Java 网络编程实战：Socket、ServerSocket 与 TCP/UDP 通信

Java java.net 包提供底层网络通信支持，核心协议：TCP（面向连接、可靠、流式）和 UDP（无连接、数据报、不可靠但低延迟）。

TCP Socket 编程模型

服务端流程

1. ServerSocket server = new ServerSocket(port); 绑定端口监听
2. Socket client = server.accept(); 阻塞等待客户端连接
3. 通过 client.getInputStream()/getOutputStream() 通信
4. 处理完关闭 client.close()，循环 accept 下一个

客户端流程

1. Socket socket = new Socket(host, port); 连接服务端
2. 获取流进行读写
3. socket.close() 断开

完整 Echo 服务端

public class EchoServer {
    public static void main(String[] args) throws IOException {
        try (ServerSocket server = new ServerSocket(8888)) {
            System.out.println("服务端启动，监听 8888...");
            while (true) {
                try (Socket client = server.accept();
                     BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream(), StandardCharsets.UTF_8));
                     PrintWriter out = new PrintWriter(new OutputStreamWriter(client.getOutputStream(), StandardCharsets.UTF_8), true)) {
                    
                    String line;
                    while ((line = in.readLine()) != null) {
                        System.out.println("收到: " + line);
                        out.println("Echo: " + line); // autoFlush=true
                        if ("bye".equalsIgnoreCase(line)) break;
                    }
                } catch (IOException e) {
                    System.err.println("客户端异常: " + e.getMessage());
                }
            }
        }
    }
}

对应客户端

public class EchoClient {
    public static void main(String[] args) throws IOException {
        try (Socket socket = new Socket("localhost", 8888);
             BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream(), StandardCharsets.UTF_8));
             PrintWriter out = new PrintWriter(new OutputStreamWriter(socket.getOutputStream(), StandardCharsets.UTF_8), true);
             BufferedReader console = new BufferedReader(new InputStreamReader(System.in))) {
            
            String input;
            while ((input = console.readLine()) != null) {
                out.println(input);
                System.out.println("服务端回复: " + in.readLine());
                if ("bye".equalsIgnoreCase(input)) break;
            }
        }
    }
}

核心类 API 速查

ServerSocket

Socket

InetAddress

InetAddress local = InetAddress.getLocalHost(); // 本机地址
InetAddress google = InetAddress.getByName("www.google.com"); // DNS 解析
System.out.println(google.getHostAddress()); // IP 字符串

UDP 编程

基于 DatagramSocket 和 DatagramPacket，无连接、面向数据报。

UDP 服务端

try (DatagramSocket server = new DatagramSocket(9999)) {
    byte[] buf = new byte[1024];
    DatagramPacket packet = new DatagramPacket(buf, buf.length);
    while (true) {
        server.receive(packet); // 阻塞接收
        String msg = new String(packet.getData(), 0, packet.getLength(), StandardCharsets.UTF_8);
        System.out.println("来自 " + packet.getAddress() + ":" + packet.getPort() + " -> " + msg);
        
        // 回复
        byte[] reply = ("ACK: " + msg).getBytes(StandardCharsets.UTF_8);
        DatagramPacket replyPacket = new DatagramPacket(reply, reply.length, packet.getAddress(), packet.getPort());
        server.send(replyPacket);
    }
}

UDP 客户端

try (DatagramSocket client = new DatagramSocket()) {
    InetAddress serverAddr = InetAddress.getByName("localhost");
    String msg = "Hello UDP";
    byte[] buf = msg.getBytes(StandardCharsets.UTF_8);
    DatagramPacket packet = new DatagramPacket(buf, buf.length, serverAddr, 9999);
    client.send(packet);
    
    // 接收回复
    byte[] replyBuf = new byte[1024];
    DatagramPacket reply = new DatagramPacket(replyBuf, replyBuf.length);
    client.receive(reply);
    System.out.println("服务端回复: " + new String(reply.getData(), 0, reply.getLength(), StandardCharsets.UTF_8));
}

TCP vs UDP 选型

高并发服务端架构演进

1. 单线程阻塞：一次处理一个，吞吐极低
2. 多线程/线程池：每连接一线程，C10K 问题（线程开销大）
3. NIO 非阻塞：Selector + ServerSocketChannel，单线程管理多连接
4. Netty 框架：生产级 NIO 框架，解决粘包/半包、内存管理、编解码
5. 虚拟线程：Java 21+ Thread.ofVirtual()，轻量级线程，阻塞式写法达成高并发

最佳实践

• 显式指定字符集（UTF-8），避免平台默认编码差异
• 使用 try-with-resources 确保 Socket/流关闭
• 设置合理超时（连接超时、读超时），防止死锁
• 应用层协议设计：定长头部 + 长度字段 或 分隔符/换行符，解决 TCP 粘包
• 心跳机制检测死连接
• 优雅关闭：半关闭（shutdownOutput()）发送 FIN，继续接收对端数据