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id="post-info"><h1 class="post-title">Java并发实现原理-多线程基础-volatile关键字(5)</h1><div id="post-meta"><div class="meta-firstline"><span class="post-meta-date"><i class="far fa-calendar-alt fa-fw post-meta-icon"></i><span class="post-meta-label">发表于</span><time class="post-meta-date-created" datetime="2022-04-09T08:43:59.000Z" title="发表于 2022-04-09 16:43:59">2022-04-09</time><span class="post-meta-separator">|</span><i class="fas fa-history fa-fw post-meta-icon"></i><span class="post-meta-label">更新于</span><time class="post-meta-date-updated" datetime="2022-04-10T14:30:29.173Z" title="更新于 2022-04-10 22:30:29">2022-04-10</time></span><span class="post-meta-categories"><span class="post-meta-separator">|</span><i class="fas fa-inbox fa-fw post-meta-icon"></i><a class="post-meta-categories" href="/categories/Java%E5%B9%B6%E5%8F%91%E5%AE%9E%E7%8E%B0%E5%8E%9F%E7%90%86/">Java并发实现原理</a></span></div><div class="meta-secondline"><span class="post-meta-separator">|</span><span class="post-meta-wordcount"><i class="far fa-file-word fa-fw post-meta-icon"></i><span class="post-meta-label">字数总计:</span><span class="word-count">981</span><span class="post-meta-separator">|</span><i class="far fa-clock fa-fw post-meta-icon"></i><span class="post-meta-label">阅读时长:</span><span>3分钟</span></span><span class="post-meta-separator">|</span><span class="post-meta-pv-cv" id="" data-flag-title="Java并发实现原理-多线程基础-volatile关键字(5)"><i class="far fa-eye fa-fw post-meta-icon"></i><span class="post-meta-label">阅读量:</span><span id="busuanzi_value_page_pv"></span></span></div></div></div><article class="post-content" id="article-container"><blockquote>
<p>volatile这个关键字很不起眼，其使用场景和语义不像synchronized、wait()和notify()那么明显。正因为其隐晦，volatile关键字可能是多线程编程中被误解最多的一个。而关键字越隐晦，背后隐含的往往越复杂、越深刻。接下来将进一步由浅入深地从使用场景讨论到其底层实现。</p>
</blockquote>
<h2 id="1-64位写入的原子性（Half-Write）"><a href="#1-64位写入的原子性（Half-Write）" class="headerlink" title="1. 64位写入的原子性（Half Write）"></a>1. 64位写入的原子性（Half Write）</h2><p>举一个简答的例子，对于一个long类型变量的赋值和取值操作而言，在多线程场景下，线程A调用set(100),线程调用B调用get(),在某些场景下，返回值可能不是100.</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Example1</span><span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">long</span> a <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token comment">// 线程A调用set(100)</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">set</span><span class="token punctuation">(</span><span class="token keyword">long</span> a<span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token operator">=</span> a<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 线程B调用get(),返回值是不是一定为100?</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">get</span><span class="token punctuation">(</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>a<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>这有点反直觉，如此简单的一个赋值和取值操作，在多线程下面为什么会不对呢？这是因为JVM的规范并没有要求64位的long或者double的写入是原子的。在32位的机器上，一个64位变量的写入可能被拆分成两个32位的写操作来执行。这样一来，读取的线程就可能读到“一般的值”。解决办法也很简单，在long前面加上volatile关键字。</p>
<h2 id="2-内存的可见性"><a href="#2-内存的可见性" class="headerlink" title="2.内存的可见性"></a>2.内存的可见性</h2><p>不仅64位，32位或者位数更小的赋值和取值操作，其实也有问题。在之前讲过，线程关闭的标志位stopped为例，它是一个boolean类型的数组，也可能出现主线程把它设置成true,而工作线程读到的却还是false的情形，这就更反直觉了。</p>
<p>注意，这里并不是说永远读到的都是false,而是说一个线程写完之后，另外一个线程立即去读，读到的是false,但之后能读到true,也就是“最终一致性”，不是“强一致性”。这种特性，对于之前的例子没有太大影响，但是如果想实现无锁算法，例如实现一把自旋锁，就会出现一个线程把状态设置为true,另外一个线程读到的却还是false,然后两个线程都会拿到这把锁的问题。</p>
