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CountDownLatch.html-7cfa05ec.js
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CountDownLatch.html-7cfa05ec.js
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import{_ as e}from"./plugin-vue_export-helper-c27b6911.js";import{r as c,o,c as l,a as s,d as n,b as t,e as p}from"./app-dea8ffe7.js";const i={},u=p(`<h1 id="第二十九节-通信工具类" tabindex="-1"><a class="header-anchor" href="#第二十九节-通信工具类" aria-hidden="true">#</a> 第二十九节：通信工具类</h1><p>JDK 中提供了一些并发编程中常用的通信工具类以供我们开发者使用，比如说 CountDownLatch,Semaphore,Exchanger,CyclicBarrier,Phaser。</p><p>它们都在 JUC 包下。先总体概括一下都有哪些工具类，它们有什么作用，然后再分别介绍它们的主要使用方法和原理。</p><table><thead><tr><th>类</th><th>作用</th></tr></thead><tbody><tr><td>Semaphore</td><td>限制线程的数量</td></tr><tr><td>Exchanger</td><td>两个线程交换数据</td></tr><tr><td>CountDownLatch</td><td>线程等待直到计数器减为 0 时开始工作</td></tr><tr><td>CyclicBarrier</td><td>作用跟 CountDownLatch 类似，但是可以重复使用</td></tr><tr><td>Phaser</td><td>增强的 CyclicBarrier</td></tr></tbody></table><h2 id="semaphore" tabindex="-1"><a class="header-anchor" href="#semaphore" aria-hidden="true">#</a> Semaphore</h2><p>Semaphore 翻译过来是信号的意思。顾名思义，这个工具类提供的功能就是多个线程彼此“传信号”。而这个“信号”是一个<code>int</code>类型的数据，也可以看成是一种“资源”。</p><p>可以在构造方法中传入初始资源总数，以及是否使用“公平”的同步器。默认情况下，是非公平的。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token comment">// 默认情况下使用非公平</span>
<span class="token keyword">public</span> <span class="token class-name">Semaphore</span><span class="token punctuation">(</span><span class="token keyword">int</span> <span class="token keyword">permits</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
sync <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">NonfairSync</span><span class="token punctuation">(</span><span class="token keyword">permits</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">Semaphore</span><span class="token punctuation">(</span><span class="token keyword">int</span> <span class="token keyword">permits</span><span class="token punctuation">,</span> <span class="token keyword">boolean</span> fair<span class="token punctuation">)</span> <span class="token punctuation">{</span>
sync <span class="token operator">=</span> fair <span class="token operator">?</span> <span class="token keyword">new</span> <span class="token class-name">FairSync</span><span class="token punctuation">(</span><span class="token keyword">permits</span><span class="token punctuation">)</span> <span class="token operator">:</span> <span class="token keyword">new</span> <span class="token class-name">NonfairSync</span><span class="token punctuation">(</span><span class="token keyword">permits</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>最主要的方法是 acquire 方法和 release 方法。<code>acquire()</code>方法会申请一个 permit，而 release 方法会释放一个 permit。当然，你也可以申请多个 <code>acquire(int permits)</code>或者释放多个 <code>release(int permits)</code>。</p><p>每次 acquire，permits 就会减少一个或者多个。如果减少到了 0，再有其他线程来 acquire，那就要阻塞这个线程直到有其它线程 release permit 为止。</p><h3 id="semaphore-使用案例" tabindex="-1"><a class="header-anchor" href="#semaphore-使用案例" aria-hidden="true">#</a> Semaphore 使用案例</h3><p>Semaphore 往往用于资源有限的场景中，去限制线程的数量。举个例子，我想限制同时只能有 3 个线程在工作：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">SemaphoreDemo</span> <span class="token punctuation">{</span>
<span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">MyThread</span> <span class="token keyword">implements</span> <span class="token class-name">Runnable</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">int</span> value<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token class-name">Semaphore</span> semaphore<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">MyThread</span><span class="token punctuation">(</span><span class="token keyword">int</span> value<span class="token punctuation">,</span> <span class="token class-name">Semaphore</span> semaphore<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>value <span class="token operator">=</span> value<span class="token punctuation">;</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>semaphore <span class="token operator">=</span> semaphore<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">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
