PingClient.java

import org.java_websocket.client.WebSocketClient;
import org.java_websocket.handshake.ServerHandshake;

import java.net.URI;
import java.net.URISyntaxException;

public class PingClient extends WebSocketClient {
    public PingClient(URI uri) {
        super(uri);
    }

    @Override
    public void onOpen(ServerHandshake handshakedata) {
        send(Long.toString(System.currentTimeMillis()));
    }

    @Override
    public void onMessage(String message) {
        long current = System.currentTimeMillis();
        try {
            long client = Long.parseLong(message);
            System.out.println(current-client);
        } catch (Exception e) {
            //To handle
        }
    }

    @Override
    public void onClose(int code, String reason, boolean remote) {

    }

    @Override
    public void onError(Exception ex) {

    }

    public static void main(String[] args) throws URISyntaxException {
        WebSocketClient client = new PingClient(new URI("ws://localhost:8887"));
        client.connect();
    }
}

PingServer.java

import org.java_websocket.WebSocket;
import org.java_websocket.handshake.ClientHandshake;
import org.java_websocket.server.WebSocketServer;

import java.net.InetSocketAddress;

public class PingServer extends WebSocketServer {
    public PingServer(InetSocketAddress inetSocketAddress) {
        super(inetSocketAddress);
    }

    @Override
    public void onOpen(WebSocket conn, ClientHandshake handshake) {
        conn.send(Long.toString(System.currentTimeMillis()));
    }

    @Override
    public void onClose(WebSocket conn, int code, String reason, boolean remote) {

    }

    @Override
    public void onMessage(WebSocket conn, String message) {
        long current = System.currentTimeMillis();
        try {
            long client = Long.parseLong(message);
            System.out.println(current-client);
        } catch (Exception e) {
            //To handle
        }
    }

    @Override
    public void onError(WebSocket conn, Exception ex) {

    }

    @Override
    public void onStart() {

    }

    public static void main(String[] args) {
        String host = "localhost";
        int port = 8887;

        WebSocketServer server = new PingServer(new InetSocketAddress(host, port));
        server.run();

    }
}