Firebase JsonMappingException：无法从 START_ARRAY 令牌中反序列化 java.util.LinkedHashMap 的实例

Question

我运行它以将一些数据模拟到我的 firebase 参考中：

    String id = firebase.push().getKey();
    FirebaseSchool school = new FirebaseSchool(id, "Test school", "Test city", "Test address");
    firebase = firebase.child(id);
    firebase.setValue(school);

    id = firebase.child("rooms").push().getKey();
    FirebaseRoomReference roomReference = new FirebaseRoomReference(id, "Test room", "Test owner");
    firebase.child("rooms").child(id).setValue(roomReference);

    id = firebase.child("rooms").push().getKey();
    FirebaseRoomReference roomReference2 = new FirebaseRoomReference(id, "Test room 2", "Test owner 2");
    firebase.child("rooms").child(id).setValue(roomReference2);

这会创建以下 json：

"schools" : {
    "-K9LoPw-o0dOGAdWYqBF" : {
      "address" : "Test city",
      "city" : "Test address",
      "id" : "-K9LoPw-o0dOGAdWYqBF",
      "name" : "Test school",
      "rooms" : {
        "-K9LoPwDuzW-d19432kD" : {
          "id" : "-K9LoPwDuzW-d19432kD",
          "name" : "Test room",
          "ownerName" : "Test owner"
        },
        "-K9LoPwFPTrwSpUs0dZS" : {
          "id" : "-K9LoPwFPTrwSpUs0dZS",
          "name" : "Test room 2",
          "ownerName" : "Test owner 2"
        }
      }
    }
  }

按照建议，我在响应类中将 Map 更改为 List：

public class FirebaseSchool {
    private String id;
    private String name;
    private String address;
    private String city;
    private List<FirebaseUserReference> admins;
    private List<FirebaseRoomReference> rooms;


    public List<FirebaseUserReference> getAdmins() {
        return admins;
    }

    public void setAdmins(List<FirebaseUserReference> admins) {
        this.admins = admins;
    }

    public List<FirebaseRoomReference> getRooms() {
        return rooms;
    }

    public void setRooms(List<FirebaseRoomReference> rooms) {
        this.rooms = rooms;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }
}

但是当我尝试使用 snapshot.getValue(FirebaseSchool.class) 阅读学校时，我收到以下错误：

com.firebase.client.FirebaseException: Failed to bounce to type
Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token at [Source: java.io.StringReader@dbf0e0f; line: 1, column: 44] (through reference chain: com.usehomeroom.vasuki.data.entities.firebase.FirebaseSchool["rooms"])

Answer 1

看起来您的数据结构被映射到数组，而不是映射。我对此感到有些惊讶，因为您的第一个索引是 1（不是 0）。但是错误消息非常明确地说明了它要做什么。

所以要解决这个问题，请将房间映射到 List 或数组：

private List<FirebaseRoomReference> rooms;

为防止此类问题，强烈建议不要使用数字索引。要么使用push() 生成的值，要么使用固定字符串作为数字前缀，这样就不会混淆它是什么类型：room_1、room_2 等。

更新

我可以用这个读取你更新的 JSON：

public static class FirebaseRoomReference {
    private String id;
    private String name;
    private String ownerName;

    public String getId() { return id; }
    public String getName() { return name; }
    public String getOwnerName() { return ownerName; }
}

public static class FirebaseSchool {
    private String id;
    private String name;
    private String address;
    private String city;
    //private List<FirebaseUserReference> admins;
    private Map<String, FirebaseRoomReference> rooms;

    //public List<FirebaseUserReference> getAdmins() { return admins; }

    public Map<String, FirebaseRoomReference> getRooms() {
        return rooms;
    }

    public String getId() { return id; }
    public String getName() { return name; }
    public String getAddress() { return address; }
    public String getCity() { return city; }
}

public static void main(String[] args) throws Exception {
    Firebase ref = new Firebase("https://stackoverflow.firebaseio.com/35109413");
    ref.child("schools").addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot snapshot, String s) {
            System.out.println(snapshot.getValue(FirebaseSchool.class));
        }
    });

我改变的地方：

• 我将 POJO 类标记为静态，因为我将它们作为内部类包含在我的测试程序中。
• 我删除了用户管理员，因为它们不在您的 JSON 中
• 我将您的列表更改为地图，因为您现在使用的是字符串键

在我更改最后一点之前，我收到了与您相同的错误消息。