semaphore<span class="token punctuation">.</span><span class="token function">acquire</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 获取permit</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">format</span><span class="token punctuation">(</span><span class="token string">"当前线程是%d, 还剩%d个资源，还有%d个线程在等待"</span><span class="token punctuation">,</span>
value<span class="token punctuation">,</span> semaphore<span class="token punctuation">.</span><span class="token function">availablePermits</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> semaphore<span class="token punctuation">.</span><span class="token function">getQueueLength</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>
<span class="token comment">// 睡眠随机时间，打乱释放顺序</span>
<span class="token class-name">Random</span> random <span class="token operator">=</span><span class="token keyword">new</span> <span class="token class-name">Random</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span>random<span class="token punctuation">.</span><span class="token function">nextInt</span><span class="token punctuation">(</span><span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">format</span><span class="token punctuation">(</span><span class="token string">"线程%d释放了资源"</span><span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><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>
semaphore<span class="token punctuation">.</span><span class="token function">release</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 释放permit</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Semaphore</span> semaphore <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Semaphore</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> <span class="token number">10</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">MyThread</span><span class="token punctuation">(</span>i<span class="token punctuation">,</span> semaphore<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</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>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>输出：</p><blockquote><p>当前线程是 1, 还剩 2 个资源，还有 0 个线程在等待<br> 当前线程是 0, 还剩 1 个资源，还有 0 个线程在等待<br> 当前线程是 6, 还剩 0 个资源，还有 0 个线程在等待<br> 线程 6 释放了资源<br> 当前线程是 2, 还剩 0 个资源，还有 6 个线程在等待<br> 线程 2 释放了资源<br> 当前线程是 4, 还剩 0 个资源，还有 5 个线程在等待<br> 线程 0 释放了资源<br> 当前线程是 7, 还剩 0 个资源，还有 4 个线程在等待<br> 线程 1 释放了资源<br> 当前线程是 8, 还剩 0 个资源，还有 3 个线程在等待<br> 线程 7 释放了资源<br> 当前线程是 5, 还剩 0 个资源，还有 2 个线程在等待<br> 线程 4 释放了资源<br> 当前线程是 3, 还剩 0 个资源，还有 1 个线程在等待<br> 线程 8 释放了资源<br> 当前线程是 9, 还剩 0 个资源，还有 0 个线程在等待<br> 线程 9 释放了资源<br> 线程 5 释放了资源<br> 线程 3 释放了资源</p></blockquote><p>可以看到，在这次运行中，最开始是 1, 0, 6 这三个线程获得了资源，而其它线程进入了等待队列。然后当某个线程释放资源后，就会有等待队列中的线程获得资源。</p><p>当然，Semaphore 默认的 acquire 方法是会让线程进入等待队列，且抛出异常中断。但它还有一些方法可以忽略中断或不进入阻塞队列：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token comment">// 忽略中断</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">acquireUninterruptibly</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">acquireUninterruptibly</span><span class="token punctuation">(</span><span class="token keyword">int</span> <span class="token keyword">permits</span><span class="token punctuation">)</span>
<span class="token comment">// 不进入等待队列，底层使用CAS</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> tryAcquire
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">tryAcquire</span><span class="token punctuation">(</span><span class="token keyword">int</span> <span class="token keyword">permits</span><span class="token punctuation">)</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">tryAcquire</span><span class="token punctuation">(</span><span class="token keyword">int</span> <span class="token keyword">permits</span><span class="token punctuation">,</span> <span class="token keyword">long</span> timeout<span class="token punctuation">,</span> <span class="token class-name">TimeUnit</span> unit<span class="token punctuation">)</span>
