北野

Jackson: Json Polymorphism

Jackson provides a way to maintain sub type information while serializing java objects. It is possible to recreate the exact sub type. The type information can be embedded into the json as a property. In the example below we create a zoo, that has a list of animals. The animal may be an elephant or a lion, and they both extend the Animal abstract class. While deserializing we want to create the exact animal type. We also demonstrate the use of @JsonTypeInfo and @JsonSubTypes annotations.

Data Serialization and Polymorphism Example

package com.studytrails.json.jackson;

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class SerializeExample1 {
    private static String outputFile = "zoo.json";

    public static void main(String[] args) throws JsonGenerationException, JsonMappingException, IOException {
        // let start creating the zoo
        Zoo zoo = new Zoo("Samba Wild Park", "Paz");
        Lion lion = new Lion("Simba");
        Elephant elephant = new Elephant("Manny");
        List<Animal> animals = new ArrayList<>();
        animals.add(lion);
        animals.add(elephant);
        zoo.setAnimals(animals);

        ObjectMapper mapper = new ObjectMapper();
        mapper.writerWithDefaultPrettyPrinter().writeValue(new FileWriter(new File(outputFile)), zoo);
    }
}

Before we look at the various classes, lets also see how to deserialize this

package com.studytrails.json.jackson;

import java.io.File;
import java.io.IOException;

import org.apache.commons.io.FileUtils;

import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class DeSerializeExample1 {

    public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {
        ObjectMapper mapper = new ObjectMapper();
        Zoo zoo = mapper.readValue(FileUtils.readFileToByteArray(new File("zoo.json")), Zoo.class);
        System.out.println(zoo);
    }
}

Zoo class

package com.studytrails.json.jackson;

import java.util.List;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.annotation.JsonTypeInfo.As;

@JsonTypeInfo(use = JsonTypeInfo.Id.MINIMAL_CLASS, include = As.PROPERTY, property = "@class")
public class Zoo {

    public String name;
    public String city;
    public List<Animal> animals;

    @JsonCreator
    public Zoo(@JsonProperty("name") String name, @JsonProperty("city") String city) {
        this.name = name;
        this.city = city;
    }

    public void setAnimals(List<animal> animals) {
        this.animals = animals;
    }

    @Override
    public String toString() {
        return "Zoo [name=" + name + ", city=" + city + ", animals=" + animals + "]";
    }

}

Animal Abstract class

package com.studytrails.json.jackson;

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonSubTypes.Type;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.annotation.JsonTypeInfo.As;

@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = As.PROPERTY, property = "@class")
@JsonSubTypes({ @Type(value = Lion.class, name = "lion"), @Type(value = Elephant.class, name = "elephant") })
public abstract class Animal {
    @JsonProperty("name")
    String name;
    @JsonProperty("sound")
    String sound;
    @JsonProperty("type")
    String type;
    @JsonProperty("endangered")
    boolean endangered;

}

Lion class

package com.studytrails.json.jackson;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;

public class Lion extends Animal {

    private String name;

    @JsonCreator
    public Lion(@JsonProperty("name") String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public String getSound() {
        return "Roar";
    }

    public String getType() {
        return "carnivorous";
    }

    public boolean isEndangered() {
        return true;
    }

    @Override
    public String toString() {
        return "Lion [name=" + name + ", getName()=" + getName() + ", getSound()=" + getSound() + ", getType()=" + getType() + ", isEndangered()="
                + isEndangered() + "]";
    }

}

Elephant class

package com.studytrails.json.jackson;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;

public class Elephant extends Animal {

    @JsonProperty
    private String name;

    @JsonCreator
    public Elephant(@JsonProperty("name") String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public String getSound() {
        return "trumpet";
    }

    public String getType() {
        return "herbivorous";
    }

    public boolean isEndangered() {
        return false;
    }

    @Override
    public String toString() {
        return "Elephant [name=" + name + ", getName()=" + getName() + ", getSound()=" + getSound() + ", getType()=" + getType()
                + ", isEndangered()=" + isEndangered() + "]";
    }

}

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