一个null指针引发的思考

场景

首页有个websocket的连接，初始化的时候需要去后台获取数据，然后会定时5s一次获取后端的数据。

代码

SpringBoot入口类

扫描注册到spring中

package com;import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.ApplicationRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;@ComponentScan(basePackages = {com.xxx})//扫描com.xxx及子包
@EnableScheduling//开启定时任务
@SpringBootApplication //启动springboot
public class Application implements ApplicationRunner {public static void main(String[] args) throws Exception {SpringApplication.run(Application.class, args);}@Overridepublic void run(ApplicationArguments args) throws Exception {}}

配置类

package com.xxx.config;import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.socket.server.standard.ServerEndpointExporter;@Configuration
public class WebSocketConfig {@Beanpublic ServerEndpointExporter serverEndpointExporter() {return new ServerEndpointExporter();}
}

websocket类

实现和web前端的交互，获取web后端的数据

package com.xxx.webSocket;import cn.hutool.extra.spring.SpringUtil;
import cn.hutool.json.JSONObject;
import cn.hutool.json.JSONUtil;
import com.xxx.service.IOverviewService;
import lombok.extern.slf4j.Slf4j;
import org.springframework.scheduling.annotation.Scheduled;
import org.springframework.stereotype.Component;import javax.annotation.Resource;
import javax.websocket.*;
import javax.websocket.server.ServerEndpoint;
import java.io.IOException;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Map;
import java.util.Objects;
import java.util.concurrent.ConcurrentHashMap;/*** 首页概况** @author 阿明* @date 2023/11/30-15:39*/
@Slf4j
@ServerEndpoint("/websocket/v3/overview")
@Component
public class WebSocketServerOverview {private static final int CARD = 5;private static final Map<String, DataWrap<OverviewVo>> dataWrapMap = new ConcurrentHashMap<>();@Resourceprivate IOverviewService overviewService;@Scheduled(cron = "*/5 * * * * ? ")public void sendCard() throws InterruptedException {try {for (DataWrap<OverviewVo> value : dataWrapMap.values()) {OverviewVo overviewData = overviewService.card();//new CardVo(totalDatas, cls, toDay, history, totalRules, use, cpu, hardDisk, memory, traffic);value.getSession().getBasicRemote().sendText(JSONUtil.toJsonStr(overviewData));}} catch (IOException e) {log.error("websocket overview错误：", e);}}/*** 连接建立成功调用的方法*/@OnOpenpublic void onOpen(Session session) {log.info("连接成功");}/*** 连接关闭调用的方法*/@OnClosepublic void onClose(Session session) {Iterator<DataWrap<OverviewVo>> iterator = dataWrapMap.values().iterator();while (iterator.hasNext()) {DataWrap<OverviewVo> value = iterator.next();if (value.getSession().equals(session)) {//清除缓存数据 value.getType()iterator.remove();}}log.info("连接关闭");}/*** 收到客户端消息后调用的方法,第一次连接成功，这里会发送一次消息** @param message 客户端发送过来的消息*/@OnMessagepublic void onMessage(String message, Session session) {JSONObject jsonObject = JSONUtil.parseObj(message);//无效消息，避免发送心跳解析失败if (jsonObject.isNull("userId")) {return;}String userId = (String) jsonObject.get("userId");Integer type = (Integer) jsonObject.get("type");Integer dataId = null;if (!jsonObject.isNull("dataId")) {dataId = (Integer) jsonObject.get("dataId");}if (!dataWrapMap.containsKey(userId + "_" + type) || dataWrapMap.containsKey(userId + "_" + type) && !Objects.equals(dataWrapMap.get(userId + "_" + type).getDataId(), dataId)) {DataWrap<OverviewVo> dataWrap = new DataWrap<>(userId, type, dataId, session, new LinkedList<>());try {switch (type) {case CARD:dataWrapMap.put(userId + "_" + type, dataWrap);OverviewVo overviewData = overviewService.card();//overviewService会是null;// OverviewVo overviewData = SpringUtil.getBean(IOverviewService.class).card();session.getBasicRemote().sendText(JSONUtil.toJsonStr(overviewData));break;default:break;}} catch (IOException e) {log.error("websocket overview错误：", e);}}log.info("收到..消息：" + message);}/*** @param session* @param error*/@OnErrorpublic void onError(Session session, Throwable error) {Iterator<DataWrap<OverviewVo>> iterator = dataWrapMap.values().iterator();while (iterator.hasNext()) {DataWrap value = iterator.next();if (value.getSession().equals(session)) {iterator.remove();}}log.error("发生错误", error);}
}

问题

小A问我他项目遇到个问题，前端页面先和后端建立连接，等连接成功后，前端会发送个{“type”:5,“userId”:“admin-uuid”}给到后端，开始的时候用的@Scheduled注解的sendCard方法5s一执行，执行到overviewService正常返回数据，今天测试说，你这个一进来没数据，等5s才有数据，想让首页一进来就有数据，所以他就打算前端发送上来数据直接去查库，返回一次数据，返回的时候OnMessage注解的onMessage方法中overviewService属性一直为空，想不到为啥为null。

