How can we achieve message push functionality in Java?

To enable message push functionality, you can utilize WebSocket technology in Java. WebSocket is a bidirectional communication protocol that can deliver real-time push messages to clients.

Firstly, you need to import WebSocket libraries in your Java project, such as Spring WebSocket or Java-WebSocket.

Next, create a WebSocket server-side using either Spring Boot or a standalone Java class. On the server side, it is necessary to define a WebSocket handler class to handle incoming messages and changes in connection status. You can inherit from the WebSocketHandler class and override methods such as onOpen, onClose, onMessage, etc.

In the processor class, you can define a static Set to store all WebSocket connections, which is used to send messages to all connected clients. You can use ConcurrentHashMap to achieve thread-safe Set.

Here is a simple example:

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

import org.springframework.web.socket.CloseStatus;
import org.springframework.web.socket.TextMessage;
import org.springframework.web.socket.WebSocketHandler;
import org.springframework.web.socket.WebSocketMessage;
import org.springframework.web.socket.WebSocketSession;

public class MyWebSocketHandler implements WebSocketHandler {
    private static final Set<WebSocketSession> sessions = Collections.synchronizedSet(new HashSet<>());

    @Override
    public void afterConnectionEstablished(WebSocketSession session) throws Exception {
        sessions.add(session);
    }

    @Override
    public void handleMessage(WebSocketSession session, WebSocketMessage<?> message) throws Exception {
        // 处理收到的消息
        String receivedMessage = (String) message.getPayload();
        System.out.println("Received message: " + receivedMessage);
        
        // 向所有连接发送消息
        for (WebSocketSession s : sessions) {
            s.sendMessage(new TextMessage("Server: " + receivedMessage));
        }
    }

    @Override
    public void afterConnectionClosed(WebSocketSession session, CloseStatus closeStatus) throws Exception {
        sessions.remove(session);
    }

    @Override
    public void handleTransportError(WebSocketSession session, Throwable exception) throws Exception {
        // 处理连接错误
    }

    @Override
    public boolean supportsPartialMessages() {
        return false;
    }
}

Finally, configure the address of the WebSocket server and call the WebSocket API in the client to connect to the server. In the client side, you can use the WebSocket object in JavaScript to receive and send messages.

This is just a simple implementation example, which would need to be modified and expanded according to specific business requirements.