Java Jackson - Json Polymorphism

from://http://www.studytrails.com/java/json/java-jackson-Serialization-polymorphism.jsp

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>

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() +  "]" ;
    }

}