<span class="token keyword">throws</span> <span class="token class-name">InterruptedException</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">tryAcquire</span><span class="token punctuation">(</span><span class="token keyword">long</span> timeout<span class="token punctuation">,</span> <span class="token class-name">TimeUnit</span> unit<span class="token punctuation">)</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="semaphore-原理" tabindex="-1"><a class="header-anchor" href="#semaphore-原理" aria-hidden="true">#</a> Semaphore 原理</h3>`,19),r={href:"https://javabetter.cn/thread/aqs.html",target:"_blank",rel:"noopener noreferrer"},k=s("code",null,"tryAcquireShared",-1),d=p(`<p>如果获取失败（想要的资源数量小于目前已有的资源数量），就会返回一个负数（代表尝试获取资源失败）。然后当前线程就会进入 AQS 的等待队列。</p><h2 id="exchanger" tabindex="-1"><a class="header-anchor" href="#exchanger" aria-hidden="true">#</a> Exchanger</h2><p>Exchanger 类用于两个线程交换数据。它支持泛型，也就是说你可以在两个线程之间传送任何数据。先来一个案例看看如何使用，比如两个线程之间想要传送字符串：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ExchangerDemo</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">InterruptedException</span> <span class="token punctuation">{</span>
<span class="token class-name">Exchanger</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> exchanger <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Exchanger</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"这是线程A，得到了另一个线程的数据："</span>
<span class="token operator">+</span> exchanger<span class="token punctuation">.</span><span class="token function">exchange</span><span class="token punctuation">(</span><span class="token string">"这是来自线程A的数据"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</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><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"这个时候线程A是阻塞的，在等待线程B的数据"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"这是线程B，得到了另一个线程的数据："</span>
<span class="token operator">+</span> exchanger<span class="token punctuation">.</span><span class="token function">exchange</span><span class="token punctuation">(</span><span class="token string">"这是来自线程B的数据"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</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><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</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>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>输出：</p><blockquote><p>这个时候线程 A 是阻塞的，在等待线程 B 的数据<br> 这是线程 B，得到了另一个线程的数据：这是来自线程 A 的数据<br> 这是线程 A，得到了另一个线程的数据：这是来自线程 B 的数据</p></blockquote><p>可以看到，当一个线程调用 exchange 方法后，会处于阻塞状态，只有当另一个线程也调用了 exchange 方法，它才会继续执行。</p>`,7),v={href:"https://javabetter.cn/thread/cas.html",target:"_blank",rel:"noopener noreferrer"},m=p(`<p>因为 Exchanger 支持泛型，所以我们可以传输任何的数据，比如 IO 流或者 IO 缓存。根据 JDK 里面注释的说法，可以总结为一下特性：</p><ul><li>此类提供对外的操作是同步的；</li><li>用于成对出现的线程之间交换数据；</li><li>可以视作双向的同步队列；</li><li>可应用于基因算法、流水线设计等场景。</li></ul><p>Exchanger 类还有一个有超时参数的方法，如果在指定时间内没有另一个线程调用 exchange，就会抛出一个超时异常。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">V</span> <span class="token function">exchange</span><span class="token punctuation">(</span><span class="token class-name">V</span> x<span class="token punctuation">,</span> <span class="token keyword">long</span> timeout<span class="token punctuation">,</span> <span class="token class-name">TimeUnit</span> unit<span class="token punctuation">)</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>那么问题来了，Exchanger 只能是两个线程交换数据吗？那三个调用同一个实例的 exchange 方法会发生什么呢？答案是只有前两个线程会交换数据，第三个线程会进入阻塞状态。</p><p>需要注意的是，exchange 是可以重复使用的。也就是说。两个线程可以使用 Exchanger 在内存中不断地再交换数据。</p><h2 id="countdownlatch" tabindex="-1"><a class="header-anchor" href="#countdownlatch" aria-hidden="true">#</a> CountDownLatch</h2><p>先来解读一下 CountDownLatch 这个类名的意义。CountDown 代表计数递减，Latch 是“门闩”的意思。也有人把它称为“屏障”。而 CountDownLatch 这个类的作用也很贴合这个名字的意义，假设某个线程在执行任务之前，需要等待其它线程完成一些前置任务，必须等所有的前置任务都完成，才能开始执行本线程的任务。</p><p>CountDownLatch 的方法也很简单，如下：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token comment">// 构造方法：</span>
<span class="token keyword">public</span> <span class="token class-name">CountDownLatch</span><span class="token punctuation">(</span><span class="token keyword">int</span> count<span class="token punctuation">)</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">await</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 等待</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">await</span><span class="token punctuation">(</span><span class="token keyword">long</span> timeout<span class="token punctuation">,</span> <span class="token class-name">TimeUnit</span> unit<span class="token punctuation">)</span> <span class="token comment">// 超时等待</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">countDown</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// count - 1</span>
