Question

引起：com.fasterxml.jackson.databind.JsonMappingException：无法从String值实例化类型[simple type，class net.we4x4.we4x4.userInformation]的值;没有单字符串构造函数/工厂方法

此错误令我感到困惑，我试图按照firebase文档中提供的几条说明操作，例如：https://www.firebase.com/docs/android/guide/retrieving-data.html

另一个关于查询数据的问题，这里还有另一个教程： https://www.simplifiedcoding.net/firebase-android-tutorial-writing-firebase-data/

并且我无法以正确的方式解决问题，我想要做的就是从这个JASON获取数据：

  {
 "data" : {
"ratingPic" : 0,
"ratingVClip" : 36
 },
"users" : {
"3c42f867-8c3a-423b-89e8-3fb777ab76f8" : {
  "email" : "k@k.com",
  "username" : "kkk"
},
"50347968-cedb-4648-8025-f737fea8d4c9" : {
  "email" : "a@a.com",
  "username" : "aaa"
  }
 }
}

通过childEventListner的singleValueEventListner，我仍然是同样的错误，我创建了一个单独的粗心来处理从firebase中提取信息：

public class userInformation {
private String email;
private String username;

public userInformation(){

}

public userInformation(String username){
    this.username = username;
}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getUsername() {
    return username;
}

public void setUsername(String username) {
    this.username = username;
}

}

这是令人困惑的部分，正如firebase社区的好人所建议的那样，我不需要一个二传手，但是当我不在这个类中放置一个二传手时我得到了错误，我只是避免得到这个类的构造错误，只提取一个字符串，当我添加更多字符串时，我仍然得到错误，所以当我添加：

    public userInformation(String username,  String email){
    this.username = username;
    this.email = email;
}

我收到错误，在下面的堆栈跟踪中？

我的主要方法是：

     public void readingData() {
    Firebase ref = new Firebase("https://wi4x4.firebaseio.com/users/" + UserID);
    ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for(DataSnapshot postSnapshot: dataSnapshot.getChildren()){
                userInformation userInformation = postSnapshot.getValue(userInformation.class);
                String username = userInformation.getUsername();
                String email = userInformation.getEmail();
                usernameDisplay.setText(email);
            }
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            firebaseError.getMessage();
        }
    });
}

***与addChildEventListner相同的错误累积;

     public void readingData(){
    Firebase ref = new Firebase("https://wi4x4.firebaseio.com/users/" + UserID);
    Query queryRef = ref.orderByChild("email");
    queryRef.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String email) {
            // Map<String, Objects> extractedEmail = (Map<String, Objects>) dataSnapshot.getValue();
            userInformation userInformation = dataSnapshot.getValue(userInformation.class);
            email = userInformation.getEmail();
            usernameDisplay.setText(email);
        }
        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String previousChild) {
        }
        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {
        }
        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String previousChild) {
        }
        @Override
        public void onCancelled(FirebaseError firebaseError) {
            Toast.makeText(getApplicationContext(),
                    "The read failed: " + firebaseError.getMessage(),
                    Toast.LENGTH_LONG).show();
        }
    });
}

我为这篇长篇文章道歉，但我想提供尽可能多的信息，以便在这里得到一些帮助来理解这些混乱。

03-26 07:30:42.654 8560-8560/net.we4x4.we4x4 E/AndroidRuntime: FATAL EXCEPTION: main
  Process: net.we4x4.we4x4, PID: 8560
  com.firebase.client.FirebaseException: Failed to bounce to type
     at com.firebase.client.DataSnapshot.getValue(DataSnapshot.java:183)
     at net.we4x4.we4x4.MainActivity$2.onDataChange(MainActivity.java:112)
     at com.firebase.client.core.ValueEventRegistration.fireEvent(ValueEventRegistration.java:45)
     at com.firebase.client.core.view.DataEvent.fire(DataEvent.java:45)
     at com.firebase.client.core.view.EventRaiser$1.run(EventRaiser.java:38)
     at android.os.Handler.handleCallback(Handler.java:733)
     at android.os.Handler.dispatchMessage(Handler.java:95)
     at android.os.Looper.loop(Looper.java:136)
     at android.app.ActivityThread.main(ActivityThread.java:5584)
     at java.lang.reflect.Method.invokeNative(Native Method)
     at java.lang.reflect.Method.invoke(Method.java:515)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1268)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1084)
     at dalvik.system.NativeStart.main(Native Method)
  Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not instantiate value of type [simple type, class net.we4x4.we4x4.userInformation] from String value; no single-String constructor/factory method
     at com.fasterxml.jackson.databind.deser.std.StdValueInstantiator._createFromStringFallbacks(StdValueInstantiator.java:428)
     at com.fasterxml.jackson.databind.deser.std.StdValueInstantiator.createFromString(StdValueInstantiator.java:299)
     at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromString(BeanDeserializerBase.java:1056)
     at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:136)
     at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:123)
     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:181)
     at net.we4x4.we4x4.MainActivity$2.onDataChange(MainActivity.java:112) 
     at com.firebase.client.core.ValueEventRegistration.fireEvent(ValueEventRegistration.java:45) 
     at com.firebase.client.core.view.DataEvent.fire(DataEvent.java:45) 
     at com.firebase.client.core.view.EventRaiser$1.run(EventRaiser.java:38) 
     at android.os.Handler.handleCallback(Handler.java:733) 
     at android.os.Handler.dispatchMessage(Handler.java:95) 
     at android.os.Looper.loop(Looper.java:136) 
     at android.app.ActivityThread.main(ActivityThread.java:5584) 
     at java.lang.reflect.Method.invokeNative(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java:515) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1268) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1084) 
     at dalvik.system.NativeStart.main(Native Method)

Answer 1

您正在阅读单个用户对象，因此它没有孩子：

public void readingData() {
  Firebase ref = new Firebase("https://wi4x4.firebaseio.com/users/" + UserID);
  ref.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot postSnapshot: dataSnapshot.getChildren()){
            userInformation userInformation = postSnapshot.getValue(userInformation.class);
            String username = userInformation.getUsername();
            String email = userInformation.getEmail();
            usernameDisplay.setText(email);
        }

因此for循环将对用户的{strong>子属性进行迭代，email和username。那些不能被反序列化为userInformation对象。

解决方案只是不循环：

public void readingData() {
  Firebase ref = new Firebase("https://wi4x4.firebaseio.com/users/" + UserID);
  ref.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        userInformation userInformation = dataSnapshot.getValue(userInformation.class);
        String username = userInformation.getUsername();
        String email = userInformation.getEmail();
        usernameDisplay.setText(email);
    }

Firebase不是从String值实例化类型的值

1 个答案: