a) If the server includes this header:

    Cache-Control &quot;max-age=3600, must-revalidate&quot;

it is telling both client caches and proxy caches that once the content is stale (older than 3600 seconds) they must revalidate at the origin server before they can serve the content.  This should be the default behavior of caching systems, but the `must-revalidate` directive makes this requirement unambiguous. 

b) The client should revalidate.  It might revalidate using the `If-Match` or `If-None-Match` headers with an ETag, or it might use the `If-Modified-Since` or `If-Unmodified-Since` headers with a date.

a. Look at the ‘Stats’ tab on [this page][1] and see what happens.

b. After expiration the browser will check at the server if the file is updated. If not, the server will respond with a `304 Not Modified` header and nothing is downloaded.

You can check this behaviour yourself by looking at the ‘Net’ panel in Firebug or similar tools. Just re-enter the URL in the address bar and compare the number of HTTP requests with the number of requests when your cache is empty.


  [1]: http://www.codinghorror.com/blog/2007/08/yslow-yahoos-problems-are-not-your-problems.html

The given answers are incorrect, at least for web browsers in 2019.

&quot;After expiration the browser will check at the server if the file is updated&quot; &lt;- not true

I have a static file served with &quot;Cache-Control: public,must-revalidate,max-age=864000&quot; and both Chrome and Firefox do a request every time (and get a 304 Not Modified back every time).

I have the class name stored in a property file. I know that the classes store will implement IDynamicLoad. How do I instantiate the class dynamically?

Right now I have

         Properties foo = new Properties();
        foo.load(new FileInputStream(new File(&quot;ClassName.properties&quot;)));
        String class_name = foo.getProperty(&quot;class&quot;,&quot;DefaultClass&quot;);
        //IDynamicLoad newClass = Class.forName(class_name).newInstance();


Does the newInstance only load compiled .class files? How do I load a Java Class that is not compiled?

How do I programmatically compile and instantiate a Java class?

