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1. 在pom文件中添加依赖

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-websocket</artifactId>
</dependency>

2. java后台

@Component
@ServerEndpoint(value = "/webSocketToUser/{id}", configurator = WebSocketConfig.class)
public class WebSocketToUser {

    //静态变量，用来记录当前在线连接数。应该把它设计成线程安全的。
    private static int onlineCount = 0;

    private static ConcurrentHashMap<String, WebSocketToUser> webSocketSet = new ConcurrentHashMap<>();
    //与某个客户端的连接会话，需要通过它来给客户端发送数据
    private static Session session;

    //指定的sid，具有唯一性
    private static String sid = "";


    /**
     * 连接建立成功调用的方法
     */
    @OnOpen
    public void onOpen(@PathParam("id") String id, Session session, EndpointConfig config) {
        //获取WebsocketConfig.java中配置的“sessionId”信息值
        String httpSessionId = (String) config.getUserProperties().get("sessionId");
        this.session = session;
        this.sid = id;

        webSocketSet.put(sid, this);     //加入set中
        addOnlineCount();           //在线数加1
        System.out.println("用戶" + sid + "加入！当前在线人数为" + getOnlineCount());
    }

    /**
     * 连接关闭调用的方法
     */
    @OnClose
    public void onClose() {
        webSocketSet.remove(this);  //从set中删除
        subOnlineCount();           //在线数减1
    }

    /**
     * 收到客户端消息后调用的方法
     *
     * @param message 客户端发送过来的消息
     */
    @OnMessage
    public void onMessage(String message, Session session) {
        System.err.println("来自客户端的消息:" + message);
    }

    /**
     * @param session
     * @param error
     */
    @OnError
    public void onError(Session session, Throwable error) {
        error.printStackTrace();
    }

    /**
     * 实现服务器主动推送
     */
    public void sendMessage(String message) throws IOException {
        this.session.getBasicRemote().sendText(message);
    }
    
    public boolean sendToUser(String message, String sendUserId) throws IOException {
        if (webSocketSet.get(sendUserId) != null) {
            if (!sid.equals(sendUserId)) {
                return false;
                //webSocketSet.get(sendUserId).sendMessage("用户" + sendUserId + "发来消息：" + message);
            } else {
                webSocketSet.get(sendUserId).sendMessage(message);
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                return true;
            }
        } else {
            //如果用户不在线则返回不在线信息给自己
            return false;
        }
    }


    public static synchronized int getOnlineCount() {
        return onlineCount;
    }

    public static synchronized void addOnlineCount() {
        WebSocketToUser.onlineCount++;
    }

    public static synchronized void subOnlineCount() {
        WebSocketToUser.onlineCount--;
    }
}

3. 配置类

@Component
public class WebSocketConfig extends ServerEndpointConfig.Configurator{

    @Override
    /**
     * 修改握手信息
     */
    public void modifyHandshake(ServerEndpointConfig sec, HandshakeRequest request, HandshakeResponse response) {
        StandardSessionFacade ssf = (StandardSessionFacade) request.getHttpSession();
        if (ssf != null) {
            HttpSession httpSession = (HttpSession) request.getHttpSession();
            //关键操作
            sec.getUserProperties().put("sessionId", httpSession.getId());
            //log.info("获取到的SessionID：" + httpSession.getId());
        }
        super.modifyHandshake(sec, request, response);

    }

    @Bean
    public ServerEndpointExporter serverEndpointConfigurator(){
        return new ServerEndpointExporter();
    }
}

4. 安全框架拦截器放行webScoket
   
5. 前端部分

mounted () {
   // 初始化
   this.init()
 },
 
init: function () {
      if (typeof (WebSocket) === "undefined") {
        alert("您的浏览器不支持socket")
      } else {
        // 实例化socket
        //userId是你要发送的用户id
        this.socket = new WebSocket("ws://localhost:8080/webSocketToUser/" + userId)
        // 监听socket连接
        this.socket.onopen = this.open
        // 监听socket错误信息
        this.socket.onerror = this.error
        // 监听socket消息
        this.socket.onmessage = this.getMessage
      }
    },
    open: function () {
      console.log("socket连接成功")
    },
    error: function () {
      console.log("连接错误")
    },
    getMessage: function (msg) {
      this.openMsg(msg.data)
    },
    send: function () {
      this.socket.send(params)
    },
    close: function () {
      console.log("socket已经关闭")
    },
    openMsg (msg) {
      let _self = this;
      this.$notify({
        title: '提示',
        message: msg,
        duration: 0,
        type: 'warning',
        onClose: function () {
          }))
        }
      });
    }

6.注入webSocket, 调用方法

@Autowired
private WebSocketToUser webSocketToUser;

webSocketToUser.sendToUser(自定义消息, 发送用户id);