排查

• 这个时候的思路肯定是执行的OnMessage注解的onMessage方法没走spring的bean，具体是为啥，开始分析，经过排查发现

问题1：

Tomcat 在 org.apache.tomcat.websocket.server.WsServerContainer 里注册 @ServerEndpoint：

@Override
public void addEndpoint(Class<?> clazz) throws DeploymentException {ServerEndpoint annotation = clazz.getAnnotation(ServerEndpoint.class);if (annotation == null) {throw new DeploymentException("Missing @ServerEndpoint annotation");}ServerEndpointConfig sec = ServerEndpointConfig.Builder.create(clazz, annotation.value()).build();addEndpoint(sec);
}

📌 关键点：

• Tomcat 通过 clazz.getAnnotation(ServerEndpoint.class) 发现 @ServerEndpoint 标注的类
• Tomcat 直接 new 这个类（它不会用 Spring 方式去创建 Bean）
• Spring 的 @Component、@Autowired 机制不会生效

问题2：

其实这个不算问题，算是为什么。

@Scheduled 是由 Spring 管理的，所以 @Autowired 可以用。
❌ @ServerEndpoint 由 Tomcat 创建，不是 Spring Bean，所以 @Autowired 不能用。
💡 如果想在 WebSocket 里用 @Autowired 的 Bean，可以用 static 或 SpringContextUtil 获取。

Spring 类加载器加载的@Scheduled 注解的类，WebSocket 容器类加载器加载的@ServerEndpoint 注解的类。

反思

• 做java的一直用spring，虽然spring帮我们做了很多事，但是也让我们养成了懒惰的思维，习惯了spring的依赖注入，然后想当然的认为都可以直接依赖使用，这种惯性思维导致了这种问题。

解决

方案一：

可以注释掉overviewService使用SpringUtil.getBean(IOverviewService.class)替代，这种直接使用spring的上下文直接取IOverviewService类对象，肯定是取到了，直接用就行。

方案二：

使用SpringConfigurator 来注入 Spring Bean

package com.xxx.autoconfig;import cn.hutool.extra.spring.SpringUtil;
import org.springframework.context.ApplicationContext;import javax.websocket.server.ServerEndpointConfig;public class SpringConfigurator extends ServerEndpointConfig.Configurator {@Overridepublic <T> T getEndpointInstance(Class<T> clazz) throws InstantiationException {ApplicationContext applicationContext = SpringUtil.getApplicationContext();if (applicationContext == null) {throw new InstantiationException("Spring ApplicationContext is null");}return applicationContext.getBean(clazz);}
}

@ServerEndpoint("/websocket/v3/overview")改为
@ServerEndpoint(value = "/websocket/v3/overview", configurator = SpringConfigurator.class)

方案三：

• 使用 @Component + ServerEndpointExporter没管用，证明WebSocket 端点的实例仍然不是由 Spring 管理的，而是由 Tomcat管理的。最终看源码：

  package org.springframework.web.socket.server.standard;
  public class ServerEndpointExporter extends WebApplicationObjectSupportimplements InitializingBean, SmartInitializingSingleton {protected void registerEndpoints() {Set<Class<?>> endpointClasses = new LinkedHashSet<>();if (this.annotatedEndpointClasses != null) {endpointClasses.addAll(this.annotatedEndpointClasses);}ApplicationContext context = getApplicationContext();if (context != null) {String[] endpointBeanNames = context.getBeanNamesForAnnotation(ServerEndpoint.class);for (String beanName : endpointBeanNames) {//这里虽然会执行，但是最终的这个bean是放到tomcat里边了endpointClasses.add(context.getType(beanName));}}for (Class<?> endpointClass : endpointClasses) {registerEndpoint(endpointClass);}if (context != null) {Map<String, ServerEndpointConfig> endpointConfigMap = context.getBeansOfType(ServerEndpointConfig.class);for (ServerEndpointConfig endpointConfig : endpointConfigMap.values()) {registerEndpoint(endpointConfig);}}}
  

