</span> <span class="title">AbstractChannel</span><span class="params">(Channel parent)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.parent = parent;</span><br><span class="line"> id = newId();</span><br><span class="line"> unsafe = newUnsafe();</span><br><span class="line"> pipeline = newChannelPipeline();</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>newChannelPipeline方法会返回一个DefaultPipline</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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">protected</span> DefaultChannelPipeline <span class="title">newChannelPipeline</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> DefaultChannelPipeline(<span class="keyword">this</span>);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>下面来看看DefaultChannelPipline的具体内容：</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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">protected</span> <span class="title">DefaultChannelPipeline</span><span class="params">(Channel channel)</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>.channel = ObjectUtil.checkNotNull(channel, <span class="string">"channel"</span>);</span><br><span class="line"> succeededFuture = <span class="keyword">new</span> SucceededChannelFuture(channel, <span class="keyword">null</span>);</span><br><span class="line"> voidPromise = <span class="keyword">new</span> VoidChannelPromise(channel, <span class="keyword">true</span>);</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 在初始的时候创建tail，head这两个节点</span></span><br><span class="line"> tail = <span class="keyword">new</span> TailContext(<span class="keyword">this</span>);</span><br><span class="line"> head = <span class="keyword">new</span> HeadContext(<span class="keyword">this</span>);</span><br><span class="line"></span><br><span class="line"> head.next = tail;</span><br><span class="line"> tail.prev = head;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>可以看出pipline是一个含有头节点的双向队列（和AQS底层维护的队列类似）</p>
<p>头节点的存在使得添加/删除一个节点都变得方便。</p>
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<h1 id="概述"><a href="#概述" class="headerlink" title="概述"></a>概述</h1><p>Netty中Channel接口的描述：</p>
<blockquote>
<p>A nexus to a network socket or a component which is capable of I/O operations such as read, write, connect, and bind.</p>
</blockquote>
<p>即是一个对socket连接的抽象，在都满足这一抽象的前提下，Netty对Channel的实现也有很多种类，本篇将主要记录对NioServerSocketChannel和NioSocketChannel这两种Channel实现的学习。</p>
<h1 id="Channel的继承关系"><a href="#Channel的继承关系" class="headerlink" title="Channel的继承关系"></a>Channel的继承关系</h1><p><img src="/2020/10/15/Netty%E6%BA%90%E7%A0%81%E9%98%85%E8%AF%BB%E7%AC%94%E8%AE%B0%EF%BC%88%E4%B8%89%EF%BC%89Netty%E7%9A%84%E4%B8%A4%E7%A7%8DChannel/channel.png" alt="channel"></p>
<p>基于上面的这个这个继承关系图，来一层一层分析每一层的抽象<font color="red"><strong>主要</strong></font>都是做了什么事情.</p>
<h2 id="1-Channel"><a href="#1-Channel" class="headerlink" title="1. Channel"></a>1. Channel</h2><p>提供了接口规范，是对连接的抽象。 Channel接口中还定义了Unsafe接口，简单来说的话，是要使用Unsafe对象来去实现Channel的一些操作(例如read, write等)</p>
<h2 id="2-AbstractChannel"><a href="#2-AbstractChannel" class="headerlink" title="2. AbstractChannel"></a>2. AbstractChannel</h2><ul>
<li><p>作用：</p>
<blockquote>
<p>A skeletal {@link Channel} implementation.</p>
</blockquote>
</li>
<li><p><strong>主要</strong>做了什么：</p>
</li>
</ul>
<ol>
<li>定义了一系列的变量(如id, unsafe, pipline等等)</li>
<li><font color="red"><strong>对id, pipline和unsafe对象进行了初始化</strong></font>, 其中unsafe对象的初始化使用了模版模式，让子类去实现</li>
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<h1 id="什么是JDK空轮询Bug"><a href="#什么是JDK空轮询Bug" class="headerlink" title="什么是JDK空轮询Bug"></a>什么是JDK空轮询Bug</h1><p><strong>JDK NIO的空轮询BUG其实是JDK NIO在Linux系统下的epoll空轮询问题。</strong></p>
<p>epoll是Linux下一种高效的IO复用方式，相较于select和poll机制来说。其高效的原因是将基于事件的fd放到内核中来完成，在内核中基于红黑树+链表数据结构来实现，链表存放有事件发生的fd集合，然后在调用epoll_wait时返回给应用程序，由应用程序来处理这些fd事件。</p>
<p>使用IO多路复用，Linux下一般默认就是epoll，Java NIO在Linux下默认也是epoll机制，但是JDK中epoll的实现却是有漏洞的。其中一个就是Epoll的空轮询Bug, 就是即使是关注的select轮询事件返回数量为0，NIO照样不断的从select本应该阻塞的Selector.select()/Selector.select(timeout)中wake up出来，导致CPU飙到100%问题。</p>
