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

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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

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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

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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

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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

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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

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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() + "]";     }   }