WebSockets and real-time communication in Java network programming

WebSocket is a two-way communication protocol that allows a low-latency, persistent connection between a web client and a server and is widely used to build real-time communication applications such as chat and online games. In Java, you can use the WebSocket API or a third-party library such as Spring WebSocket. Establishing a connection using the WebSocket API involves the following steps: establishing an HTTP connection, handshake, and upgrading to WebSocket. A common use case is building multi-player chat applications where the server endpoint handles messages via a Servlet and clients can connect to the server via JavaScript WebSocket objects and send and receive messages.

WebSocket and real-time communication in Java network programming

WebSocket is a two-way full-duplex communication protocol that allows web clients to Establish a low-latency, persistent connection between the client and the server. It is widely used to build real-time communication applications such as chat, real-time collaboration, and online gaming.

Introduction to WebSocket Protocol

WebSocket is a protocol that runs on top of an HTTP-based protocol (usually WebSocket). Establishing a WebSocket connection involves the following steps:

1. Establishing an HTTP connection: The client sends an HTTP GET request with the Upgrade: websocket header.
2. Handshake: The server returns a 101 Switching Protocols response, which contains the WebSocket key.
3. Upgrade to WebSocket: The client sends an HTTP response with a WebSocket key to upgrade to a WebSocket connection.

Using WebSocket in Java

To use WebSocket in Java, you can leverage the Java WebSocket API or a third-party library such as Spring WebSocket. This section explores examples of using the WebSocket API.

// 创建 WebSocket 服务器端点
WebSocketServer ws = new WebSocketServer() {

    @Override
    public void onOpen(WebSocket websocket, ClientEndpointConfig cec) {
        System.out.println("客户端已连接");
    }

    @Override
    public void onMessage(String message, WebSocket websocket) {
        System.out.println("收到客户端消息：" + message);
    }

    @Override
    public void onClose(WebSocket websocket) {
        System.out.println("客户端已断开连接");
    }

    @Override
    public void onError(WebSocket websocket, Throwable cause) {
        System.err.println("WebSocket 发生错误：" + cause.getMessage());
    }
};

ws.start();

// 创建 WebSocket 客户端端点
WebSocketClient wsClient = new WebSocketClient() {

    @Override
    public void onOpen(ServerEndpointConfig sec) {
        System.out.println("连接到服务器");
    }

    @Override
    public void onMessage(String message, Session session) {
        System.out.println("收到服务器消息：" + message);
    }

    @Override
    public void onClose(Session session, CloseReason closeReason) {
        System.out.println("连接已关闭");
    }

    @Override
    public void onError(Session session, WebSocketError error) {
        System.err.println("WebSocket 发生错误：" + error.getMessage());
    }
};

wsClient.open("ws://localhost:8080/websocket");

Practical case: Chat application

Using WebSocket, you can easily build a multi-person chat application. Here is an example:

// 服务器端：
@WebServlet("/chat")
public class ChatServlet extends HttpServlet {

    private Set<WebSocket> webSockets = new HashSet<>();

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) {
        String message = req.getParameter("message");
        for (WebSocket ws : webSockets) {
            ws.sendString(message);
        }
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setAttribute("ws", new WebSocket(req, resp));
        req.getRequestDispatcher("/chat.jsp").forward(req, resp);
    }

    public void onOpen(WebSocket socket) {
        webSockets.add(socket);
    }

    public void onClose(WebSocket socket) {
        webSockets.remove(socket);
    }
}

// 客户端：
<script type="text/javascript">
    var ws = new WebSocket("ws://localhost:8080/chat");

    ws.onopen = function() {
        console.log("连接已建立");
    };

    ws.onmessage = function(event) {
        console.log("收到消息：" + event.data);
    };

    ws.onclose = function() {
        console.log("连接已关闭");
    };

    function sendMessage() {
        var message = document.getElementById("message").value;
        ws.send(message);
    }
</script>

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