<h1 id="官方给的Bug复现方法"><a href="#官方给的Bug复现方法" class="headerlink" title="官方给的Bug复现方法:"></a><a href="https://bugs.java.com/bugdatabase/view_bug.do?bug_id=6670302" target="_blank" rel="noopener">官方给的Bug复现方法</a>:</h1><p>A DESCRIPTION OF THE PROBLEM :<br>The NIO selector wakes up infinitely in this situation..<br>0. server waits for connection</p>
<ol>
<li>client connects and write message</li>
<li>server accepts and register OP_READ</li>
<li>server reads message and remove OP_READ from interest op set</li>
<li>client close the connection</li>
<li>server write message (without any reading.. surely OP_READ is not set)</li>
<li>server’s select wakes up infinitely with return value 0</li>
</ol>
<h1 id="产生这一Bug的原因："><a href="#产生这一Bug的原因：" class="headerlink" title="产生这一Bug的原因："></a>产生这一Bug的原因：</h1><p>因为poll和epoll对于突然中断的连接socket会对返回的eventSet事件集合置为EPOLLHUP或者EPOLLERR，<font color="red"><strong>eventSet事件集合发生了变化</strong></font>，这就导致Selector会被唤醒，如果仅仅是因为这个原因唤醒且没有感兴趣的时间发生的话，就会变成空轮询。 </p>
<p><font size="4"><strong>epoll感兴趣的事件集合</strong></font></p>
<table>
<thead>
<tr>
<th>符号</th>
<th>描述</th>
</tr>
</thead>
<tbody><tr>
<td>EPOLLIN</td>
<td>表示对应的文件描述符可以读（包括对端SOCKET正常关闭)</td>
</tr>
<tr>
<td>EPOLLOUT</td>
<td>表示对应的文件描述符可以写；</td>
</tr>
<tr>
<td>EPOLLPRI</td>
<td>表示对应的文件描述符有紧急的数据可读（这里应该表示有带外数据到来）；</td>
</tr>
<tr>
<td><font color="red">EPOLLERR</font></td>
<td>表示对应的文件描述符发生错误；</td>
</tr>
<tr>
<td><font color="red">EPOLLHUP</font></td>
<td>表示对应的文件描述符被挂断；</td>
</tr>
<tr>
<td>EPOLLET</td>
<td>将 EPOLL设为边缘触发(Edge Triggered)模式（默认为水平触发），这是相对水平触发(Level Triggered)来说的。</td>
</tr>
<tr>
<td>EPOLLONESHOT</td>
<td>只监听一次事件，当监听完这次事件之后，如果还需要继续监听这个socke的话，需要再次把这个socket加入到EPOLL队列里</td>
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<h1 id="NioEventLoopGroup和NioEventLoop的创建"><a href="#NioEventLoopGroup和NioEventLoop的创建" class="headerlink" title="NioEventLoopGroup和NioEventLoop的创建"></a>NioEventLoopGroup和NioEventLoop的创建</h1><p>NioEventLoop的创建是在创建<code>EventLoopGroup</code>中进行的，因此，先来快速过一遍<code>EventLoopGroup</code>的创建. </p>
<p>EventLoopGroup的创建中，主要是做了3件事:</p>
<ol>
<li>创建<code>ThreadPerTaskExcutor</code></li>
<li>创建<code>EventLoop</code></li>
<li>创建<code>Chooser</code></li>
</ol>
<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></pre></td><td class="code"><pre><span class="line"><span class="comment">// 对于无参构造器，默认创建 2*cpu 个NioEventLoop</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="title">NioEventLoopGroup</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">this</span>(<span class="number">0</span>);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>经过多个构造器之后，到达<code>MultithreadEventExecutorGroup</code>, 这是NioEventLoopGroup的基类</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><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">abstract</span> <span class="class"><span class="keyword">class</span> <span class="title">MultithreadEventExecutorGroup</span> <span class="keyword">extends</span> <span class="title">AbstractEventExecutorGroup</span> </span>{</span><br><span class="line"> ...</span><br><span class="line"> ...</span><br><span class="line"> <span class="function"><span class="keyword">protected</span> <span class="title">MultithreadEventExecutorGroup</span><span class="params">(<span class="keyword">int</span> nThreads, Executor executor,</span></span></span><br><span class="line"><span class="function"><span class="params"> EventExecutorChooserFactory chooserFactory, Object... args)</span> </span>{</span><br><span class="line"> <span class="keyword">if</span> (nThreads <= <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> IllegalArgumentException(String.format(<span class="string">"nThreads: %d (expected: > 0)"</span>, nThreads));</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// 1. 创建Executor</span></span><br><span class="line"> <span class="keyword">if</span> (executor == <span class="keyword">null</span>) {</span><br><span class="line"> <span class="comment">// 对于execute的每一个task，都会创建一个新的线程去执行它</span></span><br><span class="line"> <span class="comment">// 并且创建的线程是使用同一个线程工厂来创建的，有着同一个命名格式：nioEventLoop-x-yy</span></span><br><span class="line"> executor = <span class="keyword">new</span> ThreadPerTaskExecutor(newDefaultThreadFactory());</span><br><span class="line"> }</span><br><span class="line"> children = <span class="keyword">new</span> EventExecutor[nThreads];</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 2. 