Why is NPE thrown when finishing Activity?

问题

I have an error that has been driving me crazy for days. Unfortunately, I can not show you the code for two reason, my boss will not appreciate it and the codebase is too large to share. The error occurs whenever the Activity is finishing. Weather it be because I call finish() or because the os destroys it.

The question is, what is (or could) cause execPendingActions() in FragmentManagerImpl to throw a NPE at line 1196.

Here is the stacktrace:

FragmentManagerImpl.execPendingActions() line: 1196 
FragmentManagerImpl$1.run() line: 375   
Handler.handleCallback(Message) line: 587   
Handler.dispatchMessage(Message) line: 92   
Looper.loop() line: 126 
ActivityThread.main(String[]) line: 3997    
Method.invokeNative(Object, Object[], Class, Class[], Class, int, boolean) line: not available [native method]  
Method.invoke(Object, Object...) line: 491  
ZygoteInit$MethodAndArgsCaller.run() line: 841  
ZygoteInit.main(String[]) line: 599 
NativeStart.main(String[]) line: not available [native method]  

回答1:

You could try using the compat library and debugging that.

Alternatively try and reproduce in a cut down sample app.

回答2:

When talking about finishing the activities: Are you trying to finish() the activity from a View? Are correctly sending MyClass.this.finish()?

It looks to me that OS force it to close due NPE. Something is wrong with the fragments. Something is Null among them. Could you at least paste the line where it crashes?

回答3:

You may be hitting a bug in fragment manager where it doesn't clear its list of pending transactions before being destroyed. This can happen if you commit a transaction after your state has been saved, using FragmentTransaction.commitAllowingStateLoss(). You may be able to work around it with this code in your activity:

@Override void onDestroy() {
    getFragmentManager().executePendingTransactions();
    super.onDestroy();
}

回答4:

The NullPointerException is caused by the fact that Activity's Handler is unset from the FragmentManager, so a "solution" that will prevent the crash is the following:

public void onDestroy(){ 
        super.onDestroy(); 
        try { 
            Field mActivityField = getFragmentManager().getClass().getDeclaredField("mActivity");
            mActivityField.setAccessible(true);
            mActivityField.set(getFragmentManager(), this);

            Field mPendingActionsField = getFragmentManager().getClass().getDeclaredField("mPendingActions");
            mPendingActionsField.setAccessible(true);
            mPendingActionsField.set(getFragmentManager(), null);


            Field f = Activity.class.getDeclaredField("mHandler");
            f.setAccessible(true);
            Handler handler = (Handler) f.get(this);
            handler.close(); 
        } catch (Throwable e) {

        } 
} 

来源：https://stackoverflow.com/questions/6774505/why-is-npe-thrown-when-finishing-activity