在AsyncTask中没有调用onPostExecute(Handler运行时异常)

Question

我有一个AsyncTask获取一些数据,然后用这个新数据更新UI.它已经工作好几个月,但我最近添加了一个功能,当有新数据时显示通知.现在,当我的应用程序通过通知启动时,有时我会得到此异常并且onPostExecute不会被调用.

这是应用程序启动时发生的情况:

1)展开UI并查找视图

2)取消AlarmManager检查新数据并重置警报的警报(通过).(这样,如果用户禁用警报,则在下次重新启动之前将其取消.)

3)开始AsyncTask.如果应用程序是从通知启动的,请传入一些数据,然后取消通知.

我被困在导致这个异常的原因上.似乎异常来自AsyncTask代码,所以我不确定如何解决它.

谢谢!

这是一个例外:

I/My App(  501): doInBackground exiting
W/MessageQueue(  501): Handler{442ba140} sending message to a Handler on a dead thread
W/MessageQueue(  501): java.lang.RuntimeException: Handler{442ba140} sending message to a Handler on a dead thread
W/MessageQueue(  501):  at android.os.MessageQueue.enqueueMessage(MessageQueue.java:179)
W/MessageQueue(  501):  at android.os.Handler.sendMessageAtTime(Handler.java:457)
W/MessageQueue(  501):  at android.os.Handler.sendMessageDelayed(Handler.java:430)
W/MessageQueue(  501):  at android.os.Handler.sendMessage(Handler.java:367)
W/MessageQueue(  501):  at android.os.Message.sendToTarget(Message.java:348)
W/MessageQueue(  501):  at android.os.AsyncTask$3.done(AsyncTask.java:214)
W/MessageQueue(  501):  at java.util.concurrent.FutureTask$Sync.innerSet(FutureTask.java:252)
W/MessageQueue(  501):  at java.util.concurrent.FutureTask.set(FutureTask.java:112)
W/MessageQueue(  501):  at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:310)
W/MessageQueue(  501):  at java.util.concurrent.FutureTask.run(FutureTask.java:137)
W/MessageQueue(  501):  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1068)
W/MessageQueue(  501):  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:561)
W/MessageQueue(  501):  at java.lang.Thread.run(Thread.java:1096)

编辑:这是我的onCreate主要活动中的方法(通知打开的方法).onClickListeners为了节省空间,我省略了一些.我不认为它们会产生任何影响,因为它们所连接的按钮没有按下.

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState); // Call the parent

    setContentView(R.layout.main); // Create the UI from the XML file

    // Find the UI elements
    controls = (SlidingDrawer) findViewById(R.id.drawer); // Contains the
    // buttons
    // comic = (ImageView) findViewById(R.id.comic); // Displays the comic
    subtitle = (TextView) findViewById(R.id.subtitleTxt); // Textbox for the
    // subtitle
    prevBtn = (Button) findViewById(R.id.prevBtn); // The previous button
    nextBtn = (Button) findViewById(R.id.nextBtn); // The next button
    randomBtn = (Button) findViewById(R.id.randomBtn); // The random button
    fetchBtn = (Button) findViewById(R.id.comicFetchBtn); // The go to specific id button
    mostRecentBtn = (Button) findViewById(R.id.mostRecentBtn); // The button to go to the most recent comic
    comicNumberEdtTxt = (EditText) findViewById(R.id.comicNumberEdtTxt); // The text box to Zooming image view setup
    zoomControl = new DynamicZoomControl();

    zoomListener = new LongPressZoomListener(this);
    zoomListener.setZoomControl(zoomControl);

    zoomComic = (ImageZoomView) findViewById(R.id.zoomComic);
    zoomComic.setZoomState(zoomControl.getZoomState());
    zoomComic.setImage(BitmapFactory.decodeResource(getResources(), R.drawable.defaultlogo));
    zoomComic.setOnTouchListener(zoomListener);

    zoomControl.setAspectQuotient(zoomComic.getAspectQuotient());

    resetZoomState();

    // enter the new id
    imm = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE); // Used to hide the soft keyboard

