Java JSON Stringifier with the Nashorn API

I have written a Java JSON stringifier with the Nashorn API. Since this code is slow, I wanted to ask if I can improve it. My target is the highest possible performance to make it an alternative to other JSON libraries.

JSON.java

package com.example.json;

import java.util.Map;

import javax.script.Invocable;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

import com.example.json.util.TextFile;

public class JSON {

    private static final Invocable inv = create();

    private static final Invocable create() {
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");
        try {
            engine.eval(TextFile.readFromResource("/js/json-stringify.js"));
        } catch (ScriptException e) {
            e.printStackTrace();
        }
        return (Invocable) engine;
    }

    public static final String stringify(Map<String, Object> json) {
        try {
            return (String) inv.invokeFunction("stringifyJSON", json);
        } catch (ScriptException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
        return null;
    }

}

JSONObject.java

package com.example.json.stringify;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class JSONObject {

    private Map<String, Object> map = new HashMap<String, Object>();

    public void appendChild(String key, Object object) {
        map.put(key, object);
    }

    public static final Map<String, Object> convertToJavaScript(JSONObject obj) {
        return obj.map;
    }

}

JSONArray.java

package com.example.json.stringify;

public class JSONArray {

    private Object[] array;

    public JSONArray(int length) {
        array = new Object[length];
    }

    public void set(int index, Object object) {
        if (index >= 0 && index <= array.length - 1) array[index] = object;
    }

    public static final Object[] convertToJavaScript(JSONArray arr) {
        return arr.array;
    }

}

/js/json-stringify.js

function stringifyJSON(json) {

    var JSONObject = Java.type("com.example.json.stringify.JSONObject");
    var JSONArray = Java.type("com.example.json.stringify.JSONArray");

    return JSON.stringify(convertJSONObject(json));

    function convert(json) {
        if (isJSONObject(json)) return convertJSONObject(JSONObject.convertToJavaScript(json));
        if (isJSONArray(json)) return convertJSONArray(JSONArray.convertToJavaScript(json));
        return json;
    }

    function convertJSONObject(object) {
        var jsobject = {};
        for (key in object) {
            jsobject[key] = convert(object.get(key));
        }
        return jsobject;
    }

    function convertJSONArray(array) {
        var jsarray = [];
        for (var index = 0, len = array.length; index < len; index++) {
            jsarray.push(convert(array[index]));
        }
        return jsarray;
    }

    function isJSONObject(json) Object.prototype.toString.call(json) === "[object com.example.json.stringify.JSONObject]";
    function isJSONArray(json) Object.prototype.toString.call(json) === "[object com.example.json.stringify.JSONArray]";

}

TextFile.java (from this post)

Example

I used the same example JSON from my JSON parser.

Map<String, Object> json = new HashMap<String, Object>();
JSONArray employees = new JSONArray(3);

JSONObject employee1 = new JSONObject();
employee1.appendChild("firstName", "John");
employee1.appendChild("lastName", "Doe");
employees.set(0, employee1);

JSONObject employee2 = new JSONObject();
employee2.appendChild("firstName", "Anna");
employee2.appendChild("lastName", "Smith");
employees.set(1, employee2);

JSONObject employee3 = new JSONObject();
employee3.appendChild("firstName", "Peter");
employee3.appendChild("lastName", "Jones");
employees.set(2, employee3);

json.put("employees", employees);

System.out.println(JSON.stringify(json));

Result

{"employees":[{"firstName":"John","lastName":"Doe"},{"firstName":"Anna","lastName":"Smith"},{"firstName":"Peter","lastName":"Jones"}]}