我正在编写代码以从firebase数据库中获取数据：
private void displayMessages(){
//Suppose you want to retrieve "chats" in your Firebase DB:
DatabaseReference ref=FirebaseDatabase.getInstance().getReference(Constants.USERS_CHAT).child(chatId);
Log.d(TAG,"Inside query"+ref);
ref.addListenerForSingleValueEvent(new ValueEventListener(){
@Override
public void onDataChange(DataSnapshot dataSnapshot){
Log.d(TAG,"Inside query Inside onDataChange");
if(dataSnapshot.exists()) {
queryforDisplayMessages=mDbRefusers_chat.child(chatId);
}else{
queryforDisplayMessages=mDbRefusers_chat.child(revchatId);
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
但是在运行它时，我得到以下异常：
流程：com.iworld.daxi，PID：20438 java.lang.RuntimeException：无法启动活动ComponentInfo {com.iworld.daxi / com.iworld.daxi.UserChatActivity}：
java.lang.RuntimeException: Failed to call observer method
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2946)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3046)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1688)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6809)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Caused by: java.lang.RuntimeException: Failed to call observer method
at android.arch.lifecycle.ClassesInfoCache$MethodReference.invokeCallback(ClassesInfoCache.java:225)
at android.arch.lifecycle.ClassesInfoCache$CallbackInfo.invokeMethodsForEvent(ClassesInfoCache.java:193)
at android.arch.lifecycle.ClassesInfoCache$CallbackInfo.invokeCallbacks(ClassesInfoCache.java:184)
at android.arch.lifecycle.ReflectiveGenericLifecycleObserver.onStateChanged(ReflectiveGenericLifecycleObserver.java:36)
at android.arch.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:354)
at android.arch.lifecycle.LifecycleRegistry.forwardPass(LifecycleRegistry.java:292)
at android.arch.lifecycle.LifecycleRegistry.sync(LifecycleRegistry.java:332)
at android.arch.lifecycle.LifecycleRegistry.moveToState(LifecycleRegistry.java:137)
at android.arch.lifecycle.LifecycleRegistry.handleLifecycleEvent(LifecycleRegistry.java:123)
at android.arch.lifecycle.ReportFragment.dispatch(ReportFragment.java:121)
at android.arch.lifecycle.ReportFragment.onStart(ReportFragment.java:81)
at android.app.Fragment.performStart(Fragment.java:2637)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1312)
at android.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1549)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1611)
at android.app.FragmentManagerImpl.dispatchMoveToState(FragmentManager.java:3039)
at android.app.FragmentManagerImpl.dispatchStart(FragmentManager.java:2996)
at android.app.FragmentController.dispatchStart(FragmentController.java:189)
at android.app.Activity.performStart(Activity.java:7021)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2909)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3046)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1688)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6809)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.ChildEventListener com.google.firebase.database.Query.addChildEventListener(com.google.firebase.database.ChildEventListener)' on a null object reference
at com.firebase.ui.database.FirebaseArray.onCreate(FirebaseArray.java:53)
at com.firebase.ui.common.BaseObservableSnapshotArray.addChangeEventListener(BaseObservableSnapshotArray.java:97)
at com.firebase.ui.database.FirebaseListAdapter.startListening(FirebaseListAdapter.java:48)
at java.lang.reflect.Method.invoke(Native Method)
at android.arch.lifecycle.ClassesInfoCache$MethodReference.invokeCallback(ClassesInfoCache.java:215)
at android.arch.lifecycle.ClassesInfoCache$CallbackInfo.invokeMethodsForEvent(ClassesInfoCache.java:193)
at android.arch.lifecycle.ClassesInfoCache$CallbackInfo.invokeCallbacks(ClassesInfoCache.java:184)
at android.arch.lifecycle.ReflectiveGenericLifecycleObserver.onStateChanged(ReflectiveGenericLifecycleObserver.java:36)
at android.arch.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:354)
at android.arch.lifecycle.LifecycleRegistry.forwardPass(LifecycleRegistry.java:292)
at android.arch.lifecycle.LifecycleRegistry.sync(LifecycleRegistry.java:332)
at android.arch.lifecycle.LifecycleRegistry.moveToState(LifecycleRegistry.java:137)
at android.arch.lifecycle.LifecycleRegistry.handleLifecycleEvent(LifecycleRegistry.java:123)
at android.arch.lifecycle.ReportFragment.dispatch(ReportFragment.java:121)
at android.arch.lifecycle.ReportFragment.onStart(ReportFragment.java:81)
at android.app.Fragment.performStart(Fragment.java:2637)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1312)
at android.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1549)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1611)
at android.app.FragmentManagerImpl.dispatchMoveToState(FragmentManager.java:3039)
at android.app.FragmentManagerImpl.dispatchStart(FragmentManager.java:2996)
at android.app.FragmentController.dispatchStart(FragmentController.java:189)
at android.app.Activity.performStart(Activity.java:7021)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2909)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3046)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1688)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6809)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
03-08 10:22:40.020 20438-20438/com.iworld.daxi D/AppTracker: App Event: crash
我还观察到onDataChange（）方法没有被触发。