  源码主要是这里tomcat-embed-websocket-9.0.39.jar

  package org.apache.tomcat.websocket.server; 
  public class WsServerContainer extends WsWebSocketContainerimplements ServerContainer {void addEndpoint(Class<?> pojo, boolean fromAnnotatedPojo) throws DeploymentException {if (deploymentFailed) {throw new DeploymentException(sm.getString("serverContainer.failedDeployment",servletContext.getContextPath(), servletContext.getVirtualServerName()));}ServerEndpointConfig sec;try {ServerEndpoint annotation = pojo.getAnnotation(ServerEndpoint.class);if (annotation == null) {throw new DeploymentException(sm.getString("serverContainer.missingAnnotation",pojo.getName()));}String path = annotation.value();// Validate encodersvalidateEncoders(annotation.encoders());// ServerEndpointConfig 这里使用了Configurator，这里就是为啥方案二能成功的原因。Class<? extends Configurator> configuratorClazz =annotation.configurator();Configurator configurator = null;if (!configuratorClazz.equals(Configurator.class)) {try {configurator = annotation.configurator().getConstructor().newInstance();} catch (ReflectiveOperationException e) {throw new DeploymentException(sm.getString("serverContainer.configuratorFail",annotation.configurator().getName(),pojo.getClass().getName()), e);}}//这里直接拿到这个类从tomcat生成了sec = ServerEndpointConfig.Builder.create(pojo, path).decoders(Arrays.asList(annotation.decoders())).encoders(Arrays.asList(annotation.encoders())).subprotocols(Arrays.asList(annotation.subprotocols())).configurator(configurator).build();} catch (DeploymentException de) {failDeployment();throw de;}addEndpoint(sec, fromAnnotatedPojo);}
  

方案四：

• 见网上好多用@EnableWebSocket的我是没成功，去官网找的用@EnableWebSocketMessageBroker这种更好，格式更规范

• 用spring自己的websocketGetting Started | Using WebSocket to build an interactive web application

• git clone https://github.com/spring-guides/gs-messaging-stomp-websocket.git

接受实体类HelloMessage

package com.example.messagingstompwebsocket;public class HelloMessage {private String name;public HelloMessage() {}public HelloMessage(String name) {this.name = name;}public String getName() {return name;}public void setName(String name) {this.name = name;}
}

发送实体类Greeting

package com.example.messagingstompwebsocket;public class Greeting {private String content;public Greeting() {}public Greeting(String content) {this.content = content;}public String getContent() {return content;}}

控制层GreetingController

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 delayreturn new Greeting("Hello, " + HtmlUtils.htmlEscape(message.getName()) + "!");}}

配置类WebSocketConfig

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 {@Overridepublic void configureMessageBroker(MessageBrokerRegistry config) {config.enableSimpleBroker("/topic");//配置订阅消息的目标前缀config.setApplicationDestinationPrefixes("/app");//配置应用程序的目的地前缀}@Overridepublic void registerStompEndpoints(StompEndpointRegistry registry) {registry.addEndpoint("/gs-guide-websocket");//配置端点}}

启动类MessagingStompWebsocketApplication

package com.example.messagingstompwebsocket;import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;@SpringBootApplication
public class MessagingStompWebsocketApplication {public static void main(String[] args) {SpringApplication.run(MessagingStompWebsocketApplication.class, args);}
}

主页面index.html

<!DOCTYPE html>
<html>
<head><title>Hello WebSocket</title><link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous"><link href="/main.css" rel="stylesheet"><script src="https://code.jquery.com/jquery-3.1.1.min.js"></script><script src="https://cdn.jsdelivr.net/npm/@stomp/stompjs@7.0.0/bundles/stomp.umd.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 beingenabled. Please enableJavascript 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>

app.js

const stompClient = new StompJs.Client({brokerURL: 'ws://localhost:8080/gs-guide-websocket'
});stompClient.onConnect = (frame) => {setConnected(true);console.log('Connected: ' + frame);stompClient.subscribe('/topic/greetings', (greeting) => {showGreeting(JSON.parse(greeting.body).content);});
};stompClient.onWebSocketError = (error) => {console.error('Error with websocket', error);
};stompClient.onStompError = (frame) => {console.error('Broker reported error: ' + frame.headers['message']);console.error('Additional details: ' + frame.body);
};function setConnected(connected) {$("#connect").prop("disabled", connected);$("#disconnect").prop("disabled", !connected);if (connected) {$("#conversation").show();}else {$("#conversation").hide();}$("#greetings").html("");
}function connect() {stompClient.activate();
}function disconnect() {stompClient.deactivate();setConnected(false);console.log("Disconnected");
}function sendName() {stompClient.publish({destination: "/app/hello",body: JSON.stringify({'name': $("#name").val()})});
}function showGreeting(message) {$("#greetings").append("<tr><td>" + message + "</td></tr>");
}$(function () {$("form").on('submit', (e) => e.preventDefault());$( "#connect" ).click(() => connect());$( "#disconnect" ).click(() => disconnect());$( "#send" ).click(() => sendName());
});

main.css

body {background-color: #f5f5f5;
}#main-content {max-width: 940px;padding: 2em 3em;margin: 0 auto 20px;background-color: #fff;border: 1px solid #e5e5e5;-webkit-border-radius: 5px;-moz-border-radius: 5px;border-radius: 5px;
}

启动项目

访问 http://localhost:8080/
点击connect
What is your name?输入名字点击send

Greetings下显示响应结果