Tomcat中的WebSocket是如何实现的？

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-WebSocket</artifactId>
    <version>3.0.4</version>
</dependency>

@Configuration
public class WebSocketConfig {
    @Bean
    public ServerEndpointExporter serverEndpointExporter() {
        return new ServerEndpointExporter();
    }
}

@ServerEndpoint("/ws/{id}")
@Component
public class WebSocketServer {

    private static final Map<Long, Session> map = new ConcurrentHashMap<>();

    @OnOpen
    public void open(@PathParam("id") Long id, Session session) {
        map.put(id, session);
        System.out.println(id + " 建立连接");
    }

    @OnClose
    public void close(@PathParam("id") Long id) {
        map.remove(id);
        System.out.println(id + " 关闭连接");
    }

    @OnMessage
    public void msg(String msg, Session session) {
        session.getAsyncRemote().sendText("收到消息:" + msg);
    }

    @OnError
    public void error(String error) {
        System.out.println(error);
    }

}

server:
  port: 8080
  servlet:
    context-path: /caicai

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

@Override
public void afterSingletonsInstantiated() {
    registerEndpoints();
}

protected void registerEndpoints() {
    //收集WebSocket处理类
    Set<Class<?>> endpointClasses = new LinkedHashSet<>();
    if (this.annotatedEndpointClasses != null) {
       endpointClasses.addAll(this.annotatedEndpointClasses);
    }

    ApplicationContext context = getApplicationContext();
    if (context != null) {
       //从容器中找到@ServerEndpoint注解标识的WebSocket处理类
       String[] endpointBeanNames = context.getBeanNamesForAnnotation(ServerEndpoint.class);
       for (String beanName : endpointBeanNames) {
          endpointClasses.add(context.getType(beanName));
       }
    }
	//注册
    for (Class<?> endpointClass : endpointClasses) {
       registerEndpoint(endpointClass);
    }

    if (context != null) {
       Map<String, ServerEndpointConfig> endpointConfigMap = context.getBeansOfType(ServerEndpointConfig.class);
       for (ServerEndpointConfig endpointConfig : endpointConfigMap.values()) {
          registerEndpoint(endpointConfig);
       }
    }
}

private void registerEndpoint(Class<?> endpointClass) {
    ServerContainer serverContainer = getServerContainer();
    Assert.state(serverContainer != null,
          "No ServerContainer set. Most likely the server's own WebSocket ServletContainerInitializer " +
          "has not run yet. Was the Spring ApplicationContext refreshed through a " +
          "org.springframework.web.context.ContextLoaderListener, " +
          "i.e. after the ServletContext has been fully initialized?");
    try {
       if (logger.isDebugEnabled()) {
          logger.debug("Registering @ServerEndpoint class: " + endpointClass);
       }
       //加入容器 
       serverContainer.addEndpoint(endpointClass);
    }
    catch (DeploymentException ex) {
       throw new IllegalStateException("Failed to register @ServerEndpoint class: " + endpointClass, ex);
    }
}