java在使用Firebase的Android上检索数据时出现问题

Question

我正在学习如何在Android中设置Firebase数据。我可以写入数据库，但我无法读取它并将其显示回我的应用程序。我的应用程序非常简单（目前只有一个主屏幕），我不在乎数据如何显示在页面上，因为我只是在学习——我现在正在尝试在文本视图中显示它

以下是我的主要活动：

package com.example.ispotrachel.myapplication;

import 安卓.support.v7.app.ActionBarActivity;
import 安卓.os.Bundle;
import 安卓.view.Menu;
import 安卓.view.MenuItem;
import 安卓.view.View;
import 安卓.widget.TextView;

import com.firebase.client.DataSnapshot;
import com.firebase.client.Firebase;
import com.firebase.client.FirebaseError;
import com.firebase.client.ValueEventListener;

import java.util.HashMap;
import java.util.Map;


public class MainActivity extends ActionBarActivity {

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

    Firebase ref = new Firebase("https://blistering-torch-9015.firebaseio.com");

    class User {
        private int birthYear;
        private String fullName;

        public User() {
        }

        public User(String fullName, int birthYear) {
            this.fullName = fullName;
            this.birthYear = birthYear;
        }

        public long getBirthYear() {
            return birthYear;
        }

        public String getFullName() {
            return fullName;
        }
    }

     User user1 = new User("Name 1", 1982);
     User user2 = new User("Name 2", 1972);

     Firebase usersRef = ref.child("users");

     Map<String, User> users = new HashMap<String, User>();
     users.put("user1", user1);
     users.put("user 2", user2);

    usersRef.setValue(users);

    Firebase postRef = ref.child("posts");

    Map<String, String> post1 = new HashMap<String, String>();
    post1.put("author", "user1");
    post1.put("title", "Announcing COBOL, a New Programming Language");
    postRef.push().setValue(post1);

    Map<String, String> post2 = new HashMap<String, String>();
    post2.put("author", "user2");
    post2.put("title", "The Turing Machine");
    postRef.push().setValue(post2);
    ref.addValueEventListener(new ValueEventListener() {


    @Override
    public void onDataChange(DataSnapshot arg0) {
            //System.out.println(arg0.getValue());
            TextView textViewSample = (TextView) findViewById(R.id.sampleTextView);
            textViewSample.setText(arg0.getValue(String.class));
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            System.out.println("the read failed");
        }

    });
}}

这是我的主要活动。xml:

<LinearLayout xmlns:安卓="http://schemas.安卓.com/apk/res/安卓"
 xmlns:tools="http://schemas.安卓.com/tools" 安卓:layout_width="match_parent"
 安卓:layout_height="match_parent"           安卓:paddingLeft="@dimen/activity_horizontal_margin"
安卓:paddingRight="@dimen/activity_horizontal_margin"
安卓:paddingTop="@dimen/activity_vertical_margin"
安卓:paddingBottom="@dimen/activity_vertical_margin"
tools:context=".MainActivity">


<TextView
    安卓:id="@+id/sampleTextView"
    安卓:layout_width="wrap_content"
    安卓:layout_height="wrap_content"
    安卓:text="@string/hello_world"/>


