承接前文springboot情操陶冶-web配置(三)，本文将在DispatcherServlet应用的基础上谈下websocket的使用
websocket
websocket的简单了解可见维基百科WebSocket，在笔者看来其大多数应用在web浏览器上用于与服务端的持续性通信，并大多用于接收服务器的推送内容
简单例子
spring很友好的向我们展示了如何在springboot上整合websocket，并给出了一个hello例子。读者可参照官方例子走一遍便可对websocket有一定的了解。附上官方部分源码
Controller响应层
package hello;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()) + "!");
}}
客户端HTML界面
<!DOCTYPE html>
<html>
<head>
<title>Hello WebSocket</title>
<link href="/webjars/bootstrap/css/bootstrap.min.css" rel="stylesheet">
<link href="/main.css" rel="stylesheet">
<script src="/webjars/jquery/jquery.min.js"></script>
<script src="/webjars/sockjs-client/sockjs.min.js"></script>
<script src="/webjars/stomp-websocket/stomp.min.js"></script>
<script src="/app.js"></script>
</head>
<body>
<noscript><h2 style="color: #ff0000">Seems your browser doesn't support Javascript! Websocket relies on Javascript being
enabled. Please enable
Javascript and reload this page!</h2></noscript>
<div id="main-content" class="container">
<div class="row">
<div class="col-md-6">
<form class="form-inline">
<div class="form-group">
<label for="connect">WebSocket connection:</label>
<button id="connect" class="btn btn-default" type="submit">Connect</button>
<button id="disconnect" class="btn btn-default" type="submit" disabled="disabled">Disconnect
</button>
</div>
</form>
</div>
<div class="col-md-6">
<form class="form-inline">
<div class="form-group">
<label for="name">What is your name?</label>
<input type="text" id="name" class="form-control" placeholder="Your name here...">
</div>
<button id="send" class="btn btn-default" type="submit">Send</button>
</form>
</div>
</div>
<div class="row">
<div class="col-md-12">
<table id="conversation" class="table table-striped">
<thead>
<tr>
<th>Greetings</th>
</tr>
</thead>
<tbody id="greetings">
</tbody>
</table>
</div>
</div>
</div>
</body>
</html>
在阅读完官方的demo例子之后，读者务必再去阅览下WebSocket在springboot的基本概念>>>Web on Servlet Stack。虽然文档很长，但还是需要理解下其的工作原理，大致上和rabbitmq类似，采取的是订阅推送的模式。
源码层分析
笔者优先关注下@EnableWebSocketMessageBroker注解，其用于开启websocket服务
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(DelegatingWebSocketMessageBrokerConfiguration.class)
public @interface EnableWebSocketMessageBroker {}
由上可知，引入的DelegatingWebSocketMessageBrokerConfiguration类用于加载websocket的相关配置。
本文不进行详细的源码分析，笔者则会根据上图的原理图寻找在springboot中的配置，这样应该会起到事半功倍的效果。
RequestChannel和ResponseChannel
请求与响应处理通道
@Bean
public AbstractSubscribableChannel clientInboundChannel() {
ExecutorSubscribableChannel channel = new ExecutorSubscribableChannel(clientInboundChannelExecutor());
// 拦截器，用户也可通过WebSocketMessageBrokerConfigurer接口增加拦截器
ChannelRegistration reg = getClientInboundChannelRegistration();
if (reg.hasInterceptors()) {
channel.setInterceptors(reg.getInterceptors());
}
return channel;
}protected final ChannelRegistration getClientInboundChannelRegistration() {
