Android应用程序在空对象引用上退出'boolean com.google.firebase.firestore.DocumentSnapshot.exists（）'问题

Question

I have a problems with my application and I really need your help. I am new on Android Studio and I don't know why my apps crashed on SignOut. I think is because of Query SnapShot and this line of code:

I speak just a little bit in english, so I am very sorry for my poor english :)

Here's my code for HomeFragment

 Query firstQuery =
 firebaseFirestore.collection("Posts").orderBy("timestamp",
 Query.Direction.DESCENDING).limit(3);
         firstQuery.addSnapshotListener(getActivity(), new EventListener<QuerySnapshot>() {
             @Override
             public void onEvent(QuerySnapshot documentSnapshots, FirebaseFirestoreException e) {
                 if (!documentSnapshots.isEmpty()) {
                     if (isFirstPageFirstLoad) {
                         lastVisible = documentSnapshots.getDocuments().get(documentSnapshots.size() - 1);
                         blog_list.clear();
                     }

                     for (DocumentChange doc : documentSnapshots.getDocumentChanges()) {

                         if (doc.getType() == DocumentChange.Type.ADDED) {

                             String blogPostId = doc.getDocument().getId();
                             BlogPost blogPost = doc.getDocument().toObject(BlogPost.class).withId(blogPostId);

                             if (isFirstPageFirstLoad) {
                                 blog_list.add(blogPost);
                             } else {
                                 blog_list.add(0, blogPost);
                             }
                             blogRecyclerAdapter.notifyDataSetChanged();
                         }
                     }
                     isFirstPageFirstLoad = false;
                 }
             }
         });
     }
     // Inflate the layout for this fragment
     return view;
 }

 public void loadMorePost(){
     if(firebaseAuth.getCurrentUser() != null) {
         final Query nextQuery = firebaseFirestore.collection("Posts")
                 .orderBy("timestamp", Query.Direction.DESCENDING)
                 .startAfter(lastVisible)
                 .limit(3);

         nextQuery.addSnapshotListener(getActivity(), new EventListener<QuerySnapshot>() {
             @Override
             public void onEvent(QuerySnapshot documentSnapshots, FirebaseFirestoreException e) {
                 if (!documentSnapshots.isEmpty()) {
                     lastVisible = documentSnapshots.getDocuments().get(documentSnapshots.size() - 1);
                     for (DocumentChange doc : documentSnapshots.getDocumentChanges()) {
                         if (doc.getType() == DocumentChange.Type.ADDED) {
                             String blogPostId = doc.getDocument().getId();
                             BlogPost blogPost = doc.getDocument().toObject(BlogPost.class).withId(blogPostId);
                             blog_list.add(blogPost);
                             blogRecyclerAdapter.notifyDataSetChanged();
                         }
                     }
                 }
             }
         });

And for MainActivity I use this only:

private void logOut() {
    mAuth.signOut();
    sendToLogin();
}

BlogRecyclerAdapter.java

    firebaseFirestore.collection("Posts/" + blogPostId + "/Likes").addSnapshotListener( new EventListener<QuerySnapshot>() {
        @Override
        public void onEvent(QuerySnapshot documentSnapshots, FirebaseFirestoreException e) {
            if(!documentSnapshots.isEmpty()){
                int count = documentSnapshots.size();
                holder.updateLikesCount(count);

            } else {
                holder.updateLikesCount(0);
            }
        }
    });

And my Fatal Error : On BlogRecyclerAdapter but if I remove all code on BlogRecyclerAdapter my apps still crash! I think the real problems is on HomeFragment!

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: learnorburn.lob_application, PID: 15909
                  java.lang.NullPointerException: Attempt to invoke virtual method 'boolean com.google.firebase.firestore.QuerySnapshot.isEmpty()' on a null object reference
                      at learnorburn.lob_application.BlogRecyclerAdapter$2.onEvent(BlogRecyclerAdapter.java:117)
                      at learnorburn.lob_application.BlogRecyclerAdapter$2.onEvent(BlogRecyclerAdapter.java:113)
                      at com.google.firebase.firestore.zzi.onEvent(Unknown Source)
                      at com.google.android.gms.internal.zzevc.zza(Unknown Source)
                      at com.google.android.gms.internal.zzevd.run(Unknown Source)
                      at android.os.Handler.handleCallback(Handler.java:751)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:154)
                      at android.app.ActivityThread.main(ActivityThread.java:6776)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1510)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1400)

Thank you in advance for your help

Answer 1

You have error somwehere in your query or db setup. Check the documentation of EventListener . It says:

onEvent will be called with the new value or the error if an error occurred. It's guaranteed that exactly one of value or error will be non-null.

So in your case if QuerySnapshot documentSnapshots is null , it means FirebaseFirestoreException e is not null . However, you swallow this exception. Instead you should log it and check what is written there:

Log.e("MyTag", "Firebase exception", e);

Answer 2

I found a solution but I'm not sure that is a good solution! On Firebase Database, I change the rule allow read, write: if request.auth != null; to if true; and that work. I think the problems is because when I logOut, I don't have any permission so my apps crashed. Now, I want to know if there's any security or something else who can cause a big problems with this change on Database? Also thank you very much to help me. You don know how i appreciate your help