    Log.i(LOG_TAG, "beginning loading of first comic");
    int notificationComicNumber = getIntent().getIntExtra("comic", -1);
    Log.i(LOG_TAG, "comic number from intent: " + notificationComicNumber);
    if (notificationComicNumber == -1) {
        fetch = new MyFetcher(this, zoomComic, subtitle, controls, comicNumberEdtTxt, imm, zoomControl);
        fetch.execute(MyFetcher.LAST_DISPLAYED_COMIC);
    } else {
        fetch = new MyFetcher(this, zoomComic, subtitle, controls, comicNumberEdtTxt, imm, zoomControl);
        fetch.execute(notificationComicNumber);
        ((NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE)).cancelAll();
    }
    Log.i(LOG_TAG, "ending loading of new comic");

    Log.i(LOG_TAG, "first run checks beginning");
    // Get SharedPreferences
    prefs = getSharedPreferences("prefs", Context.MODE_PRIVATE);

    // Check if this is the first run of the app for this version
    if (prefs.getBoolean("firstRun-" + MAJOR_VERSION_NUMBER, true)) {
        prefs.edit().putBoolean("firstRun-" + MAJOR_VERSION_NUMBER, false).commit();
        firstRunVersionDialog();
    }

    // Check if this is the first run of the app
    if (prefs.getBoolean("firstRun", true)) {
        prefs.edit().putBoolean("firstRun", false).commit();
        firstRunDialog();
    }
    Log.i(LOG_TAG, "First run checks done");

            // OnClickListener s for the buttons omitted to save space

编辑2:我一直在挖掘Android源代码,追踪异常的来源.这是线456和457 sendMessageAtTime中Handler:

msg.target = this;
sent = queue.enqueueMessage(msg, uptimeMillis);

这是enqueueMessage来自MessageQueue:

    final boolean enqueueMessage(Message msg, long when) {
        if (msg.when != 0) {
            throw new AndroidRuntimeException(msg
                    + " This message is already in use.");
        }
        if (msg.target == null && !mQuitAllowed) {
            throw new RuntimeException("Main thread not allowed to quit");
        }
        synchronized (this) {
            if (mQuiting) {
                RuntimeException e = new RuntimeException(
                    msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                return false;
            } else if (msg.target == null) {
                mQuiting = true;
            }

            msg.when = when;
            //Log.d("MessageQueue", "Enqueing: " + msg);
            Message p = mMessages;
            if (p == null || when == 0 || when < p.when) {
                msg.next = p;
                mMessages = msg;
                this.notify();
            } else {
                Message prev = null;
                while (p != null && p.when <= when) {
                    prev = p;
                    p = p.next;
                }
                msg.next = prev.next;
                prev.next = msg;
                this.notify();
            }
        }
        return true;
    }

我对什么mQuiting是有点困惑,但看起来前一次enqueueMessage被称为msg.targetnull.

Answer 1

这是由于Android框架中的AsyncTask中存在错误.AsyncTask.java具有以下代码:

private static final InternalHandler sHandler = new InternalHandler();

它期望在主线程上初始化它,但是这不能保证,因为它将在导致类运行其静态初始化器的任何线程上初始化.我重现了Handler引用工作线程的问题.

导致这种情况发生的常见模式是使用IntentService类.C2DM示例代码执行此操作.

一个简单的解决方法是将以下代码添加到应用程序的onCreate方法:

Class.forName("android.os.AsyncTask");

这将强制在主线程中初始化AsyncTask.我在android bug数据库中提交了一个bug.请参阅http://code.google.com/p/android/issues/detail?id=20915.

Answer 2

为了概括Jonathan Perlow针对他明确指出的错误的解决方案,我在任何使用AsyncTask的类中使用以下内容.looper/handler/post是如何在Android应用程序中的任何位置运行UI线程而不传递活动或其他上下文句柄的方法.在类中添加此静态初始化块:

{ // https://stackoverflow.com/questions/4280330/onpostexecute-not-being-called-in-asynctask-handler-runtime-exception
    Looper looper = Looper.getMainLooper();
    Handler handler = new Handler(looper);
    handler.post(new Runnable() {
      public void run() {
        try {
          Class.forName("android.os.AsyncTask");
        } catch (ClassNotFoundException e) {
          e.printStackTrace();
        }
      }
    });
}

我们在尝试运行单元测试时遇到了问题.我找到了解决方法,但没有具体确定问题.我们只知道尝试在Android JUnit测试中使用AsyncTask <>导致onPostExecute()不被调用.现在我们知道为什么.

