【原】浅谈Firefox下的js、css、图片阻塞现象（…

2018-06-18 05:09:28来源：未知阅读 ()

外部js文件阻塞body中的图片

以如下html为例：

 1 <!DOCTYPE HTML>
 2 <html>
 3 <head>
 4 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
 5 <title> 外部JS文件阻塞图片 </title>
 6 <script src="https://95598.gd.csg.cn/javascript/jquery-1.7.2.min.js"  type="text/javascript"></script>
 7 </head>
 8 <body>
 9 <img src="https://95598.gd.csg.cn/images/secImages/logo.gif" alt="" />
10 <img src="https://95598.gd.csg.cn/images/secImages/header_city_turn.png" width="50" height="21" alt="切换城市" />
11 <img src="https://95598.gd.csg.cn/images/secImages/customer_service.jpg" width="150" height="150" />
12 <img src="https://95598.gd.csg.cn/activity/images/wel_act.jpg" alt="" />
13 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_2.png" width="48" height="48"/> 
14 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_3.png" width="48" height="48"/>
15 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_top.png" width="48" height="48"/>
16 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t1.png" width="169" height="53" alt="" />
17 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t2.png" width="169" height="53" alt="" />
18 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t3.png" width="169" height="53" alt="" />
19 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a1.png" width="64" height="64" alt="" />
20 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a2.png" width="64" height="64" alt="" />
21 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a3.png" width="60" height="60" alt=""/>
22 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a4.png" width="64" height="64" alt="" />
23 </body>
24 </html>

script 在<head>中

经过firebug网络分析：

图 1-1

观察图1-1可以得出如下两个结论：

1.浏览器针对请求（如图片）是并发处理的，在IP、端口号相同时，Firefox的默认并发请求数是6（HTTP1.1和HTTP1.0的并发请求数因浏览器版本的不同而数量不同），可以看到6张图片是同时从服务器下载

2.js文件加载完后，中间约有0.1s的空闲时间，这段时间浏览器没有进行任何操作，这一现象可以称作阻塞，外部js文件阻塞了body中的img请求（在IE下不存在阻塞）

如果将<script>块置于img中间，也会产生阻塞（在IE下不存在阻塞）

 1 <!DOCTYPE HTML>
 2 <html>
 3 <head>
 4 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
 5 <title> 外部JS文件阻塞图片 </title>
 6 </head>
 7 <body>
 8 <img src="https://95598.gd.csg.cn/images/secImages/logo.gif" alt="" />
 9 <img src="https://95598.gd.csg.cn/images/secImages/header_city_turn.png" width="50" height="21" alt="切换城市" />
10 <script src="https://95598.gd.csg.cn/javascript/jquery-1.7.2.min.js" type="text/javascript"></script>
11 <img src="https://95598.gd.csg.cn/images/secImages/customer_service.jpg" width="150" height="150" />
12 <img src="https://95598.gd.csg.cn/activity/images/wel_act.jpg" alt="" />
13 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_2.png" width="48" height="48"/> 
14 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_3.png" width="48" height="48"/>
15 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_top.png" width="48" height="48"/>
16 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t1.png" width="169" height="53" alt="" />
17 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t2.png" width="169" height="53" alt="" />
18 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t3.png" width="169" height="53" alt="" />
19 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a1.png" width="64" height="64" alt="" />
20 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a2.png" width="64" height="64" alt="" />
21 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a3.png" width="60" height="60" alt=""/>
22 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a4.png" width="64" height="64" alt="" />
23 </body>
24 </html>

scipt置于中间

图 1-2

如果将外部js文件置于</body>前，则可以消除阻塞现象

 1 <!DOCTYPE HTML>
 2 <html>
 3 <head>
 4 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
 5 <title> 外部JS文件阻塞图片 </title>
 6 </head>
 7 <body>
 8 <img src="https://95598.gd.csg.cn/images/secImages/logo.gif" alt="" />
 9 <img src="https://95598.gd.csg.cn/images/secImages/header_city_turn.png" width="50" height="21" alt="切换城市" />
10 <img src="https://95598.gd.csg.cn/images/secImages/customer_service.jpg" width="150" height="150" />
11 <img src="https://95598.gd.csg.cn/activity/images/wel_act.jpg" alt="" />
12 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_2.png" width="48" height="48"/> 
13 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_3.png" width="48" height="48"/>
14 <img src="https://95598.gd.csg.cn/images/secImages/sidebar_top.png" width="48" height="48"/>
15 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t1.png" width="169" height="53" alt="" />
16 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t2.png" width="169" height="53" alt="" />
17 <img src="https://95598.gd.csg.cn/images/secImages/electricity_t3.png" width="169" height="53" alt="" />
18 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a1.png" width="64" height="64" alt="" />
19 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a2.png" width="64" height="64" alt="" />
20 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a3.png" width="60" height="60" alt=""/>
21 <img src="https://95598.gd.csg.cn/images/secImages/icon_big_a4.png" width="64" height="64" alt="" />
22 <script src="https://95598.gd.csg.cn/javascript/jquery-1.7.2.min.js"  type="text/javascript"></script>
23 </body>
24 </html>