<span class="token keyword">public</span> <span class="token keyword">long</span> <span class="token function">getCount</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 获取当前还有多少count</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="countdownlatch-案例" tabindex="-1"><a class="header-anchor" href="#countdownlatch-案例" aria-hidden="true">#</a> CountDownLatch 案例</h3><p>我们知道，玩游戏的时候，在游戏真正开始之前，一般会等待一些前置任务完成，比如“加载地图数据”，“加载人物模型”，“加载背景音乐”等等。只有当所有的东西都加载完成后，玩家才能真正进入游戏。下面我们就来模拟一下这个 demo。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">CountDownLatchDemo</span> <span class="token punctuation">{</span>
<span class="token comment">// 定义前置任务线程</span>
<span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">PreTaskThread</span> <span class="token keyword">implements</span> <span class="token class-name">Runnable</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token class-name">String</span> task<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token class-name">CountDownLatch</span> countDownLatch<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token class-name">String</span> task<span class="token punctuation">,</span> <span class="token class-name">CountDownLatch</span> countDownLatch<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>task <span class="token operator">=</span> task<span class="token punctuation">;</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>countDownLatch <span class="token operator">=</span> countDownLatch<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">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">Random</span> random <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Random</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span>random<span class="token punctuation">.</span><span class="token function">nextInt</span><span class="token punctuation">(</span><span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>task <span class="token operator">+</span> <span class="token string">" - 任务完成"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
countDownLatch<span class="token punctuation">.</span><span class="token function">countDown</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</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>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// 假设有三个模块需要加载</span>
<span class="token class-name">CountDownLatch</span> countDownLatch <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">CountDownLatch</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 主任务</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"等待数据加载..."</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">format</span><span class="token punctuation">(</span><span class="token string">"还有%d个前置任务"</span><span class="token punctuation">,</span> countDownLatch<span class="token punctuation">.</span><span class="token function">getCount</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>
countDownLatch<span class="token punctuation">.</span><span class="token function">await</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"数据加载完成，正式开始游戏！"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</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><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 前置任务</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token string">"加载地图数据"</span><span class="token punctuation">,</span> countDownLatch<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token string">"加载人物模型"</span><span class="token punctuation">,</span> countDownLatch<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token string">"加载背景音乐"</span><span class="token punctuation">,</span> countDownLatch<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</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>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>输出：</p><blockquote><p>等待数据加载...