杰克逊：如何反序列化我自己课程的实例字段

Question

I'm writing RESTful web service for my android app. I have my POJO class named ServiceResponse:

private int responseCode;
private String objectType;
private ArrayList<Object> data;

public int getResponseCode() {
    return responseCode;
}

public void setResponseCode(int responseCode) {
    this.responseCode = responseCode;
}

public ArrayList<Object> getData() {
    return data;
}

public void setArrayData(ArrayList<Object> data) {
    this.data = data;
}

public void setData(Object data) {
    if(this.data == null) this.data = new ArrayList<Object>();
    this.data.add(data);
}

public String getObjectType() {
    return objectType;
}

public void setObjectType(String objectType) {
    this.objectType = objectType;
}

It's parsed to JSON using Jackson, example here: http://pastebin.com/pu8792a1 There will be many classes passed using this arraylist. My question is, how do I make it ServiceResponse object again from the JSON?

{
  "responseCode" : 2,
  "objectType" : "com.example.User",
  "data" : [ {
    "name" : "name",
    "password" : "pass",
    "email" : "mail@gmail.com",
    "device" : "ABC",
    "level" : 10,
    "gold" : 82,
    "delois" : 0,
    "uranium" : 0,
    "attack" : 5,
    "speed" : 5,
    "armor" : 5,
    "controllability" : 5,
    "exp" : 100,
    "hp" : 100,
    "hpMax" : 300,
    "deuter" : 21,
    "deuterMax" : 120,
    "research" : 1413360596907,
    "id" : 1
  } ]

I tried: (skipped try & catch)

    ObjectMapper mapper = new ObjectMapper();
    ServiceResponse response;
    response = mapper.readValue(output, ServiceResponse.class);

    User user =  (User) ( response.getData().get(0) );
    System.out.println(user.toString());

but I got ClassCastException: java.util.ArrayList cannot be cast into com.example.User

Answer 1

A couple problems:

Problem 1:

"data" : [ {
  "name" : "name",
  "password" : "pass"
  ...
} ]

private ArrayList<Object> data;

Since you define the type of the list as Object the parser doesn't know to map it to a User type, so it will map it to an array of LinkedHashMap . So basically you're left with ArrayList<LinkedHashMap> data . Change all the Object to User .

Problem 2:

private ArrayList<User> data;
...
public void setData(User data) {
    if (this.data == null) {
        this.data = new ArrayList<User>();
    }
    this.data.add(data);
}

The parser will look for JavaBean properties that follow JavaBean naming convention ie property / setProperty / getProperty . In your case the parser thinks the setData is a setter to set the value of the data field. As a setter , it will expect an ArrayList<User> and not a User as the argument. So better to change the name to something like addUser(User user) . While you're at it, you should change the name of setArrayData to follow the name convention also.

public void setData(ArrayList<User> data) {
    this.data = data;
}

public void addUser(User data) {
    if (this.data == null) {
        this.data = new ArrayList<User>();
    }
    this.data.add(data);
}

Making these few changes should get it working (tested).

---

UPDATE: Problem 1 is actually not a problem

I know I could make it work if I used User instead of Object, but I'd like to send different objects in data array (sometimes it would be User, other time, for example Item). Is there a way to do it? Here They say I can use this: POJO pojo = mapper.convertValue(singleObject, POJO.class); To convert from LinkedHashMap to my object, but when I try it: User user = mapper.convertValue(response.getData().get(0), User.class); User is still null

As the OP brought to my attention, we can convert the map the parser created by using ObjectMapper.convertValue( map , User.class); . Not sure why the OP is getting null, but when I tested, changing the User types back to Object type, it works fine

public class Test {
    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        InputStream is = Test.class.getResourceAsStream("test.json");
        ServiceResponse response = mapper.readValue(is, ServiceResponse.class);
        User user = (User) (mapper.convertValue(response.getData().get(0), User.class));
        System.out.println(user.toString());
        is.close();
    }
}

Result test.json.User@366e2eef

---

UPDATE 2:

The OP wants to use the String objectType to convert the the map by type:

ServiceResponse response = ...
String objectType = response.getObjectType();
Class clazz = Class.forName(objectType);      // catch exception
ArrayList user = (mapper.convertValue(response.getData(), 
            TypeFactory.defaultInstance().constructCollectionType(
                    ArrayList.class, clazz)));