<p>所以，我们所说的“内存可见性”，指的是“写完之后立即对其他线程可见”，它的反面不是“不可见”，而是“稍后才能可见”。解决这个问题很容易，给变量加上volatile关键字即可。</p>
<p>“内存可见性”问题的出现，跟现代CPU的架构密切相关，后面会详细探讨。</p>
<h2 id="3-重排序-DCL问题"><a href="#3-重排序-DCL问题" class="headerlink" title="3. 重排序:DCL问题"></a>3. 重排序:DCL问题</h2><p>单例模式的线程安全写法不止一种，常用写法为DCL(Double Checking Locking)，如下所示：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Singleton</span><span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token class-name">Singleton</span> instance<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">SIngleton</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">if</span><span class="token punctuation">(</span>instance <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token comment">// 为了性能，延迟使用synchronized</span>
<span class="token keyword">synchronized</span><span class="token punctuation">(</span><span class="token class-name">Singleton</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">if</span><span class="token punctuation">(</span>instance <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token comment">// 有问题的代码！！！</span>
instance <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Instance</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">return</span> instance<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>上述的**instance = new Instance()**代码会有问题：其底层会分为三个操作：</p>
<p>1.分配一块内存</p>
<p>2.在内存上初始化成员变量</p>
<p>3.把instance引用指向内存</p>
<p>在这三个操作中，操作2和操作3可能重排序，即先把instance指向内存，再初始化成员变量，因为二者并没有先后的依赖关系。此时，另外一个线程可能拿到一个未完成初始化的对象。这时，直接访问里面的成员变量，就可能出错。这就是典型的”构造函数溢出“问题。解决办法很简单，就是为instance变量加上volatile修饰。</p>
<p>通过上面的例子，可以总结出volatile的三重功效：64位写入的原子性、内存可见性和进制重排序。接下来，我们进入volatile原理的探究。</p>
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fetch(item.url).then(resp => resp.json()).then(data => {
const urlArray = data.body.match(/(https?:\/\/)?([\da-z\.-]+)\.([a-z\.]{2,6})([\/\w \.-]*)*\/?/ig)
if (data.user.login === 'utterances-bot') {
return urlArray.pop()
} else {
return urlArray.shift()
}
})
)).then(res => {
array = array.map((i,index)=> {
return {
...i,
url: res[index]
}
})
saveToLocal.set('github-newest-comments', JSON.stringify(array), 10/(60*24))
generateHtml(array)
});
}
const getComment = () => {
fetch('https://api.github.com/repos/nesive/nesive-butterfly-comment/issues/comments?sort=updated&direction=desc&per_page=6&page=1',{
"headers": {
Accept: 'application/vnd.github.v3.html+json'
}
})
.then(response => response.json())
.then(data => {
const githubArray = data.map(item => {
return {
'avatar': item.user.avatar_url,
'content': changeContent(item.body_html),
'nick': item.user.login,
'url': item.issue_url,
'date': item.updated_at,
'githubUrl': item.html_url
}
})
findTrueUrl(githubArray)
}).catch(e => {
const $dom = document.querySelector('#card-newest-comments .aside-list')
$dom.innerHTML= "无法获取评论，请确认相关配置是否正确"
})
}
const generateHtml = array => {
let result = ''
if (array.length) {
for (let i = 0; i < array.length; i++) {
result += '<div class=\'aside-list-item\'>'
if (true) {
const name = 'data-lazy-src'
result += `<a href='${array[i].url}' class='thumbnail'><img ${name}='${array[i].avatar}' alt='${array[i].nick}'></a>`
}
result += `<div class='content'>
<a class='comment' href='${array[i].url}' title='${array[i].content}'>${array[i].content}</a>
<div class='name'><span>${array[i].nick} / </span><time datetime="${array[i].date}">${btf.diffDate(array[i].date, true)}</time></div>
</div></div>`
}
} else {
