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java - 使用 Firebase 在 Android 上检索数据时出现问题

转载 作者:太空狗 更新时间:2023-10-29 13:21:51 24 4
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我刚刚学习如何在 Android 中设置 Firebase 数据。我可以写入数据库,但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(它现在只有一个主屏幕)而且我不关心数据在页面上的显示方式,因为我只是在学习 - 我现在正在尝试在 TextView 中显示它。

这是我的主要 Activity java:

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:

<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,它崩溃了。这是 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

最佳答案

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 客户端取回了一些数据,但无法将其转换为您指定的类型。

问题详情

您要在此处将 ValueEventListener 添加到 Firebase 的根目录:

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

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

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

如果您有一个仅包含单个字符串的非常简单的 Firebase,这会很好地工作。但是,根据您的其余代码,您的 Firebase 中似乎有不止一个 String :)

如何修复您有几个选择:

  1. 将您的事件监听器附加到树的更下方。您可以获得字符串形式的用户全名:

    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,您可以获得整个用户,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());
    }
    ...
    }

关于java - 使用 Firebase 在 Android 上检索数据时出现问题,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/27663300/

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