在Android中执行一次异步任务
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【中文标题】在Android中执行一次异步任务【英文标题】：executing Async task in Android once 【发布时间】：2017-08-21 01:43:12 【问题描述】：嗨，我在这里需要帮助onCreate 方法
我已经实现了这一点，并将数据保存在 onSaveInstanceState 但它不断下载设备旋转上的数据
这是我的 MainActivity 代码
package com.ksamj.top10downloader;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ListView;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
public class MainActivity extends AppCompatActivity
private static final String TAG = "MainActivity";
private ListView listApps;
private String feedURL = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=%d/xml";
private int feedLimit = 10;
private String feedCachedURL = "INVALID";
public static final String STATE_URL = "feedURL";
public static final String STATE_LIMIT = "feedLimit";
@Override
protected void onCreate(Bundle savedInstanceState)
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
listApps = (ListView) findViewById(R.id.xmlListView);
if(savedInstanceState != null) // if null means run at 1st time
feedURL = savedInstanceState.getString(STATE_URL);
feedLimit = savedInstanceState.getInt(STATE_LIMIT);
Log.d(TAG, "from Bundle feedURL: "+ feedURL);
Log.d(TAG, "from Bundle feedLimit: "+ feedLimit);
downloadURL(String.format(feedURL,feedLimit));
@Override
public boolean onCreateOptionsMenu(Menu menu)
getMenuInflater().inflate(R.menu.feed_menu, menu);
return true;
@Override
public boolean onOptionsItemSelected(MenuItem item)
switch (item.getItemId())
case R.id.menuFreeApps:
feedURL = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=%d/xml";
break;
case R.id.menuPaidApps:
feedURL = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/toppaidapplications/limit=%d/xml";
break;
case R.id.menuSongs:
feedURL = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topsongs/limit=%d/xml";
break;
case R.id.menu10:
feedLimit = 10;
item.setChecked(true);
break;
case R.id.menu25:
feedLimit = 25;
item.setChecked(true);
break;
case R.id.menuRefresh:
feedCachedURL = "INVALID";
break;
default:
return super.onOptionsItemSelected(item);
downloadURL(String.format(feedURL,feedLimit));
return true;
@Override
protected void onSaveInstanceState(Bundle outState)
outState.putString(STATE_URL, feedURL);
outState.putInt(STATE_LIMIT, feedLimit);
super.onSaveInstanceState(outState);
private void downloadURL(String feedURL)
if (!feedURL.equalsIgnoreCase(feedCachedURL))
Log.d(TAG, "downloadURL: Downloading...");
DownloadData downloadData = new DownloadData();
downloadData.execute(feedURL);
feedCachedURL = feedURL;
Log.d(TAG, "downloadURL: feedURL " + feedURL);
Log.d(TAG, "downloadURL: feedCachedURL " + feedCachedURL);
else
Log.d(TAG, "downloadURL: Nothing to Download");
private class DownloadData extends AsyncTask<String, Void, String>
private static final String TAG = "DownloadData";
@Override
protected void onPostExecute(String s)
super.onPostExecute(s);
//Log.d(TAG, "onPostExecute: params are" + s);
ParseApplications parseApplications = new ParseApplications();
parseApplications.parse(s);
FeedAdapter feedAdapter = new FeedAdapter(MainActivity.this, R.layout.list_record, parseApplications.getApplications());
listApps.setAdapter(feedAdapter);
@Override
protected String doInBackground(String... strings)
Log.d(TAG, "doInBackground: starts with " + strings[0]);
String rssFeed = downloadXML(strings[0]);
if (rssFeed == null)
Log.e(TAG, "doInBackground: Error Downloading");
return rssFeed;
private String downloadXML(String urlPath)
StringBuilder xmlResult = new StringBuilder();
try
URL url = new URL(urlPath);
HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
int response = httpURLConnection.getResponseCode();
Log.d(TAG, "downloadXML: The response Code is " + response);
BufferedReader reader = new BufferedReader(new InputStreamReader(httpURLConnection.getInputStream()));
int charsRead;
char[] inputBuffer = new char[5500];
while (true)
charsRead = reader.read(inputBuffer);
if (charsRead < 0)
break;
if (charsRead > 0)
xmlResult.append(String.copyValueOf(inputBuffer, 0, charsRead));
reader.close();
return xmlResult.toString();
catch (MalformedURLException ex)
Log.e(TAG, "downloadXML: Malformated URL Exp: " + ex.getMessage());
catch (IOException ex)
Log.e(TAG, "downloadXML: IO Exp: " + ex.getMessage());
return null;
这是我的 logcat，用于执行后的简单轮换
03-28 10:01:56.373 21047-21047/com.ksamj.top10downloader W/art: Verification of java.lang.Object com.ksamj.top10downloader.MainActivity.access$super(com.ksamj.top10downloader.MainActivity, java.lang.String, java.lang.Object[]) took 100.789ms
03-28 10:01:56.515 21047-21047/com.ksamj.top10downloader W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
03-28 10:01:56.805 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: Downloading...
