android - 缓存 HTTP 响应 (HttpResponseCache)

Question

我正在尝试使用 HttpResponseCache 来缓存使用 HttpURLConnection 发送的每个 HTTP 请求。
但即使请求第二次到达资源，命中计数也始终为零。为什么它没有命中缓存？

命中数：

import android.net.http.HttpResponseCache;
...
HttpResponseCache.getInstalled().getHitCount()

用法：

for(byte i = 0; i < 2; i++){
   // Cache-Control:max-age=300
    HttpURLConnection conn = (HttpURLConnection) new URL("http://redbot.org/").openConnection(); 
    conn.setUseCaches(true);
    if(i == 1){
        try {
            conn.addRequestProperty("Cache-Control", "only-if-cached");
            InputStream cached = conn.getInputStream();
            // the resource was cached! show it
            Log.i(TAG, "cached");
        } catch (FileNotFoundException e) {
            // the resource was not cached
            Log.i(TAG, "NOT cached");
        }
    }
}

这就是我启用缓存的方式：

File httpCacheDir = new File(this.getCacheDir(), "http");
long httpCacheSize = 10 * 1024 * 1024; // 10 MiB
HttpResponseCache.install(httpCacheDir, httpCacheSize);