I need to decode HTML entities, e.g. from &amp;amp;#246; to &amp;#246;, and &amp;amp;amp; to &amp;amp;.

````URLEncoder.decode(str)```` does not do the job (convert from % notations). TextUtils has a HTMLencode, but not a HTMLdecode.

Are there any function for decoding HTML entities?

Decode HTML entities in android

I have  a couple of queries related to Cache-Control.

If I specify Cache-Control `max-age=3600, must-revalidate` for a static html/js/images/css file, with Last Modified Header defined in HTTP header:
 
 
1. Does browser/proxy cache(like Squid/Akamai) go all the way to origin server to validate **before max-age expires**? Or will it serve content from cache till max-age expires?
2. After max-age expiry (that is expiry from cache), is there a If-Modified-Since check or is content re-downloaded from origin server w/o If-Modified-Since check?


HTTP Cache Control max-age, must-revalidate

I have a couple of queries related to Cache-Control.
If I specify Cache-Control <code>max-age=3600, must-revalidate</code> for a static html/js/images/css file, with Last Modified Header defined in HTTP header:
<ol>
<li>Does browser/proxy cache(like Squid/Akamai) go all the way to origin server to validate before max-age expires? Or will it serve content from cache till max-age expires?</li>
<li>After max-age expiry (that is expiry from cache), is there a If-Modified-Since check or is content re-downloaded from origin server w/o If-Modified-Since check?</li>
</ol>

I want to send an image from the android client to the Django server using Http Post. The image is chosen from the gallery. At present, I am using list value name Pairs to send the necessary data to the server and receiving responses from Django in JSON. Can the same approach be used for images (with urls for images embedded in JSON responses)? 

Also, which is a better method: accessing images remotely without downloading them from the server or downloading and storing them in a Bitmap array and using them locally? The images are few in number (&lt;10) and small in size (50*50 dip).

Any tutorial to tackle these problems would be much appreciated. 

Edit: The images chosen from the gallery are sent to the server after scaling it to required size.  

Sending images using Http Post

If I understand right, applications sometimes use HTTP to send messages, since using other ports is liable to cause firewall problems. But how does that work without conflicting with other applications such as web-browsers? In fact how do multiple browsers running at once not conflict? Do they all monitor the port and get notified... can you share a port in this way?

I have a feeling this is a dumb question, but not something I ever thought of before, and in other cases I&#39;ve seen problems when 2 apps are configured to use the same port.

how can an application use port 80/HTTP without conflicting with browsers?

What are the correct content-types for XML, HTML and XHTML documents?

I need to write a simple crawler that only fetches these kinds of files.

Nowadays http://example.net/index.html can serve for example a JPEG file due to mod_rewrite, so I need to check the content-type from response header and compare it with a list of allowed content-types.

Where can I get such a list from?

Valid content-type for XML, HTML and XHTML documents

I want to find a minimal set of headers, that work with &quot;all&quot; caches and browsers (also when using **HTTPS**!)

On my web site, I&#39;ll have three kinds of resources:

&lt;h3&gt;(1) Forever cacheable (public / equal for all users)&lt;/h3&gt;
&lt;sup&gt;Example: 0A470E87CC58EE133616F402B5DDFE1C.cache.html (&lt;a href=&quot;http://code.google.com/webtoolkit/doc/latest/FAQ_DebuggingAndCompiling.html#Compiling_for_Web_Mode&quot;&gt;auto generated by GWT&lt;/a&gt;)&lt;/sup&gt; 

- These files are automatically assigned a new name, when they change content (based on the MD5).

- They should get cached as much as possible, even when using HTTPS (so I assume, I should set `Cache-Control: public`, especially for Firefox?)

- They shouldn&#39;t require the client to make a round-trip to the server to validate, if the content has changed.

&lt;h3&gt;(2) Changing occasionally (public / equal for all users)&lt;/h3&gt;
&lt;sup&gt;Examples: index.html, mymodule.nocache.js&lt;/sup&gt;

- These files change their content without changing the URL, when a new version of the site is deployed.

- They can be cached, but probably need a round-trip to be revalidated every time.


&lt;h3&gt;(3) Individual for each request (private / user specific)&lt;/h3&gt;
&lt;sup&gt;Example: JSON responses&lt;/sup&gt;

- These resources should never be cached unencrypted to disk under no circumstances. (Except maybe I&#39;ll have a few specific requests that could be cached.)

---
I have a general idea on which headers I would probably use for each type, but there&#39;s always something I could be missing.

Ideal HTTP cache control headers for different types of resources

I would like to make BIT (Built in tests) to a number of server in my cloud. I need the request to fail on large timeout.

How should I do this with java?

Trying something like the below does not seem to work.

    public class TestNodeAliveness {
     public static NodeStatus nodeBIT(String elasticIP) throws ClientProtocolException, IOException {
      HttpClient client = new DefaultHttpClient();
      client.getParams().setIntParameter(&quot;http.connection.timeout&quot;, 1);
      
      HttpUriRequest request = new HttpGet(&quot;http://192.168.20.43&quot;);
      HttpResponse response = client.execute(request);
      
      System.out.println(response.toString());
