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在当今互联的世界中，网络编程已成为Java开发者必备的技能之一。无论是构建微服务架构、实现实时通信应用，还是开发分布式系统，网络编程都扮演着核心角色。本文将带你走进Java网络编程的世界，从基础概念到实际应用，一步步构建你的网络编程能力。

一、网络编程基础概念

1.1 TCP vs UDP

• TCP：面向连接、可靠传输、数据顺序保证

• UDP：无连接、快速、可能丢包

1.2 核心概念

• IP地址：设备的网络标识

• 端口号：应用程序的通信端点（0-65535）

• Socket：网络通信的端点抽象

二、简单的TCP客户端/服务器示例

2.1 基础TCP服务器

import java.io.*; import java.net.*; public class SimpleTCPServer { public static void main(String[] args) { final int PORT = 8888; try (ServerSocket serverSocket = new ServerSocket(PORT)) { System.out.println("服务器启动，监听端口: " + PORT); while (true) { // 等待客户端连接 Socket clientSocket = serverSocket.accept(); System.out.println("客户端连接: " + clientSocket.getInetAddress()); // 处理客户端请求 handleClient(clientSocket); } } catch (IOException e) { e.printStackTrace(); } } private static void handleClient(Socket clientSocket) { try (BufferedReader in = new BufferedReader( new InputStreamReader(clientSocket.getInputStream())); PrintWriter out = new PrintWriter( clientSocket.getOutputStream(), true)) { String request; while ((request = in.readLine()) != null) { System.out.println("收到: " + request); out.println("服务器回复: " + request.toUpperCase()); } } catch (IOException e) { System.out.println("客户端断开连接"); } } }

2.2 基础TCP客户端

import java.io.*; import java.net.*; public class SimpleTCPClient { public static void main(String[] args) { final String SERVER_ADDRESS = "localhost"; final int PORT = 8888; try (Socket socket = new Socket(SERVER_ADDRESS, PORT); BufferedReader in = new BufferedReader( new InputStreamReader(socket.getInputStream())); PrintWriter out = new PrintWriter( socket.getOutputStream(), true); BufferedReader userInput = new BufferedReader( new InputStreamReader(System.in))) { System.out.println("已连接到服务器"); String userInputLine; while ((userInputLine = userInput.readLine()) != null) { out.println(userInputLine); System.out.println("服务器回复: " + in.readLine()); if ("exit".equalsIgnoreCase(userInputLine)) { break; } } } catch (UnknownHostException e) { System.err.println("未知主机: " + SERVER_ADDRESS); } catch (IOException e) { System.err.println("IO异常: " + e.getMessage()); } } }

三、多线程服务器实现

单线程服务器无法处理并发连接，让我们升级为多线程版本：

import java.io.*; import java.net.*; import java.util.concurrent.*; public class MultiThreadedServer { private static final int PORT = 8888; private static final ExecutorService threadPool = Executors.newCachedThreadPool(); public static void main(String[] args) { try (ServerSocket serverSocket = new ServerSocket(PORT)) { System.out.println("多线程服务器启动，端口: " + PORT); while (true) { Socket clientSocket = serverSocket.accept(); threadPool.execute(new ClientHandler(clientSocket)); } } catch (IOException e) { e.printStackTrace(); } finally { threadPool.shutdown(); } } static class ClientHandler implements Runnable { private final Socket clientSocket; public ClientHandler(Socket socket) { this.clientSocket = socket; } @Override public void run() { try (BufferedReader in = new BufferedReader( new InputStreamReader(clientSocket.getInputStream())); PrintWriter out = new PrintWriter( clientSocket.getOutputStream(), true)) { out.println("欢迎连接到服务器！当前线程: " + Thread.currentThread().getName()); String inputLine; while ((inputLine = in.readLine()) != null) { if ("exit".equalsIgnoreCase(inputLine)) { out.println("再见!"); break; } out.println("回声: " + inputLine); } } catch (IOException e) { System.out.println("客户端异常断开"); } finally { try { clientSocket.close(); } catch (IOException e) { e.printStackTrace(); } } } } }

