网上有很多方法，这里写一下我使用到的方法。
官方文档：
https://docs.spring.io/spring/docs/5.2.5.RELEASE/spring-framework-reference/web.html#websocket

环境

1）在build.gradle添加引用compile("org.springframework.boot:spring-boot-starter-websocket")
2）还要有2个JS文件：sockjs.min.js和stomp.min.js

配置

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

    @Override
    public void configureMessageBroker(MessageBrokerRegistry registry) {
        registry.enableSimpleBroker("/topic", "/queue");
        registry.setApplicationDestinationPrefixes("/app");
        registry.setUserDestinationPrefix("/user/");
    }

    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
 registry.addEndpoint("/myEndPoint").setAllowedOrigins().withSockJS();
    }
}

js文件

if('WebSocket' in window){
                var socket = new SockJS('/myEndPoint');
                websocket = Stomp.over(socket);
                websocket.connect({},function(frame){
                    send();
websocket.subscribe('/topic/newMsg',function(response){
                        var responseData = JSON.parse(response.body);
                        // do your sth.
                    });
                },function(err){
                    console.log(err);
                });
            }
            
           function send(){
               websocket.send("/app/msg",{}, "");
           }
           websocket.onopen = function (e) {
              console.log(e);
           };
           websocket.onmessage = function (e) {
              console.log(e)
           };
           websocket.onerror = function (e) {
              console.log(e);
           };
           websocket.onclose = function (e) {
              console.log(e);
           }

接收端

@MessageMapping("/msg") 
public void msg() {
    // do your sth.
}

发送端

@Autowired
private SimpMessagingTemplate messagingTemplate;

//.....
messagingTemplate.convertAndSend("/topic/newMsg", obj);

上面这些配置是所有人都收到，如果只想发给某个人呢？
https://stackoverflow.com/questions/22367223/sending-message-to-specific-user-on-spring-websocket

发给某个人的配置

【如果要发给个人，一定是配置了Security，这样才有用户的概念】
JS（要注意url加了前缀“/user”）
websocket.subscribe('/user/topic/newMsg',function(response){});
接收端

@MessageMapping("/msg") 
public void msg(Principal principal) {
String userId = principal.getName();
// 将这个userId在convertAndSendToUser方法中要用到
// do your sth.
}

发送端

// 这里的userId就是上面的那个，或者根据自己的实际情况用其他方法传递
messagingTemplate.convertAndSendToUser(userId,"/topic/newMsg", obj);