Right way to write JSON deserializer in Spring or extend it

Question

I am trying to write a custom JSON deserializer in Spring. I want to use default serializer for most part of fields and use a custom deserializer for few properties. Is it possible? I am trying this way because, most part of properties are values, so for these I can let Jackson use default deserializer; but few properties are references, so in the custom deserializer I have to query a database for reference name and get reference value from database.

I'll show some code if needed.

Answer 1

I've searched a lot and the best way I've found so far is on this article:

Class to serialize

package net.sghill.example;

import net.sghill.example.UserDeserializer
import net.sghill.example.UserSerializer
import org.codehaus.jackson.map.annotate.JsonDeserialize;
import org.codehaus.jackson.map.annotate.JsonSerialize;

@JsonDeserialize(using = UserDeserializer.class)
public class User {
    private ObjectId id;
    private String   username;
    private String   password;

    public User(ObjectId id, String username, String password) {
        this.id = id;
        this.username = username;
        this.password = password;
    }

    public ObjectId getId()       { return id; }
    public String   getUsername() { return username; }
    public String   getPassword() { return password; }
}

Deserializer class

package net.sghill.example;

import net.sghill.example.User;
import org.codehaus.jackson.JsonNode;
import org.codehaus.jackson.JsonParser;
import org.codehaus.jackson.ObjectCodec;
import org.codehaus.jackson.map.DeserializationContext;
import org.codehaus.jackson.map.JsonDeserializer;

import java.io.IOException;

public class UserDeserializer extends JsonDeserializer<User> {

    @Override
    public User deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
        ObjectCodec oc = jsonParser.getCodec();
        JsonNode node = oc.readTree(jsonParser);
        return new User(null, node.get("username").getTextValue(), node.get("password").getTextValue());
    }
}

Edit: Alternatively you can look at this article which uses new versions of com.fasterxml.jackson.databind.JsonDeserializer.

Answer 2

I was trying to @Autowire a Spring-managed service into my Deserializer. Somebody tipped me off to Jackson using the new operator when invoking the serializers/deserializers. This meant no auto-wiring of Jackson's instance of my Deserializer. Here's how I was able to @Autowire my service class into my Deserializer:

context.xml

<mvc:annotation-driven>
  <mvc:message-converters>
    <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
      <property name="objectMapper" ref="objectMapper" />
    </bean>
  </mvc:message-converters>
</mvc>
<bean id="objectMapper" class="org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean">
    <!-- Add deserializers that require autowiring -->
    <property name="deserializersByType">
        <map key-type="java.lang.Class">
            <entry key="com.acme.Anchor">
                <bean class="com.acme.AnchorDeserializer" />
            </entry>
        </map>
    </property>
</bean>

Now that my Deserializer is a Spring-managed bean, auto-wiring works!

AnchorDeserializer.java

public class AnchorDeserializer extends JsonDeserializer<Anchor> {
    @Autowired
    private AnchorService anchorService;
    public Anchor deserialize(JsonParser parser, DeserializationContext context)
             throws IOException, JsonProcessingException {
        // Do stuff
    }
}

AnchorService.java

@Service
public class AnchorService {}

---

Update: While my original answer worked for me back when I wrote this, @xi.lin's response is exactly what is needed. Nice find!

Answer 3

With Spring MVC 4.2.1.RELEASE, you need to use the new Jackson2 dependencies as below for the Deserializer to work.

Dont use this

<dependency>  
            <groupId>org.codehaus.jackson</groupId>  
            <artifactId>jackson-mapper-asl</artifactId>  
            <version>1.9.12</version>  
        </dependency>  

Use this instead.

<dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-annotations</artifactId>
            <version>2.2.2</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
            <version>2.2.2</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.2.2</version>
        </dependency>  

Also use com.fasterxml.jackson.databind.JsonDeserializer and com.fasterxml.jackson.databind.annotation.JsonDeserialize for the deserialization and not the classes from org.codehaus.jackson