JSON-RPC with NodeMCU and ArduinoJSON

NodeMCU_JSON-RPC.ino

#include <ArduinoJson.h>
#include <ESP8266WiFi.h>          //ESP8266 Core WiFi Library (you most likely already have this in your sketch)
#include <DNSServer.h>            //Local DNS Server used for redirecting all requests to the configuration portal
#include <ESP8266WebServer.h>     //Local WebServer used to serve the configuration portal
#include <WiFiManager.h>          //https://github.com/tzapu/WiFiManager WiFi Configuration Magic

// change the IP address in the following line, should return "hello world" wrapped in JSON-RPC boilerplate
// curl -d '{"jsonrpc": "2.0", "method": "concatenate", "params": ["hello", "world"], "id": 5}' -H "Content-Type: application/json" -X POST http://192.168.0.24/api

ESP8266WebServer http_server(80);
volatile int counter=0;

void setup() {
  Serial.begin(115200);
  Serial.println("Configuring WiFi...");

  WiFiManager wifiManager;
  //wifiManager.resetSettings();
  wifiManager.autoConnect("SetupDevice");

  Serial.println("Setting up JSON-RPC server...");
  http_server.on("/api", HTTP_POST, handle_json );
  http_server.begin();
}

void loop() {
  // put your main code here, to run repeatedly:
  http_server.handleClient();
  counter++;
  delay(100);
}

void handle_json(){
  DynamicJsonBuffer jsonBuffer;
  JsonObject& root = jsonBuffer.parseObject(http_server.arg("plain"));

  Serial.println( "=====================" );
  Serial.println( String("jsonrpc: ") + root["jsonrpc"].as<String>() );
  Serial.println( String("id: ") + root["id"].as<String>() );
  Serial.println( String("method: ") + root["method"].as<String>() );
  Serial.println( String("params: ") + root["params"].as<String>() );

  int id = root["id"].as<int>();
  String k1 = root["params"][0].as<String>();
  String k2 = root["params"][1].as<String>();

  JsonObject& resp = jsonBuffer.createObject();
  resp["jsonrpc"] = "2.0";
  resp["id"] = id;
  resp["result"] = k1 + " " + k2;
  Serial.println("Response:");
  resp.printTo(Serial);

  String out;
  resp.printTo(out);
  http_server.send( 200, "application/json", out.c_str() );
}