validate json by reflection before using gson

AttributesExample.java

public class AttributesExample {
    private String str = "foo";
    private double dbl = 0.8;
    private boolean bool = false;
}

Main.java

import com.google.gson.*;
import java.lang.reflect.Field;


public class Main {

    public static void main(String[] args) {
        Gson gson = new GsonBuilder().create();

        String json = "{" +
                "\"str\": \"hi\", " +
                "\"dbl\": 2.0, " +
                "\"bool\": true " +
                "}";

        if (isValid(AttributesExample.class, json)) {
            AttributesExample attributesExample = gson.fromJson(json, AttributesExample.class);
            // ...
        } else {
            // json not valid
        }
    }

    private static boolean isValid(Class cls, String jsonString) {
        JsonObject jsonObj = new JsonParser().parse(jsonString).getAsJsonObject();
        
        if (jsonObj.entrySet().size() > cls.getDeclaredFields().length) { // if the json-string has more entries than class has fields, we're already out
            return false;
        }

        for(Field field : cls.getDeclaredFields()) {
            if (!jsonObj.has(field.getName())) { // if the json-string has a key that doesn't have a corresponding field in the class, we're out
                return false;
            }

            JsonElement value = jsonObj.get(field.getName());

            switch (field.getType().getTypeName()) {
                case "java.lang.String":
                    // nothing to check, everything is ok as string
                    break;
                case "java.lang.Double":
                    // check if its a double by try-parsing value, if not we're out
                    break;
                case "java.lang.Boolean":
                    // check if its a boolean by try-parsing value, if not we're out
                    break;
            }
        }

        return true;
    }
}

Hi @Vinod-Kumar-S, thanks, I am glad it's useful for you. Regarding your question, I wrote this 6 years ago, I don't even remember anymore what I created this for back then 😉 I am not sure what you mean about giving examples. The switch case seems to speak for itself by going through the different types that the field can have 🤔 I guess my intention with those comments was to try parsing these values as Double/Boolean to see if that works or creates an exception.

Thank You for the help Benjamin.