How does Java Websocket implement real-time map display function?

How does Java Websocket implement real-time map display function?

Real-time map display function plays an important role in many real-time applications. For example, common applications such as taxi locating applications, applications for tracking transportation materials, and social applications for sharing locations in real time all require real-time map display capabilities. To implement these real-time map display functions, we can use Java Websocket technology to easily build a real-time server to implement these functions.

Java Websocket allows us to establish a real-time, two-way, persistent connection between the server and the client. This allows us to create a real-time data channel that can flow data between the server and the client. Using Java Websockets, we can update the node locations on the client's map screen in real time and move them to the correct location. Below we will introduce how to use Java Websocket to build a real-time map display function and provide some sample code.

Step 1: Establish a WebSocket server

We can quickly establish a WebSocket server using the WebSocket API provided in Java. In this example, we will use the Jetty WebSocket API to provide sample code. The following steps will guide you how to set up a WebSocket server:

1. Import the following Maven dependencies:

<dependency>
    <groupId>org.eclipse.jetty.websocket</groupId>
    <artifactId>websocket-server</artifactId>
    <version>9.4.1.v20170120</version>
</dependency>
<dependency>
    <groupId>org.eclipse.jetty.websocket</groupId>
    <artifactId>websocket-servlet</artifactId>
    <version>9.4.1.v20170120</version>
</dependency>

2. Create a WebSocket server class:

import org.eclipse.jetty.server.Server;
import org.eclipse.jetty.server.handler.ResourceHandler;
import org.eclipse.jetty.servlet.ServletContextHandler;
import org.eclipse.jetty.servlet.ServletHolder;
import org.eclipse.jetty.websocket.servlet.WebSocketServletFactory;

public class WebSocketServer {

    public static void main(String[] args) throws Exception {
        // 建立服务器
        Server server = new Server(8080);

        // 设置静态资源处理器
        ResourceHandler resourceHandler = new ResourceHandler();
        resourceHandler.setDirectoriesListed(true);
        resourceHandler.setResourceBase("web");

        // 设置WebSocketServlet处理器
        ServletContextHandler contextHandler = new ServletContextHandler(ServletContextHandler.SESSIONS);
        contextHandler.setContextPath("/");
        server.setHandler(contextHandler);
        ServletHolder holder = new ServletHolder("echo", WebSocketServlet.class);
        contextHandler.addServlet(holder, "/echo/*");
        holder.setInitParameter("maxIdleTime", "60000");
        WebSocketServletFactory factory = holder.getServletFactory();
        factory.register(MyWebSocketHandler.class);

        server.start();
        server.join();
    }
}

3. Create a WebSocket processing class:

import org.eclipse.jetty.websocket.api.Session;
import org.eclipse.jetty.websocket.api.annotations.*;
import java.io.IOException;

@WebSocket
public class MyWebSocketHandler {

    // 打开WebSocket连接时调用
    @OnWebSocketConnect
    public void onConnect(Session session) {
        System.out.println("连接成功: " + session.getRemoteAddress().getAddress());
    }

    // 关闭WebSocket连接时调用
    @OnWebSocketClose
    public void onClose(Session session, int statusCode, String reason) {
        System.out.println("断开连接: " + session.getRemoteAddress().getAddress());
    }

    // 接收WebSocket消息时调用
    @OnWebSocketMessage
    public void onMessage(Session session, String message) throws IOException {
        System.out.println("接收到消息: " + message);
        session.getRemote().sendString(message);
    }
}

The above is an example of a simple Jetty WebSocket server. When the client connects to the server, the server outputs a connection success message. When a client disconnects from a server, the server also outputs a disconnect message. When the server receives a message from the client, it sends the same message back to the client.

Step 2: Transmit map data to the client

When we receive the latest map data, we need to transmit the data to the client in order to update the map in real time. This can be achieved with the following code:

// 将地图数据转换为JSON格式
String mapData = "{"nodes":[{"id":1,"x":0.1,"y":0.1},{"id":2,"x":0.5,"y":0.5}],"edges":[]}";
// 向所有WebSocket终端发送地图消息
for (Session session : sessions) {
    if (session.isOpen()) {
        session.getRemote().sendString(mapData);
    }
}

In this code, we convert the map data into JSON format and send it to all open WebSocket endpoints.

Step 3: Display the map on the client

When we receive the latest map data sent by the server, we need to use JavaScript code to display it on the client. Please see the following sample code:

<!DOCTYPE html>
<html>
<head>
    <title>实时地图展示</title>
    <meta charset="UTF-8"/>
    <style>
        #container {
            width: 800px;
            height: 600px;
            position: relative;
            margin: auto;
            border: 1px solid black;
            overflow: hidden;
        }
        .node {
            width: 20px;
            height: 20px;
            border-radius: 10px;
            position: absolute;
            background-color: red;
        }
    </style>
</head>
<body>
<div id="container">
</div>
<script>
    var nodes = [];
    var edges = [];
    var nodeMap = new Map();
    var edgeMap = new Map();
    // 创建WebSocket
    var webSocket = new WebSocket("ws://localhost:8080/echo");

    // 监听WebSocket打开事件
    webSocket.onopen = function (event) {
        console.log("连接成功");
    };

    // 监听WebSocket消息事件
    webSocket.onmessage = function (event) {
        console.log("接收到消息: " + event.data);
        var data = JSON.parse(event.data);
        nodes = data.nodes;
        edges = data.edges;
        drawMap();
    };

    // 绘制地图节点
    function drawMap() {
        var container = document.getElementById("container");
        for (var i = 0; i < nodes.length; i++) {
            var node = nodes[i];
            var div = document.createElement("div");
            div.classList.add("node");
            div.style.left = (node.x * container.clientWidth - 10) + "px";
            div.style.top = (node.y * container.clientHeight - 10) + "px";
            nodeMap[node.id] = div;
            container.appendChild(div);
        }
    }
</script>
</body>
</html>

In this example, we create a WebSocket object and listen to its open and message events. When we receive the map data via WebSocket, we will draw the map nodes into the HTML DOM. The code for drawing map nodes uses JavaScript to calculate the positions of all nodes and place them within the display area.

Conclusion

Java WebSocket technology can realize the real-time map display function very easily. By setting up a WebSocket server and using the Jetty WebSocket API, we can establish a real-time, two-way, persistent connection. Once the connection is established, we can flow data between the server and client. By converting map data into JSON format and sending it to all open WebSocket endpoints, we enable clients to update map node locations in real time. Through JavaScript code, we can display it on the client. This article provides some simple sample code for reference.

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