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使用Firebase在安卓设备上检索数据时遇到的问题

javaandroidfirebase
3
我正在学习如何在Android中设置Firebase数据。我能够写入数据库，但是我无法读取并在我的应用程序中显示它。我的应用程序非常简单（目前只有一个主屏幕），我不关心数据在页面上如何显示，因为我只是在学习 - 目前我正在尝试在文本视图中显示它。
这是我的MainActivity Java代码：
package com.example.ispotrachel.myapplication;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.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 activity_main.xml 文件：

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


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


</LinearLayout>

当我运行应用程序时，它会崩溃。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$1.run(ValueEventRegistration.java:48)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.android.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$1.run(ValueEventRegistration.java:48)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)device not found
发生了什么问题呢？（例如，错误消息。） - Nathan Tuggy
@ispotrachel：请在您的问题下方点击编辑链接，将堆栈跟踪添加到其中。这也将允许您将它们格式化为更易读的内容。 - Frank van Puffelen
大家好，@FrankvanPuffelen，我已经更新了我的问题并附上了错误信息。谢谢！ - ispotrachel
1
如果您在导致崩溃的那一行设置断点，我猜测您会发现arg0.getValue()返回一个Map<String, String>。如果是这种情况，您可以使用类似于((Map<String,String>)arg0.getValue()).get("title")的方式获取帖子的标题。请参考此处的示例：https://www.firebase.com/docs/android/guide/retrieving-data.html#section-types - Frank van Puffelen
嗨@FrankvanPuffelen。当我按照教程操作时，我能够使用system.out.print。问题出现在我尝试在应用程序中打印列表或文本框中的数据时。 - ispotrachel
2个回答
2

简而言之

看起来你正在尝试将一个JSON对象加载到一个字符串中。

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

问题详情

您正在此处向Firebase的根添加一个ValueEventListener：

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

在该值事件监听器中，您告诉Firebase将整个Firebase作为 字符串 加载：

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

如果您的Firebase只包含单个字符串，那么这将很好地工作。但是，根据您代码的其余部分，看起来您的Firebase不仅有一个String :)

如何修复它 您有几个选项：

  1. Attach your event listener further down the tree. You could get user's full name as a string:

    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. Or, you can get a whole user, but parse it as a User instead of a 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());
    }
    ...
}
0

我有类似的问题。

这是我从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) {

        }
    });

它在我没有使用push()添加新项目之前是有效的。我的Firebase控制台中的数据看起来像here。

异常文本：

07-05 16:45:07.097 29997-29997/com.skl.bingofire E/AndroidRuntime: FATAL EXCEPTION: 主线程 进程: com.skl.bingofire, PID: 29997 com.google.firebase.database.DatabaseException: 在反序列化时期望一个列表，但得到了一个 java.util.HashMap 类 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: 加载模块描述符类失败：在路径 DexPathList[[zip 文件 "/data/app/com.skl.bingofire-1/base.apk"],nativeLibraryDirectories=[/data/app/com.skl.bingofire-1/lib/arm, /vendor/lib, /system/lib]] 上未找到类 "com.google.android.gms.dynamite.descriptors.com.google.firebase.auth.ModuleDescriptor"

我使用了这个模型：

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“-KLdskjdkjkjdjksjdksjdjsdjsjk”），Firebase 会将整个列表识别为“HashMap”，否则识别为“ArrayList”。因此，我将我的模型更改为以下结构：

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

并将其添加到其他聊天消息中，使用类似“-KLdskjdkjkjdjksjdksjdjsdjsjk”的键。现在我的代码可以工作了。但我认为这不是很明显。

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