how to send message to particular websocket connection using java server

You have to design an architecture for that.

When a client establishes a connection with the server (opens the WebSocket), the server has to keep the connection somewhere (howsoever you're identifying a specific connection with the Java backend you're using), in a data structure that will depend on what you're trying to do. A good identifier would be an ID the user provides (like a nickname that's not already picked by another peer connected to the same server). Otherwise, simply use the socket object as a unique identifier and, when listing other users on the frontend, associate them with their unique identifier so that a client can send a message to a specific peer.

A HashMap would be a good choice for a data structure if a client is going to chat with another specific client, as you can map the unique ID of a client to the socket and find an entry with in O(1) in a hash table.

If you want to broadcast a message from a client to all other clients, although a HashMap would also work pretty well (with something like HashMap.values()), you may use a simple List, sending the incoming message to all connected clients except the original sender.

Of course, you also want to remove a client from the data structure when you lose connection with it, which is easy using a WebSocket (the Java framework you are using should call you back when a socket closes).

Here's an (almost complete) example using a Jetty 9 WebSocket (and JDK 7):

package so.example;
import java.io.IOException;
import java.util.HashMap;

import org.eclipse.jetty.websocket.api.Session;
import org.eclipse.jetty.websocket.api.annotations.OnWebSocketClose;
import org.eclipse.jetty.websocket.api.annotations.OnWebSocketConnect;
import org.eclipse.jetty.websocket.api.annotations.OnWebSocketMessage;
import org.eclipse.jetty.websocket.api.annotations.WebSocket;

@WebSocket
public class MyWebSocket {
    private final static HashMap<String, MyWebSocket> sockets = new HashMap<>();
    private Session session;
    private String myUniqueId;

    private String getMyUniqueId() {
        // unique ID from this class' hash code
        return Integer.toHexString(this.hashCode());
    }

    @OnWebSocketConnect
    public void onConnect(Session session) {
        // save session so we can send
        this.session = session;

        // this unique ID
        this.myUniqueId = this.getMyUniqueId();

        // map this unique ID to this connection
        MyWebSocket.sockets.put(this.myUniqueId, this);

        // send its unique ID to the client (JSON)
        this.sendClient(String.format("{\"msg\": \"uniqueId\", \"uniqueId\": \"%s\"}",
                this.myUniqueId));

        // broadcast this new connection (with its unique ID) to all other connected clients
        for (MyWebSocket dstSocket : MyWebSocket.sockets.values()) {
            if (dstSocket == this) {
                // skip me
                continue;
            }
            dstSocket.sendClient(String.format("{\"msg\": \"newClient\", \"newClientId\": \"%s\"}",
                    this.myUniqueId));
        }
    }

    @OnWebSocketMessage
    public void onMsg(String msg) {
        /*
         * process message here with whatever JSON library or protocol you like
         * to get the destination unique ID from the client and the actual message
         * to be sent (not shown). also, make sure to escape the message string
         * for further JSON inclusion. 
         */
        String destUniqueId = ...;
        String escapedMessage = ...;

        // is the destination client connected?
        if (!MyWebSocket.sockets.containsKey(destUniqueId)) {
            this.sendError(String.format("destination client %s does not exist", destUniqueId));
            return;
        }

        // send message to destination client
        this.sendClient(String.format("{\"msg\": \"message\", \"destId\": \"%s\", \"message\": \"%s\"}",
                destUniqueId, escapedMessage));
    }

    @OnWebSocketClose
    public void onClose(Session session, int statusCode, String reason) {
        if (MyWebSocket.sockets.containsKey(this.myUniqueId)) {
            // remove connection
            MyWebSocket.sockets.remove(this.myUniqueId);

            // broadcast this lost connection to all other connected clients
            for (MyWebSocket dstSocket : MyWebSocket.sockets.values()) {
                if (dstSocket == this) {
                    // skip me
                    continue;
                }
                dstSocket.sendClient(String.format("{\"msg\": \"lostClient\", \"lostClientId\": \"%s\"}",
                        this.myUniqueId));
            }
        }
    }

    private void sendClient(String str) {
        try {
            this.session.getRemote().sendString(str);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void sendError(String err) {
        this.sendClient(String.format("{\"msg\": \"error\", \"error\": \"%s\"}", err));
    }
}

The code is self explanatory. About JSON formatting and parsing, Jetty has some interesting utilities within package org.eclipse.jetty.util.ajax.

Also note that if your WebSocket server framework is not thread-safe, you will need to synchronize the data structure to make sure there's no data corruption (here MyWebSocket.sockets).