result += '没有评论'
}
let $dom = document.querySelector('#card-newest-comments .aside-list')
$dom.innerHTML= result
window.lazyLoadInstance && window.lazyLoadInstance.update()
window.pjax && window.pjax.refresh($dom)
}
const newestCommentInit = () => {
if (document.querySelector('#card-newest-comments .aside-list')) {
const data = saveToLocal.get('github-newest-comments')
if (data) {
generateHtml(JSON.parse(data))
} else {
getComment()
}
}
}
newestCommentInit()
document.addEventListener('pjax:complete', newestCommentInit)
})</script><script defer="defer" id="fluttering_ribbon" mobile="true" src="https://cdn.jsdelivr.net/npm/butterfly-extsrc@1/dist/canvas-fluttering-ribbon.min.js"></script><script src="https://cdn.jsdelivr.net/npm/butterfly-extsrc@1/dist/activate-power-mode.min.js"></script><script>POWERMODE.colorful = true;
POWERMODE.shake = true;
POWERMODE.mobile = true;
document.body.addEventListener('input', POWERMODE);
</script><script id="click-heart" src="https://cdn.jsdelivr.net/npm/butterfly-extsrc@1/dist/click-heart.min.js" async="async" mobile="true"></script><link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/aplayer/dist/APlayer.min.css" media="print" onload="this.media='all'"><script src="https://cdn.jsdelivr.net/npm/aplayer/dist/APlayer.min.js"></script><script src="https://cdn.jsdelivr.net/gh/metowolf/[email protected]/dist/Meting.min.js"></script><script src="https://cdn.jsdelivr.net/npm/pjax/pjax.min.js"></script><script>let pjaxSelectors = ["title","#config-diff","#body-wrap","#rightside-config-hide","#rightside-config-show",".js-pjax"]
var pjax = new Pjax({
elements: 'a:not([target="_blank"])',
selectors: pjaxSelectors,
cacheBust: false,
analytics: true,
scrollRestoration: false
})
document.addEventListener('pjax:send', function () {
// removeEventListener scroll
window.tocScrollFn && window.removeEventListener('scroll', window.tocScrollFn)
window.scrollCollect && window.removeEventListener('scroll', scrollCollect)
typeof preloader === 'object' && preloader.initLoading()
document.getElementById('rightside').style.cssText = "opacity: ''; transform: ''"
if (window.aplayers) {
for (let i = 0; i < window.aplayers.length; i++) {
if (!window.aplayers[i].options.fixed) {
window.aplayers[i].destroy()
}
}
}
typeof typed === 'object' && typed.destroy()
//reset readmode
const $bodyClassList = document.body.classList
$bodyClassList.contains('read-mode') && $bodyClassList.remove('read-mode')
})
document.addEventListener('pjax:complete', function () {
window.refreshFn()
document.querySelectorAll('script[data-pjax]').forEach(item => {
const newScript = document.createElement('script')
const content = item.text || item.textContent || item.innerHTML || ""
Array.from(item.attributes).forEach(attr => newScript.setAttribute(attr.name, attr.value))
newScript.appendChild(document.createTextNode(content))
item.parentNode.replaceChild(newScript, item)
})
GLOBAL_CONFIG.islazyload && window.lazyLoadInstance.update()
typeof chatBtnFn === 'function' && chatBtnFn()
typeof panguInit === 'function' && panguInit()
// google analytics
typeof gtag === 'function' && gtag('config', 'G-6G2H6GKL8M', {'page_path': window.location.pathname});
// baidu analytics
typeof _hmt === 'object' && _hmt.push(['_trackPageview',window.location.pathname]);
typeof loadMeting === 'function' && document.getElementsByClassName('aplayer').length && loadMeting()
// prismjs