这篇文章展示了如何在Android JUnit测试中运行多线程异步代码:

在基于Android AsyncTask的JUnit测试中使用CountDownLatch

为了与非UI单元测试一起使用,我创建了一个简单的android.test.InstrumentationTestCase子类.它有一个"ok"标志和一个CountDownLatch.reset()或reset(count)创建一个新的CountDownLatch({1,count}).good()在latch上设置ok = true,count--和calls.countDown().bad()设置ok = false,并一直倒计时.waitForIt(seconds)等待超时或coundown latch为零.然后它调用assertTrue(ok).

那么测试就像:

someTest() {
  reset();
  asyncCall(args, new someListener() {
    public void success(args) { good(); }
    public void fail(args) { bad(); }
  });
  waitForIt();
}

由于AsyncTask静态初始化错误,我们必须在传递给runTestOnUiThread()的Runnable中运行我们的实际测试.如上所述进行适当的静态初始化,除非正在测试的调用需要在UI线程上运行,否则这不是必需的.

我现在使用的另一个习惯是测试当前线程是否是UI线程,然后在适当的线程上运行所请求的操作,无论如何.有时,允许调用者请求同步与异步,在必要时覆盖是有意义的.例如,网络请求应始终在后台线程上运行.在大多数情况下,AsyncTask线程池是完美的.只是意识到只有一个数字会同时运行,阻止其他请求.要测试当前线程是否是UI线程:

boolean onUiThread = Looper.getMainLooper().getThread() == Thread.currentThread();

然后,使用一个简单的子类的AsyncTask的(只是doInBackground()和onPostExecute()需要)<>到在非UI线程或handler.post()或postDelayed()运行到UI线程上运行.

为调用者提供运行同步或异步的选项(获取本地未显示的本地有效onUiThread值;如上所述添加本地布尔值):

void method(final args, sync, listener, callbakOnUi) {
  Runnable run = new Runnable() { public void run() {
    // method's code... using args or class members.
    if (listener != null) listener(results);
    // Or, if the calling code expects listener to run on the UI thread:
    if (callbackOnUi && !onUiThread)
      handler.post(new Runnable() { public void run() {listener()}});
    else listener();
  };
  if (sync) run.run(); else new MyAsync().execute(run);
  // Or for networking code:
  if (sync && !onUiThread) run.run(); else new MyAsync().execute(run);
  // Or, for something that has to be run on the UI thread:
  if (sync && onUiThread) run.run() else handler.post(run);
}

此外,使用AsyncTask可以非常简单和简洁.使用下面的RunAsyncTask.java定义,然后编写如下代码:

    RunAsyncTask rat = new RunAsyncTask("");
    rat.execute(new Runnable() { public void run() {
        doSomethingInBackground();
        post(new Runnable() { public void run() { somethingOnUIThread(); }});
        postDelayed(new Runnable() { public void run() { somethingOnUIThreadInABit(); }}, 100);
    }});

或者简单地说:new RunAsyncTask("").execute(new Runnable(){public void run(){doSomethingInBackground();}});

RunAsyncTask.java:

package st.sdw;
import android.os.AsyncTask;
import android.util.Log;
import android.os.Debug;

public class RunAsyncTask extends AsyncTask<Runnable, String, Long> {
    String TAG = "RunAsyncTask";
    Object context = null;
    boolean isDebug = false;
    public RunAsyncTask(Object context, String tag, boolean debug) {
      this.context = context;
      TAG = tag;
      isDebug = debug;
    }
    protected Long doInBackground(Runnable... runs) {
      Long result = 0L;
      long start = System.currentTimeMillis();
      for (Runnable run : runs) {
        run.run();
      }
      return System.currentTimeMillis() - start;
    }
    protected void onProgressUpdate(String... values) {        }
    protected void onPostExecute(Long time) {
      if (isDebug && time > 1) Log.d(TAG, "RunAsyncTask ran in:" + time + " ms");
      v = null;
    }
    protected void onPreExecute() {        }
    /** Walk heap, reliably triggering crash on native heap corruption.  Call as needed. */  
    public static void memoryProbe() {
      System.gc();
      Runtime runtime = Runtime.getRuntime();
      Double allocated = new Double(Debug.getNativeHeapAllocatedSize()) / 1048576.0;
      Double available = new Double(Debug.getNativeHeapSize()) / 1048576.0;
      Double free = new Double(Debug.getNativeHeapFreeSize()) / 1048576.0;
      long maxMemory = runtime.maxMemory();
      long totalMemory = runtime.totalMemory();
      long freeMemory = runtime.freeMemory();
     }
 }