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class="sidebar-heading clickable active"><span class="title">2.11 并发编程</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><a href="/thread/wangzhe-thread.html" class="nav-link sidebar-link sidebar-page" aria-label="创建Java线程的3种方式">创建Java线程的3种方式</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/thread-state-and-method.html" class="nav-link sidebar-link sidebar-page" aria-label="线程的6种状态及切换">线程的6种状态及切换</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/thread-group-and-thread-priority.html" class="nav-link sidebar-link sidebar-page" aria-label="线程组和线程优先级">线程组和线程优先级</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/why-need-thread.html" class="nav-link sidebar-link sidebar-page" aria-label="进程与线程的区别">进程与线程的区别</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/thread-bring-some-problem.html" class="nav-link sidebar-link sidebar-page" aria-label="并发编程带来了哪些问题">并发编程带来了哪些问题</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/jmm.html" class="nav-link sidebar-link sidebar-page" aria-label="Java内存模型">Java内存模型</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/volatile.html" class="nav-link sidebar-link sidebar-page" aria-label="volatile">volatile</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/synchronized.html" class="nav-link sidebar-link sidebar-page" aria-label="synchronized">synchronized</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/cas.html" class="nav-link sidebar-link sidebar-page" aria-label="CAS的原理">CAS的原理</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/aqs.html" class="nav-link sidebar-link sidebar-page" aria-label="AQS详解">AQS详解</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/lock.html" class="nav-link sidebar-link sidebar-page" aria-label="锁">锁</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/reentrantLock.html" class="nav-link sidebar-link sidebar-page" aria-label="重入锁ReentrantLock">重入锁ReentrantLock</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/ReentrantReadWriteLock.html" class="nav-link sidebar-link sidebar-page" aria-label="读写锁ReentrantReadWriteLock">读写锁ReentrantReadWriteLock</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/condition.html" class="nav-link sidebar-link sidebar-page" aria-label="线程协作类Condition">线程协作类Condition</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/LockSupport.html" class="nav-link sidebar-link sidebar-page" aria-label="线程阻塞唤醒类LockSupport">线程阻塞唤醒类LockSupport</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/map.html" class="nav-link sidebar-link sidebar-page" aria-label="并发集合容器">并发集合容器</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/ConcurrentHashMap.html" class="nav-link sidebar-link sidebar-page" aria-label="ConcurrentHashMap">ConcurrentHashMap</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/ConcurrentLinkedQueue.html" class="nav-link sidebar-link sidebar-page" aria-label="ConcurrentLinkedQueue">ConcurrentLinkedQueue</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/CopyOnWriteArrayList.html" class="nav-link sidebar-link sidebar-page" aria-label="CopyOnWriteArrayList">CopyOnWriteArrayList</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/ThreadLocal.html" class="nav-link sidebar-link sidebar-page" aria-label="ThreadLocal">ThreadLocal</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/BlockingQueue.html" class="nav-link sidebar-link sidebar-page" aria-label="BlockingQueue">BlockingQueue</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/pool.html" class="nav-link sidebar-link sidebar-page" aria-label="线程池">线程池</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/ScheduledThreadPoolExecutor.html" class="nav-link sidebar-link sidebar-page" aria-label="计划任务">计划任务</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/atomic.html" class="nav-link sidebar-link sidebar-page" aria-label="原子操作类">原子操作类</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/thread/CountDownLatch.html" class="nav-link sidebar-link sidebar-page" aria-label="通信工具类CountDownLatch">通信工具类CountDownLatch</a><ul class="sidebar-sub-headers"></ul></li><li><a aria-current="page" href="/thread/fork-join.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="Fork/Join框架">Fork/Join框架</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/fork-join.html#什么是fork-join" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="什么是Fork/Join">什么是Fork/Join</a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/fork-join.html#工作窃取算法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="工作窃取算法">工作窃取算法</a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/fork-join.html#fork-join的具体实现" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="Fork/Join的具体实现">Fork/Join的具体实现</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/fork-join.html#forkjointask" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="ForkJoinTask">ForkJoinTask</a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/fork-join.html#forkjoinpool" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="ForkJoinPool">ForkJoinPool</a><ul class="sidebar-sub-headers"></ul></li></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/fork-join.html#fork-join的使用" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="Fork/Join的使用">Fork/Join的使用</a><ul class="sidebar-sub-headers"></ul></li></ul></li><li><a href="/thread/shengchanzhe-xiaofeizhe.html" class="nav-link sidebar-link sidebar-page" aria-label="生产者-消费者模式">生产者-消费者模式</a><ul class="sidebar-sub-headers"></ul></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.12 JVM</span><span class="arrow