if (this.clientInboundChannelRegistration == null) {
ChannelRegistration registration = new ChannelRegistration();
// 加载请求通道，也可新增拦截器
configureClientInboundChannel(registration);
registration.interceptors(new ImmutableMessageChannelInterceptor());
this.clientInboundChannelRegistration = registration;
}
return this.clientInboundChannelRegistration;
}@Bean
public AbstractSubscribableChannel clientOutboundChannel() {
ExecutorSubscribableChannel channel = new ExecutorSubscribableChannel(clientOutboundChannelExecutor());
// 拦截器，用户也可通过WebSocketMessageBrokerConfigurer接口增加拦截器
ChannelRegistration reg = getClientOutboundChannelRegistration();
if (reg.hasInterceptors()) {
channel.setInterceptors(reg.getInterceptors());
}
return channel;
}
不管是请求通道还是响应通道代码一模一样，使用的均是ExecutorSubscribableChannel，其内部整合了拦截器和线程池。此类基本是所有channel的公用类，笔者稍微看下里面有什么小花头
No.1 消息处理ExecutorSubscribableChannel
@Override
public boolean sendInternal(Message<?> message, long timeout) {
// 消息处理者遍历
for (MessageHandler handler : getSubscribers()) {
// 统一由SendTask类处理消息
SendTask sendTask = new SendTask(message, handler);
if (this.executor == null) {
sendTask.run();
}
else {
this.executor.execute(sendTask);
}
}
return true;
}
这里的消息处理者有直接处理注解的，也有直接返回给BorkerRelay的，读者可自行去查阅
No.2 消息任务SendTask
@Override
public void run() {
Message<?> message = this.inputMessage;
try {
// 应用拦截器
message = applyBeforeHandle(message);
if (message == null) {
return;
}
// 通过messageHandler来处理消息
this.messageHandler.handleMessage(message);
triggerAfterMessageHandled(message, null);
}
catch (Exception ex) {
triggerAfterMessageHandled(message, ex);
if (ex instanceof MessagingException) {
throw (MessagingException) ex;
}
String description = "Failed to handle " + message + " to " + this + " in " + this.messageHandler;
throw new MessageDeliveryException(message, description, ex);
}
catch (Throwable err) {
String description = "Failed to handle " + message + " to " + this + " in " + this.messageHandler;
MessageDeliveryException ex2 = new MessageDeliveryException(message, description, err);
triggerAfterMessageHandled(message, ex2);
throw ex2;
}
}
由此可得出通用的SendTask只是个消息中转器，最终的消息处理均是由MessageHandler来解决的。看来处理消息的路由核心得继续往下文分析了
注解方式消息处理器MessageHandler
即解析@MessageMapping/@SendTo等websocket注解的方法，其类似于MVC的@RequestMapping等注解。可见SimpAnnotationMethodMessageHandler类
@Override
protected SimpAnnotationMethodMessageHandler createAnnotationMethodMessageHandler() {
// 请求和响应通道、broker消息模板
return new WebSocketAnnotationMethodMessageHandler(
clientInboundChannel(), clientOutboundChannel(), brokerMessagingTemplate());
}
上述代码中的broker消息模板主要通过brokerChannel通道将注解方法返回的值经过订阅关系处理后再传入响应通道。笔者此处对WebSocketAnnotationMethodMessageHandler作下分步骤的解析
No.1 入参解析器
protected List<HandlerMethodArgumentResolver> initArgumentResolvers() {
ApplicationContext context = getApplicationContext();
ConfigurableBeanFactory beanFactory = (context instanceof ConfigurableApplicationContext ?
