我正在尝试使用Java的HttpURLConnection来执行“条件获取”，但我从未获得304状态代码

问题描述

这是我的代码：

    final HttpURLConnection conn = (HttpURLConnection) sourceURL.openConnection();
    if (cachedPage != null) {
        if (cachedPage.eTag != null) {
            conn.setRequestProperty("If-None-Match", cachedPage.eTag);
        }
        conn.setIfModifiedSince(cachedPage.pageLastModified);
    }

    conn.connect();

    if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) {

        newCachedPage.eTag = conn.getHeaderField("ETag");
        newCachedPage.pageLastModified = conn.getHeaderFieldDate("Last-Modified", 0);

    } else if (conn.getResponseCode() == HttpURLConnection.HTTP_NOT_MODIFIED) {
        // Never reaches here
    }

我似乎永远不会得到HTTP_NOT_MODIFIED响应代码，即使连续多次点击同一台服务器 - 页面肯定没有变化。此外，conn.getHeaderField（ETag）似乎总是响应null，有时conn.getHeaderFieldDate（Last-Modified，0）返回0.我已经尝试过针对各种Web服务器。

I never seem to get the HTTP_NOT_MODIFIED response code, even hitting the same server several times in quick succession - where there is definitely no change to the page. Also, conn.getHeaderField("ETag") always seems to respond null, and sometimes conn.getHeaderFieldDate("Last-Modified", 0) returns 0. I've tried this against a variety of web servers.

谁能告诉我我做错了什么？

Can anyone tell me what I'm doing wrong?

推荐答案

你们都是取决于服务器配置。

You're all dependent on the server config.

如果你得到一个 Expires 响应标题，那么它只是意味着你不要我需要请求任何东西，直到指定的到期时间。如果你得到一个 Last-Modified 响应标题，那么这意味着你应该可以使用 If-Modified-Since 测试它。如果你得到 ETag 响应标题，那么这意味着你应该可以使用 If-None-Match 来测试一下。

If you get an Expires response header, then it just means that you don't need to request anything until the specified expire time. If you get a Last-Modified response header, then it means that you should be able to use If-Modified-Since to test it. If you get an ETag response header, then it means that you should be able to use If-None-Match to test it.

让我们参加作为示例（Stackoverflow的favicon图像）：

Lets take http://cdn3.sstatic.net/stackoverflow/img/favicon.ico as an example (the Stackoverflow's favicon image):

URLConnection connection = new URL("http://cdn3.sstatic.net/stackoverflow/img/favicon.ico").openConnection();
System.out.println(connection.getHeaderFields());

这给出：

现在，做一个与：

Now, do a If-Modified-Since with the same value as Last-Modified:

URLConnection connection = new URL("http://cdn3.sstatic.net/stackoverflow/img/favicon.ico").openConnection();
connection.setRequestProperty("If-Modified-Since", "Wed, 06 Oct 2010 02:53:46 GMT");
System.out.println(connection.getHeaderFields());

这给出了预期的304：

This gives as expected a 304:

现在，做一个，其值与：

Now, do a If-None-Match with the same value as ETag:

URLConnection connection = new URL("http://cdn3.sstatic.net/stackoverflow/img/favicon.ico").openConnection();
connection.setRequestProperty("If-None-Match", "9d9bd8b1165cb1:0");
System.out.println(connection.getHeaderFields());

这给意外 200：

更令人惊讶的是，当两个标头设置为随机垃圾值为 ETag 时，服务器仍然给出304.这表明。这可能是（代理）配置问题或完全明确地完成（不明显的原因imho）。

Even more surprising, when the both headers are set with random garbage value as ETag, the server still gives a 304. This is an indication that the If-None-Match is completely ignored by the server behind http://cdn3.sstatic.net. That might be a (proxy) configuration issue or be done fully awarely (not for obvious reasons imho).

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