right"></span></button></section></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">三、Java企业级开发</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">四、数据库</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">五、计算机基础</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">六、求职面试</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">七、学习资源</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">八、学习建议</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">九、知识库搭建</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">十、联系作者</span><span class="arrow right"></span></button></section></li></ul></aside><main class="page" id="main-content"><nav class="breadcrumb disable"></nav><div class="page-title"><h1>深入理解Java并发编程之Fork/Join框架</h1><div class="page-info"><span class="author-info" aria-label="作者🖊" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></svg><span><a class="author-item" href="/about-the-author/" target="_blank" rel="noopener noreferrer">沉默王二</a></span><span property="author" content="沉默王二"></span></span><span class="date-info" aria-label="写作日期📅" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon 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18.548s-18.548-8.304-18.548-18.548V623.635c0-20.388-16.746-36.974-37.33-36.974H624.13c-20.585 0-37.331 16.747-37.331 37.33v222.086c0 20.585 16.654 37.331 37.126 37.331H730.22c10.243 0 18.547 8.304 18.547 18.547 0 10.244-8.304 18.547-18.547 18.547z"></path></svg><ul class="categories-wrapper"><li class="category category0 clickable" role="navigation">Java核心</li><li class="category category6 clickable" role="navigation">并发编程</li><meta property="articleSection" content="Java核心,并发编程"></ul></span><span aria-label="标签🏷" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon tag-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="tag icon"><path d="M939.902 458.563L910.17 144.567c-1.507-16.272-14.465-29.13-30.737-30.737L565.438 84.098h-.402c-3.215 0-5.726 1.005-7.634 2.913l-470.39 470.39a10.004 10.004 0 000 14.164l365.423 365.424c1.909 1.908 4.42 2.913 7.132 2.913s5.223-1.005 7.132-2.913l470.39-470.39c2.01-2.11 3.014-5.023 2.813-8.036zm-240.067-72.121c-35.458 0-64.286-28.828-64.286-64.286s28.828-64.285 64.286-64.285 64.286 28.828 64.286 64.285-28.829 64.286-64.286 64.286z"></path></svg><ul class="tags-wrapper"><li class="tag tag4 clickable" role="navigation">Java</li></ul><meta property="keywords" content="Java"></span><span class="reading-time-info" aria-label="阅读时间⌛" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon timer-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="timer icon"><path d="M799.387 122.15c4.402-2.978 7.38-7.897 7.38-13.463v-1.165c0-8.933-7.38-16.312-16.312-16.312H256.33c-8.933 0-16.311 7.38-16.311 16.312v1.165c0 5.825 2.977 10.874 7.637 13.592 4.143 194.44 97.22 354.963 220.201 392.763-122.204 37.542-214.893 196.511-220.2 389.397-4.661 5.049-7.638 11.651-7.638 19.03v5.825h566.49v-5.825c0-7.379-2.849-13.981-7.509-18.9-5.049-193.016-97.867-351.985-220.2-389.527 123.24-37.67 216.446-198.453 220.588-392.892zM531.16 450.445v352.632c117.674 1.553 211.787 40.778 211.787 88.676H304.097c0-48.286 95.149-87.382 213.728-88.676V450.445c-93.077-3.107-167.901-81.297-167.901-177.093 0-8.803 6.99-15.793 15.793-15.793 8.803 0 15.794 6.99 15.794 15.793 0 80.261 63.69 145.635 142.01 145.635s142.011-65.374 142.011-145.635c0-8.803 6.99-15.793 15.794-15.793s15.793 6.99 15.793 15.793c0 95.019-73.789 172.82-165.96 177.093z"></path></svg><span>大约 9 分钟</span><meta property="timeRequired" content="PT9M"></span></div><hr></div><div class="toc-place-holder"><aside id="toc"><div class="toc-header">此页内容</div><div class="toc-wrapper"><ul class="toc-list"><li class="toc-item"><a aria-current="page" href="/thread/fork-join.html#什么是fork-join" class="router-link-active router-link-exact-active toc-link level2">什么是Fork/Join</a></li><li class="toc-item"><a aria-current="page" href="/thread/fork-join.html#工作窃取算法" class="router-link-active router-link-exact-active toc-link level2">工作窃取算法</a></li><li class="toc-item"><a aria-current="page" href="/thread/fork-join.html#fork-join的具体实现" class="router-link-active router-link-exact-active toc-link level2">Fork/Join的具体实现</a></li><ul class="toc-list"><li class="toc-item"><a aria-current="page" href="/thread/fork-join.html#forkjointask" class="router-link-active router-link-exact-active toc-link level3">ForkJoinTask</a></li><li class="toc-item"><a aria-current="page" href="/thread/fork-join.html#forkjoinpool" class="router-link-active router-link-exact-active toc-link level3">ForkJoinPool</a></li></ul><li class="toc-item"><a aria-current="page" href="/thread/fork-join.html#fork-join的使用" class="router-link-active router-link-exact-active toc-link level2">Fork/Join的使用</a></li></ul></div></aside></div><div class="theme-hope-content"><h2 id="什么是fork-join" tabindex="-1"><a class="header-anchor" href="#什么是fork-join" aria-hidden="true">#</a> 什么是Fork/Join</h2><p>Fork/Join框架是一个实现了ExecutorService接口的多线程处理器，它专为那些可以通过递归分解成更细小的任务而设计，最大化的利用多核处理器来提高应用程序的性能。</p><p>与其他ExecutorService相关的实现相同的是，Fork/Join框架会将任务分配给线程池中的线程。而与之不同的是，Fork/Join框架在执行任务时使用了<strong>工作窃取算法</strong>。</p><p><strong>fork</strong>在英文里有分叉的意思，<strong>join</strong>在英文里连接、结合的意思。顾名思义，fork就是要使一个大任务分解成若干个小任务，而join就是最后将各个小任务的结果结合起来得到大任务的结果。</p><p>Fork/Join的运行流程大致如下所示：</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/thread/fork-join-ba0c0e3f-dc9b-445d-874a-5878503a98f7.png" alt="fork/join流程图"></p><p>需要注意的是，图里的次级子任务可以一直分下去，一直分到子任务足够小为止。用伪代码来表示如下：</p><div class="language-pseudocode ext-pseudocode line-numbers-mode"><pre class="language-pseudocode"><code>solve(任务):