<br> 还有 3 个前置任务<br> 加载人物模型 - 任务完成<br> 加载背景音乐 - 任务完成<br> 加载地图数据 - 任务完成<br> 数据加载完成，正式开始游戏！</p></blockquote><h3 id="countdownlatch-原理" tabindex="-1"><a class="header-anchor" href="#countdownlatch-原理" aria-hidden="true">#</a> CountDownLatch 原理</h3>`,16),b={href:"https://javabetter.cn/thread/aqs.html",target:"_blank",rel:"noopener noreferrer"},h=p(`<p>需要注意的是构造器中的<strong>计数值（count）实际上就是闭锁需要等待的线程数量</strong>。这个值只能被设置一次，而且 CountDownLatch<strong>没有提供任何机制去重新设置这个计数值</strong>。</p><h2 id="cyclicbarrier" tabindex="-1"><a class="header-anchor" href="#cyclicbarrier" aria-hidden="true">#</a> CyclicBarrier</h2><p>CyclicBarrirer 从名字上来理解是“循环屏障”的意思。前面提到了 CountDownLatch 一旦计数值<code>count</code>被降为 0 后，就不能再重新设置了，它只能起一次“屏障”的作用。而 CyclicBarrier 拥有 CountDownLatch 的所有功能，还可以使用<code>reset()</code>方法重置屏障。</p><p>如果参与者（线程）在等待的过程中，Barrier 被破坏，就会抛出 BrokenBarrierException。可以用<code>isBroken()</code>方法检测 Barrier 是否被破坏。</p><ol><li>如果有线程已经处于等待状态，调用 reset 方法会导致已经在等待的线程出现 BrokenBarrierException 异常。并且由于出现了 BrokenBarrierException，将会导致始终无法等待。</li><li>如果在等待的过程中，线程被中断，会抛出 InterruptedException 异常，并且这个异常会传播到其他所有的线程。</li><li>如果在执行屏障操作过程中发生异常，则该异常将传播到当前线程中，其他线程会抛出 BrokenBarrierException，屏障被损坏。</li><li>如果超出指定的等待时间，当前线程会抛出 TimeoutException 异常，其他线程会抛出 BrokenBarrierException 异常。</li></ol><h3 id="cyclicbarrier-案例" tabindex="-1"><a class="header-anchor" href="#cyclicbarrier-案例" aria-hidden="true">#</a> CyclicBarrier 案例</h3><p>我们同样用玩游戏的例子。如果玩一个游戏有多个“关卡”，那使用 CountDownLatch 显然不太合适，因为需要为每个关卡都创建一个实例。那我们可以使用 CyclicBarrier 来实现每个关卡的数据加载等待功能。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">CyclicBarrierDemo</span> <span class="token punctuation">{</span>
<span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">PreTaskThread</span> <span class="token keyword">implements</span> <span class="token class-name">Runnable</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token class-name">String</span> task<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token class-name">CyclicBarrier</span> cyclicBarrier<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token class-name">String</span> task<span class="token punctuation">,</span> <span class="token class-name">CyclicBarrier</span> cyclicBarrier<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>task <span class="token operator">=</span> task<span class="token punctuation">;</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>cyclicBarrier <span class="token operator">=</span> cyclicBarrier<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">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// 假设总共三个关卡</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> i <span class="token operator"><</span> <span class="token number">4</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">Random</span> random <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Random</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span>random<span class="token punctuation">.</span><span class="token function">nextInt</span><span class="token punctuation">(</span><span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">format</span><span class="token punctuation">(</span><span class="token string">"关卡%d的任务%s完成"</span><span class="token punctuation">,</span> i<span class="token punctuation">,</span> task<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
cyclicBarrier<span class="token punctuation">.</span><span class="token function">await</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> <span class="token operator">|</span> <span class="token class-name">BrokenBarrierException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</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>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">CyclicBarrier</span> cyclicBarrier <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">CyclicBarrier</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"本关卡所有前置任务完成，开始游戏..."