NullPointerException in onRestoreInstanceState

Question

下面的SwiperActivity从ActionBarSherlock 4.0.2库扩展了SherlockFragmentActivity。我只能在加载不同的应用程序之后才能得到这个结果，然后切换回来，在ICS4.0.3OTA上的华硕Eee变压器TF101上，以及ICS4.0.4OTA上的Galaxy。我不确定这是否显然是ICS4.0.3错误(它似乎与ICS源代码匹配，这里是http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/4.0.3_r1/com/android/internal/policy/impl/PhoneWindow.java#PhoneWindow.getPanelState%28int%2Cboolean%29)。

我还在ActionBarSherlock 4.0.0和4.0.1上得到了这个结果。

我没有覆盖onRestoreInstanceState或任何实例或状态。

(我还记录了每个重写活动方法(例如"SwiperAct.onCreate 0“)的第一行，最后记录了"SwiperAct.onCreate 99"，试图找出导致这种情况的方法。

04-16 14:30:57.710: D/dalvikvm(667): GC_CONCURRENT freed 2692K, 71% free 8750K/30023K, paused 2ms+4ms
04-16 14:30:58.140: D/dalvikvm(667): GC_CONCURRENT freed 120K, 69% free 9362K/30023K, paused 3ms+4ms
04-16 14:30:58.480: D/dalvikvm(667): GC_CONCURRENT freed 16K, 67% free 10207K/30023K, paused 4ms+5ms
04-16 14:31:00.700: D/dalvikvm(18313): Late-enabling CheckJNI
04-16 14:31:00.700: I/ActivityManager(557): Start proc com.company.appname for activity com.company.appname/.SwiperActivity: pid=18313 uid=10011 gids={3003, 1015}
04-16 14:31:00.720: D/WindowManager(557): readLidState, sw:1
04-16 14:31:00.720: D/WindowManager(557): adjustConfigurationLw, config:{1.0 0mcc0mnc (no locale) layoutdir=0 sw800dp w1280dp h752dp xlrg land ?uimode ?night finger qwerty/v/v -nav/v} mLidOpen:1 mHasDockFeature:true mHasHallSensorFeature:true config.hardKeyboardHidden:1
04-16 14:31:00.730: D/OpenGLRenderer(18274): Flushing caches (mode 1)
04-16 14:31:00.730: D/dalvikvm(18313): Debugger has detached; object registry had 1 entries
04-16 14:31:00.900: D/OpenGLRenderer(18274): Flushing caches (mode 0)
04-16 14:31:00.950: D/OpenGLRenderer(667): Flushing caches (mode 0)
04-16 14:31:00.970: E/SwiperAct.onCreate 0(18313): starting...
04-16 14:31:00.980: D/SwiperAct.onSingleUse 0(18313): starting...
04-16 14:31:01.050: D/SQLiteUtil(18313): create DatabaseOpenHelper instance
04-16 14:31:01.050: D/SQLiteUtil(18313): create DatabaseOpenHelper instance
04-16 14:31:01.080: V/SQLiteOpenHelper(18313): onOpen()
04-16 14:31:01.100: V/SQLiteOpenHelper(18313): onOpen()
04-16 14:31:01.100: D/SwiperAct.onSingleUse 99(18313): ending...
04-16 14:31:01.120: D/dalvikvm(18313): GC_CONCURRENT freed 209K, 5% free 6728K/7047K, paused 4ms+2ms
04-16 14:31:01.190: E/SwiperAct.onCreate 99(18313): ending...
04-16 14:31:01.200: D/AndroidRuntime(18313): Shutting down VM
04-16 14:31:01.200: W/dalvikvm(18313): threadid=1: thread exiting with uncaught exception (group=0x40a7b1f8)
04-16 14:31:01.200: E/AndroidRuntime(18313): FATAL EXCEPTION: main
04-16 14:31:01.200: E/AndroidRuntime(18313): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.company.appname/com.company.appname.SwiperActivity}: java.lang.NullPointerException
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.app.ActivityThread.access$600(ActivityThread.java:123)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.os.Handler.dispatchMessage(Handler.java:99)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.os.Looper.loop(Looper.java:137)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.app.ActivityThread.main(ActivityThread.java:4424)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at java.lang.reflect.Method.invokeNative(Native Method)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at java.lang.reflect.Method.invoke(Method.java:511)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at dalvik.system.NativeStart.main(Native Method)
04-16 14:31:01.200: E/AndroidRuntime(18313): Caused by: java.lang.NullPointerException
04-16 14:31:01.200: E/AndroidRuntime(18313):    at com.android.internal.policy.impl.PhoneWindow$PanelFeatureState.onRestoreInstanceState(PhoneWindow.java:3341)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at com.android.internal.policy.impl.PhoneWindow.restorePanelState(PhoneWindow.java:1663)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at com.android.internal.policy.impl.PhoneWindow.restoreHierarchyState(PhoneWindow.java:1619)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.app.Activity.onRestoreInstanceState(Activity.java:906)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.app.Activity.performRestoreInstanceState(Activity.java:878)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.app.Instrumentation.callActivityOnRestoreInstanceState(Instrumentation.java:1100)
04-16 14:31:01.200: E/AndroidRuntime(18313):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1934)
04-16 14:31:01.200: E/AndroidRuntime(18313):    ... 11 more
04-16 14:31:01.200: W/ActivityManager(557):   Force finishing activity com.company.appname/.SwiperActivity
04-16 14:31:01.500: D/dalvikvm(18313): GC_CONCURRENT freed 177K, 4% free 7027K/7303K, paused 1ms+3ms
04-16 14:31:01.610: D/dalvikvm(18313): GC_CONCURRENT freed 114K, 3% free 7455K/7623K, paused 6ms+3ms
04-16 14:31:01.740: D/dalvikvm(18313): GC_CONCURRENT freed 117K, 3% free 7900K/8071K, paused 3ms+3ms
04-16 14:31:01.750: D/WindowManager(557): readLidState, sw:1
04-16 14:31:01.750: D/WindowManager(557): adjustConfigurationLw, config:{1.0 0mcc0mnc (no locale) layoutdir=0 sw800dp w1280dp h752dp xlrg land ?uimode ?night finger qwerty/v/v -nav/v} mLidOpen:1 mHasDockFeature:true mHasHallSensorFeature:true config.hardKeyboardHidden:1
04-16 14:31:01.750: W/ActivityManager(557): Activity pause timeout for ActivityRecord{41568e50 com.company.appname/.SwiperActivity}