</LinearLayout>

当我运行应用程序时，它崩溃了。以下是logcat错误消息：

12-27 12:15:02.678    1895-1895/com.example.ispotrachel.myapplication I/art﹕ Not late     enabling -Xcheck:jni (already on)
12-27 12:15:02.946    1895-1907/com.example.ispotrachel.myapplication I/art﹕ Background     sticky concurrent mark sweep GC freed 3867(286KB) AllocSpace objects, 0(0B) LOS objects, 11% free, 1009KB/1135KB, paused 2.595ms total 117.910ms
12-27 12:15:03.317    1895-1926/com.example.ispotrachel.myapplication D/OpenGLRenderer﹕ Render dirty regions requested: true
12-27 12:15:03.320    1895-1895/com.example.ispotrachel.myapplication D/﹕ HostConnection::get() New Host Connection established 0xa6667e80, tid 1895
12-27 12:15:03.327    1895-1895/com.example.ispotrachel.myapplication D/Atlas﹕ Validating map...
12-27 12:15:03.391    1895-1926/com.example.ispotrachel.myapplication D/﹕ HostConnection::get() New Host Connection established 0xa66ef3b0, tid 1926
12-27 12:15:03.405    1895-1926/com.example.ispotrachel.myapplication I/OpenGLRenderer﹕ Initialized EGL, version 1.4
12-27 12:15:03.422    1895-1926/com.example.ispotrachel.myapplication D/OpenGLRenderer﹕ Enabling debug mode 0
12-27 12:15:03.439    1895-1926/com.example.ispotrachel.myapplication W/EGL_emulation﹕ eglSurfaceAttrib not implemented
12-27 12:15:03.439    1895-1926/com.example.ispotrachel.myapplication W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xa66b7880, error=EGL_SUCCESS
12-27 12:15:07.474    1895-1926/com.example.ispotrachel.myapplication W/EGL_emulation﹕ eglSurfaceAttrib not implemented
12-27 12:15:07.474    1895-1926/com.example.ispotrachel.myapplication W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xa66b7880, error=EGL_SUCCESS
12-27 12:15:13.993    1895-1895/com.example.ispotrachel.myapplication D/AndroidRuntime﹕    Shutting down VM 

--------- beginning of crash
12-27 12:15:13.993    1895-1895/com.example.ispotrachel.myapplication E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.ispotrachel.myapplication, PID: 1895 com.firebase.client.FirebaseException: Failed to bounce to type 
        at com.firebase.client.DataSnapshot.getValue(DataSnapshot.java:196)
        at com.example.ispotrachel.myapplication.MainActivity$1.onDataChange(MainActivity.java:79) 
        at com.firebase.client.core.ValueEventRegistration$1.run(ValueEventRegistration.java:48)
        at 安卓.os.Handler.handleCallback(Handler.java:739)
        at 安卓.os.Handler.dispatchMessage(Handler.java:95)
        at 安卓.os.Looper.loop(Looper.java:135)
        at 安卓.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:694)

 Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_OBJECT token
        at [Source: java.io.StringReader@c8f7297; line: 1, column: 1]
        at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:575)
        at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:46)
        at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:11)
        at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2888)
        at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2034)
        at com.firebase.client.DataSnapshot.getValue(DataSnapshot.java:192)
        at com.example.ispotrachel.myapplication.MainActivity$1.onDataChange(MainActivity.java:79)
        at com.firebase.client.core.ValueEventRegistration$1.run(ValueEventRegistration.java:48)
        at 安卓.os.Handler.dispatchMessage(Handler.java:95)
        at 安卓.os.Looper.loop(Looper.java:135)
        at 安卓.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:694)device not found

Answer 1

我也有类似的问题

以下是我如何从firebase检索数据的代码：

    DatabaseReference chatRef = database.getReference("chats");
    chatRef.keepSynced(true);
    Query queryRef = chatRef.orderByChild("id");

    queryRef.addChildEventListener(new ChildEventListener() {
        // Retrieve new posts as they are added to the database
        @Override
        public void onChildAdded(DataSnapshot snapshot, String previousChildKey) {
            ChatPost newPost = snapshot.getValue(ChatPost.class);
            ChatPost topic = new ChatPost(newPost.id, newPost.topic);
            addChatTopic(topic);
            System.out.println("NEW Child Topic: " + newPost.topic);

            chatAdapter.addItem(newPost);
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
            ChatPost newPost = dataSnapshot.getValue(ChatPost.class);
            ChatPost topic = new ChatPost(newPost.id, newPost.topic);
            addChatTopic(topic);
            System.out.println("CHANGED Child Topic: " + newPost.topic);

            chatAdapter.updateItem(newPost);
        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {
            ChatPost newPost = dataSnapshot.getValue(ChatPost.class);
            ChatPost topic = new ChatPost(newPost.id, newPost.topic);
            chatTopicsList.add(topic);
            System.out.println("REMOVED Child Topic: " + newPost.topic);

            chatAdapter.removeItem(newPost);
        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