创建EventLoop数组</span></span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i < nThreads; i ++) {</span><br><span class="line"> <span class="keyword">boolean</span> success = <span class="keyword">false</span>;</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> children[i] = newChild(executor, args);</span><br><span class="line"> success = <span class="keyword">true</span>;</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e) {</span><br><span class="line"> <span class="comment">// 抛异常</span></span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> <span class="comment">// 优雅的关闭</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// 3. 获取chooser</span></span><br><span class="line"> chooser = chooserFactory.newChooser(children);</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><br><span class="line"> ...</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2 id="1-创建Executor——ThreadPerTaskExecutor"><a href="#1-创建Executor——ThreadPerTaskExecutor" class="headerlink" title="1. 创建Executor——ThreadPerTaskExecutor"></a>1. 创建Executor——ThreadPerTaskExecutor</h2><p>ThreadPerTaskExecutor的作用就和它的名字一样，每提交一个任务都新创建一个线程去做。</p>
<p>ThreadPerTaskExecutor除了实现上面这个功能以外，还使用一个专用的ThreadFactory来去创建线程，这就保证ThreadPerTaskExecutor创建的所有线程都是以“nioEventLoopGroup-x-yy”的格式来命名的。</p>
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<h1 id="服务端启动样例"><a href="#服务端启动样例" class="headerlink" title="服务端启动样例"></a>服务端启动样例</h1><p>为了方便理解，先来看以下服务端的启动流程。</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">public</span> <span class="class"><span class="keyword">class</span> <span class="title">NettyServer</span></span>{</span><br><span class="line"> <span class="comment">// ...</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">start</span><span class="params">()</span> <span class="keyword">throws</span> Exception </span>{</span><br><span class="line"> <span class="comment">// 创建NIO的EventLoop实例</span></span><br><span class="line"> EventLoopGroup bossGroup = <span class="keyword">new</span> NioEventLoopGroup();</span><br><span class="line"> EventLoopGroup workerGroup = <span class="keyword">new</span> NioEventLoopGroup();</span><br><span class="line"> <span class="keyword">try</span>{</span><br><span class="line"> ServerBootstrap b = <span class="keyword">new</span> ServerBootstrap();</span><br><span class="line"> <span class="comment">// 配置</span></span><br><span class="line"> b.group(bossGroup,workerGroup)</span><br><span class="line"> .channel(NioServerSocketChannel<span class="class">.<span class="keyword">class</span>)</span></span><br><span class="line"><span class="class"> .<span class="title">localAddress</span>(<span class="title">new</span> <span class="title">InetSocketAddress</span>(<span class="title">port</span>))</span></span><br><span class="line"><span class="class"> .<span class="title">childHandler</span>(<span class="title">new</span> <span class="title">ChannelInitializer</span><<span class="title">SocketChannel</span>>()</span>{</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="function"><span class="keyword">protected</span> <span class="keyword">void</span> <span class="title">initChannel</span><span class="params">(SocketChannel socketChannel)</span> <span class="keyword">throws</span> Exception </span>{</span><br><span class="line"> socketChannel.pipeline().addLast(...);</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"> <span class="comment">// 启动</span></span><br><span class="line"> ChannelFuture f = b.bind().sync();</span><br><span class="line"> f.channel().closeFuture().sync();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> bossGroup.shutdownGracefully().sync();</span><br><span class="line"> workerGroup.shutdownGracefully().sync();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h1 id="概述"><a href="#概述" class="headerlink" title="概述"></a>概述</h1><p>Netty服务端在启动时，主要有以下几个流程：</p>
<ol>
<li>创建NioSocketChannel</li>
<li>初始化NioSocketChannel</li>
<li>注册NioSocketChannel到Selector中</li>
<li>做Bind操作</li>
<li>添加Read事件</li>
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