Answer 1

我不能泄露源代码，因为它是为了工作-然而，我做了一些改名，并剥夺了它。但是，在把它剥离到我怀疑导致错误的部分之后，并启用导致错误的行，bug就不会再发生了！

因此，这个源代码只是给其他经历过这种情况的人提供一个线索，让他们知道到哪里去看看。这段代码实际上并不会触发bug。

onCreate调用showSingleUse，后者生成SingleUseFragment，后者在活动中调用onSingleUse (因为它实现了一个名为OnFragmentUtilityListener的接口)。然后，onSingleUse调用一个在其onPreExecute中执行findViewById的AsyncTask。

禁用findViewById并将其移动到onCreate解决了这个问题。

SwiperActivity.java:

import android.os.AsyncTask;
import android.os.Build;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.widget.TextView;
import com.actionbarsherlock.app.SherlockFragmentActivity;

public class SwiperActivity extends SherlockFragmentActivity implements OnFragmentUtilityListener {
TextView textviewLastUpdated; // this is how the NPE was fixed
ReloadTask reloadTask;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    textviewLastUpdated = (TextView) findViewById(R.id.textviewLastUpdated); // this is how the NPE was fixed
    showSingleUse(this);
}
public static void showSingleUse(FragmentActivity activity) {
    try {
        if (activity.getSupportFragmentManager().findFragmentByTag("singleusefragment") == null) {
            activity.getSupportFragmentManager().beginTransaction().add(new SingleUseFragment(), "singleusefragment").commit();
        }
    } catch (IllegalStateException e) {}
}
@Override
public void onSingleUse() {
    startReloadTask();
}
private class ReloadTask extends AsyncTask<Void, Void, Void> {
    @Override
    protected void onPreExecute() {
        showLoading();
    }
    @Override
    protected Void doInBackground(Void... arg0) {
        return null;
    }
    @Override
    protected void onPostExecute(Void result) {
        hideLoading();
    }
}
public static boolean allowsExecutorChange() {
    return Build.VERSION.SDK_INT > 10;
}
void startReloadTask() {
    reloadTask = new ReloadTask();
    if (allowsExecutorChange()) {
        reloadTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
    } else {
        reloadTask.execute();
    }
}
void showLoading() {
    // this was causing the NPE!
    //TextView textviewLastUpdated = (TextView) findViewById(R.id.textviewLastUpdated);
    if (textviewLastUpdated!=null) textviewLastUpdated.setText("Loading...");
}
void hideLoading() {
    // this was causing the NPE!
    //TextView textviewLastUpdated = (TextView) findViewById(R.id.textviewLastUpdated);
    if (textviewLastUpdated!=null) textviewLastUpdated.setText("Loaded!");
}
@Override
public void onDismissedDialog(String tag, Object data) {}
@Override
public void onClickedPositiveDialog(String tag, Object data) {}

}

OnFragmentUtilityListener.java:

public interface OnFragmentUtilityListener {
public void onDismissedDialog(String tag, Object data);
public void onClickedPositiveDialog(String tag, Object data);
public void onSingleUse();
}

SingleUseFragment.java:

/** hack-ish way to detect rotation and not do it again */
public class SingleUseFragment extends Fragment {
boolean wasRotating = false;
@Override
public void onActivityCreated(Bundle state) {
    super.onActivityCreated(state);
    setRetainInstance(true);
}
static SingleUseFragment newInstance() {
        return new SingleUseFragment();
    }
@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    if (!wasRotating) ((OnFragmentUtilityListener) activity).onSingleUse();
}
@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    wasRotating = true;
}
}