      return null;
     }
     
     
     public static void main(String[] args) throws ClientProtocolException, IOException {
      nodeBIT(&quot;&quot;);
     }
    }

-- EDIT: Clarify what library is being used --

I&#39;m using httpclient from apache, here is the relevant pom.xml section

 

     &lt;dependency&gt;
       &lt;groupId&gt;org.apache.httpcomponents&lt;/groupId&gt;
       &lt;artifactId&gt;httpclient&lt;/artifactId&gt;
       &lt;version&gt;4.0.1&lt;/version&gt;
       &lt;type&gt;jar&lt;/type&gt;
     &lt;/dependency&gt;



Java HTTP Client Request with defined timeout

If I want to add a list of values as an HTTP Header, is there a standard way to do this?  I couldn&#39;t find anything (that I could easily understand) in RFC 822.  For example, is
comma separated values standard or semi-colon separated values.  Is there a standard at all?

Example:

    Key: value1;value2;value3

Standard for adding multiple values of a single HTTP Header to a request or response

As far as I know it is allowed by the HTTP spec to set more than one HTTP header with the same name. Is there any use case to do so (from client to server and vice versa)?

[HTTP 1.1 Section 4.2][1]:
&gt; Multiple message-header fields with
&gt; the same field-name MAY be present in
&gt; a message if and only if the entire
&gt; field-value for that header field is
&gt; defined as a comma-separated list
&gt; [i.e., #(values)]. It MUST be possible
&gt; to combine the multiple header fields
&gt; into one &quot;field-name: field-value&quot;
&gt; pair, without changing the semantics
&gt; of the message, by appending each
&gt; subsequent field-value to the first,
&gt; each separated by a comma. The order
&gt; in which header fields with the same
&gt; field-name are received is therefore
&gt; significant to the interpretation of
&gt; the combined field value, and thus a
&gt; proxy MUST NOT change the order of
&gt; these field values when a message is
&gt; forwarded.

If I&#39;m not wrong there is no case where multiple headers with the same name are needed.

  [1]: http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2

Set more than one HTTP header with the same name?

I am writing a web service that accepts only json, and also outputs only json.

So I need to return the appropriate status code if any other format is requested.

It appears that I have two choices:

1. 406 - Not Acceptable
2. 415 - Unsupported Media Type

It would be great if someone could enlighten me as to the semantics of the two codes.



HTTP 406 and 415 error codes

For a web page that exists, but for which a user does not have sufficient privileges (they are not logged in or do not belong to the proper user group), what is the proper HTTP response to serve?

`401 Unauthorized`?  
`403 Forbidden`?  
Something else?

What I&#39;ve read on each so far isn&#39;t very clear on the difference between the two. What use cases are appropriate for each response?

403 Forbidden vs 401 Unauthorized HTTP responses

Can you please describe an example indicating difference between Public and Private Cache-Control in asp.net applications hosted in IIS.

I read in [MSDN][1] that the difference is the following:

**Public:** 
Sets Cache-Control: public to specify that the response is cacheable by clients and shared (proxy) caches.

**Private:** Default value. Sets Cache-Control: private to specify that the response is cacheable only on the client and not by shared (proxy server) caches.

I am not sure I have completely understood the pros and cons from each choice. An example for when to or not to use it would be great. 

For example what should I do if i have two web servers hosting the same application? Is there anything to watch out if I choose Private or Public?


  [1]: http://msdn.microsoft.com/en-us/library/system.web.httpcacheability(VS.71).aspx

Private vs Public in Cache-Control

**Question Part A** ▉ (100 bountys, awarded)  
Main question was how to make this site, load faster. First we needed to read these waterfalls. Thanks all for your suggestions on the waterfall readout analysis. Evident from the various waterfall graphs shown here is the main bottleneck: the PHP-generated thumbnails. The protocol-less jquery loading from CDN advised by David got my bounty, albeit making my site only 3% faster overall, and while not answering the site&#39;s main bottleneck. Time for for clarification of my question, and, another bounty:

**Question Part B** ▉ (100 bountys, awarded)  
The new focus was now to solve the problem that the 6 jpg images had, which are causing the most of the loading-delay. These 6 images are PHP-generated thumbnails, tiny and only 3~5 kb, but loading relatively *very* slowly. Notice the &quot;*time to first byte*&quot; on the various graphs. The problem remained unsolved, but a bounty went to James, who fixed the header error that RedBot [underlined][1]: *&quot;An If-Modified-Since conditional request returned the full content unchanged.&quot;*.

**Question Part C** ▉ (my last bounty: 250 points)   
Unfortunately, after even REdbot.org header error was fixed, the delay caused by the PHP-generated images remained untouched. What on earth are these tiny puny 3~5Kb thumbnails thinking? All that header information can send a rocket to moon and back. Any suggestions on this bottleneck is much appreciated and treated as possible answer, since I am stuck at this bottleneckish problem for already seven months now.

[Some background info on my site: CSS is at the top. JS at the bottom (Jquery,JQuery UI, bought menu awm/menu.js engines, tabs js engine, video swfobject.js) The black lines on the second image show whats initiating what to load. The angry robot is my pet &quot;ZAM&quot;. He is harmless and often happier.]


