One fundamental difficulty is how do you know what last-modified date or ETag to send? The browser has cache information that it uses for sending requests, but it won't share that information with JavaScript. Fortunately, jQuery keeps track of the Last-Modified and ETag headers from Ajax responses, so you can use ifModified:true to have jQuery set those header values the next time it sends a request for that resource.

有兩點需要注意:

• 304 響應(yīng)不攜帶數(shù)據(jù).這是設(shè)計使然.假設(shè)是，如果您選擇使用緩存，您應(yīng)該已經(jīng)在緩存中擁有數(shù)據(jù)的副本！如果沒有從服務(wù)器獲取數(shù)據(jù)是一個問題(即，因為您還沒有該數(shù)據(jù))為什么要使用緩存?當(dāng)您手頭有舊數(shù)據(jù)并且只需要新數(shù)據(jù)時，應(yīng)該使用緩存；因此，使用 304 取回任何數(shù)據(jù)應(yīng)該不是問題.

• 304 responses do not carry data. This is by design. The assumption is that if you have elected to use caching, you should have a copy of the data already in your cache! If getting no data from the sever is a problem (i.e., because you don't already have that data) why are you using caching? Caching should be used when you have the old data on hand and only want new data; thus, getting back no data with a 304 should not be a problem.

jQuery 必須存儲上一個請求的最后修改日期或 ETag(與 If-None-Match 一起使用).流程是這樣的:

jQuery must have a last-modified date or an ETag (to use with If-None-Match) stored from a previous request. The process goes like this:

• 第一次獲取:jQuery 沒有緩存信息，所以它不會發(fā)送 If-Modified-Since 或 If-None-Match.當(dāng)響應(yīng)返回時，服務(wù)器可能會宣布最后修改的數(shù)據(jù)或 ETag，jQuery 將其存儲以供將來使用.

• First fetch: jQuery has no cache information, so it doesn't send If-Modified-Since or If-None-Match. When the response comes back, the server may announce a last-modified data or an ETag, which jQuery stores for future use.

后續(xù)提取:jQuery 擁有上次提取的緩存信息并將該數(shù)據(jù)轉(zhuǎn)發(fā)到服務(wù)器.如果資源沒有改變，Ajax 請求會得到 304 響應(yīng).如果資源已更改，Ajax 請求會收到 200 響應(yīng)，以及新的緩存信息供 jQuery 用于下一次獲取.

Subsequent fetches: jQuery has cache information from the last fetch and forwards that data to the server. If the resource has not changed, the Ajax request gets a 304 response. If the resource has changed, the Ajax request gets a 200 response, along with new cache information for jQuery to use for its next fetch.

jQuery 不在頁面重新加載之間保留緩存信息(例如，在 cookie 中).因此，頁面重新加載后第一次獲取資源永遠不會是 304，因為 jQuery 沒有要發(fā)送的緩存信息(即，我們重置回第一次獲取"情況).jQuery 沒有理由不能持久化緩存信息，但目前它沒有.

jQuery does not persist cache information (e.g., in cookies) between page reloads, however. Therefore, the first fetch of a resource after a page reload will never be a 304, because jQuery has no cache information to send (i.e., we reset back to the "first fetch" case). There is no reason why jQuery couldn't persist cache information, but at present it doesn't.

這里的底線是您可以使用緩存標(biāo)頭來獲取 JavaScript 304 響應(yīng)，但您不能訪問 瀏覽器自己的 ETag 或特定資源的上次修改日期.因此，瀏覽器本身可能知道有關(guān)資源的緩存信息，但您的 JavaScript 代碼不知道.在這種情況下，瀏覽器將使用其緩存標(biāo)頭來獲得真正的 304 響應(yīng)，但會將 200 響應(yīng)轉(zhuǎn)發(fā)到您的 JavaScript 代碼，因為 JavaScript 沒有發(fā)送任何緩存信息.

The bottom line here is that you can use cache headers to get a JavaScript 304 response, but you can't access the browser's own ETag or last-modified date for a particular resource. So, the browser itself might know caching information about a resource, but your JavaScript code does not. In that case, the browser will use its cache headers to potentially get a real 304 response, but forward a 200 response to your JavaScript code, because JavaScript didn't send any cache information.

