如何发送哈希图运行方法？

Question

I am writting java client and server in which I want to maintain hash map between multiple threads.I have written following code but I am unnable to access updated code in run method.Also are concurrent hash maps more useful in such scenario? Any tips will be very helpful.Thanks in advance

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Map;

import org.json.simple.JSONObject;


public class JavaServer extends Thread{

    //To access packet in run method
    static byte[] receiveData=new byte[2048];
    DatagramPacket receivePacket = new DatagramPacket(receiveData,
            receiveData.length);
    JavaGenericCryptoLibrary cryptoObj=new JavaGenericCryptoLibrary();
    static JavaBuisinessFunctionsServer objB=new JavaBuisinessFunctionsServer();
    static DatagramSocket ds;
    static Map<String,String>  hm = new HashMap<String,String>();
    public JavaServer(DatagramSocket ds, Map<String,String> hm1) {
        JavaServer.ds=ds;
        JavaServer.hm=hm1;
    }


    public static void main(String[] args)
    {
        @SuppressWarnings("unused")
        JSONObject receivedObj = null;
        try {
            // Port Number for login
            int serverPort_login = 3964;
            int serverPort_requests = 3764;

            DatagramSocket serverSocket = new DatagramSocket(serverPort_login);
            DatagramSocket serverSocket_clients = new DatagramSocket(serverPort_requests);
            DatagramPacket receivePacket = new DatagramPacket(receiveData,
                    receiveData.length);

            hm.put("Current_User","Blank");
            // Server initialized
            System.out.println("Intilized Server");
            while(true)
            {   
                hm.remove("Current_User");
                hm.put("Current_User","Blank");
                serverSocket.receive(receivePacket);
                hm=(HashMap<String, String>) objB.login(serverSocket,receivePacket.getAddress(),receivePacket.getPort(),(HashMap<String, String>) hm);
                System.out.println("current user at server"+hm.get("Current_User"));
                //JavaServer r = new JavaServer(serverSocket_clients,hm);
                System.out.println("before thread:"+JavaServer.hm.get("Current_User"));
                System.out.println("before thread:"+JavaServer.hm.get(JavaServer.hm.get("Current_User")));

                new JavaServer(serverSocket_clients, hm).start();
            }
        } catch (Exception e) {
            System.out.println("Server" + e);
        } 
    }

    @SuppressWarnings("unchecked")
    public void run()
    {
        try {
            // Packet received on the thread.
            // Respective function will be initialized 
            JSONObject online_user_json_obj=new JSONObject();
            while(true)
            {   
                ds.receive(receivePacket);
                JSONObject t=cryptoObj.readRecievedPacket(receivePacket);
                switch((String)t.get("Command"))
                {
                case "Chat":

                    break;
                case "Logout":

                    break;
                case "List":

                    Enumeration<String> online_users = cryptoObj.get_Online_Users();
                    online_user_json_obj.put("List", online_users);
                    String user=hm.get("Current_User");
                    System.out.println("user "+user);
                    String ip_port=hm.get(user);
                    System.out.println("ip_port"+ip_port);
                                    //Here I amnot getting updated values
                    String[] ip_port_arr=ip_port.split(" ");
                    System.out.println(ip_port_arr[0]);
                    InetAddress ia=InetAddress.getByName(ip_port_arr[0]);
                    int port=Integer.getInteger(ip_port_arr[1]);
                    ds.send(cryptoObj.udpPacketGenerator(online_user_json_obj,ia,port));
                    break;
                default:
                    System.out.println("Rogue Data");
                    break;
                }

            } 
        }catch (Exception e) {
            e.printStackTrace();
        }   

    }
}

Answer 1

I think you need to look at a book like Lea's Concurrent Programming in Java. One thread needs to become the object's monitor. Then all the servers can call a wait() on the HashMap and be woken up after the monitor is notify(). You then call notifyAll() from the HashMap's monitor thread when the HashMap is modified. See java.lang.Object notify(), wait() and notifyAll() for information on how to set this up.

Basically, you'll have to designate one thread the monitor of the object. Then, when that thread is woken by notify() you'll notify all the other threads.

Answer 2

ConcurrentHashMap will be helpful in your case. Else you can use a lock and guard the access to the hashMap that way, the changes will be visible to all threads.