WebSocket 实现消息推送

使用 WebSocket 实现消息推送是一个非常流行的方案，它允许服务器和客户端之间建立持久的双向通信通道。以下是如何在一个典型的 Java Spring Boot 项目中使用 WebSocket 实现消息推送的详细步骤。

1. 添加依赖

在 pom.xml 文件中添加 WebSocket 和 Spring Boot 相关的依赖：

<dependencies><!-- Spring Boot Starter WebSocket --><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-websocket</artifactId></dependency>
</dependencies>

2. 配置 WebSocket

创建一个 WebSocket 配置类，配置 WebSocket 的端点和处理器：

import org.springframework.context.annotation.Configuration;
import org.springframework.web.socket.config.annotation.EnableWebSocket;
import org.springframework.web.socket.config.annotation.WebSocketConfigurer;
import org.springframework.web.socket.config.annotation.WebSocketHandlerRegistry;@Configuration
@EnableWebSocket
public class WebSocketConfig implements WebSocketConfigurer {@Overridepublic void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {registry.addHandler(new MyWebSocketHandler(), "/ws/my-endpoint").setAllowedOrigins("*"); // 配置允许跨域访问}
}

3. 实现 WebSocket 处理器

创建一个 WebSocket 处理器来处理消息的发送和接收：

import org.springframework.web.socket.WebSocketSession;
import org.springframework.web.socket.handler.TextWebSocketHandler;
import org.springframework.web.socket.TextMessage;
import org.springframework.web.socket.WebSocketMessage;
import java.io.IOException;public class MyWebSocketHandler extends TextWebSocketHandler {@Overridepublic void handleTextMessage(WebSocketSession session, TextMessage message) throws IOException {String payload = message.getPayload();// 处理接收到的消息（例如：记录日志、处理业务逻辑等）System.out.println("Received message: " + payload);// 发送消息到客户端session.sendMessage(new TextMessage("Message received: " + payload));}@Overridepublic void afterConnectionEstablished(WebSocketSession session) throws Exception {System.out.println("WebSocket connection established: " + session.getId());}@Overridepublic void afterConnectionClosed(WebSocketSession session, CloseStatus status) throws Exception {System.out.println("WebSocket connection closed: " + session.getId());}
}

4. 创建 WebSocket 客户端

在前端页面中使用 JavaScript 创建 WebSocket 客户端，并与服务器进行通信。以下是一个简单的 HTML 和 JavaScript 示例：

<!DOCTYPE html>
<html>
<head><title>WebSocket Example</title>
</head>
<body><h1>WebSocket Example</h1><textarea id="messages" cols="100" rows="10" readonly></textarea><br><input type="text" id="messageInput" placeholder="Enter your message" /><button onclick="sendMessage()">Send</button><script>var socket = new WebSocket('ws://localhost:8080/ws/my-endpoint');socket.onopen = function(event) {console.log('WebSocket connection established');};socket.onmessage = function(event) {var messages = document.getElementById('messages');messages.value += 'Server: ' + event.data + '\n';};socket.onclose = function(event) {console.log('WebSocket connection closed');};function sendMessage() {var messageInput = document.getElementById('messageInput');var message = messageInput.value;socket.send(message);messageInput.value = '';}</script>
</body>
</html>

5. 推送消息到客户端

在服务器端推送消息到客户端的一个例子是使用 @Scheduled 注解定期发送消息：

import org.springframework.scheduling.annotation.Scheduled;
import org.springframework.stereotype.Component;
import org.springframework.web.socket.TextMessage;
import org.springframework.web.socket.WebSocketSession;
import java.io.IOException;
import java.util.Collection;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;@Component
public class WebSocketPushService {private final Map<String, WebSocketSession> sessions = new ConcurrentHashMap<>();public void addSession(WebSocketSession session) {sessions.put(session.getId(), session);}public void removeSession(WebSocketSession session) {sessions.remove(session.getId());}@Scheduled(fixedRate = 5000) // 每5秒推送一次public void pushMessage() {String message = "Current Time: " + System.currentTimeMillis();Collection<WebSocketSession> sessionCollection = sessions.values();for (WebSocketSession session : sessionCollection) {try {session.sendMessage(new TextMessage(message));} catch (IOException e) {e.printStackTrace();}}}
}

6. 更新 WebSocket 处理器

将 WebSocket 处理器更新为能够处理连接和断开连接事件，并将客户端会话保存到 WebSocketPushService：

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.socket.WebSocketSession;
import org.springframework.web.socket.handler.TextWebSocketHandler;
import org.springframework.web.socket.TextMessage;public class MyWebSocketHandler extends TextWebSocketHandler {@Autowiredprivate WebSocketPushService pushService;@Overridepublic void afterConnectionEstablished(WebSocketSession session) throws Exception {System.out.println("WebSocket connection established: " + session.getId());pushService.addSession(session);}@Overridepublic void afterConnectionClosed(WebSocketSession session, CloseStatus status) throws Exception {System.out.println("WebSocket connection closed: " + session.getId());pushService.removeSession(session);}@Overridepublic void handleTextMessage(WebSocketSession session, TextMessage message) throws IOException {String payload = message.getPayload();System.out.println("Received message: " + payload);session.sendMessage(new TextMessage("Message received: " + payload));}
}