javase socket

### Java SE中的Socket编程

Java SE提供了强大的网络通信功能，其中`java.net.Socket`类和`java.net.ServerSocket`类用于实现TCP/IP协议下的客户端和服务端之间的连接[^1]。

#### 创建服务器端套接字
服务端通过创建`ServerSocket`对象监听特定端口等待来自客户端的请求：

import java.io.*;
import java.net.*;

public class EchoServer {
    public static void main(String[] args) throws IOException {
        int port = 7;
        try (ServerSocket serverSocket = new ServerSocket(port)) {
            System.out.println("EchoServer started on port " + port);
            
            while(true){
                Socket clientSocket = serverSocket.accept();
                handleClient(clientSocket);
            }
        }
    }

    private static void handleClient(Socket socket) throws IOException{
        BufferedReader in = null;
        PrintWriter out = null;

        try {
            in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            out = new PrintWriter(socket.getOutputStream(), true);

            String inputLine;
            while ((inputLine = in.readLine()) != null) {
                if (".".equals(inputLine))
                    break;
                out.println(inputLine);
            }
        } finally {
            if(out!=null)out.close();
            if(in!=null)in.close();
            socket.close();
        }
    }
}

此代码片段展示了如何设置一个简单的回声服务器，它接收并返回任何发送给它的消息直到收到`.`字符为止。

#### 客户端套接字建立连接
为了与上述的服务端交互，在客户端可以利用`Socket`来发起连接尝试，并读取响应数据流：

import java.io.*;
import java.net.*;

public class EchoClient {

    public static void main(String[] args) throws IOException {
        
        String hostName = "localhost";
        int portNumber = 7;

        try (
            Socket echoSocket = new Socket(hostName, portNumber);
            PrintWriter out = new PrintWriter(echoSocket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(
                new InputStreamReader(echoSocket.getInputStream()));
            BufferedReader stdIn = new BufferedReader(
                new InputStreamReader(System.in))
        ) {
            String userInput;
            while ((userInput = stdIn.readLine()) != null) {
                out.println(userInput);
                System.out.println("echo: " + in.readLine());
            }
        } catch (UnknownHostException e) {
            System.err.println("Don't know about host " + hostName);
            System.exit(1);
        } catch (IOException e) {
            System.err.println("Couldn't get I/O for the connection to " +
                               hostName);
            System.exit(1);
        }
    }
}

这段程序实现了向指定主机名和端口号发出连接请求的功能，并能够持续输入字符串至远程服务器直至手动终止进程。