Home php教程 php手册 怎样防止从Cache中读取WML页面

怎样防止从Cache中读取WML页面

Jun 21, 2016 am 08:58 AM
cache gt http lt response

cache|页面

当WML页面下载到WAP设备后,它将保存在WAP设备内存中一段时间,直到这个时间过期。在这之后,页面将从服务器下载,而不是从WAP设备的缓存读取。这个过程被称做Cache。
但是有些时候不想让页面从缓存中读取,而是从服务器端读取。一个典型的例子就是当服务器的内容不断在更新的时候,通过在HTTP头中加入一定的cache信息,来告诉WAP设备该页面将不存储在缓存中。
可以在服务器端生成HTTP头,或者使用PHP、ASP、Perl或者其他服务端开发语言。这一行不能被包括在页面里,既然是HTTP的信息头,就不是WML元素。
对于静态页面,或许没有使用服务器端脚本语言,许多浏览器支持META标签来控制浏览器的Cache。看本部分的最后的例子。
将下面代码加入到HTTP头中,页面将马上过期:
Expires: Mon, 26 Jul 1997 05:00:00 GMT
Last-Modified: DD. month YYYY HH:MM:SS GMT
Cache-Control: no-cache, must-revalidate
Pragma: no-cache
第一行告诉微型浏览器,页面已经过期一段时间了。第二行告诉浏览器页面最后一次修改的时间。DD应该换成当天的日期,month YY HH MM SS等等类推。第三行和第四行有同样的效果。告诉浏览器页面不被Cache(第三行适用于 HTTP 1.1,第四行适用于HTTP 1.0)。
下面的是PHP的一个例子:

// set the correct MIME type
     header("Content-type: text/vnd.wap.wml");
// expires in the past
     header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
// Last modified, right now
     header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT"); 
// Prevent caching, HTTP/1.1
     header("Cache-Control: no-cache, must-revalidate");
// Prevent caching, HTTP/1.0
     header("Pragma: no-cache");
   ?>
下面是使用WebClasses(VB)的例子。使用"Response.Expires=-1",防止Cache。
 Private Sub WebClass_Start()
      'Set correct MIME type
      Response.ContentType = "text/vnd.wap.wml"
     
      'Make sure no caching
      Response.Expires = -1
      Response.AddHeader "Pragma", "no-cache"
      Response.AddHeader "Cache-Control", "no-cache, must-revalidate"
   
      'Use basicwml(my own) as template
      Set NextItem = basicwml
  End Sub 
这里有一个ASP的例子,同样使用“Response.Expires=-1”防止Cache。
    Response.ContentType = "text/vnd.wap.wml"
    Response.Expires = -1
    Response.AddHeader "Pragma", "no-cache"
    Response.AddHeader "Cache-Control", "no-cache, must-revalidate"
%> 
最后是使用META的例子:

br/>"' target=_blank>http://www.wapforum.org/DTD/wml_1.1.xml">
 
   
     
   
   
     

This deck will never be stored in the cache


   
 
下面的页面是在经过86400秒(24 hours)后过期。

br/>"' target=_blank>http://www.wapforum.org/DTD/wml_1.1.xml">
 
   
     
   
   
     

This card will live in the cache for a day


   

 

有些浏览器例如:UP.Simulator如果可以通过“返回”达到另外一个卡片,那么它将不会重新装载卡片。为了强制这个更新动作,用户必须在META标签中使用must-revalidate 参数。



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