import com.google.android.apps.picview.data.WebResponseCursor.CachedWebResponse; //导入依赖的package包/类
/**
 * Gets the response for the request with the given URL from the database.
 * 
 * @param url
 *          the request {@link URL} of the response to get
 * @return the response or <code>null</code>, if the database does not contain
 *         a response for the given URL
 */
public CachedWebResponse get(URL url) {
  if (!responseDb.isReady()) {
    return null;
  }

  WebResponseCursor c = responseDb.query(url.toString());

  if (c == null) {
    return null;
  }
  if (!c.moveToFirst()) {
    c.close();
    return null;
  }

  Log.i(TAG, "Trying to read web response from database");
  return c.getResponseAndClose();
}
 

import com.google.android.apps.picview.data.WebResponseCursor.CachedWebResponse; //导入依赖的package包/类
/**
 * Performs a cached fetch. If the response is in one of the caches
 * (file-system or in-memory), this version is returned. If the response could
 * not be found in cache, it's fetched and automatically put into both caches.
 * 
 * @param url
 *          the URL to fetch
 * @param forceFetchFromWeb
 *          whether the content should be fetched from the web, regardless of
 *          whether it is present in any of the caches
 */
public CachedResponse<String> cachedFetch(URL url, boolean forceFetchFromWeb) {

  // Make sure we have a URL object that we can synchronize on.
  url = getSynchronizableInstance(url);

  // Synchronize per URL.
  synchronized (url) {
    String responseText = null;
    boolean fromDatabase = false;

    if (!forceFetchFromWeb) {
      // Get it from in-memory cache, if we have it.
      if (cache.containsKey(url) && cache.get(url) != null) {
        return new CachedResponse<String>(CachedResponse.FROM_MEMORY,
            cache.get(url));
      }

      // If it's not in-memory, try to load it from file system.
      CachedWebResponse response = fileSystemCache.get(url);

      if (response != null) {
        responseText = response.response;
        fromDatabase = true;
      }
    }

    // If it is also not found in the file system cache, or fetching
    // from cache was intentionally skipped, try to fetch it
    // from the network.
    if (responseText == null || forceFetchFromWeb) {
      responseText = fetchFromWeb(url);
      if (responseText != null) {
        fileSystemCache.asyncPut(url, "TODO", responseText);
      }
    }
    if (responseText != null) {
      cache.put(url, responseText);
    }
    return new CachedResponse<String>(fromDatabase ? CachedResponse.FROM_FILE
        : CachedResponse.NOT_CACHED, responseText);
  }
}