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最近写了些代码，遇到一些Android的坑…真的坑

1.Android HttpURLConnection response code 204/200

Android 中 HttpURLConnection 的 response code 并不可靠，譬如服务器明明返回了HTTP 204，然而你调用 urlConnection.getResponseCode 得到的却是 HTTP 200…(如果用Java SE运行得到的是正常的204)

查看了一下 Android 源码中 isCaptivePortal 函数(/frameworks/base/services/core/java/com/android/server/connectivity/NetworkMonitor.java)，其中处理到了 HTTP response code，

    /**
     * Do a URL fetch on a known server to see if we get the data we expect.
     * Returns HTTP response code.
     */
    private int isCaptivePortal() {
        if (!mIsCaptivePortalCheckEnabled) return 204;

        HttpURLConnection urlConnection = null;
        int httpResponseCode = 599;
        try {
            URL url = new URL("http", mServer, "/generate_204");
            if (DBG) {
                log("Checking " + url.toString() + " on " +
                        mNetworkAgentInfo.networkInfo.getExtraInfo());
            }
            urlConnection = (HttpURLConnection) mNetworkAgentInfo.network.openConnection(url);
            urlConnection.setInstanceFollowRedirects(false);
            urlConnection.setConnectTimeout(SOCKET_TIMEOUT_MS);
            urlConnection.setReadTimeout(SOCKET_TIMEOUT_MS);
            urlConnection.setUseCaches(false);

            // Time how long it takes to get a response to our request
            long requestTimestamp = SystemClock.elapsedRealtime();

            urlConnection.getInputStream();

            // Time how long it takes to get a response to our request
            long responseTimestamp = SystemClock.elapsedRealtime();

            httpResponseCode = urlConnection.getResponseCode();
            if (DBG) {
                log("isCaptivePortal: ret=" + httpResponseCode +
                        " headers=" + urlConnection.getHeaderFields());
            }
            // NOTE: We may want to consider an "HTTP/1.0 204" response to be a captive
            // portal.  The only example of this seen so far was a captive portal.  For
            // the time being go with prior behavior of assuming it's not a captive
            // portal.  If it is considered a captive portal, a different sign-in URL
            // is needed (i.e. can't browse a 204).  This could be the result of an HTTP
            // proxy server.

            // Consider 200 response with "Content-length=0" to not be a captive portal.
            // There's no point in considering this a captive portal as the user cannot
            // sign-in to an empty page.  Probably the result of a broken transparent proxy.
            // See http://b/9972012.
            if (httpResponseCode == 200 && urlConnection.getContentLength() == 0) {
                if (DBG) log("Empty 200 response interpreted as 204 response.");
                httpResponseCode = 204;
            }

            sendNetworkConditionsBroadcast(true /* response received */, httpResponseCode == 204,
                    requestTimestamp, responseTimestamp);
        } catch (IOException e) {
            if (DBG) log("Probably not a portal: exception " + e);
            if (httpResponseCode == 599) {
                // TODO: Ping gateway and DNS server and log results.
            }
        } finally {
            if (urlConnection != null) {
                urlConnection.disconnect();
            }
        }
        return httpResponseCode;
    }

 
从第 625 行可以看出，Android 源码中判断当 response code=200 且 content-length 为 0 时，手动将 response code 置为 204。嗯，我也这样做，然后该问题就解决了。

总之，Android 中 HttpURLConnection 和 Java SE 中运行不大一致，除此之外，譬如如果你是 POST 请求，却设置了 urlConnection.setDoOutput(false)，将会影响响应头。

但是，至于为什么出现这些情况，暂时并没细究，改日有空再看看 Android 相关源码。

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