不可能使 JavaScript 304 請求與實際的網(wǎng)絡(luò) 304 響應(yīng)完全一致，因為您的瀏覽器知道的緩存信息和您的 JavaScript 代碼知道的緩存信息可能以不可預(yù)知的方式不同.但是，大多數(shù)情況下正確獲取 304 請求足以滿足大多數(shù)實際開發(fā)需求.

It is not possible to make JavaScript 304 requests align perfectly with actual network 304 responses, because the cache information known by your browser and the cache information known by your JavaScript code may differ in unpredictable ways. However, getting 304 requests correctly most of the time is good enough for most practical development needs.

這是一個用 Node.js 編寫的簡短服務(wù)器示例(但它應(yīng)該足夠簡單，可以移植到其他語言):

Here's a brief server example written in Node.js (but it should be simple enough to port to other langauges):

require("http").createServer(function (req, res) {
  console.log(req.headers["if-modified-since"]);

  // always send Last-Modifed header
  var lastModDate = "Fri, 13 Feb 2013 13:43:19 GMT";
  res.setHeader("Last-Modified", lastModDate);

  // if the request has a If-Modified-Since header,
  //   and it's newer than the last modification,
  //   then send a 304 response; otherwise send 200
  if(req.headers["if-modified-since"] &&
     Date.parse(lastModDate) <= Date.parse(req.headers["if-modified-since"])) {
    console.log("304 -- browser has it cached");
    res.writeHead(304, {'Content-Type': 'text/plain'});
    res.end();
  } else {
    console.log("200 -- browser needs it fresh");
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('some content');
  }

}).listen(8080);

運行此服務(wù)器時，您可以在瀏覽器中加載頁面并在瀏覽器控制臺中執(zhí)行兩種不同的測試:

When running this server, you can load the page in your browser and perform two different tests in your browser console:

var xhr = new XMLHttpRequest();
xhr.open("GET", "/");
xhr.send();
xhr.onload = function() { console.log(xhr.status); }

此腳本將始終看到 200 響應(yīng)，即使瀏覽器提供 If-Modified-Since 請求標(biāo)頭并獲得 304(這將發(fā)生在第一個請求之后，在瀏覽器看到服務(wù)器的 Last-Modifed 響應(yīng)頭之后).

This script will always see a 200 response, even if the browser supplies a If-Modified-Since request header and gets a 304 (which will happen all requests after the first, after the browser sees the server's Last-Modifed response header).

相比之下，此腳本將總是看到 304 響應(yīng):

By contrast, this script will always see 304 response:

var xhr = new XMLHttpRequest();
xhr.open("GET", "/");
xhr.setRequestHeader("If-Modified-Since", "Fri, 15 Feb 2013 13:43:19 GMT");
xhr.send();
xhr.onload = function() { console.log(xhr.status); }

腳本提供自己的 If-Modified-Since 請求標(biāo)頭(服務(wù)器最后修改日期后兩天)；它不依賴于瀏覽器為 If-Modified-Since 提供的任何內(nèi)容，因此允許(根據(jù) XHR 規(guī)范)查看 304 響應(yīng).

The script supplies its own If-Modified-Since request header (two days after the server's last-modified date); it does not rely on whatever the browser supplies for If-Modified-Since, and therefore is allowed (per the XHR spec) to see 304 responses.

最后，這個腳本總是會看到一個200:

Finally, this script will always see a 200:

var xhr = new XMLHttpRequest();
xhr.open("GET", "/");
xhr.setRequestHeader("If-Modified-Since", "Fri, 12 Feb 2013 13:43:19 GMT");
xhr.send();
xhr.onload = function() { console.log(xhr.status); }

這是因為腳本使用了早于服務(wù)器最后修改日期的 If-Modified-Since，所以服務(wù)器總是發(fā)送 200.服務(wù)器不會發(fā)送 304 因為它假定客戶端沒有最新版本的緩存副本(即，客戶端宣布自 2 月 12 日以來它看到了更改，但是有一個2 月 13 日的更改，客戶顯然沒有看到).

This is because the script uses a If-Modified-Since that is prior to the server's last-modified date, so the server always sends a 200. The server won't send a 304 because it assumes the client doesn't have a cached copy of the most recent version (i.e., the client announces that it's seen changes since Feb 12, but there was a change on Feb 13 that the client apparently hasn't seen).

這篇關(guān)于如何檢查 jQuery.ajax() 請求標(biāo)頭狀態(tài)是否為“304 未修改"?的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網(wǎng)！