Hey y’all,

Im am running a debug client in google chrome using tomcat and eclipse.
I also have an Arduino Uno with an Ethernet shield as a server. Also, my client
is written in java using GWT (Google Web Toolkit). When using
my debug client to send an API to my arduino server, i encounter to issues. 1) I get the below highlighted error 2) response.getStatusCode() r returns 0. I was wondering if someone could assist?

XMLHttpRequest cannot load http://arduinoServerAPIUSingItsIPADDY. No ‘Access-Control-Allow-Origin’ header is present on the requested resource. Origin ‘’ is therefore not allowed access.

Below is my java client code and my server code on my Arduino:

Java Client code:

RequestBuilder rb = new RequestBuilder(RequestBuilder.POST,
		rb.setHeader("Content-Type", "application/json");
		try {
			rb.sendRequest(null, new RequestCallback() {

				public void onError(Request request, Throwable exception) {
					System.out.println("Plan fail");

				public void onResponseReceived(Request request,
						Response response) {
					if (200 == response.getStatusCode()) {
						System.out.println("IT WORKED");
						if(response.getText() == "ON")
					} else {
						System.out.println("IT REPLIED BUT DIDNT WORK");
		} catch (RequestException e) {

Arduino server code sending json:

#include <SPI.h>
#include <Ethernet.h>

// MAC address from Ethernet shield sticker under board
byte mac[] = { 0xDE, 0xDE, 0xDE, 0xDE, 0xDE, 0xDE };
IPAddress ip(ARDUINOIPADDY); // IP address, may need to change depending on network
EthernetServer server(80);  // create a server at port 80
boolean incoming = 0;

String HTTP_req;          // stores the HTTP request
boolean LED_status = 0;   // state of LED, off by default

void setup()
    Ethernet.begin(mac, ip);  // initialize Ethernet device
    server.begin();           // start to listen for clients
    Serial.begin(9600);       // for diagnostics
    pinMode(2, OUTPUT);       // LED on pin 2
void loop(void) {
  // listen for incoming clients
  EthernetClient client = server.available();
  if (client) {
    Serial.println("new client");
    // an http request ends with a blank line
    boolean currentLineIsBlank = true;
    while (client.connected()) {
      if (client.available()) {
        char c =;
        // if you've gotten to the end of the line (received a newline
        // character) and the line is blank, the http request has ended,
        // so you can send a reply
     if (c == '\n' && currentLineIsBlank) {
          // send a standard http response header
          client.println("HTTP/1.1 200 OK");
          client.println("Content-Type: application/json;charset=utf-8");
          client.println("Server: Arduino");
          client.println("Connnection: close");

        if (c == '\n') {
          // you're starting a new line
          currentLineIsBlank = true;
        else if (c != '\r') {
          // you've gotten a character on the current line
          currentLineIsBlank = false;
    // give the web browser time to receive the data
    // close the connection:
    Serial.println("client disonnected");


I also wanted to add that the arduino server is receiving my clients message…it just doesn’t send a response of 200 and i get getting that “Access-Control-Allow-Origin” error.

In your JS code, you could change document.domain to the domain of the XMLHTTPRequest request. If you use Ajax in jQuery, you can use a number of methods to retrieve the file as certain scenarios are not bound by the 'same origin policy'.

Or if you have access to the remote server, send the appropriate header when you send the data:

header( 'Access-Control-Allow-Origin: *' );

This will allow cross-site access. For a single domain replace the star with the domain name ( without http & www parts ).