Java Junit test HTTP POST request

I need to test the following method with out altering the method itself. The method makes a POST method to a server. But I need to make a test case that's independent from the server.

I tested a similar method before redirecting it to a local file. But that for that I was giving protocol as file, hostname as localhost and port as -1.

My problem is that this method does a post and casts to HttpURLConnection and wr = new DataOutputStream(conn.getOutputStream()); wont work on an local txt file through http.

//the constructor

public HTTPConnector(String usr, String pwd, String protocol,
            String hostname, int port) {
        this.usr = usr;
        this.pwd = pwd;
        this.protocol = protocol;
        this.hostname = hostname;
        this.port = port;

        // connect();
    }

//the method I need to test

public String doPost(String reference, String data) throws IOException {
        URL url = null;
        HttpURLConnection conn = null;
        BufferedReader rd = null;
        DataOutputStream wr = null;
        InputStream is = null;
        String line = null;
        StringBuffer response = null;

        url = new URL(protocol, hostname, port, reference);
        conn = (HttpURLConnection) url.openConnection();

        conn.setRequestMethod("POST");

        conn.setRequestProperty("Authorization", "Basic dGVzdDphc2Rm");
        conn.setRequestProperty("Content-Type", "application/xml");

        conn.setUseCaches(false);
        conn.setDoInput(true);
        conn.setDoOutput(true);

        // Send response
        wr = new DataOutputStream(conn.getOutputStream());
        wr.writeBytes(data);
        wr.flush();
        wr.close();

        // Get response
        is = conn.getInputStream();
        rd = new BufferedReader(new InputStreamReader(is));
        response = new StringBuffer();
        while ((line = rd.readLine()) != null) {
            response.append(l