if(任务已经划分到足够小):

顺序执行任务

else:

for(划分任务得到子任务)

solve(子任务)

结合所有子任务的结果到上一层循环

return 最终结合的结果

</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>通过上面伪代码可以看出，我们通过递归嵌套的计算得到最终结果，这里有体现<strong>分而治之(divide and conquer)</strong> 的算法思想。</p><h2 id="工作窃取算法" tabindex="-1"><a class="header-anchor" href="#工作窃取算法" aria-hidden="true">#</a> 工作窃取算法</h2><p>工作窃取算法指的是在多线程执行不同任务队列的过程中，某个线程执行完自己队列的任务后从其他线程的任务队列里窃取任务来执行。</p><p>工作窃取流程如下图所示：</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/thread/fork-join-819f4ad9-25ce-4e7e-a1d7-e36a70e584a4.png" alt="工作窃取算法流程"></p><p>值得注意的是，当一个线程窃取另一个线程的时候，为了减少两个任务线程之间的竞争，我们通常使用<strong>双端队列</strong>来存储任务。被窃取的任务线程都从双端队列的<strong>头部</strong>拿任务执行，而窃取其他任务的线程从双端队列的<strong>尾部</strong>执行任务。</p><p>另外，当一个线程在窃取任务时要是没有其他可用的任务了，这个线程会进入<strong>阻塞状态</strong>以等待再次“工作”。</p><h2 id="fork-join的具体实现" tabindex="-1"><a class="header-anchor" href="#fork-join的具体实现" aria-hidden="true">#</a> Fork/Join的具体实现</h2><p>前面我们说Fork/Join框架简单来讲就是对任务的分割与子任务的合并，所以要实现这个框架，先得有<strong>任务</strong>。在Fork/Join框架里提供了抽象类<code>ForkJoinTask</code>来实现任务。</p><h3 id="forkjointask" tabindex="-1"><a class="header-anchor" href="#forkjointask" aria-hidden="true">#</a> ForkJoinTask</h3><p>ForkJoinTask是一个类似普通线程的实体，但是比普通线程轻量得多。</p><p><strong>fork()方法</strong>:使用线程池中的空闲线程异步提交任务</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 本文所有代码都引自Java 8</span>

