static String tenantId = "xxxxxxxxxxxxxxxxxxxxxxx";
static String username = "xxxxxxxxxxxxxxxxxxxxxxx";
static String password = "xxxxxxxxx";
static String clientId = "xxxxxxxxxxxxxxxxxxxxxxx";
static String resource = "https://dev.sharepoint.com";
static String userEmail = "xxxxxxxxxxxxxxxxxxxxxx";
static String clientSecret = "xxxxxxxxxxxxxxxxxxx";

public static void main(String[] args) throws MalformedURLException, IOException {
    AuthenticationContext authContext = null;
    AuthenticationResult authResult = null;
    ExecutorService service = null;

    try {
        service = Executors.newFixedThreadPool(1);
        String url = "https://login.microsoftonline.com/" + tenantId + "/oauth2/authorize";
        authContext = new AuthenticationContext(url, false, service);
        ClientCredential credential = new ClientCredential(clientId, clientSecret);
        Future<AuthenticationResult> future = authContext.acquireToken(resource, credential, null);

        authResult = future.get();
        System.out.println("get access token: n" + authResult.getAccessToken());
        System.out.println("get refresh token: n" + authResult.getRefreshToken());
    } catch (Exception ex) {
        ex.printStackTrace();
    }
    // get access token by refresh token
     getToken(authResult.getRefreshToken());
}

public static void getToken(String refreshToken) throws IOException {

    String encoding = "UTF-8";
    String params = "client_id=" + clientId + "&refresh_token=" + refreshToken
            + "&grant_type=refresh_token&resource=https%3A%2F%2Fdev.sharepoint.com";
    String path = "https://login.microsoftonline.com/" + tenantId + "/oauth2/token";
    byte[] data = params.getBytes(encoding);
    URL url = new URL(path);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setRequestMethod("POST");
    conn.setDoOutput(true);
    conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
    //conn.setRequestProperty("Content-Length", String.valueOf(data.length));
    conn.setConnectTimeout(5 * 1000);
    OutputStream outStream = conn.getOutputStream();
    outStream.write(data);
    outStream.flush();
    outStream.close();
    System.out.println(conn.getResponseCode());
    System.out.println(conn.getResponseMessage());

    BufferedReader br = null;
    if (conn.getResponseCode() != 200) {
        br = new BufferedReader(new InputStreamReader((conn.getErrorStream())));
    } else {
        br = new BufferedReader(new InputStreamReader((conn.getInputStream())));
    }
    System.out.println("Response body : " + br.readLine());
}