</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>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token string">"加载地图数据"</span><span class="token punctuation">,</span> cyclicBarrier<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token string">"加载人物模型"</span><span class="token punctuation">,</span> cyclicBarrier<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token string">"加载背景音乐"</span><span class="token punctuation">,</span> cyclicBarrier<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</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>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>输出：</p><blockquote><p>关卡 1 的任务加载地图数据完成<br> 关卡 1 的任务加载背景音乐完成<br> 关卡 1 的任务加载人物模型完成<br> 本关卡所有前置任务完成，开始游戏...<br> 关卡 2 的任务加载地图数据完成<br> 关卡 2 的任务加载背景音乐完成<br> 关卡 2 的任务加载人物模型完成<br> 本关卡所有前置任务完成，开始游戏...<br> 关卡 3 的任务加载人物模型完成<br> 关卡 3 的任务加载地图数据完成<br> 关卡 3 的任务加载背景音乐完成<br> 本关卡所有前置任务完成，开始游戏...</p></blockquote><p>注意这里跟 CountDownLatch 的代码有一些不同。CyclicBarrier 没有分为<code>await()</code>和<code>countDown()</code>，而是只有单独的一个<code>await()</code>方法。</p><p>一旦调用 await 方法的线程数量等于构造方法中传入的任务总量（这里是 3），就代表达到屏障了。CyclicBarrier 允许我们在达到屏障的时候可以执行一个任务，可以在构造方法传入一个 Runnable 类型的对象。</p><p>上述案例就是在达到屏障时，输出“本关卡所有前置任务完成，开始游戏...”。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token comment">// 构造方法</span>
<span class="token keyword">public</span> <span class="token class-name">CyclicBarrier</span><span class="token punctuation">(</span><span class="token keyword">int</span> parties<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">(</span>parties<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">CyclicBarrier</span><span class="token punctuation">(</span><span class="token keyword">int</span> parties<span class="token punctuation">,</span> <span class="token class-name">Runnable</span> barrierAction<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// 具体实现</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="cyclicbarrier-原理" tabindex="-1"><a class="header-anchor" href="#cyclicbarrier-原理" aria-hidden="true">#</a> CyclicBarrier 原理</h3>`,15),w={href:"https://javabetter.cn/thread/lock.html",target:"_blank",rel:"noopener noreferrer"},y={href:"https://javabetter.cn/thread/condition.html",target:"_blank",rel:"noopener noreferrer"},g=p(`<div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">int</span> <span class="token function">dowait</span><span class="token punctuation">(</span><span class="token keyword">boolean</span> timed<span class="token punctuation">,</span> <span class="token keyword">long</span> nanos<span class="token punctuation">)</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><h2 id="phaser" tabindex="-1"><a class="header-anchor" href="#phaser" aria-hidden="true">#</a> Phaser</h2><p>Phaser 是 Java 7 中引入的一个并发同步工具，它提供了对动态数量的线程的同步能力，这与 CyclicBarrier 和 CountDownLatch 不同，因为它们都需要预先知道等待的线程数量。Phaser 是多阶段的，意味着它可以同步不同阶段的多个操作。</p><p>前面我们介绍了 CyclicBarrier，可以发现它在构造方法里传入了“任务总量”<code>parties</code>之后，就不能修改这个值了，并且每次调用<code>await()</code>方法也只能消耗一个<code>parties</code>计数。但 Phaser 可以动态地调整任务总量！</p><p>Phaser 是阶段性的，所以它有一个内部的阶段计数器。每当我们到达一个阶段的结尾时，Phaser 会自动前进到下一个阶段。</p><p>名词解释：</p><ul><li><p>Party：Phaser 的上下文中，一个 party 可以是一个线程，也可以是一个任务。当我们在 Phaser 上注册一个 party 时，Phaser 会递增它的参与者数量。</p></li><li><p>arrive：对应一个 party 的状态，初始时是 unarrived，当调用<code>arriveAndAwaitAdvance()</code>或者 <code>arriveAndDeregister()</code>进入 arrive 状态，可以通过<code>getUnarrivedParties()</code>获取当前未到达的数量。</p></li><li><p>register：注册一个新的 party 到 Phaser。</p></li><li><p>deRegister：减少一个 party。</p></li><li><p>phase：阶段，当所有注册的 party 都 arrive 之后，将会调用 Phaser 的<code>onAdvance()</code>方法来判断是否要进入下一阶段。</p></li></ul><p>Phaser 的终止有两种途径，Phaser 维护的线程执行完毕或者<code>onAdvance()</code>返回<code>true</code>。</p><h3 id="phaser-案例" tabindex="-1"><a class="header-anchor" href="#phaser-案例" aria-hidden="true">#</a> Phaser 案例</h3><p>还是游戏的案例。假设我们游戏有三个关卡，但只有第一个关卡有新手教程，需要加载新手教程模块。但后面的第二个关卡和第三个关卡都不需要。我们可以用 Phaser 来做这个需求。</p><p>代码：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">PhaserDemo</span> <span class="token punctuation">{</span>