<span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token class-name">ForkJoinTask</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">V</span><span class="token punctuation">></span></span> <span class="token function">fork</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token class-name">Thread</span> t<span class="token punctuation">;</span>

<span class="token comment">// ForkJoinWorkerThread是执行ForkJoinTask的专有线程，由ForkJoinPool管理</span>

<span class="token comment">// 先判断当前线程是否是ForkJoin专有线程，如果是，则将任务push到当前线程所负责的队列里去</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>t <span class="token operator">=</span> <span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token keyword">instanceof</span> <span class="token class-name">ForkJoinWorkerThread</span><span class="token punctuation">)</span>

<span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token class-name">ForkJoinWorkerThread</span><span class="token punctuation">)</span>t<span class="token punctuation">)</span><span class="token punctuation">.</span>workQueue<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 如果不是则将线程加入队列</span>

<span class="token comment">// 没有显式创建ForkJoinPool的时候走这里，提交任务到默认的common线程池中</span>

<span class="token class-name">ForkJoinPool</span><span class="token punctuation">.</span>common<span class="token punctuation">.</span><span class="token function">externalPush</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">;</span>

<span class="token comment">// doJoin()方法来获取当前任务的执行状态</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>s <span class="token operator">=</span> <span class="token function">doJoin</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&</span> DONE_MASK<span class="token punctuation">)</span> <span class="token operator">!=</span> NORMAL<span class="token punctuation">)</span>

<span class="token function">reportException</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 任务正常完成，获取返回值</span>

<span class="token keyword">return</span> <span class="token function">getRawResult</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token doc-comment comment">/**

* doJoin()方法用来返回当前任务的执行状态

**/</span>

<span class="token keyword">private</span> <span class="token keyword">int</span> <span class="token function">doJoin</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">int</span> s<span class="token punctuation">;</span> <span class="token class-name">Thread</span> t<span class="token punctuation">;</span> <span class="token class-name">ForkJoinWorkerThread</span> wt<span class="token punctuation">;</span> <span class="token class-name">ForkJoinPool<span class="token punctuation">.</span>WorkQueue</span> w<span class="token punctuation">;</span>

<span class="token comment">// 先判断任务是否执行完毕，执行完毕直接返回结果（执行状态）</span>

<span class="token keyword">return</span> <span class="token punctuation">(</span>s <span class="token operator">=</span> status<span class="token punctuation">)</span> <span class="token operator"><</span> <span class="token number">0</span> <span class="token operator">?</span> s <span class="token operator">:</span>

<span class="token comment">// 如果没有执行完毕，先判断是否是ForkJoinWorkThread线程</span>

<span class="token punctuation">(</span><span class="token punctuation">(</span>t <span class="token operator">=</span> <span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token keyword">instanceof</span> <span class="token class-name">ForkJoinWorkerThread</span><span class="token punctuation">)</span> <span class="token operator">?</span>

<span class="token comment">// 如果是，先判断任务是否处于工作队列顶端（意味着下一个就执行它）</span>

<span class="token comment">// tryUnpush()方法判断任务是否处于当前工作队列顶端，是返回true</span>

<span class="token comment">// doExec()方法执行任务</span>

<span class="token punctuation">(</span>w <span class="token operator">=</span> <span class="token punctuation">(</span>wt <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">ForkJoinWorkerThread</span><span class="token punctuation">)</span>t<span class="token punctuation">)</span><span class="token punctuation">.</span>workQueue<span class="token punctuation">)</span><span class="token punctuation">.</span>

<span class="token comment">// 如果是处于顶端并且任务执行完毕，返回结果</span>

<span class="token function">tryUnpush</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span> <span class="token operator">&&</span> <span class="token punctuation">(</span>s <span class="token operator">=</span> <span class="token function">doExec</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator"><</span> <span class="token number">0</span> <span class="token operator">?</span> s <span class="token operator">:</span>

