WebSocket原理及应用
简介及原理
WebSocket是一种基于HTTP的长连接技术，能够实现客户端和服务端的双向通信，可用来构建实时应用。
因为HTTP本身是基于TCP连接的，所以，WebSocket在HTTP协议的基础上做了一个简单的升级，即建立TCP连接后，浏览器发送请求时，附带几个头：
GET /chat HTTP/1.1
Host: www.example.com
Upgrade：websocket
Connection: Upgrade
这就表示客户端希望升级连接，变成长连接的WebSocket，服务器返回升级成功的响应：
HTTP/1.1. 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
收到成功响应后表示WebSocket“握手”成功，这样，代表WebSocket的这个TCP连接不会被服务器关闭，而是一直保持，服务器可随时向浏览器推送消息，浏览器也可随时向服务器推送消息。
双方推送的消息既可以是文本消息，也可以是二进制消息，一般来说，绝大部分应用程序会推送基于JSON的文本消息。
WebSocket连接建立之后，通过该连接传输的数据报文会小很多，因为此类报文中只包含数据不包含请求头。
Java中的具体代码实现
WebSocket采用CS架构，服务端集成WebSocket之后，会成为Server，浏览器则作为Client。
在Java中，可以通过在项目pom文件中添加spring-websocket或者spring-boot-starter-websocket依赖，来集成WebSocket。
集成WebSocket之后的服务端，会暴漏出一个URL，如下述代码中的/chat，客户端使用ws协议访问该URL，即可与服务端建立连接。
@Bean
WebSocketConfigurer createWebSocketConfigurer(
@Autowired ChatHandler chatHandler,
@Autowired ChatHandshakeInterceptor chatInterceptor)
{
return new WebSocketConfigurer() {
public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
// 把URL与指定的WebSocketHandler关联，可关联多个:
registry.addHandler(chatHandler, "/chat").addInterceptors(chatInterceptor);
}
};
}
每一个WebSocket连接，会产生一个WebSocketSession，每个Session拥有唯一ID。和WebSocket相关的数据，如用户名等，均可放入关联的getAttributes()中。
@Component
public class ChatHandler extends TextWebSocketHandler {
// 保存所有Client的WebSocket会话实例:
private Map<String, WebSocketSession> clients = new ConcurrentHashMap<>();
/**
* 连接建立后
*/
@Override
public void afterConnectionEstablished(WebSocketSession session) throws Exception {
// 新会话根据ID放入Map:
clients.put(session.getId(), session);
session.getAttributes().put("name", "Guest1");
}
/**
* 连接断开后
*/
@Override
public void afterConnectionClosed(WebSocketSession session, CloseStatus status) throws Exception {
clients.remove(session.getId());
}
}
用实例clients持有当前所有的WebSocketSession是为了广播，即向所有用户推送同一消息时，可以这么写：
String json = ...
TextMessage message = new TextMessage(json);
for (String id : clients.keySet()) {
WebSocketSession session = clients.get(id);
session.sendMessage(message);
}
如果想将消息推送给部分用户，则需要过滤出这些用户的Session，然后再发送消息。
接收消息的示例代码如下：
@Component
public class ChatHandler extends TextWebSocketHandler {
...
@Override
protected void handleTextMessage(WebSocketSession session, TextMessage message) throws Exception {
String s = message.getPayload();
String r = ... // 根据输入消息构造待发送消息
broadcastMessage(r); // 推送给所有用户
}
}
STOMP Over WebSocket
STOMP：Simple (or Streaming) Text Oriented Message Protocol, formely known as TTMP, is a simple text-based protocol, designed for working with message-origented middleware (MOM). It provides an interoperable with format that allows STOMP clients to talk with any message borker supporting the protocol.
Overview
The protocol is broadly similar to HTTP, and works over TCP using the following commands:
- CONNECT
- SEND
- SUBSCRIBE
- UNSUBSCRIBE
- BEGIN
- COMMIT
- ABORT
- ACK
- NACK
- DISCONNECT
Communication between client and server is through a “frame” consisting of a number of lines. The first line contains the command, followed by headers in the form : (one per line), followed by a blank line and then the body content, ending in a null character. Communication between server and client is through a MESSAGE, RECEIPT or ERROR frame with a similar format of headers and body content.
Example
SEND
destination:/queue/a
content-type:text/plain
hello queue a
^@
Implementations
These are some MOM products that support STOMP:
- Apache ActiveMQ,
- Fuse Message Broker
- HornetQ
- Open Message Queue (OpenMQ)
- RabbitMQ (message broker, has support for STOMP)
- syslog-ng through its STOMP destination plugin
A list of implementations is also maintained on the STOMP web site.
STOMP is also supported by the Spring Framework in module org.springframework:spring-websocket. [1]
External links
- STOMP website
Spring集成STOMP
-
引入依赖
dependency> <groupId>org.springframework.bootgroupId> <artifactId>spring-boot-starter-websocketartifactId> dependency> <dependency> <groupId>org.webjarsgroupId> <artifactId>stomp-websocketartifactId> <version>2.3.3version> dependency> -
配置类
package com.example.messagingstompwebsocket; import org.springframework.context.annotation.Configuration; import org.springframework.messaging.simp.config.MessageBrokerRegistry; import org.springframework.web.socket.config.annotation.EnableWebSocketMessageBroker; import org.springframework.web.socket.config.annotation.StompEndpointRegistry; import org.springframework.web.socket.config.annotation.WebSocketMessageBrokerConfigurer; @Configuration @EnableWebSocketMessageBroker public class WebSocketConfig implements WebSocketMessageBrokerConfigurer { @Override public void configureMessageBroker(MessageBrokerRegistry config) { // 服务端给客户端推送消息时，客户端订阅的destination前缀 config.enableSimpleBroker("/topic"); // 客户端给服务端发送消息时，服务端的destination前缀，配合注解MessageMapping使用 config.setApplicationDestinationPrefixes("/app"); } @Override public void registerStompEndpoints(StompEndpointRegistry registry) { // 客户端和服务端建立WebSocket连接的endpoint registry.addEndpoint("/gs-guide-websocket").withSockJS(); } } -
收发消息controller
package com.example.messagingstompwebsocket; import org.springframework.messaging.handler.annotation.MessageMapping; import org.springframework.messaging.handler.annotation.SendTo; import org.springframework.stereotype.Controller; import org.springframework.web.util.HtmlUtils; @Controller public class GreetingController { @MessageMapping("/hello") @SendTo("/topic/greetings") public Greeting greeting(HelloMessage message) throws Exception { Thread.sleep(1000); // simulated delay return new Greeting("Hello, " + HtmlUtils.htmlEscape(message.getName()) + "!"); } }配合配置类，客户端通过destination：/app/hello 发送消息给服务端，服务端则推送消息给订阅destination：/topic/greetings 的客户端。