How does Jackson deserialize json into a generic type?

Question

class Person:

@Data
public class Person<T extends Parent> implements Serializable {
    private static final long serialVersionUID = 7822965786010192978L;

    private static final ObjectMapper objectMapper = new ObjectMapper();  
    private String id;    
    private T people; // change (String peopleInfo) to object extends Parent after read data from database
    private String peopleInfo; // change (T people) to string and save in the dataBase as an string

    @SneakyThrows
    public void setPeople(T people) {
        this.people = people;
        peopleInfo = objectMapper.writeValueAsString(people);
    }

    @SneakyThrows
    public void setPeopleInfo(String peopleInfo) {
        this.peopleInfo = peopleInfo;
        if (!Strings.isNullOrEmpty(peopleInfo)) {
            people = objectMapper.readValue(peopleInfo, new TypeReference<T>() {});
        }
    }
}

class Parent:

@Data
public class Parent implements Serializable {
    private static final long serialVersionUID = 2092353331893381153L;
    private String name;
}

class Child:

@Data
public class Child extends Parent {
    private static final long serialVersionUID = 3318503314202792570L;
    private String pocketMoney;
}

and the test function: I want to save the persnInfo to the database as String and change this string to an object people automatically after reading data from database.

@Test
public void testReadData() throws Exception {
    Child child = new Child();
    child.setName("_child");
    child.setPocketMoney("10$");

    Person<Child> person = new Person<>();
    person.setId("1");
    person.setPeople(child);

    // assume this json was read from database
    String json = person.getPeopleInfo();
    System.out.println(json);

    Person<Child> readPerson = new Person<>();
    readPerson.setId("1");
    readPerson.setPeopleInfo(json);

    Child readChild = readPerson.getPeople();
    System.out.println(readChild.getPocketMoney());
}

The error happened:

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "pocketMoney" (class pers.test.common.objmapper.Parent), not marked as ignorable (one known property: "name"])

at [Source: {"name":"_child","pocketMoney":"10$"}; line: 1, column: 33] (through reference chain: pers.test.common.objmapper.Parent["pocketMoney"])

at com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:51)

I have no idea to solve this, anyone can help me make it pass without changing the testReadData()' but change the modelPeople`.

@GhostCat in the `Person.setPeopleInfo ()` `people = objectMapper.readValue(peopleInfo, new TypeReference() {});` — Jarvis, Jul 18 '17 at 08:46

score 5 · Accepted Answer · answered Jul 18 '17 at 08:46

5

Your problem is here:

new TypeReference<T>()

This doesn't do what you expect it to do. Java generics are erased at runtime; therefore the above statement is basically new TypeReference<Object>.

In other words - the fact that you declared

Person<Child> readPerson = new Person<>();

to expect Child objects is simply not sufficient!

You probably have to pass the specific class Child.class to the code that maps JSON strings back. For further information, look here.

answered Jul 18 '17 at 08:46

GhostCat

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I know that pass in the test fun as : `ObjectMapper objectMapper = new ObjectMapper(); Child readChild = objectMapper.readValue(readPerson.getPeopleInfo(), Child.class);` and this will work, but still I want do it in the pojo `People` self dynamically. – Jarvis Jul 18 '17 at 09:07
As said: then you **have** to somehow pass down the specific type, like Child.class. That is why I added that second link! – GhostCat Jul 18 '17 at 09:10
I do think so. And in case I am mistaken, and somebody has a **better** solution (which I doubt, because, as said: those generic types are **erased** at runtime) - feel free to accept that other answer. – GhostCat Jul 18 '17 at 09:13

How does Jackson deserialize json into a generic type?

1 Answers1