((ConfigurableApplicationContext) context).getBeanFactory() : null);List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>();// @Header和@Headers参数注解解析
resolvers.add(new HeaderMethodArgumentResolver(this.conversionService, beanFactory));
resolvers.add(new HeadersMethodArgumentResolver());
// @DestinationVariable注解参数解析
resolvers.add(new DestinationVariableMethodArgumentResolver(this.conversionService));// 读取Principal类型的参数,读取的为头部的simpUser属性
resolvers.add(new PrincipalMethodArgumentResolver());
// 读取Message类型的参数
resolvers.add(new MessageMethodArgumentResolver(this.messageConverter));resolvers.addAll(getCustomArgumentResolvers());//用户自定义，可扩展
// @Payload注解的参数解析
resolvers.add(new PayloadArgumentResolver(this.messageConverter, this.validator));return resolvers;
}
No.2 反参解析器
@Override
protected List<? extends HandlerMethodReturnValueHandler> initReturnValueHandlers() {
List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>();// Single-purpose return value types
handlers.add(new ListenableFutureReturnValueHandler());
handlers.add(new CompletableFutureReturnValueHandler());// @SendTo和@SendToUser注解解析器
SendToMethodReturnValueHandler sendToHandler =
new SendToMethodReturnValueHandler(this.brokerTemplate, true);
if (this.headerInitializer != null) {
sendToHandler.setHeaderInitializer(this.headerInitializer);
}
handlers.add(sendToHandler);// @SubscribeMapping注解解析器
SubscriptionMethodReturnValueHandler subscriptionHandler =
new SubscriptionMethodReturnValueHandler(this.clientMessagingTemplate);
subscriptionHandler.setHeaderInitializer(this.headerInitializer);
handlers.add(subscriptionHandler);// custom return value types
handlers.addAll(getCustomReturnValueHandlers());// 默认处理
sendToHandler = new SendToMethodReturnValueHandler(this.brokerTemplate, false);
sendToHandler.setHeaderInitializer(this.headerInitializer);
handlers.add(sendToHandler);return handlers;
}
No.3 HandlerMethod对象创建
@Override
public void afterPropertiesSet() {
// 入参和反参解析器配置
if (this.argumentResolvers.getResolvers().isEmpty()) {
this.argumentResolvers.addResolvers(initArgumentResolvers());
}if (this.returnValueHandlers.getReturnValueHandlers().isEmpty()) {
this.returnValueHandlers.addHandlers(initReturnValueHandlers());
}ApplicationContext context = getApplicationContext();
if (context == null) {
return;
}
for (String beanName : context.getBeanNamesForType(Object.class)) {
if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
Class<?> beanType = null;
try {
beanType = context.getType(beanName);
}
catch (Throwable ex) {
// An unresolvable bean type, probably from a lazy bean - let's ignore it.
if (logger.isDebugEnabled()) {
logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex);
}
}
// 查找被@Controller注解下修饰的带有@MessageMapping和@SubscribeMapping注解的方法集合并存放至handlerMethods映射集合中
if (beanType != null && isHandler(beanType)) {
detectHandlerMethods(beanName);
}
}
}
}
主要是搜索带有@MessageMapping和@SubscribeMapping注解的方法注册至MessageHandler对象中的handlerMethods属性，方便后续对请求的路由
No.4 请求通道注册SimpAnnotationMethodMessageHandler处理类
@Override
public final void start() {
synchronized (this.lifecycleMonitor) {
// 请求通道注入此处理器
this.clientInboundChannel.subscribe(this);
this.running = true;
}
}
针对注解方式的消息路由处理我们基本知道了，那这个针对websocket的发过来的请求如何被路由至相应的HandlerMethod中呢？
HandlerMapping
笔者此处找寻下针对websocket的请求的路由
@Bean
public HandlerMapping stompWebSocketHandlerMapping() {
WebSocketHandler handler = decorateWebSocketHandler(subProtocolWebSocketHandler());
WebMvcStompEndpointRegistry registry = new WebMvcStompEndpointRegistry(
handler, getTransportRegistration(), messageBrokerTaskScheduler());
ApplicationContext applicationContext = getApplicationContext();
if (applicationContext != null) {
registry.setApplicationContext(applicationContext);
}
registerStompEndpoints(registry);
// 返回的是AbstractUrlHandlerMapping的继承类
return registry.getHandlerMapping();
}
具体的读者可查询源码，内容还是很多的，笔者只知道上述代码返回的是AbstractUrlHandlerMapping的继承类，其存储的urlMap中的key值为websocket的端点路径，比如/ws-demo/**，而value值则是HttpRequestHandler接口的实现类，其主要处理基于HTTP的websocket请求。
@FunctionalInterface
public interface HttpRequestHandler {/**
* Process the given request, generating a response.