直到我没有使用push（）添加新项目，它才开始工作。我在firebase控制台中的数据看起来像here

例外情况文本：

07-05 16:45:07.097 29997-29997/com.skl.bingofire E/AndroidRuntime: FATAL EXCEPTION: main Process: com.skl.bingofire, PID: 29997 com.google.firebase.database.DatabaseException: Expected a List while deserializing, but got a class java.util.HashMap at com.google.android.gms.internal.zzaix.zza(Unknown Source) at com.google.android.gms.internal.zzaix.zza(Unknown Source) at com.google.android.gms.internal.zzaix.zzb(Unknown Source) at com.google.android.gms.internal.zzaix$zza.zze(Unknown Source) at com.google.android.gms.internal.zzaix$zza.zzaC(Unknown Source) at com.google.android.gms.internal.zzaix.zzd(Unknown Source) at com.google.android.gms.internal.zzaix.zzb(Unknown Source) at com.google.android.gms.internal.zzaix.zza(Unknown Source) at com.google.firebase.database.DataSnapshot.getValue(Unknown Source) at com.skl.bingofire.activities.MainActivity$7.onChildAdded(MainActivity.java:228) at com.google.android.gms.internal.zzaer.zza(Unknown Source) at com.google.android.gms.internal.zzagp.zzSu(Unknown Source) at com.google.android.gms.internal.zzags$1.run(Unknown Source) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5417) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 07-05 16:45:16.924 29997-30029/com.skl.bingofire E/DynamiteModule: Failed to load module descriptor class: Didn't find class "com.google.android.gms.dynamite.descriptors.com.google.firebase.auth.ModuleDescriptor" on path: DexPathList[[zip file "/data/app/com.skl.bingofire-1/base.apk"],nativeLibraryDirectories=[/data/app/com.skl.bingofire-1/lib/arm, /vendor/lib, /system/lib]]

我用了这个模型：

class ChatPost {
    public String id;
    public String topic;
    public String color;
    public ArrayList<Message> messages;
}

我发现，如果数据列表中包含键，例如“-kldskjdkjdjdjdjdjdjdjdjdjdjjsjk”firebase，则会将孔列表识别为“HashMap”，否则识别为“ArrayList”。我将我的模型改为这种结构：

class ChatPost {
    public String id;
    public String topic;
    public String color;
    public HashMap<String, Message> messages;
}

并在聊天信息的其余部分添加了类似“-kldskjdkjdkjdksjk”的键。我的代码现在正在运行。但我认为这并不明显

Answer 2

tl；dr

看起来您正在尝试将JSON对象加载到字符串中

错误消息Failed to bounce to type表示Firebase客户端返回了一些数据，但无法将其转换为您指定的类型

有关问题的详细信息

您正在将ValueEventListener添加到Firebase的根目录：

ref.addValueEventListener(new ValueEventListener() { ...

在该值事件侦听器中，您告诉Firebase将整个Firebase作为String加载：

textViewSample.setText(arg0.getValue(String.class));

如果你有一个非常简单的Firebase，它只包含一个字符串，这就可以了。然而，从代码的其余部分来看，Firebase中似乎不止一个String：）

如何修复它 你有几个选择：

1. 将事件侦听器连接到树的更下方。您可以通过字符串获取用户的全名：

   ref.child("users/user1/fullName").addValueEventListener(new ValueEventListener() {
   
       @Override
       public void onDataChange(DataSnapshot arg0) {
           TextView textViewSample = (TextView) findViewById(R.id.sampleTextView);
           textViewSample.setText(arg0.getValue(String.class));
       }
       ...
   }
   

2. 或者，您可以获得一个完整的用户，但将其解析为User，而不是String：

   ref.child("users/user1/fullName").addValueEventListener(new ValueEventListener() {
   
       @Override
       public void onDataChange(DataSnapshot arg0) {
           User tempUser = arg0.getValue(User.class);
           TextView textViewSample = (TextView) findViewById(R.id.sampleTextView);
           textViewSample.setText(tempUser.getFullName());
       }
       ...
   }