<span class="token comment">// 如果不在顶端或者在顶端却没未执行完毕，那就调用awitJoin()执行任务</span>

<span class="token comment">// awaitJoin()：使用自旋使任务执行完成，返回结果</span>

wt<span class="token punctuation">.</span>pool<span class="token punctuation">.</span><span class="token function">awaitJoin</span><span class="token punctuation">(</span>w<span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">,</span> <span class="token number">0L</span><span class="token punctuation">)</span> <span class="token operator">:</span>

<span class="token comment">// 如果不是ForkJoinWorkThread线程，执行externalAwaitDone()返回任务结果</span>

<span class="token function">externalAwaitDone</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></div><p>我们在之前介绍过说Thread.join()会使线程阻塞，而ForkJoinPool.join()会使线程免于阻塞，下面是ForkJoinPool.join()的流程图： <img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/thread/fork-join-8e03485d-efe0-4edf-8516-a9b10dea6e7f.png" alt="join流程图"></p><p><strong>RecursiveAction和RecursiveTask</strong></p><p>通常情况下，在创建任务的时候我们一般不直接继承ForkJoinTask，而是继承它的子类<strong>RecursiveAction</strong>和<strong>RecursiveTask</strong>。</p><p>两个都是ForkJoinTask的子类，<strong>RecursiveAction可以看做是无返回值的ForkJoinTask，RecursiveTask是有返回值的ForkJoinTask</strong>。</p><p>此外，两个子类都有执行主要计算的方法compute()，当然，RecursiveAction的compute()返回void，RecursiveTask的compute()有具体的返回值。</p><h3 id="forkjoinpool" tabindex="-1"><a class="header-anchor" href="#forkjoinpool" aria-hidden="true">#</a> ForkJoinPool</h3><p>ForkJoinPool是用于执行ForkJoinTask任务的执行（线程）池。</p><p>ForkJoinPool管理着执行池中的线程和任务队列，此外，执行池是否还接受任务，显示线程的运行状态也是在这里处理。</p><p>我们来大致看下ForkJoinPool的源码：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token annotation punctuation">@sun.misc.Contended</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ForkJoinPool</span> <span class="token keyword">extends</span> <span class="token class-name">AbstractExecutorService</span> <span class="token punctuation">{</span>

<span class="token keyword">volatile</span> <span class="token class-name">WorkQueue</span><span class="token punctuation">[</span><span class="token punctuation">]</span> workQueues<span class="token punctuation">;</span>

<span class="token comment">// 线程的运行状态</span>

<span class="token keyword">volatile</span> <span class="token keyword">int</span> runState<span class="token punctuation">;</span>

<span class="token comment">// 创建ForkJoinWorkerThread的默认工厂，可以通过构造函数重写</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">ForkJoinWorkerThreadFactory</span> defaultForkJoinWorkerThreadFactory<span class="token punctuation">;</span>

<span class="token comment">// 公用的线程池，其运行状态不受shutdown()和shutdownNow()的影响</span>

<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">ForkJoinPool</span> common<span class="token punctuation">;</span>

<span class="token comment">// 私有构造方法，没有任何安全检查和参数校验，由makeCommonPool直接调用</span>

<span class="token comment">// 其他构造方法都是源自于此方法</span>

<span class="token comment">// parallelism: 并行度，</span>

<span class="token comment">// 默认调用java.lang.Runtime.availableProcessors() 方法返回可用处理器的数量</span>

<span class="token keyword">private</span> <span class="token class-name">ForkJoinPool</span><span class="token punctuation">(</span><span class="token keyword">int</span> parallelism<span class="token punctuation">,</span>

<span class="token class-name">ForkJoinWorkerThreadFactory</span> factory<span class="token punctuation">,</span> <span class="token comment">// 工作线程工厂</span>

<span class="token class-name">UncaughtExceptionHandler</span> handler<span class="token punctuation">,</span> <span class="token comment">// 拒绝任务的handler</span>

<span class="token class-name">String</span> workerNamePrefix<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 线程名prefix</span>

<span class="token keyword">this</span><span class="token punctuation">.</span>workerNamePrefix <span class="token operator">=</span> workerNamePrefix<span class="token punctuation">;</span>