* @param request current HTTP request
* @param response current HTTP response
* @throws ServletException in case of general errors
* @throws IOException in case of I/O errors
*/
void handleRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException;}
感兴趣的读者可进行深入的研究，其中有包括对ajax/sockJs/handshake等的支持。
消息流
主要是由AbstractHttpReceivingTransportHandler接收客户端的请求，然后通过StompSubProtocolHandler类将消息发送至ExecutorSubscribableChannel，由其调用sendInternal()方法遍历注册的MessageHandlers，由后者去真正的处理消息并回包。具体的代码逻辑还是很复杂的，笔者此处罗列下注解方式的处理以及响应给客户端的消息处理
No.1 注解消息处理AbstractMethodMessageHandler
@Override
public void handleMessage(Message<?> message) throws MessagingException {
// 获取目的地址
String destination = getDestination(message);
if (destination == null) {
return;
}
// 确保请求的发过来的地址是指定的前缀，否则消息就会被直接丢弃
String lookupDestination = getLookupDestination(destination);
if (lookupDestination == null) {
return;
}MessageHeaderAccessor headerAccessor = MessageHeaderAccessor.getMutableAccessor(message);
headerAccessor.setHeader(DestinationPatternsMessageCondition.LOOKUP_DESTINATION_HEADER, lookupDestination);
headerAccessor.setLeaveMutable(true);
message = MessageBuilder.createMessage(message.getPayload(), headerAccessor.getMessageHeaders());if (logger.isDebugEnabled()) {
logger.debug("Searching methods to handle " +
headerAccessor.getShortLogMessage(message.getPayload()) +
", lookupDestination='" + lookupDestination + "'");
}
// 找寻注解进行相应的方法响应
handleMessageInternal(message, lookupDestination);
headerAccessor.setImmutable();
}
此处需要注意的是请求的路径前缀必须是指定的前缀，此前缀可通过WebSocketMessageBrokerConfigurer#configureMessageBroker()方法来设置，如下
@Override
public void configureMessageBroker(MessageBrokerRegistry registry) {
// response destination prefix
registry.enableSimpleBroker("/topic");
// request destination prefix
registry.setApplicationDestinationPrefixes("/app");
}
No.2 注解消息响应处理SimpleBrokerMessageHandler
@Override
protected void handleMessageInternal(Message<?> message) {
MessageHeaders headers = message.getHeaders();
SimpMessageType messageType = SimpMessageHeaderAccessor.getMessageType(headers);
String destination = SimpMessageHeaderAccessor.getDestination(headers);
String sessionId = SimpMessageHeaderAccessor.getSessionId(headers);updateSessionReadTime(sessionId);// 此处确保回包的路径是以指定的BrokerPath作为前缀，否则则会被丢弃，配置同上
if (!checkDestinationPrefix(destination)) {
return;
}// 针对消息的发送，会根据多个订阅者进行广播发送
if (SimpMessageType.MESSAGE.equals(messageType)) {
logMessage(message);
sendMessageToSubscribers(destination, message);
}
// 连接请求响应
else if (SimpMessageType.CONNECT.equals(messageType)) {
....
}
}
// 关闭请求响应
else if (SimpMessageType.DISCONNECT.equals(messageType)) {
....
}
// 订阅请求响应
else if (SimpMessageType.SUBSCRIBE.equals(messageType)) {
...
}
// 取消订阅请求响应
else if (SimpMessageType.UNSUBSCRIBE.equals(messageType)) {
....
}
}
No.3 消息响应处理StompBrokerRelayMessageHandler，其作为真实的处理响应的出处
@Override
protected void handleMessageInternal(Message<?> message) {
String sessionId = SimpMessageHeaderAccessor.getSessionId(message.getHeaders());.....
