Android 成功 gradle 构建后，应用程序在运行时崩溃

Question

我是 android 和 firestore 的新手。我构建了一个小型应用程序，每当我运行我的应用程序时，即使 gradle 构建成功，应用程序也会在运行时崩溃。下面主要是activity和Logcat的代码，方便大家理解。

package com.example.make;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.widget.TextView;

import com.google.firebase.firestore.DocumentChange;
import com.google.firebase.firestore.EventListener;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.FirebaseFirestoreException;
import com.google.firebase.firestore.QuerySnapshot;

import java.util.ArrayList;
import java.util.List;

import javax.annotation.Nullable;

import io.opencensus.tags.Tag;

public class MainActivity extends AppCompatActivity {

private static final String Tag = "Firelog";

private List<Users> usersList;
private UsersListAdapter usersListAdapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    RecyclerView mMainList = findViewById(R.id.main_list);

    usersList  = new ArrayList<>();
    usersListAdapter = new UsersListAdapter(usersList);
    mMainList.setLayoutManager(new LinearLayoutManager(MainActivity.this));
    mMainList.setAdapter(usersListAdapter);


    TextView mName = findViewById(R.id.item_name);
    TextView mStatus = findViewById(R.id.item_status);

    try {

        FirebaseFirestore db = FirebaseFirestore.getInstance();

        db.collection("Users").addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot queryDocumentSnapshots, @Nullable FirebaseFirestoreException e) {

                if (e != null) {

                    Log.d(Tag, "Error : " + e.getMessage(), new NullPointerException());


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

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

                            doc.getDocument().getReference().collection("Users").addSnapshotListener(new EventListener<QuerySnapshot>() {
                                @Override
                                public void onEvent(@Nullable QuerySnapshot queryDocumentSnapshots, @Nullable FirebaseFirestoreException e) {
                                    if (e != null) {
                                        Log.d("", "Error :" + e.getMessage(), new NullPointerException());
                                    }

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

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

                                            Log.d("USer Name :", doc.getDocument().getId());
                                        }
                                    }
                                }
                            });

                            Users users = doc.getDocument().toObject(Users.class);
                            usersList.add(users);
                            usersListAdapter.notifyDataSetChanged();

                        }

                    }
                }
            }
        });
    } catch (NullPointerException error) {
        mName.setText(error.getMessage());
    }
}

}

这是我得到的错误，

12-03 13:59:30.576 26036-26036/com.example.make E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.make, PID: 26036 java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.google.firebase.firestore.QuerySnapshot.getDocumentChanges()' on a null object reference at com.example.make.MainActivity.onEvent(MainActivity.java:65) at com.example.make.MainActivity.onEvent(MainActivity.java:56) at com.google.firebase.firestore.Query.lambda$addSnapshotListenerInternal(com.google.firebase:firebase-firestore@@17.1.3:885) at com.google.firebase.firestore.Query$$Lambda.onEvent(com.google.firebase:firebase-firestore@@17.1.3) at com.google.firebase.firestore.util.ExecutorEventListener.lambda$onEvent[=17=](com.google.firebase:firebase-firestore@@17.1.3:42) at com.google.firebase.firestore.util.ExecutorEventListener$$Lambda.run(com.google.firebase:firebase-firestore@@17.1.3) at android.os.Handler.handleCallback(Handler.java:754) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:163) at android.app.ActivityThread.main(ActivityThread.java:6383) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794)

我还放置了 try catch 块来捕获空指针异常，但它没有 work.Any 帮助表示赞赏。谢谢。

Answer 1

错误指出：

java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.google.firebase.firestore.QuerySnapshot.getDocumentChanges()' on a null object reference

在您的代码中：

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

queryDocumentSnapshots 对象为空（可以通过注释推断）。

Answer 2

public void onEvent(@Nullable QuerySnapshot queryDocumentSnapshots, @Nullable FirebaseFirestoreException e) {
    if(queryDocumentSnapshots != null){
       for (DocumentChange doc : queryDocumentSnapshots.getDocumentChanges()) {
           if (doc.getType() == DocumentChange.Type.ADDED) {
              Log.d("USer Name :", doc.getDocument().getId());
           }
       }
    }
}

方法中有明确注释，QuerySnapshot是可以为nullable的，所以要添加null检查。

Android 成功 gradle 构建后，应用程序在运行时崩溃

Android app crashes at runtime after successful gradle build

android

nullpointerexception

adapter

firebase

android-recyclerview