[JAVA] Corresponds to a property whose type is an array when empty using JsonDeserializer

Overview

There is an API that returns JSON with properties of object type if there is a value and array type if it is empty, and by default the types do not match and cannot be bound to the class.

Caused by: com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `net.yotama.sample.resource.Resource$Member` out of START_ARRAY token
 at [Source: (String)"{"member": []}"; line: 1, column: 12](through reference chain: net.yotama.sample.resource.Resource["member"])

Use Jackson's JsonDeserializer and replace it with null when it is an array so that it can be bound to the resource class.

Assumed JSON response

If there is a value

{
    "member": {
        "name": "yotama"
    }
}

If there is no value

{
    "member": []
}

environment

• Java 8
• Jackson 2.9.4
• Lombok 1.16.20

Implementation

Resource class

Resource.java

package net.yotama.sample.resource;

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.TreeNode;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import lombok.Value;

import java.io.IOException;

@Value
public class Resource {
    @JsonDeserialize(using = MemberDeserializer.class)
    Member member;

    @Value
    public static class Member {
        String name;
    }

    public static class MemberDeserializer extends JsonDeserializer<Member> {
        private ObjectMapper objectMapper = new ObjectMapper();

        @Override
        public Member deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
            TreeNode treeNode = p.getCodec().readTree(p);
            return treeNode.isObject() ? objectMapper.readValue(treeNode.toString(), Member.class) : null;
        }
    }
}

lombok.config

#I'm using Lombok for simplicity,
# 1.16.Used by Jackson from 20@ConstructorProperties will not be assigned, so set it provisionally
lombok.anyConstructor.addConstructorProperties=true

Operation check

package net.yotama.sample.resource;

import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.RequiredArgsConstructor;
import org.junit.experimental.theories.DataPoints;
import org.junit.experimental.theories.Theories;
import org.junit.experimental.theories.Theory;
import org.junit.runner.RunWith;

import java.io.IOException;

import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;

@RunWith(Theories.class)
public class ResourceTest {
    private ObjectMapper objectMapper = new ObjectMapper();

    @DataPoints
    public static Fixture[] data = {
            new Fixture("{\"member\": []}", new Resource(null)),
            new Fixture("{\"member\": null}", new Resource(null)),
            new Fixture("{\"member\": {\"name\": \"hoge\"}}", new Resource(new Resource.Member("hoge"))),
    };

    @Theory
    public void test(Fixture fixture) throws IOException {
        assertThat(objectMapper.readValue(fixture.json, Resource.class), is(fixture.expected));
    }

    @RequiredArgsConstructor
    public static class Fixture {
        final String json;
        final Resource expected;
    }
}