<span class="token keyword">this</span><span class="token punctuation">.</span>factory <span class="token operator">=</span> factory<span class="token punctuation">;</span>

<span class="token keyword">this</span><span class="token punctuation">.</span>ueh <span class="token operator">=</span> handler<span class="token punctuation">;</span>

<span class="token keyword">this</span><span class="token punctuation">.</span>config <span class="token operator">=</span> <span class="token punctuation">(</span>parallelism <span class="token operator">&</span> SMASK<span class="token punctuation">)</span> <span class="token operator">|</span> mode<span class="token punctuation">;</span>

<span class="token keyword">long</span> np <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">long</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token operator">-</span>parallelism<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// offset ctl counts</span>

<span class="token keyword">this</span><span class="token punctuation">.</span>ctl <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>np <span class="token operator"><<</span> AC_SHIFT<span class="token punctuation">)</span> <span class="token operator">&</span> AC_MASK<span class="token punctuation">)</span> <span class="token operator">|</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>np <span class="token operator"><<</span> TC_SHIFT<span class="token punctuation">)</span> <span class="token operator">&</span> TC_MASK<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></div><h4 id="workqueue" tabindex="-1"><a class="header-anchor" href="#workqueue" aria-hidden="true">#</a> WorkQueue</h4><p>双端队列，ForkJoinTask存放在这里。</p><p>当工作线程在处理自己的工作队列时，会从队列首取任务来执行（FIFO）；如果是窃取其他队列的任务时，窃取的任务位于所属任务队列的队尾（LIFO）。</p><p>ForkJoinPool与传统线程池最显著的区别就是它维护了一个<strong>工作队列数组</strong>（volatile WorkQueue[] workQueues，ForkJoinPool中的<strong>每个工作线程都维护着一个工作队列</strong>）。</p><h4 id="runstate" tabindex="-1"><a class="header-anchor" href="#runstate" aria-hidden="true">#</a> runState</h4><p>ForkJoinPool的运行状态。<strong>SHUTDOWN</strong>状态用负数表示，其他用2的幂次表示。</p><h2 id="fork-join的使用" tabindex="-1"><a class="header-anchor" href="#fork-join的使用" aria-hidden="true">#</a> Fork/Join的使用</h2><p>上面我们说ForkJoinPool负责管理线程和任务，ForkJoinTask实现fork和join操作，所以要使用Fork/Join框架就离不开这两个类了，只是在实际开发中我们常用ForkJoinTask的子类RecursiveTask 和RecursiveAction来替代ForkJoinTask。</p><p>下面我们用一个计算斐波那契数列第n项的例子来看一下Fork/Join的使用：</p><blockquote><p>斐波那契数列数列是一个线性递推数列，从第三项开始，每一项的值都等于前两项之和：</p><p>1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89······</p><p>如果设f(n）为该数列的第n项（n∈N*），那么有：f(n) = f(n-1) + f(n-2)。</p></blockquote><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">FibonacciTest</span> <span class="token punctuation">{</span>

<span class="token keyword">class</span> <span class="token class-name">Fibonacci</span> <span class="token keyword">extends</span> <span class="token class-name">RecursiveTask</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> <span class="token punctuation">{</span>

<span class="token keyword">public</span> <span class="token class-name">Fibonacci</span><span class="token punctuation">(</span><span class="token keyword">int</span> n<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token comment">// 主要的实现逻辑都在compute()里</span>

<span class="token annotation punctuation">@Override</span>

<span class="token keyword">protected</span> <span class="token class-name">Integer</span> <span class="token function">compute</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token comment">// 这里先假设 n >= 0</span>

<span class="token keyword">return</span> n<span class="token punctuation">;</span>

<span class="token class-name">Fibonacci</span> f1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Fibonacci</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

f1<span class="token punctuation">.</span><span class="token function">fork</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">Fibonacci</span> f2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Fibonacci</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

f2<span class="token punctuation">.</span><span class="token function">fork</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">return</span> f1<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> f2<span class="token punctuation">.</span><span class="token function">join</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">void</span> <span class="token function">testFib</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">ExecutionException</span><span class="token punctuation">,</span> <span class="token class-name">InterruptedException</span> <span class="token punctuation">{</span>

