Can't serialize/deserialize classes implementing interface with GSON

Question

I'm trying to serialize a class that has a list of interface objects. I have added a type register that should get it to work, however I'm out of luck as it's still not working. Code below.

Rule:

public interface Rule extends Serializable {

    public String getName();

    public boolean setData(String data);
    public String getData();

}

RuleAbstract:

public abstract class RuleAbstract implements Rule {

    private static transient final long serialVersionUID = 1L;

    public RuleAbstract(){ }

}

ActionRule:

public interface ActionRule extends Rule {

    public void doAction(Player player);

}

One of the ActionRule classes:

public class DoCmd extends RuleAbstract implements ActionRule {

    private static transient final long serialVersionUID = 1L;

    private static DoCmd i = new DoCmd();
    public static DoCmd get() { return i; }

    private String cmd;

    public DoCmd(){

    }

    @Override
    public void doAction(Player player) {
        if(this.cmd == null) return;
        MixinCommand.get().dispatchCommand(player, this.cmd);
    }

    @Override
    public boolean setData(String data) {
        /** Must start with leading slash */
        if(!data.startsWith("/")) return false;
        this.cmd = data;
        return true;
    }

    @Override
    public String getData() {
        return this.cmd;
    }

}

And finally, the class that must be serialized & deserialized:

public class RuleList {

    List<Rule> rules = new ArrayList<Rule>();

}

All of this should work with my type adapter:

public class AdapterRule implements JsonDeserializer<Rule>, JsonSerializer<Rule> {

    private static AdapterRule i = new AdapterRule();
    public static AdapterRule get() { return i; }

    @Override
    public JsonElement serialize(Rule src, Type typeOf, JsonSerializationContext context) {
        if(src.getData() != null || src.getData() != ""){
            return new JsonPrimitive(String.format("%s %s", src.getName(), src.getData()));
        }
        return new JsonPrimitive(src.getName());
    }

    @Override
    public Rule deserialize(JsonElement src, Type typeOf, JsonDeserializationContext context) throws JsonParseException {
        String splitted[] = src.getAsJsonPrimitive().getAsString().split(" ");
        if(splitted.length == 1){
            return RuleAbstract.getRule(splitted[0]);
        } else {
            String tempData = "";
            for(String data : splitted){
                tempData = tempData + " " + data;
            }
            Rule rule = RuleAbstract.getRule(splitted[0]);
            rule.setData(tempData);
            return rule;
        }
    }

}

Could someone PLEASE point out what I'm doing wrong? Error I'm receiving:

java.lang.RuntimeException: Unable to invoke no-args constructor for interface some.package.Rule. Register an InstanceCreator with Gson for this type may fix this problem.

Answer 1

The GSON deserializer needs a default constructor on the class being deserialized, ie RuleList .

If you can't provide a default constructor then you need to register an InstanceCreator that will provide a new RuleList to the deserializer.