03-28 10:01:56.809 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: feedURL http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:01:56.809 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: feedCachedURL http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:01:56.809 21047-21272/com.ksamj.top10downloader D/DownloadData: doInBackground: starts with http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:01:56.813 21047-21272/com.ksamj.top10downloader D/NetworkSecurityConfig: No Network Security Config specified, using platform default
03-28 10:01:56.953 21047-21274/com.ksamj.top10downloader I/OpenGLRenderer: Initialized EGL, version 1.4
03-28 10:01:56.953 21047-21274/com.ksamj.top10downloader D/OpenGLRenderer: Swap behavior 1
03-28 10:01:56.957 21047-21274/com.ksamj.top10downloader W/OpenGLRenderer: Failed to choose config with EGL_SWAP_BEHAVIOR_PRESERVED, retrying without...
03-28 10:01:56.957 21047-21274/com.ksamj.top10downloader D/OpenGLRenderer: Swap behavior 0
03-28 10:01:57.002 21047-21272/com.ksamj.top10downloader D/DownloadData: downloadXML: The response Code is 200
03-28 10:01:57.098 21047-21047/com.ksamj.top10downloader W/art: Before Android 4.1, method int android.support.v7.widget.ListViewCompat.lookForSelectablePosition(int, boolean) would have incorrectly overridden the package-private method in android.widget.ListView
03-28 10:01:57.403 21047-21047/com.ksamj.top10downloader W/InputMethodManager: Ignoring onBind: cur seq=94, given seq=93
03-28 10:02:02.331 21047-21047/com.ksamj.top10downloader D/MainActivity: from Bundle feedURL: http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=%d/xml
03-28 10:02:02.332 21047-21047/com.ksamj.top10downloader D/MainActivity: from Bundle feedLimit: 10
03-28 10:02:02.332 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: Downloading...
03-28 10:02:02.333 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: feedURL http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:02:02.333 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: feedCachedURL http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:02:02.336 21047-21345/com.ksamj.top10downloader D/DownloadData: doInBackground: starts with http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:02:02.354 21047-21345/com.ksamj.top10downloader D/DownloadData: downloadXML: The response Code is 200
你能帮忙找出我做错了什么或遗漏了什么吗？
【问题讨论】：
您的应用程序支持两个方向？如果是，那么您需要在保存的实例中管理方向更改保存数据。 【参考方案1】：使用加载器模式。加载器允许你注册一个异步进程，这样你就不会启动额外的任务，所以当它完成时存在的 Activity 会得到实际的结果。详情请见https://developer.android.com/reference/android/app/LoaderManager.html。
【讨论】：
【参考方案2】：每次旋转设备时feedCachedURL变量设置为INVALID，你也应该在旋转前保存它
【讨论】：
【参考方案3】：当 savedInstance 为 null 时调用 downloadUrl 函数。 定义一个变量 rssFeed，您将在异步任务的 POstExecute 上获得该变量。
private String mRssFeed;
if(savedInstanceState != null) // if null means run at 1st time
feedURL = savedInstanceState.getString(STATE_URL);
feedLimit = savedInstanceState.getInt(STATE_LIMIT);
Log.d(TAG, "from Bundle feedURL: "+ feedURL);
Log.d(TAG, "from Bundle feedLimit: "+ feedLimit);
mRssFeed = savedInstanceState.getString("RSS_FEED");
if(mRssFeed != null && !mRssFeed.equals("")