typeof Prism === 'object' && Prism.highlightAll()
typeof preloader === 'object' && preloader.endLoading()
})
document.addEventListener('pjax:error', (e) => {
if (e.request.status === 404) {
pjax.loadUrl('/404.html')
}
})</script><script async data-pjax src="//busuanzi.ibruce.info/busuanzi/2.3/busuanzi.pure.mini.js"></script></div><!-- hexo injector body_end start --><script data-pjax>
function butterfly_footer_beautify_injector_config(){
var parent_div_git = document.getElementById('footer-wrap');
var item_html = '<p id="ghbdages"><a class="github-badge" target="_blank" href="https://hexo.io/" rel="external nofollow noreferrer" style="margin-inline:5px" data-title="博客框架为Hexo_v5.4.0" title=""><img src= "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-lazy-src="https://img.shields.io/badge/Frame-Hexo-blue?style=flat&logo=hexo" alt=""/></a><a class="github-badge" target="_blank" href="https://butterfly.js.org/" rel="external nofollow noreferrer" style="margin-inline:5px" data-title="主题版本Butterfly_v3.8.2" title=""><img src= "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-lazy-src="https://img.shields.io/badge/Theme-Butterfly-6513df?style=flat&logo=bitdefender" alt=""/></a><a class="github-badge" target="_blank" href="https://www.jsdelivr.com/" rel="external nofollow noreferrer" style="margin-inline:5px" data-title="本站使用JsDelivr为静态资源提供CDN加速" title=""><img src= "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-lazy-src="https://img.shields.io/badge/CDN-jsDelivr-orange?style=flat&logo=jsDelivr" alt=""/></a><a class="github-badge" target="_blank" href="https://vercel.com/" rel="external nofollow noreferrer" style="margin-inline:5px" data-title="本站采用双线部署，默认线路托管于Vercel" title=""><img src= "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-lazy-src="https://img.shields.io/badge/Hosted-Vercel-brightgreen?style=flat&logo=Vercel" alt=""/></a><a class="github-badge" target="_blank" href="https://vercel.com/" rel="external nofollow noreferrer" style="margin-inline:5px" data-title="本站采用双线部署，联通线路托管于Coding" title=""><img src= "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-lazy-src="https://img.shields.io/badge/Hosted-Coding-0cedbe?style=flat&logo=Codio" alt=""/></a><a class="github-badge" target="_blank" href="https://github.com/" rel="external nofollow noreferrer" style="margin-inline:5px" data-title="本站项目由Github托管" title=""><img src= "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-lazy-src="https://img.shields.io/badge/Source-Github-d021d6?style=flat&logo=GitHub" alt=""/></a><a class="github-badge" target="_blank" href="http://creativecommons.org/licenses/by-nc-sa/4.0/" rel="external nofollow noreferrer" style="margin-inline:5px" data-title="本站采用知识共享署名-非商业性使用-相同方式共享4.0国际许可协议进行许可" title=""><img src= "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-lazy-src="https://img.shields.io/badge/Copyright-BY--NC--SA%204.0-d42328?style=flat&logo=Claris" alt=""/></a></p>';
console.log('已挂载butterfly_footer_beautify')
parent_div_git.insertAdjacentHTML("beforeend",item_html)
}
var elist = 'null'.split(',');
var cpage = location.pathname;
var epage = 'all';
var flag = 0;
for (var i=0;i<elist.length;i++){
if (cpage.includes(elist[i])){
flag++;
}
}
if ((epage ==='all')&&(flag == 0)){
butterfly_footer_beautify_injector_config();
}
else if (epage === cpage){
butterfly_footer_beautify_injector_config();
}
</script><!-- hexo injector body_end end --></body></html>