Question

I have never done much with serialization, but am trying to use Google’s gson to serialize a Java object to a file. Here is an example of my issue:

public interface Animal {
    public String getName();
}


 public class Cat implements Animal {

    private String mName = "Cat";
    private String mHabbit = "Playing with yarn";

    public String getName() {
        return mName;
    }

    public void setName(String pName) {
        mName = pName;
    }

    public String getHabbit() {
        return mHabbit;
    }

    public void setHabbit(String pHabbit) {
        mHabbit = pHabbit;
    }

}

public class Exhibit {

    private String mDescription;
    private Animal mAnimal;

    public Exhibit() {
        mDescription = "This is a public exhibit.";
    }

    public String getDescription() {
        return mDescription;
    }

    public void setDescription(String pDescription) {
        mDescription = pDescription;
    }

    public Animal getAnimal() {
        return mAnimal;
    }

    public void setAnimal(Animal pAnimal) {
        mAnimal = pAnimal;
    }

}

public class GsonTest {

public static void main(String[] argv) {
    Exhibit exhibit = new Exhibit();
    exhibit.setAnimal(new Cat());
    Gson gson = new Gson();
    String jsonString = gson.toJson(exhibit);
    System.out.println(jsonString);
    Exhibit deserializedExhibit = gson.fromJson(jsonString, Exhibit.class);
    System.out.println(deserializedExhibit);
}
}

So this serializes nicely — but understandably drops the type information on the Animal:

{"mDescription":"This is a public exhibit.","mAnimal":{"mName":"Cat","mHabbit":"Playing with yarn"}}

This causes real problems for deserialization, though:

Exception in thread "main" java.lang.RuntimeException: No-args constructor for interface com.atg.lp.gson.Animal does not exist. Register an InstanceCreator with Gson for this type to fix this problem.

I get why this is happening, but am having trouble figuring out the proper pattern for dealing with this. I did look in the guide but it didn’t address this directly.

Answer 1

Put the animal as transient, it will then not be serialized.

Or you can serialize it yourself by implementing defaultWriteObject(...) and defaultReadObject(...) (I think thats what they were called…)

EDIT See the part about “Writing an Instance Creator” here.

Gson cant deserialize an interface since it doesnt know which implementing class will be used, so you need to provide an instance creator for your Animal and set a default or similar.