ParseApplications parseApplications = new ParseApplications();
parseApplications.parse(mRssFeed);
FeedAdapter feedAdapter = new FeedAdapter(MainActivity.this, R.layout.list_record, parseApplications.getApplications());
listApps.setAdapter(feedAdapter);
else
downloadURL(String.format(feedURL,feedLimit));
else
downloadURL(String.format(feedURL,feedLimit));
您还需要在 OnSavedInstanceMethod 中保存您获得的提要，并在 postExecute 中设置 mRssFeed。
@Override
protected void onSaveInstanceState(Bundle outState)
outState.putString(STATE_URL, feedURL);
outState.putInt(STATE_LIMIT, feedLimit);
//save the rssFeed here
outState.putString("RSS_FEED", mRssFeed);
super.onSaveInstanceState(outState);
private class DownloadData extends AsyncTask<String, Void, String>
private static final String TAG = "DownloadData";
@Override
protected void onPostExecute(String s)
super.onPostExecute(s);
//Log.d(TAG, "onPostExecute: params are" + s);
mRssFeed = s;
ParseApplications parseApplications = new ParseApplications();
parseApplications.parse(mRssFeed);
FeedAdapter feedAdapter = new FeedAdapter(MainActivity.this, R.layout.list_record, parseApplications.getApplications());
listApps.setAdapter(feedAdapter);
@Override
protected String doInBackground(String... strings)
Log.d(TAG, "doInBackground: starts with " + strings[0]);
String rssFeed = downloadXML(strings[0]);
if (rssFeed == null)
Log.e(TAG, "doInBackground: Error Downloading");
return rssFeed;
private String downloadXML(String urlPath)
StringBuilder xmlResult = new StringBuilder();
try
URL url = new URL(urlPath);
HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
int response = httpURLConnection.getResponseCode();
Log.d(TAG, "downloadXML: The response Code is " + response);
BufferedReader reader = new BufferedReader(new InputStreamReader(httpURLConnection.getInputStream()));
int charsRead;
char[] inputBuffer = new char[5500];
while (true)
charsRead = reader.read(inputBuffer);
if (charsRead < 0)
break;
if (charsRead > 0)
xmlResult.append(String.copyValueOf(inputBuffer, 0, charsRead));
reader.close();
return xmlResult.toString();
catch (MalformedURLException ex)
Log.e(TAG, "downloadXML: Malformated URL Exp: " + ex.getMessage());
catch (IOException ex)
Log.e(TAG, "downloadXML: IO Exp: " + ex.getMessage());
return null;
即使您正在从 savedInstance 恢复数据，您的 downloadUrl 也会被调用。将其放入 else 将确保不会调用它。
但正如 Gabe 所说，如果您正在下载并且状态发生变化，则可能会出现一些问题。在这种情况下，我建议您进一步使用 RxAndroid。与 AsyncTasks 相比，这为您提供了很多控制权。
以下是有关 android 中一些最佳异步编程实践的好读物：
http://code.hootsuite.com/asynchronous-android-programming-the-good-the-bad-and-the-ugly/
【讨论】：
这个解决方案的问题是只有第一个会运行。第一个是原始 Activity 的一部分，因此它在 onPostExecute 中所做的任何 UI 更改都将转到原始 Activity，而不是当前 Activity。 对不起，我没有明白你的意思。但是如果我们检查 rssFeed 是否为空，然后触发 downLoadUrl 方法，它应该可以正常工作。问题是如果应用程序已经有数据，那么不需要在状态变化时重新下载。如果没有数据，它将下载。你的想法？ 这只是问题的一半。其他大厅是什么，如果它正在下载。然后，您可能会进行六次浪费的下载。由于同步任务共享同一个线程，那些无用的下载将在最后一个影响正确活动的下载之前发生。停止那是重要的事情。装载机就是解决这个问题的答案 是的，我同意 AsyncTask 在这方面很痛苦。这就是为什么而不是使用 Loader RxAndroid 是要走的路。它使我们对这些事情有很大的控制权。在这种情况下，我更喜欢使用 Rx。但既然问题是关于 AsyncTask 的，那么这就是它的答案。 Rx 为您提供了对状态更改的大量控制，因此是完成后台进程的最佳方式之一。不过 loader 也是一个不错的选择。 Rx android 可能是最常用的库。并非一切都是钉子。没有理由添加所有这些开销并最终得到 rx android 为此提供的更难调试的代码库。以上是关于在Android中执行一次异步任务的主要内容，如果未能解决你的问题，请参考以下文章