网络编程基础：连接Java的秘密网络

1 网络编程的重要性

网络编程允许Java应用程序与其他计算机或设备进行通信。这包括从简单的数据传输到复杂的分布式系统和Web服务。

2 Java网络编程的核心类

Java提供了多个类来支持网络编程：

InetAddress：表示网络上的IP地址。
URL：表示统一资源定位符，用于访问网络资源。
URLConnection：处理从URL指定的网络资源的通信。
Socket：用于TCP网络通信。
ServerSocket：等待客户端连接的服务器端Socket。
DatagramSocket：用于UDP网络通信。
DatagramPacket：包含发送或接收数据的UDP数据包。

3 创建客户端-服务器应用

Java网络编程通常涉及创建客户端和服务器应用程序，它们通过Socket进行通信。

示例代码（服务器端）：

import java.io.*;
import java.net.*;public class Server {public static void main(String[] args) throws IOException {ServerSocket serverSocket = new ServerSocket(6789);System.out.println("Server is listening on port 6789");try (Socket socket = serverSocket.accept()) {InputStreamReader input = new InputStreamReader(socket.getInputStream());BufferedReader bufRead = new BufferedReader(input);PrintWriter output = new PrintWriter(socket.getOutputStream(), true);String line;while ((line = bufRead.readLine()) != null) {System.out.println("Client: " + line);output.println("Server: " + line);}}}
}

示例代码（客户端）：

import java.io.*;
import java.net.*;public class Client {public static void main(String[] args) throws IOException {Socket socket = new Socket("localhost", 6789);InputStreamReader input = new InputStreamReader(socket.getInputStream());BufferedReader bufRead = new BufferedReader(input);PrintWriter output = new PrintWriter(socket.getOutputStream(), true);BufferedReader stdInput = new BufferedReader(new InputStreamReader(System.in));String line;while ((line = stdInput.readLine()) != null) {output.println(line);System.out.println("Server: " + bufRead.readLine());}}
}

4 UDP通信

UDP是一种无连接的协议，适用于需要快速传输但不一定要求可靠到达的场景。

示例代码（UDP服务器）：

import java.net.*;public class UDPServer {public static void main(String[] args) throws Exception {DatagramSocket serverSocket = new DatagramSocket(9876);byte[] receiveData = new byte[1024];DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);serverSocket.receive(receivePacket);String sentence = new String(receivePacket.getData(), 0, receivePacket.getLength());System.out.println("RECEIVED: " + sentence);serverSocket.close();}
}

示例代码（UDP客户端）：

import java.net.*;public class UDPClient {public static void main(String[] args) throws Exception {DatagramSocket clientSocket = new DatagramSocket();String sentence = "Hello from client";byte[] buf = sentence.getBytes();InetAddress IPAddress = InetAddress.getByName("localhost");DatagramPacket sendPacket = new DatagramPacket(buf, buf.length, IPAddress, 9876);clientSocket.send(sendPacket);clientSocket.close();}
}

5 多线程网络服务器

为了处理多个客户端，服务器通常需要使用多线程。

示例代码（多线程服务器）：

import java.io.*;
import java.net.*;public class MultiThreadedServer {public static void main(String[] args) throws IOException {ServerSocket serverSocket = new ServerSocket(6789);while (true) {Socket clientSocket = serverSocket.accept();new ClientHandler(clientSocket).start();}}private static class ClientHandler extends Thread {private Socket clientSocket;public ClientHandler(Socket socket) {this.clientSocket = socket;}public void run() {try {InputStreamReader input = new InputStreamReader(clientSocket.getInputStream());BufferedReader bufRead = new BufferedReader(input);PrintWriter output = new PrintWriter(clientSocket.getOutputStream(), true);String line;while ((line = bufRead.readLine()) != null) {System.out.println("Client: " + line);output.println("Server: " + line);}} catch (IOException e) {e.printStackTrace();} finally {try {clientSocket.close();} catch (IOException e) {e.printStackTrace();}}}}
}