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JavaScript 的性能优化：加载和执行

概览无论当前JavaScript代码是内嵌还是在外链文件中，页面的下载和渲染都必须停下来等待脚本执行完成。JavaScript执行过程耗时越久，浏览器等待响应用户输入的时间就越长。浏览器在下载和执行脚本时出现阻塞的原因在于，脚本可能会改变页面或JavaScript的命名空间，它们对后面页面内容造成影响。一个典型的例子就是在页面中使用document.write()。例如清单1清单1JavaScript代码内嵌示例Sou</span> <p class="auth-span"> <span class="muted"><i class="fa fa-user"></i><span class="zj-icon-text">系统</span></span> <span class="muted"><i class="fa fa-clock-o"></i><span class="zj-icon-text">2019-08-29 22:50:45</span></span> <span class="muted"><i class="fa fa-eye"></i><span class="zj-icon-text">3687</span></span>  </p> </article><article class="excerpt"> <header> <a title="Android" class="label label-important zj-del-target" href="/category/57">Android<i class="label-arrow"></i></a> <h2> <a class="zj-titles zj-del-target" href="/article/7718" title="8.无法訪问developer.android.com的解决方式。" target="_blank" onclick="return recordId('',)">8.无法訪问developer.android.com的解决方式。 </a> </h2> </header> <div class="focus"> <a class="zj-del-target" href="/article/7718" target="_blank"><img style="width: 200px; height: 123px;" class="thumb" src="/Public/static/images/article/19.jpg" alt="" title=""></a> </div> <span class="note" onclick="if (jq.browser.versions.mobile) {window.location.href='/article/7718';}else{window.open('/article/7718');}" style="cursor: pointer;">问题：无法訪问developer.android.com，就无法知道Android的最新信息。解决的方法：寻找国外的代理ip，比方http://www.xici.net.co/上面的国外代理ip。设置方法：以谷歌浏览器（Chrome）为例。step1：点击“设置”。step2：点击“显示高级设置”。step3：点击“网络”标签下的“更改代理server设置”。step4：在“连接”标签下，点击“局域网设置”。step5：勾选“为LAN使用代理server”</span> <p class="auth-span"> <span class="muted"><i class="fa fa-user"></i><span class="zj-icon-text">系统</span></span> <span class="muted"><i class="fa fa-clock-o"></i><span class="zj-icon-text">2019-08-12 01:54:25</span></span> <span class="muted"><i class="fa fa-eye"></i><span class="zj-icon-text">3687</span></span>  </p> </article><article class="excerpt"> <header> <a title="DB2" class="label label-important zj-del-target" href="/category/48">DB2<i class="label-arrow"></i></a> <h2> <a class="zj-titles zj-del-target" href="/article/7375" title="db2 快照 SNAPSHOT" target="_blank" onclick="return recordId('',)">db2 快照 SNAPSHOT </a> </h2> </header> <div class="focus"> <a class="zj-del-target" href="/article/7375" target="_blank"><img style="width: 200px; height: 123px;" class="thumb" src="/Public/static/images/article/18.jpg" alt="" title=""></a> </div> <span class="note" onclick="if (jq.browser.versions.mobile) {window.location.href='/article/7375';}else{window.open('/article/7375');}" style="cursor: pointer;">打开和关闭快照缺省情况不打开DB2监控，必须在连接或实例级别上进行设置。有一系列监视器开关来决定是否监控某种数据元素。还预留了一个内存堆，用于包含为监控而存储的信息。1：在instance级别上设置监视器开关在实例级别上设置监视器开关会影响连接到该实例中任何数据库的所有用户。下面是监视器开关DFT_MON_STMT：语句监视器（用于动态SQL）DFT_MON_TABLE：表监视器DFT_MON_LOCK：锁监视器DFT_MON_BUFPOOL：缓冲池监视器</span> <p class="auth-span"> <span class="muted"><i class="fa fa-user"></i><span class="zj-icon-text">系统</span></span> <span class="muted"><i class="fa fa-clock-o"></i><span class="zj-icon-text">2019-08-12 01:54:05</span></span> <span class="muted"><i class="fa fa-eye"></i><span class="zj-icon-text">3687</span></span>  </p> </article><article class="excerpt"> <header> <a title="Ubuntu" class="label label-important zj-del-target" href="/category/55">Ubuntu<i class="label-arrow"></i></a> <h2> <a class="zj-titles zj-del-target" href="/article/1618" title="Ubuntu下deb包的安装方法" target="_blank" onclick="return recordId('',)">Ubuntu下deb包的安装方法 </a> </h2> </header> <div class="focus"> <a class="zj-del-target" href="/article/1618" target="_blank"><img style="width: 200px; height: 123px;" class="thumb" src="/Public/static/images/article/20.jpg" alt="" title=""></a> </div> <span class="note" onclick="if (jq.browser.versions.mobile) {window.location.href='/article/1618';}else{window.open('/article/1618');}" style="cursor: pointer;">deb是debianlinus的安装格式，跟redhat的rpm很类似，最主要的安装命令是：dpkg-ifile.debdpkg是DebianPackage的简写，是为Debian专门开发的套件管理系统，方便软件的安装、更新及移除。全部源自Debian的Linux发行版都使用dpkg，比如Ubuntu、Knoppix等。下面是一些Dpkg的普通使用方法：1、dpkg-i<package.deb>安装一个Debian软件包，如你手动下载的文件。2、dpkg-</span> <p class="auth-span"> <span class="muted"><i class="fa fa-user"></i><span class="zj-icon-text">系统</span></span> <span class="muted"><i class="fa fa-clock-o"></i><span class="zj-icon-text">2019-08-12 01:32:34</span></span> <span class="muted"><i class="fa fa-eye"></i><span class="zj-icon-text">3687</span></span>  </p> </article><article class="excerpt"> <header> <a title="Python" class="label label-important zj-del-target" href="/category/44">Python<i class="label-arrow"></i></a> <h2> <a class="zj-titles zj-del-target" href="/article/30729" title="Python计算机视觉编程第六章——图像聚类(K-means聚类，DBSCAN" target="_blank" onclick="return recordId('',)">Python计算机视觉编程第六章——图像聚类(K-means聚类，DBSCAN </a> </h2> </header> <div class="focus"> <a class="zj-del-target" href="/article/30729" target="_blank"><img style="width: 200px; height: 123px;" class="thumb" src="/Public/static/images/article/5.jpg" alt="" title=""></a> </div> <span class="note" onclick="if (jq.browser.versions.mobile) {window.location.href='/article/30729';}else{window.open('/article/30729');}" style="cursor: pointer;">Python计算机视觉编程图像聚类（一）K-means聚类1.1SciPy聚类包1.2图像聚类1.1在主成分上可视化图像1.1像素聚类（二）层次聚类（三）谱聚类图像聚类聚类概念无监督学习：没有标签。（对于监督学习问题中，我们会被告知什么是正确答案，在无监督学习中，没有任何标签，或者都具有相同的标签，得到的数据如下图，图上有一系列点，但是它们没有标签，因此训练集可以写成{x(1),x(1),x(1),...,x(m)}\left\{x^{(1)},x^{(1</span> <p class="auth-span"> <span class="muted"><i class="fa fa-user"></i><span class="zj-icon-text">系统</span></span> <span class="muted"><i class="fa fa-clock-o"></i><span class="zj-icon-text">2019-09-27 17:56:55</span></span> <span class="muted"><i class="fa fa-eye"></i><span class="zj-icon-text">3686</span></span>  </p> </article><div class="pagination" style="display:;"> <ul><li><a href="/search/spark/1/p/1">首页</a></li> <li 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