<span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">PreTaskThread</span> <span class="token keyword">implements</span> <span class="token class-name">Runnable</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token class-name">String</span> task<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token class-name">Phaser</span> phaser<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token class-name">String</span> task<span class="token punctuation">,</span> <span class="token class-name">Phaser</span> phaser<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>task <span class="token operator">=</span> task<span class="token punctuation">;</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>phaser <span class="token operator">=</span> phaser<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">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> i <span class="token operator"><</span> <span class="token number">4</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token comment">// 第二次关卡起不加载NPC，跳过</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>i <span class="token operator">>=</span> <span class="token number">2</span> <span class="token operator">&&</span> <span class="token string">"加载新手教程"</span><span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>task<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">continue</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token class-name">Random</span> random <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Random</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span>random<span class="token punctuation">.</span><span class="token function">nextInt</span><span class="token punctuation">(</span><span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">format</span><span class="token punctuation">(</span><span class="token string">"关卡%d，需要加载%d个模块，当前模块【%s】"</span><span class="token punctuation">,</span>
i<span class="token punctuation">,</span> phaser<span class="token punctuation">.</span><span class="token function">getRegisteredParties</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> task<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 从第二个关卡起，不加载NPC</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>i <span class="token operator">==</span> <span class="token number">1</span> <span class="token operator">&&</span> <span class="token string">"加载新手教程"</span><span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>task<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"下次关卡移除加载【新手教程】模块"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
phaser<span class="token punctuation">.</span><span class="token function">arriveAndDeregister</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 移除一个模块</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
phaser<span class="token punctuation">.</span><span class="token function">arriveAndAwaitAdvance</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> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</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>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Phaser</span> phaser <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Phaser</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">protected</span> <span class="token keyword">boolean</span> <span class="token function">onAdvance</span><span class="token punctuation">(</span><span class="token keyword">int</span> phase<span class="token punctuation">,</span> <span class="token keyword">int</span> registeredParties<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">format</span><span class="token punctuation">(</span><span class="token string">"第%d次关卡准备完成"</span><span class="token punctuation">,</span> phase <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> phase <span class="token operator">==</span> <span class="token number">3</span> <span class="token operator">||</span> registeredParties <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token string">"加载地图数据"</span><span class="token punctuation">,</span> phaser<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token string">"加载人物模型"</span><span class="token punctuation">,</span> phaser<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token string">"加载背景音乐"</span><span class="token punctuation">,</span> phaser<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PreTaskThread</span><span class="token punctuation">(</span><span class="token string">"加载新手教程"</span><span class="token punctuation">,</span> phaser<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</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>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>输出：</p><blockquote><p>关卡 1，需要加载 4 个模块，当前模块【加载背景音乐】<br> 关卡 1，需要加载 4 个模块，当前模块【加载新手教程】<br> 下次关卡移除加载【新手教程】模块<br> 关卡 1，需要加载 3 个模块，当前模块【加载地图数据】<br> 关卡 1，需要加载 3 个模块，当前模块【加载人物模型】<br> 第 1 次关卡准备完成<br> 关卡 2，需要加载 3 个模块，当前模块【加载地图数据】<br> 关卡 2，需要加载 3 个模块，当前模块【加载背景音乐】<br> 关卡 2，需要加载 3 个模块，当前模块【加载人物模型】<br> 第 2 次关卡准备完成<br> 关卡 3，需要加载 3 个模块，当前模块【加载人物模型】<br> 关卡 3，需要加载 3 个模块，当前模块【加载地图数据】<br> 关卡 3，需要加载 3 个模块，当前模块【加载背景音乐】<br> 第 3 次关卡准备完成</p></blockquote><p>这里要注意关卡 1 的输出，在“加载新手教程”线程中调用了<code>arriveAndDeregister()</code>减少一个 party 之后，后面的线程使用<code>getRegisteredParties()</code>得到的是已经被修改后的 parties 了。