javascript 如何使用java服务器向特定的websocket连接发送消息

how to send message to particular websocket connection using java server

提问by manankhh

I am new to WebSockets.

我是 WebSockets 的新手。

I have already made a simple server-client chat in WebSockets.

我已经在 WebSockets 中进行了简单的服务器-客户端聊天。

And now I am trying to make client-server-client chat application.

现在我正在尝试制作客户端-服务器-客户端聊天应用程序。

I have a question that in java server how can we send a message to particular WebSocket connection.

我有一个问题，在 Java 服务器中，我们如何向特定的 WebSocket 连接发送消息。

If user-A want to send a message to User-B.

如果用户 A 想向用户 B 发送消息。

Then how can I manage that User-B is using this or that connection or send a message to that particular connection?

那么我如何管理用户 B 正在使用这个或那个连接或者向那个特定连接发送消息？

I am searching too much for this on google but could not find anything good.

我在谷歌上搜索了太多，但找不到任何好的东西。

回答by eepp

You have to design an architecture for that.

你必须为此设计一个架构。

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.

当客户端建立与服务器的连接（打开 WebSocket）时，服务器必须将连接保持在某处（无论您标识与您正在使用的 Java 后端的特定连接），数据结构将取决于你想做什么。一个好的标识符应该是用户提供的 ID（例如连接到同一服务器的另一个对等点尚未选择的昵称）。否则，只需使用套接字对象作为唯一标识符，并在前端列出其他用户时，将他们与他们的唯一标识符相关联，以便客户端可以向特定对等方发送消息。

A HashMapwould 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.

HashMap如果客户端要与另一个特定客户端聊天，A将是数据结构的不错选择，因为您可以将客户端的唯一 ID 映射到套接字并在哈希表中找到 O(1) 中的条目。

If you want to broadcast a message from a client to all other clients, although a HashMapwould 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.

如果您想从一个客户端向所有其他客户端广播消息，尽管 aHashMap也可以很好地工作（使用类似HashMap.values()），您可以使用一个简单的List，将传入消息发送到除原始发件人之外的所有连接的客户端。

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).

当然，您还希望在失去与数据结构的连接时从数据结构中删除客户端，这很容易使用 WebSocket（您使用的 Java 框架应该在套接字关闭时回调您）。

Here's an (almost complete) example using a Jetty9 WebSocket (and JDK 7):

这是一个使用Jetty9 WebSocket（和 JDK 7）的（几乎完整的）示例：

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.

该代码是不言自明的。关于 JSON 格式化和解析，Jetty 在 package 中有一些有趣的实用程序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).

另请注意，如果您的 WebSocket 服务器框架不是线程安全的，您将需要同步数据结构以确保没有数据损坏（此处MyWebSocket.sockets）。