Jackson 链自定义解串器

Question

If you have a class User :

class User {
    private String firstName;
    private String lastName;
    private Address address; //object
}

And a class Address respectively:

class Address {
    private String streetName;
    private String postCode;
    private AddressType addressType; //enum
}

And custom Jackson deserializers for both, is there a good way to link them? eg

class UserDeserialiser extends JsonDeserializer<User> {
    public User deserialize(JsonParser jp, DeserializationContext ctxt) {
        ObjectNode node = jp.getCodec().readTree(jp);

        User user = fetchUser();
        user.setFirstName(node.get("firstName").asText());
        user.setFirstName(node.get("lastName").asText());
        user.setAddress(???); // delegate to AddressDeserialiser here
    }
}

Similarly in the Address deserialiser, is it possible to delegate to the default one to handle the enum (as it might have custom mappings)?

class AddressDeserialiser extends JsonDeserializer<Address> {
    public User deserialize(JsonParser jp, DeserializationContext ctxt) {
        ObjectNode node = jp.getCodec().readTree(jp);

        Address user = fetchAddress();
        user.setStreetName(node.get("streetName").asText());
        user.setAddressType(???); // delegate to jackson default object mapper?
    }
}

Answer 1

_{I had the same requirement a while ago, and figured out a solution. I don't think it's a "nice" solution, because it's a bit clumsy, but maybe this encourages someone to write a better answer. That would be great.}

---

As mentioned in a comment, this can usually be solved by using annotations. Specifically, the @JacksonDeserialize annotations.

But if (for whatever reason - and there are many possible reasons for that) the use of annotations is not desired, it is necessary to explicitly create a parser for the relevant subtree of the JSON tree. This will pick up either the default deserializers or the custom deserializers that have been registered for the respective type in the module of the object mapper.

The relevant part/pattern referring to your example, namely to the point that contained the ??? question marks, is this:

JsonNode addressNode = node.get("address");
if (addressNode != null)
{
    JsonParser parser = addressNode.traverse();
    parser.setCodec(jp.getCodec());
    Address address = parser.readValueAs(Address.class);
    user.setAddress(address);
}

Here is a MCVE showing this approach in action:

import java.io.IOException;

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.node.ObjectNode;

class UserDeserialiser extends JsonDeserializer<User>
{
    @Override
    public User deserialize(JsonParser jp, DeserializationContext ctxt)
        throws IOException, JsonProcessingException
    {
        System.out.println("Deserializing User...");

        ObjectNode node = jp.getCodec().readTree(jp);

        User user = new User();
        user.setFirstName(node.get("firstName").asText());
        user.setLastName(node.get("lastName").asText());

        JsonNode addressNode = node.get("address");
        if (addressNode != null)
        {
            JsonParser parser = addressNode.traverse();
            parser.setCodec(jp.getCodec());
            Address address = parser.readValueAs(Address.class);
            user.setAddress(address);
        }
        return user;
    }
}

class AddressDeserialiser extends JsonDeserializer<Address>
{
    @Override
    public Address deserialize(JsonParser jp, DeserializationContext ctxt)
        throws IOException, JsonProcessingException
    {
        System.out.println("Deserializing Address...");

        ObjectNode node = jp.getCodec().readTree(jp);

        Address address = new Address();
        address.setStreetName(node.get("streetName").asText());
        address.setPostCode(node.get("postCode").asText());

        JsonNode addressTypeNode = node.get("addressType");
        if (addressTypeNode != null)
        {
            JsonParser parser = addressTypeNode.traverse();
            parser.setCodec(jp.getCodec());
            Address.AddressType addressType = 
                parser.readValueAs(Address.AddressType.class);
            address.setAddressType(addressType);
        }
        return address;
    }
}

public class NestedDeserializers
{
    public static void main(String[] args) throws IOException
    {

        User user = new User();
        user.setFirstName("A");
        user.setLastName("B");

        Address address = new Address();
        address.setStreetName("C");
        address.setPostCode("D");
        address.setAddressType(Address.AddressType.X);
        user.setAddress(address);

        ObjectMapper mapper = createObjectMapper();
        String jsonString = mapper.writeValueAsString(user);
        System.out.println("JSON string representation:\n" + jsonString);

        User readUser = mapper.readValue(jsonString, User.class);

        System.out.println("User     : " + user);
        System.out.println("Read user: " + readUser);
    }

    private static ObjectMapper createObjectMapper()
    {
        ObjectMapper mapper = new ObjectMapper();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);
        SimpleModule module = new SimpleModule();
        module.addDeserializer(User.class, new UserDeserialiser());
        module.addDeserializer(Address.class, new AddressDeserialiser());
        mapper.registerModule(module);
        return mapper;
    }

}


//=============================================================================
// Dummy User/Address classes below

class User
{
    private String firstName;
    private String lastName;
    private Address address;

    public String getFirstName()
    {
        return firstName;
    }

    public void setFirstName(String firstName)
    {
        this.firstName = firstName;
    }

    public String getLastName()
    {
        return lastName;
    }

    public void setLastName(String lastName)
    {
        this.lastName = lastName;
    }

    public Address getAddress()
    {
        return address;
    }

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

    @Override
    public String toString()
    {
        return "User [firstName=" + getFirstName() + ", lastName="
            + getLastName() + ", address=" + getAddress() + "]";
    }

}

class Address
{
    enum AddressType
    {
            X, Y;
    }

    private String streetName;
    private String postCode;
    private AddressType addressType;

    public String getStreetName()
    {
        return streetName;
    }

    public void setStreetName(String streetName)
    {
        this.streetName = streetName;
    }

    public String getPostCode()
    {
        return postCode;
    }

    public void setPostCode(String postCode)
    {
        this.postCode = postCode;
    }

    public AddressType getAddressType()
    {
        return addressType;
    }

    public void setAddressType(AddressType addressType)
    {
        this.addressType = addressType;
    }

    @Override
    public String toString()
    {
        return "Address [streetName=" + getStreetName() + ", postCode="
            + getPostCode() + ", addressType=" + getAddressType() + "]";
    }

}