但是当前这个阶段(phase)，仍然是需要 4 个 parties 都 arrive 才触发屏障的。从下一个阶段开始，才需要 3 个 parties 都 arrive 就触发屏障。</p><p>Phaser 类用来控制某个阶段的线程数量很有用，但它并不在意这个阶段具体有哪些线程 arrive，只要达到它当前阶段的 parties 值，就触发屏障。所以我这里的案例虽然制定了特定的线程（加载新手教程）来更直观地表述 Phaser 的功能，但其实 Phaser 是没有分辨具体是哪个线程的功能的，它在意的只是数量，这一点需要大家注意。</p><h3 id="phaser-原理" tabindex="-1"><a class="header-anchor" href="#phaser-原理" aria-hidden="true">#</a> Phaser 原理</h3>`,17),f={href:"https://javabetter.cn/thread/fork-join.html",target:"_blank",rel:"noopener noreferrer"},q=p(`<div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">AtomicReference</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">QNode</span><span class="token punctuation">></span></span> evenQ<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">AtomicReference</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">QNode</span><span class="token punctuation">></span></span> oddQ<span class="token punctuation">;</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">QNode</span> <span class="token keyword">implements</span> <span class="token class-name">ForkJoinPool<span class="token punctuation">.</span>ManagedBlocker</span> <span class="token punctuation">{</span>
<span class="token comment">// 实现代码</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>有兴趣的小伙伴可以去看看 JDK 源代码，这里不做过多叙述。</p><h2 id="小结" tabindex="-1"><a class="header-anchor" href="#小结" aria-hidden="true">#</a> 小结</h2><p>总的来说，CountDownLatch，CyclicBarrier，Phaser 是一个比一个强大，但也一个比一个复杂，需要根据自己的业务需求合理选择。</p>`,4),x={href:"http://concurrent.redspider.group/",target:"_blank",rel:"noopener noreferrer"},S=s("hr",null,null,-1),T={href:"https://github.com/itwanger/toBeBetterJavaer",target:"_blank",rel:"noopener noreferrer"},_={href:"https://javabetter.cn/thread/",target:"_blank",rel:"noopener noreferrer"},C={href:"https://javabetter.cn/thread/",target:"_blank",rel:"noopener noreferrer"},B={href:"https://javabetter.cn/thread/",target:"_blank",rel:"noopener noreferrer"},P={href:"https://javabetter.cn/thread/",target:"_blank",rel:"noopener noreferrer"},D=s("figure",null,[s("img",{src:"https://cdn.tobebetterjavaer.com/stutymore/wangzhe-thread-20230904125125.png",alt:"",tabindex:"0",loading:"lazy"}),s("figcaption")],-1);function j(L,A){const a=c("ExternalLinkIcon");return o(),l("div",null,[u,s("p",null,[n("Semaphore 内部有一个继承了 "),s("a",r,[n("AQS"),t(a)]),n(" 的同步器 Sync，重写了"),k,n("方法。在这个方法里，会去尝试获取资源。")]),d,s("p",null,[n("看源码可以发现它是使用 park/unpark 来实现等待状态切换的，但是在使用 park/unpark 方法之前，使用了 "),s("a",v,[n("CAS"),t(a)]),n(" 检查，估计是为了提高性能。")]),m,s("p",null,[n("其实 CountDownLatch 类的原理挺简单的，内部同样是一个继承了 "),s("a",b,[n("AQS"),t(a)]),n(" 的实现类 Sync，且实现起来还很简单，可能是 JDK 里面 AQS 的子类中最简单的实现了，有兴趣的小伙伴可以去看看这个内部类的源码。")]),h,s("p",null,[n("CyclicBarrier 虽说功能与 CountDownLatch 类似，但是实现原理却完全不同，CyclicBarrier 内部使用的是 "),s("a",w,[n("Lock"),t(a)]),n(" + "),s("a",y,[n("Condition"),t(a)]),n(" 实现的等待/通知模式。详情可以查看这个方法的源码：")]),g,s("p",null,[n("Phaser 类的原理相比起来要复杂得多。它内部使用了两个基于 "),s("a",f,[n("Fork-Join 框架"),t(a)]),n("的原子类辅助：")]),q,s("blockquote",null,[s("p",null,[n("编辑：沉默王二，部分内容来源于朋友小七萤火虫开源的这个仓库："),s("a",x,[n("深入浅出 Java 多线程"),t(a)])])]),S,s("p",null,[n("GitHub 上标星 10000+ 的开源知识库《"),s("a",T,[n("二哥的 Java 进阶之路"),t(a)]),n("》第二份 PDF 《"),s("a",_,[n("并发编程小册"),t(a)]),n("》终于来了！包括线程的基本概念和使用方法、Java的内存模型、sychronized、volatile、CAS、AQS、ReentrantLock、线程池、并发容器、ThreadLocal、生产者消费者模型等面试和开发必须掌握的内容，共计 15 万余字，200+张手绘图，可以说是通俗易懂、风趣幽默……详情戳："),s("a",C,[n("太赞了，二哥的并发编程进阶之路.pdf"),t(a)])]),s("p",null,[s("a",B,[n("加入二哥的编程星球"),t(a)]),n("，在星球的第二个置顶帖「"),s("a",P,[n("知识图谱"),t(a)]),n("」里就可以获取 PDF 版本。")]),D])}const I=e(i,[["render",j],["__file","CountDownLatch.html.vue"]]);export{I as default};