<span class="token class-name">ForkJoinPool</span> forkJoinPool <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ForkJoinPool</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">"CPU核数："</span> <span class="token operator">+</span> <span class="token class-name">Runtime</span><span class="token punctuation">.</span><span class="token function">getRuntime</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">availableProcessors</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">long</span> start <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">Fibonacci</span> fibonacci <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Fibonacci</span><span class="token punctuation">(</span><span class="token number">40</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">Future</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> future <span class="token operator">=</span> forkJoinPool<span class="token punctuation">.</span><span class="token function">submit</span><span class="token punctuation">(</span>fibonacci<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">long</span> end <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">currentTimeMillis</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">"耗时：%d millis"</span><span class="token punctuation">,</span> end <span class="token operator">-</span> start<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

计算结果：102334155

耗时：9490 millis

</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></div><p>需要注意的是，上述计算时间复杂度为<code>O(2^n)</code>，随着n的增长计算效率会越来越低，这也是上面的例子中n不敢取太大的原因。</p><p>此外，也并不是所有的任务都适合Fork/Join框架，比如上面的例子任务划分过于细小反而体现不出效率，下面我们试试用普通的递归来求f(n)的值，看看是不是要比使用Fork/Join快：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 普通递归，复杂度为O(2^n)</span>

<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">plainRecursion</span><span class="token punctuation">(</span><span class="token keyword">int</span> n<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>

<span class="token keyword">return</span> <span class="token function">plainRecursion</span><span class="token punctuation">(</span>n <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token function">plainRecursion</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">2</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">testPlain</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">int</span> result <span class="token operator">=</span> <span class="token function">plainRecursion</span><span class="token punctuation">(</span><span class="token number">40</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">"耗时：%d millis"</span><span class="token punctuation">,</span> end <span class="token operator">-</span>start<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

耗时：436 millis

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div></div></div><p>通过输出可以很明显的看出来，使用普通递归的效率都要比使用Fork/Join框架要高很多。</p><p>这里我们再用另一种思路来计算：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 通过循环来计算，复杂度为O(n)</span>

<span class="token keyword">private</span> <span class="token keyword">int</span> <span class="token function">computeFibonacci</span><span class="token punctuation">(</span><span class="token keyword">int</span> n<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">return</span> n<span class="token punctuation">;</span>

<span class="token keyword">int</span> first <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>

<span class="token keyword">int</span> second <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>

<span class="token keyword">int</span> third <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>

<span class="token comment">// 第三个数是前两个数之和</span>

third <span class="token operator">=</span> first <span class="token operator">+</span> second<span class="token punctuation">;</span>

<span class="token comment">// 前两个数右移</span>

first <span class="token operator">=</span> second<span class="token punctuation">;</span>

second <span class="token operator">=</span> third<span class="token punctuation">;</span>

<span class="token keyword">return</span> third<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">testComputeFibonacci</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">int</span> result <span class="token operator">=</span> <span class="token function">computeFibonacci</span><span class="token punctuation">(</span><span class="token number">40</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">"耗时：%d millis"</span><span class="token punctuation">,</span> end <span class="token operator">-</span>start<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

耗时：0 millis

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div></div></div><p>这里耗时为0不代表没有耗时，是表明这里计算的耗时几乎可以忽略不计，大家可以在自己的电脑试试，即使是n取大很多量级的数据（注意int溢出的问题）耗时也是很短的，或者可以用System.nanoTime()统计纳秒的时间。</p><p>为什么在这里普通的递归或循环效率更快呢？因为Fork/Join是使用多个线程协作来计算的，所以会有线程通信和线程切换的开销。</p><p>如果要计算的任务比较简单（比如我们案例中的斐波那契数列），那当然是直接使用单线程会更快一些。但如果要计算的东西比较复杂，计算机又是多核的情况下，就可以充分利用多核CPU来提高计算速度。</p><p>另外，Java 8 Stream的并行操作底层就是用到了Fork/Join框架，下一章我们将从源码及案例两方面介绍Java 8 Stream的并行操作。</p><hr><blockquote><p>编辑：沉默王二，内容大部分来源以下三个开源仓库：</p><ul><li><a href="http://concurrent.redspider.group/" target="_blank" rel="noopener noreferrer">深入浅出 Java 多线程<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon 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