测试环境:windows/chrome
实例1:页面中仅有图片
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title></title> </head> <body> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <img src="1.jpg" width="200" height="300" /> </body> </html>
1、上图中红色框框出来的蓝色的线就是DOMContentLoaded event的时间,而此时图片(1.jpg)还未加载,正好才Send Request(1.jpg)。由此可以看出DOMContentLoaded事件并没有等待图片加载完成。
2、图片1.jpg一加载完,就立刻解码了(Image Decode)。
实例2:页面中有css和图片
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title></title> <link href="style.css" rel="stylesheet" type="text/css" /> </head> <body> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <img src="1.jpg" width="200" height="300" /> </body> </html>
从上图中可以看出:1、DOMContentLoaded事件不直接等待CSS文件和图片的加载完成。
2、CSS阻塞了图片的解码。可以从图片加载好之后有Recalculate Style和Layout,理论上也可以理解,因为CSS中可能对图片进行了样式上的修改之类的。所以要等到CSS加载和执行完才知道。
实例3:页面中有JS、CSS和图片
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title></title> <script type="text/javascript" src="jquery1.js"></script> <link href="style.css" rel="stylesheet" type="text/css" /> <script type="text/javascript" src="jquery2.js"></script> </head> <body> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <img src="1.jpg" width="200" height="300" /> <script type="text/javascript" src="jquery3.js"></script> </body> </html>
jquery1.js
console.timeStamp(‘Inline script before link in head‘); //console.timeStamp() 可以向Timeline中添加一条记录,并对应上方的一条黄线。
jquery2.js
console.log(‘test‘);
从上图可以看出:1、DOMContentLoaded事件要等到所有JS执行完才触发。
2、在CSS之前的jquery1.js一加载完了就立刻执行。而在CSS后的JS需要在CSS加载完之后才执行,比如说jquery3.js很早就加载完了,但是它等到style.css加载完了之后才执行,而且它还是在jquery2.js执行了之后才执行的,这是由于jquery3.js放在了jquery2.js之后。虽然对于现代浏览器,这些CSS、JS资源是并发请求的(从Send Request可以看出)。
总结:现代浏览器会并发的预加载CSS, JS,也就是一开始就并发的请求这些资源,但是,执行CSS和JS的顺序还是按原来的依赖顺序(JS的执行要等待位于其前面的CSS和JS加载、执行完)。先加载完成的资源,如果其依赖还没加载、执行完,就只能等着。
实例4:只有JS和图片
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title></title> <script type="text/javascript" src="jquery.js"></script> </head> <body> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <div>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</div> <img src="1.jpg" width="200" height="300" /> <script type="text/javascript" src="jqueryE.js"></script> </body> </html>
其中jquery.js很大,有900+KB。jqueryE.js更大,1.8MB。目的就是为了测试两个JS是否会阻塞图片的解码。
从上图可以看出,jquery.js加载并执行了之后,才进行图片解码的(即使图片在很早之前就加载完成了),而jqueryE.js是在图片解码之后才执行的。说明<head>标签里的script会阻塞图片的展现。而放在<body>最下面的JS则不会。
此外我还测试了,如果jqueryE.js比较小的话,它还是会在图片解码前加载并执行完成的,所以说,讨论了这么多,在实际应用中,还是要具体情况具体分析!而且chrome在这方面也一直在不断的更新优化。这个探讨可以说只是一个参考,让大家有这么一个思想,关键是要结合自己的业务、需求场景,有针对性的做分析和优化。
Javascript、CSS和IMG之网页执行探索,布布扣,bubuko.com
原文:http://www.cnblogs.com/EricaMIN1987_IT/p/3701025.html