[Jackson] Json to Java Conversion with Ploymorphism

Converting Json String to Java Object is never straight forward, especially when polymorphism is involved. Consider the below interface:

public interface Animal {
    int getLegs();
    String getSpecies();
    String getCommonName();
}

It has two sub classes:

public class Bird implements Animal {
    private final String species;
    private final String commonName;
    private final int legs;
    public Bird(String species, String commonName, int legs) {
	this.species = species;
	this.commonName = commonName;
	this.legs = legs;
    }
    public Bird() {
	this(null, null, 0);
    }
    @Override
    public int getLegs() {
	return legs;
    }
    @Override
    public String getSpecies() {
	return species;
    }
    @Override
    public String getCommonName() {
	return commonName;
    }
    @Override
    public int hashCode() {
	final int prime = 31;
	int result = 1;
	result = prime * result + ((commonName == null) ? 0 : commonName.hashCode());
	result = prime * result + legs;
	result = prime * result + ((species == null) ? 0 : species.hashCode());
	return result;
    }
    @Override
    public boolean equals(Object obj) {
	if (this == obj)
	    return true;
	if (obj == null)
	    return false;
	if (getClass() != obj.getClass())
	    return false;
	Bird other = (Bird) obj;
	if (commonName == null) {
	    if (other.commonName != null)
		return false;
	} else if (!commonName.equals(other.commonName))
	    return false;
	if (legs != other.legs)
	    return false;
	if (species == null) {
	    if (other.species != null)
		return false;
	} else if (!species.equals(other.species))
	    return false;
	return true;
    }
}

public class Lizard implements Animal {
    private final String species;
    private final String commonName;
    private final int legs;
    public Lizard(String species, String commonName, int legs) {
	this.species = species;
	this.commonName = commonName;
	this.legs = legs;
    }
    public Lizard() {
	this(null, null, 0);
    }
    @Override
    public int getLegs() {
	return legs;
    }
    @Override
    public String getSpecies() {
	return species;
    }
    @Override
    public String getCommonName() {
	return commonName;
    }
    @Override
    public int hashCode() {
	final int prime = 31;
	int result = 1;
	result = prime * result + ((commonName == null) ? 0 : commonName.hashCode());
	result = prime * result + legs;
	result = prime * result + ((species == null) ? 0 : species.hashCode());
	return result;
    }
    @Override
    public boolean equals(Object obj) {
	if (this == obj)
	    return true;
	if (obj == null)
	    return false;
	if (getClass() != obj.getClass())
	    return false;
	Lizard other = (Lizard) obj;
	if (commonName == null) {
	    if (other.commonName != null)
		return false;
	} else if (!commonName.equals(other.commonName))
	    return false;
	if (legs != other.legs)
	    return false;
	if (species == null) {
	    if (other.species != null)
		return false;
	} else if (!species.equals(other.species))
	    return false;
	return true;
    }
}

We now will try to convert a Json String to a Java Object:

	String lizardJson = "{"species":"Varanus bengalensis","commonName":"Bengal monitor","legs":4}";
	ObjectMapper mapper = new ObjectMapper();
	    mapper.readValue(lizardJson, Animal.class);

The full test can be found here: https://github.com/paawak/blog/blob/master/code/json-inheritance-demo/src/test/java/com/swayam/demo/json/polymorphic/noannotation/PolymorphicJsonTest.java
This will immediately throw the below exception:

com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of com.swayam.demo.json.polymorphic.noannotation.Animal, problem: abstract types either need to be mapped to concrete types, have custom deserializer, or be instantiated with additional type information
 at [Source: {"species":"Copsychus saularis","commonName":"Doel, Oriental magpie-robin","legs":2}; line: 1, column: 1]
	at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148)
	at com.fasterxml.jackson.databind.DeserializationContext.instantiationException(DeserializationContext.java:857)
	at com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:139)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3564)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2580)

The reason is, the Json String does not have any information about the specific subclass. This can be very easily overcome by forcing the type information while serializing Java to Json, by using the @JsonTypeInfo annotation. This should be placed on the interface or the abstract class as shown below:

@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY, property = "type")
public interface Animal {
    int getLegs();
    String getSpecies();
    String getCommonName();
}

Now, the below code would work:

Animal expectedLizard = new Lizard("Varanus bengalensis", "Bengal monitor", 4);
	String lizardJson = "{"type":"com.swayam.demo.json.polymorphic.withannotation.Lizard","species":"Varanus bengalensis","commonName":"Bengal monitor","legs":4}";
	ObjectMapper mapper = new ObjectMapper();
	Animal lizard = mapper.readValue(lizardJson, Animal.class);
	assertEquals(expectedLizard, lizard);

The full test can be found here: https://github.com/paawak/blog/blob/master/code/json-inheritance-demo/src/test/java/com/swayam/demo/json/polymorphic/withannotation/PolymorphicJsonTest.java
The entire source code for this can be found here: https://github.com/paawak/blog/tree/master/code/json-inheritance-demo