----------

**Load Waterfall: Chronological** | http://webpagetest.org
![enter image description here][2]

----------

**Parallel Domains Grouped** | http://webpagetest.org
![enter image description here][3]

----------

**Site-Perf Waterfall** | [http://site-perf.com][4]
![enter image description here][5]

----------

**Pingdom Tools Waterfall**  | [http://tools.pingdom.com][6]

![enter image description here][7]

----------

**GTmetrix Waterfall**  | [http://gtmetrix.com][8]

![enter image description here][9]

----------

  [1]: http://i.stack.imgur.com/FLWNX.png
  [2]: http://i.stack.imgur.com/0LdLq.png
  [3]: http://i.stack.imgur.com/KqdKq.png
  [4]: http://site-perf.com
  [5]: http://i.stack.imgur.com/DuZzJ.png
  [6]: http://tools.pingdom.com
  [7]: http://i.stack.imgur.com/pTbCb.png
  [8]: http://gtmetrix.com
  [9]: http://i.stack.imgur.com/DzIR3.png

Cached, PHP generated Thumbnails load slowly

What’s the difference between `Expires` and `Cache-Control` headers?


what’s the difference between Expires and Cache-Control headers?

I&#39;m using Amazon S3 to serve static assets for my website. I want to have browsers cache these assets for as long as possible. What meta-data headers should I include with my assets

    Cache-Control: max-age=???

Max value for cache control header in HTTP

I&#39;m trying to use http caching. In my controller I&#39;m setting a response as follows: 

    $response-&gt;setPublic();
    $response-&gt;setMaxAge(120);
    $response-&gt;setSharedMaxAge(120);
    $response-&gt;setLastModified($lastModifiedAt);

**dev mode**

In dev environment first response is a 200 with following headers:

    cache-control:max-age=120, public, s-maxage=120
    last-modified:Wed, 29 Feb 2012 19:00:00 GMT

For next 2 minutes every response is a 304 with following headers:

    cache-control:max-age=120, public, s-maxage=120

This is basically what I expect it to be.

**prod mode**

In prod mode response headers are different. Note that in app.php I wrap the kernel in AppCache.

First response is a 200 with following headers:

    cache-control:must-revalidate, no-cache, private
    last-modified:Thu, 01 Mar 2012 11:17:35 GMT

So it&#39;s a private no-cache response.

Every next request is pretty much what I&#39;d expect it to be; a 304 with following headers:

    cache-control:max-age=120, public, s-maxage=120

**Should I worry about it? Is it an expected behaviour?** 

**What will happen if I put Varnish or Akamai server in front of it?**


I did a bit of debugging and I figured that response is private because of last-modified header. HttpCache kernel [uses EsiResponseCacheStrategy][1] to update the cached response ([HttpCache::handle()][2] method). 

    if (HttpKernelInterface::MASTER_REQUEST === $type) {
        $this-&gt;esiCacheStrategy-&gt;update($response);
    }

EsiResponseCacheStrategy [turns a response into non cacheable][3] if it uses either Last-Response or ETag ([EsiResponseCacheStrategy::add()][4] method):

    if ($response-&gt;isValidateable()) {
        $this-&gt;cacheable = false;
    } else {
        // ... 
    }

[Response::isValidateable()][5] returns true if Last-Response or ETag header is present.

It results in [overwriting the Cache-Control header][6] ([EsiResponseCacheStrategy::update()][7] method):

    if (!$this-&gt;cacheable) {
        $response-&gt;headers-&gt;set(&#39;Cache-Control&#39;, &#39;no-cache, must-revalidate&#39;);
    
        return;
    }

I asked this question on Symfony2 user group but I didn&#39;t get an answer so far: https://groups.google.com/d/topic/symfony2/6lpln11POq8/discussion

**Update.**

Since I no longer have access to the original code I tried to [reproduce the scenario with the latest Symfony standard edition][8].

Response headers are more consistent now, but still seem to be wrong. 

As soon as I set a `Last-Modified` header on the response, the first response made by a browser has a: 

    Cache-Control:must-revalidate, no-cache, private

Second response has an expected:

    Cache-Control:max-age=120, public, s-maxage=120

If I avoid sending `If-Modified-Since` header, every request returns `must-revalidate, no-cache, private`.

It doesn&#39;t matter if the request was made in `prod` or `dev` environment anymore.


  [1]: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/HttpCache/HttpCache.php#L210
  [2]: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/HttpCache/HttpCache.php#L171
  [3]: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/HttpCache/EsiResponseCacheStrategy.php#L42
  [4]: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/HttpCache/EsiResponseCacheStrategy.php#L40
  [5]: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/Response.php#L407
  [6]: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/HttpCache/EsiResponseCacheStrategy.php#L62
  [7]: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/HttpCache/EsiResponseCacheStrategy.php#L55
  [8]: https://github.com/jakzal/symfony-standard/blob/cache/src/Acme/DemoBundle/Controller/DemoController.php#L62

Content Type	Original Author	Original Content on Stackoverflow
Question	nyb	View Question on Stackoverflow
Solution 1 - Http	james.garriss	View Answer on Stackoverflow
Solution 2 - Http	Marcel Korpel	View Answer on Stackoverflow
Solution 3 - Http	mhenry1384	View Answer on Stackoverflow

HTTP Cache Control max-age, must-revalidate

Http Problem Overview

Http Solutions

Solution 1 - Http

Solution 2 - Http

Solution 3 - Http

Decode HTML entities in android

How do I programmatically compile and instantiate a Java class?

Attributions