四、UDP编程示例

import java.net.*; import java.io.*; public class UDPExample { // UDP 服务器 static class UDPServer { public static void main(String[] args) throws IOException { DatagramSocket socket = new DatagramSocket(8888); byte[] buffer = new byte[1024]; System.out.println("UDP服务器启动..."); while (true) { DatagramPacket packet = new DatagramPacket(buffer, buffer.length); socket.receive(packet); String received = new String(packet.getData(), 0, packet.getLength()); System.out.println("收到: " + received); // 发送响应 String response = "UDP响应: " + received.toUpperCase(); byte[] responseData = response.getBytes(); DatagramPacket responsePacket = new DatagramPacket( responseData, responseData.length, packet.getAddress(), packet.getPort() ); socket.send(responsePacket); } } } // UDP 客户端 static class UDPClient { public static void main(String[] args) throws IOException { DatagramSocket socket = new DatagramSocket(); InetAddress address = InetAddress.getByName("localhost"); BufferedReader userInput = new BufferedReader( new InputStreamReader(System.in)); while (true) { System.out.print("输入消息: "); String message = userInput.readLine(); // 发送消息 byte[] sendData = message.getBytes(); DatagramPacket sendPacket = new DatagramPacket( sendData, sendData.length, address, 8888); socket.send(sendPacket); // 接收响应 byte[] receiveData = new byte[1024]; DatagramPacket receivePacket = new DatagramPacket( receiveData, receiveData.length); socket.receive(receivePacket); String response = new String(receivePacket.getData(), 0, receivePacket.getLength()); System.out.println("服务器响应: " + response); if ("exit".equalsIgnoreCase(message)) { break; } } socket.close(); } } }

五、实战：简单的聊天室服务器

import java.io.*; import java.net.*; import java.util.*; import java.util.concurrent.*; public class ChatRoomServer { private static final int PORT = 9999; private static final Set<ClientHandler> clients = ConcurrentHashMap.newKeySet(); public static void main(String[] args) { ExecutorService pool = Executors.newCachedThreadPool(); try (ServerSocket serverSocket = new ServerSocket(PORT)) { System.out.println("聊天室服务器启动，端口: " + PORT); while (true) { Socket clientSocket = serverSocket.accept(); ClientHandler client = new ClientHandler(clientSocket); clients.add(client); pool.execute(client); } } catch (IOException e) { e.printStackTrace(); } } static class ClientHandler implements Runnable { private Socket socket; private PrintWriter out; private String clientName; public ClientHandler(Socket socket) { this.socket = socket; } @Override public void run() { try { out = new PrintWriter(socket.getOutputStream(), true); BufferedReader in = new BufferedReader( new InputStreamReader(socket.getInputStream())); // 获取客户端名称 out.println("请输入你的名字:"); clientName = in.readLine(); broadcast(clientName + " 加入了聊天室"); String message; while ((message = in.readLine()) != null) { if ("/exit".equalsIgnoreCase(message)) { break; } broadcast(clientName + ": " + message); } } catch (IOException e) { System.out.println(clientName + " 异常断开"); } finally { try { socket.close(); } catch (IOException e) { e.printStackTrace(); } clients.remove(this); broadcast(clientName + " 离开了聊天室"); } } private void broadcast(String message) { System.out.println(message); for (ClientHandler client : clients) { if (client != this) { client.out.println(message); } } } } }

六、最佳实践与注意事项

6.1 资源管理

• 始终使用try-with-resources确保资源关闭

• 注意处理各种IO异常

6.2 性能考虑

• 使用线程池管理连接

• 考虑使用NIO处理大量并发连接

• 合理设置缓冲区大小

6.3 安全性

• 验证输入数据

• 防止缓冲区溢出

• 使用SSL/TLS进行加密通信

七、下一步学习方向

1. Java NIO：学习非阻塞IO，处理高并发场景

2. Netty框架：企业级网络应用框架

3. HTTP客户端：Java 11+的HttpClient API

4. WebSocket：实现实时双向通信

5. RPC框架：如gRPC、Dubbo等

结语

Java网络编程是一个既基础又强大的技能。从简单的Socket编程开始，逐步深入到多线程、NIO，再到使用成熟的网络框架，这个过程充满挑战但也极具价值。掌握网络编程不仅能让你更好地理解网络通信的本质，还能为构建分布式系统打下坚实基础。