AsyncTask在OnPostExecute中返回null

Question

我正在尝试将文本从服务器上的.txt转换为字符串（feed_str）我的代码：

public class getFeedData extends AsyncTask<String, Integer, String>
{

  @Override
  protected String doInBackground(String... params)
  {
    String feed_str = null;
    try
    {
        // Create a URL for the desired page
        URL url = new URL("http://www.this.is/a/server/file.txt");

        // Read all the text returned by the server
        BufferedReader in = new BufferedReader(new InputStreamReader(url.openStream()));

        while ((feed_str = in.readLine()) != null)
        {
            // str is one line of text; readLine() strips the newline character(s)
        }
        in.close();
    }
    catch (MalformedURLException e)
    {
        System.out.println("AsyncError: " + e);
    }
    catch (IOException e)
    {
        System.out.println("AsyncError: " + e);
    }
    catch (NullPointerException e)
    {
        System.out.println("AsyncError: " + e);
    }
    return feed_str;
  }

  @Override
  protected void onPostExecute(String feed_str)
  {
    super.onPostExecute(feed_str);
    System.out.println("onPostExecute " + feed_str);
  }

使用此代码，logcat应该输出类似以下内容： "onPostExecute text from server"而不是输出"onPostExecute null"

任何想法如何解决？

顺便说一句：已使用浏览器检查了该URL，并显示了文本，因此该URL并非问题。

Answer 1

直到feed_str == null该循环才会退出。 因此，它的最后一个值为null，即返回的值。

while ((feed_str = in.readLine()) != null)
{
    // str is one line of text; readLine() strips the newline character(s)
}

如果您要返回该字符串，则还需要保留一个“总计”字符串。

String entireFeed = "";
while ((feed_str = in.readLine()) != null)
{
    entireFeed += feedStr + "\n";
    // whatever else you're doing
}
...
return entireFeed;