Use activateDefaultTyping( objectMapper.getPolymorphicTypeValidator(), ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE, JsonTypeInfo.As.PROPERTY); After serialization, deserialize error

[superwen0001]

package com.miyo.product.qingkong.host;

import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.AllArgsConstructor;
import lombok.Data;

import java.util.ArrayList;
import java.util.List;

public class Test {
    public static void main(String[] args) throws JsonProcessingException {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.activateDefaultTyping(
                objectMapper.getPolymorphicTypeValidator(),
                ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE,
                JsonTypeInfo.As.PROPERTY);
        List<Person> list = new ArrayList<>();
        list.add(new Person("Alice"));


        String json = objectMapper.writeValueAsString(list);
        System.out.println(json);
        JavaType javaType = objectMapper.getTypeFactory().constructParametricType(ArrayList.class, Person.class);
        List o = objectMapper.readValue(json, javaType);
        System.out.println(o);
    }
    @Data
    @AllArgsConstructor
    public static class Person {
        private String name;
    }
}

After serialization, deserialize

[{"@class":"com.miyo.product.qingkong.host.Test$Person","name":"Alice"}]
Disconnected from the target VM, address: 'localhost:56649', transport: 'socket'
Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `com.miyo.product.qingkong.host.Test$Person` (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)
 at [Source: (String)"[{"@class":"com.miyo.product.qingkong.host.Test$Person","name":"Alice"}]"; line: 1, column: 3] (through reference chain: java.util.ArrayList[0])
	at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
	at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1728)
	at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1353)
	at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1415)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:352)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:185)
	at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer._deserializeFromArray(CollectionDeserializer.java:355)
	at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:244)
	at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:28)
	at com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:323)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4674)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3629)
	at com.miyo.product.qingkong.host.Test.main(Test.java:30)

[JooHyukKim]

Could you fix the markdown code-block format? with proper indents and such

[cowtowncoder]

I'll fix those.

[cowtowncoder]

This could be the usual FAQ wrt type erasure: serialization of root-level generic type (List).
But I think this:

ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE,

is wrong choice IF intended base type is List<Person> as used for readValue() -- Person is neither java.lang.Object nor "non-concrete" -- it is a concrete class (unless there's some odd Lombok magic).

So may be fixed by various alternatives:

1. Use readValue target of List<Object> so base type for elements is java.lang.Object and Type Id is expected
2. Change DefaultTyping to DefaultTyping.NON_FINAL
3. Use writeValue() target type of List<Person> via ObjectReader: something like ObjectWriter w = mapper.writerFor(new TypeReference<List<Person>>() { })

But also: if type used is List<Person>, why do you enable "default typing" at all?

[superwen0001]

import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.*;

import java.util.ArrayList;
import java.util.List;

public class Test {
    public static void main(String[] args) throws JsonProcessingException {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.activateDefaultTyping(
                objectMapper.getPolymorphicTypeValidator(),
                ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE,
                JsonTypeInfo.As.PROPERTY);
        Grade grade = new Grade();
        List<Person> list = new ArrayList<>();
        list.add(new Child("Alice","man"));
        grade.setPersons(list);
        String json = objectMapper.writeValueAsString(grade);
        System.out.println(json);
        Grade o = objectMapper.readValue(json, Grade.class);
        System.out.println(o);
    }


    @Data
    @AllArgsConstructor
    @NoArgsConstructor
    public static class Grade {
        private List<Person> persons;
    }

    @Data
    @AllArgsConstructor
    public static class Person {
        private String name;
    }


    @Getter
    @Setter
    public static class Child extends Person {
        private String sex;

        public Child(String name, String sex) {
            super(name);
            this.sex = sex;
        }
    }
}

That's my variant of what I need. I'm going to save a generic list and then retrieve it again, and I want to save it in a simpler json format. Instead of a specialized set of two series

[cowtowncoder]

@superwen0001 So you didn't bother read (or use) any of my answer, and expect me or someone to help?

Good luck; maybe someone else wants to waste their time here.

[superwen0001]

When I use ObjectMapper.DefaultTyping.NON_FINAL, there are issues with serializing and deserializing a plain List. Is there a compatible way to handle this?

public class Test {
    public static void main(String[] args) throws JsonProcessingException {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.activateDefaultTyping(
                objectMapper.getPolymorphicTypeValidator(),
                ObjectMapper.DefaultTyping.NON_FINAL,
                JsonTypeInfo.As.PROPERTY);

        List<String> list1 = List.of("1", "2");
        String s = objectMapper.writeValueAsString(list1);
        System.out.println(s);
        list1 = objec

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Could not resolve type id '1' as a subtype of java.util.List<java.lang.Object>: no such class found
at [Source: (String)"["1","2"]"; line: 1, column: 6]

[superwen0001]

In my actual scenario, when I deserialize, I don't know what type I need to deserialize into. I am actually implementing MyBatis Plus's AbstractJsonTypeHandler.

@Override
public Object parse(String json) {
    ObjectMapper objectMapper = getObjectMapper();

    TypeFactory typeFactory = objectMapper.getTypeFactory();
    JavaType javaType = typeFactory.constructType(getFieldType());
    try {
        return objectMapper.readValue(json, javaType);
    } catch (JacksonException e) {
        log.error("deserialize json: " + json + " to " + javaType + " error ", e);
        throw new RuntimeException(e);
    }
}

[JooHyukKim]

Check above for answers