.....// 回包路径，默认以用户设定的BrokerPath为前缀;不满足就将包丢弃
String destination = stompAccessor.getDestination();
if (command != null && command.requiresDestination() && !checkDestinationPrefix(destination)) {
return;
}// 连接请求
if (StompCommand.CONNECT.equals(command)) {
if (logger.isDebugEnabled()) {
logger.debug(stompAccessor.getShortLogMessage(EMPTY_PAYLOAD));
}
stompAccessor = (stompAccessor.isMutable() ? stompAccessor : StompHeaderAccessor.wrap(message));
stompAccessor.setLogin(this.clientLogin);
stompAccessor.setPasscode(this.clientPasscode);
if (getVirtualHost() != null) {
stompAccessor.setHost(getVirtualHost());
}
StompConnectionHandler handler = new StompConnectionHandler(sessionId, stompAccessor);
this.connectionHandlers.put(sessionId, handler);
this.stats.incrementConnectCount();
Assert.state(this.tcpClient != null, "No TCP client available");
this.tcpClient.connect(handler);
}
// 关闭请求
else if (StompCommand.DISCONNECT.equals(command)) {
StompConnectionHandler handler = this.connectionHandlers.get(sessionId);
if (handler == null) {
if (logger.isDebugEnabled()) {
logger.debug("Ignoring DISCONNECT in session " + sessionId + ". Connection already cleaned up.");
}
return;
}
stats.incrementDisconnectCount();
handler.forward(message, stompAccessor);
}
else {
StompConnectionHandler handler = this.connectionHandlers.get(sessionId);
if (handler == null) {
if (logger.isDebugEnabled()) {
logger.debug("No TCP connection for session " + sessionId + " in " + message);
}
return;
}
// 直接调用连接返回，内含sessionId以及订阅者id等等
handler.forward(message, stompAccessor);
}
}
小结
先了解websocket的原理，然后再结合源码加深对原理的理解，这便是了解一个新技术的必要步骤。笔者此处针对官方的例子作以下小贴士
1.配置websocket的请求响应前缀以及端点配置，务必实现WebSocketMessageBrokerConfigurer接口
@Configuration
@EnableWebSocketMessageBroker
public class WebsocketConfig implements WebSocketMessageBrokerConfigurer { @Override
public void registerStompEndpoints(StompEndpointRegistry registry) {
// register web socket contextPath and allow any origin
registry.addEndpoint("/ws-demo").setAllowedOrigins("*").withSockJS();
} @Override
public void configureMessageBroker(MessageBrokerRegistry registry) {
// response destination prefix
registry.enableSimpleBroker("/topic");
// request destination prefix
registry.setApplicationDestinationPrefixes("/app");
}
}
2.针对回包处理时，一般我们需要指定路径，如果采用注解方式，默认情况下@SendTo不指定的时候，会采用用户设置的回包路径前缀，比如@MessageMapping("/app/hello")–>/topic/hello。
当然用户也可以采用SimpMessageTemplate#convertAndSend()方法直接发送至指定的回包路径
3.客户端采用sockJs相关API时，其支持通过HTTP/HTTPS协议连接指定的websocket端点，但是务必在订阅或者发送消息的时候，指定的目的地址必须以/为开头，否则发送不成功
4.客户端采用sockJs时，针对发起的subscribe请求作如下总结
// 当服务端采取@SubscribeMapping注解时，则会对/app/subscribe直接请求有返回值
stompClient.subscribe('/app/subscribe', function (greeting) {
showGreeting(JSON.parse(greeting.body));
});// 当服务端没有采取@SubsribeMapping注解时，下述代码则实现对/topic/subscribe的消息接收
stompClient.subscribe('/topic/subscribe', function (greeting) {
showGreeting(JSON.parse(greeting.body).content);
});
5.本文的例子读者也可访问该地址获取，建议了解原理再去阅读源码会事半功倍的
